ADDIS ABABA UNIVERSITY
SCHOOL OF GRADUATE STUDIES
DEPARTMENT OF EARTH SCIENCES
GROUNDWATER POTENTIAL INVESTIGATION OF
UPPER WABE RIVER CATCHMENT,
SOUTH EASTERN CENTRAL ETHIOPIA
Bokoji
38.8
38.9
39
39.1
39.2
39.3
820000
39.4
39.5
39.6
39.7
39.8
39.9
Meraro
7.4
Gimbite
810000
7.3
800000
Shashamane
7.2
790000
Asassa
be
Wa
er
Riv
Dinsho
Kofele
780000
7.1
Adaba
Dodola
7
770000
6.9
760000
750000
6.8
0
20000
40000
60000
460000 470000 480000 490000 500000 510000 520000 530000 540000 550000 560000 570000
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR
THE DEGREE OF MASTERS OF SCIENCE IN HYDROGEOLOGY
HABTAMU GIZACHEW DEMISSIE
June 2009
GROUNDWATER POTENTIAL INVESTIGATION OF
UPPER WABE RIVER CATCHMENT,
SOUTH EASTERN CENTRAL ETHIOPIA
A THESIS SUBMITTED TO
THE SCHOOL OF GRADUATE STUDIES
ADDIS ABABA UNIVERSITY
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR
THE DEGREE OF MASTERS OF SCIENCE IN HYDROGEOLOGY
BY
HABTAMU GIZACHEW DEMISSIE
JUNE 2009
Addis Ababa University
School of Graduate Studies
Groundwater Potential Investigation of
Upper Wabe River Catchment
South Eastern Central Ethiopia
By
Habtamu Gizachew Demissie
Department of Earth Sciences
Approved by Board of Examiners:-
Dr. Balemwal Atnafu
____________________
Chairman
Dr. Seifu Kebede
____________________
Adivisor
Dr. Tenalem Ayenew
___________________
Examiner
Dr. Dereje Ayalew
Examiner
____________________
ACKNOWLEDGMENT
I gratefully acknowledge Oromia Water Works Construction Enterprise for allowing me to
pursue my postgraduate studies, assisting me by providing financial and material resources and
giving me long leave of absence during my study.
I am deeply indebted to my instructor and advisor Dr.Seifu Kebede for his supervision and
unreserved guidance, valuable suggestions and comments he has provided me throughout my
research.
My deepest gratitude goes to my instructor Dr.Tenalem Ayenew for his kind support,
encouragement and the knowledge he has shared me through his nice teaching method since
my under graduate and post graduate study. I also would like to thank my co-advisor
Dr.Gezahegn Yirgu for his valuable consultation and providing me important reference
material.
Immeasurable and special thanks deserve to my long and all time friend Ato Kassahun Aberra
who his support will never forget for his advice, encouragement and provision of important
reference materials.
I gratefully acknowledge the following colleagues: Ato Aberra Taye, Ato Tesfaye Wube, Ato
Elias Mitiku, Ato Tesfaye Bogale and also to all whose name is not mentioned here for their
insightful discussions, technical assistance, general sharing of wisdom and ideas, their
encouragement and material support. I also wish to extend my appreciation to all my
classmates for their support, socialization and help each other. I thankfully acknowledge
Oromia Water Works Design and Supervision Enterprise, the National Meteorological Service
Agency, the Ministry of Water Resources, Ethiopian Institute of Geological Survey, for their
unreserved and kind cooperation in delivering me the most valuable data. I also would like to
forward my heart felt thanks to all staffs of Earth Sciences of Addis Ababa University.
Finally, I would like to thank my wife W/ro Aberrash W/Tsadik, my son Kiya Habtamu, my
mother W/ro Asegedech Legesse and my brothers and sisters for their love and moral support
which gave me great strength to carry out the work.
i
ABSTRACT
The highlands falling within the project area represent part of the southeastern central
Ethiopian plateau including Arsi, Bale and parts of southwestern margin of the project area
also extend to adjoining areas of Sidama Zone in South Nations, Nationalities and Peoples
Regional State. The Wabe river originates just east of Wetera Resa Village in South Nations,
Nationalities and Peoples Regional State.
All the study area is covered by Tertiary and Quaternary volcanics. Tertiary volcanics of the
pre-rift and post-pift cover large part of the area. The Pre-Rift succession represented by Alaji
basalt, alkali trachyte flows, and alkali trachyte and basalt flows and the Post-Rift volcanic
succession including alkaline to peralkaline basalts and trachytes. The Quaternary Volcanics
represented by Ginir Volcanics and basalt, often scoriaceous, with minor cinder cones and
vitric tuffs. These volcanic rocks are topped by Plio-Pleistocene fluvio-lacustrine sedimentary
sequence. Most central part of the area covered by Post-Rrift succession of the Nazarath group
volcanic rocks.
Groundwater recharge is estimated by applying three methods; base flow separation, water
balance and soil moisture balance methods. Drainage-area ratio extrapolating method between
the drainage area of the gauged sub-catchment and ungauged sub catchment is employed to
estimate the discharge of the river at the delineated mouth of the river. Recharge estimated by
flow separation method is more than seven fold greater than recharge estimated by soil
moisture and water balance methods.
The composition of the majority of the highland volcanic plateau is silicate minerals of mostly
plagioclase feldspars of the albite and anorthite group and pyroxene composition. These
minerals are rich in Ca, Mg and Na. Hydrolysis, decomposition and/or leaching of these
silicate minerals enriches the water in the highlands by Ca, Mg, and Na cations. In the study
area most water types obtained from laboratory analyses have low TDS and based on cation
composition Ca-Na-HCO3 is the dominant water type followed by Na-Ca-HCO3 evolving
down the flow path to dominantly Na-Ca-HCO3 water. Concerning water quality criteria for
drinking and irrigation purposes the water from the area fits the standard quality.
ii
TABLE OF CONTENTS
CONTENTS
PAGE
ACKNOWLEDGMENT ............................................................................................................I
ABSTRACT............................................................................................................................... II
CHAPTER ONE ........................................................................................................................ 1
INTRODUCTION ................................................................................................................... 1
1.1) General ......................................................................................................................... 1
1.2) Previous Works ............................................................................................................ 2
1.3) Objective of the study................................................................................................... 4
1.4) Methodology ................................................................................................................ 5
CHAPTER TWO ....................................................................................................................... 6
DESCRIPTION OF THE STUDY AREA ............................................................................... 6
2.1) Location, Arial extent and Accessibility ...................................................................... 6
2.2) Population..................................................................................................................... 8
2.3) Climate ......................................................................................................................... 8
2.4) Physiography ................................................................................................................ 8
2.5) Land use and Land cover............................................................................................ 11
2.7) Drainage pattern ......................................................................................................... 15
2.8) Geology and Structure................................................................................................ 16
2.8.1) Regional Geology................................................................................................ 16
2.8.2) Local Geology ..................................................................................................... 19
2.8.2.1) Cenozoic Rocks............................................................................................ 19
2.8.2.2) Tertiary Volcanics ........................................................................................ 19
2.8.3) Quaternary Volcanics and Sediments.................................................................. 22
2.8.3.1) Quaternary Volcanics ................................................................................... 22
2.8.3.2) Quaternary Sediments .................................................................................. 22
2.8.3.3) Structures...................................................................................................... 25
CHAPTER THREE................................................................................................................. 26
RECHARGE ESTIMATION................................................................................................. 26
3.1) Introduction ................................................................................................................ 26
3.2) Precipitation............................................................................................................... 27
3.3) Eastimation of Evapotranspiration (ET)..................................................................... 34
3.3.1.) Common hydrometeorological factors affecting evapotranspiration................. 34
3.3.1.1) Temperature.................................................................................................. 34
3.3.1.2) Relative Humidity ........................................................................................ 36
3.3.1.3) Wind speed ................................................................................................... 37
Figure 3.7:-Monthly average wind speed .................................................................. 38
3.3.1.4) Sunshine hours ............................................................................................ 38
3.3.2) Estimation of evapotranspiration (ET) ................................................................ 39
3.3.2.1) Potential evapotranspiration(PET) ............................................................... 39
3.3.3) Actual evapotranspiration(AET) ......................................................................... 43
3.4) Ground Water Recharge Estimation........................................................................... 47
iii
3.4.1) Recharge Estimation from soil moisture balance approach ................................ 47
3.4.2) Recharge Estimation from water balance method............................................... 47
3.4.3) Recharge Estimation from Base Flow Separation............................................... 48
CHAPTER FOUR ................................................................................................................... 51
HYDROGEOLOGY.............................................................................................................. 51
4.1) General ....................................................................................................................... 51
4.2) Hydrogeologic units and groundwater movement ..................................................... 51
4.2.1) Aquifer types and their Yield .............................................................................. 54
4.3) Ground water flow Conceptual Model....................................................................... 61
CHAPTER FIVE ..................................................................................................................... 69
HYDROGEOCHEMISTRY.................................................................................................. 69
5.1) General ....................................................................................................................... 69
5.1.1) pH, EC and TDS.................................................................................................. 70
5.2) Graphical presentation, Classification and interpretation of Analytical Results of
laboratory measured parameters ........................................................................................ 73
5.3) Major ion evolutions and their Controlling Factors ................................................... 75
5.3.1) Calcium, Magnesium and Sodium ...................................................................... 75
5.3.2) HCO3-, SO4=, Cl-, F-, and NO3- ............................................................................ 79
5.4) Water Quality Criteria ................................................................................................ 80
CHAPTER SIX ........................................................................................................................ 83
CONCLUSION AND RECOMENDATION ........................................................................ 83
6.1) Conclusion.................................................................................................................. 83
6.2) Recommendations ...................................................................................................... 87
REFERENCES ........................................................................................................................ 89
ANNEXES ................................................................................................................................ 93
iv
Lists of Figures
Figure 2.1:- Location Map of the Study Areaarea
7
Figure 2.2:-Elevation range of the study area
10
Figure 2.3:-Cross-section from A-B from Kaka mt.to Bale mts. area
10
Figure 2.4:-Land use land cover map of the area
12
Figure 2.5:- soil map of the study area.
14
Figure 2.6:-: Drainage pattern of Upper Wabe River Sub Basin
15
Figure 2.7:-Regional lithostratigraphy
18
Figure 2.8:- columnar joints of ignimbrite cliff
21
Figure 2.9:- Geological map of the study area & geographical distribution of
the main lineaments.
25
Figure 3.1:-Mean monthly precipitation trend from stations in and around the
catchment area
28
Figure 3.2:-Relation ship between surface elevation and depth of precipitation
28
Figure 3.3:-Aerial rainfall using the Thiessen polygon method
31
Figure 3.4:-Aerial mean depth of rainfall using the Isohyetal method
32
Figure 3.5:-Mean Maximum, Minimum and Average monthly temperature
36
Figure 3.6:- Average monthly mean trend of relative humidity (%)
37
Figure 3.7:-Monthly average wind speed
38
Figure 3.8:-Mean sunshine hrs of the study area
39
Figure 3.9:- Base flow Separation of Upper Wabe River using Time- plot
50
Figure 4.1:-Semira Kolba spring through fractured ignimbritic rock
56
Figure 4.2:-Lithologocal log of Hinja Burkitu BH-1
59
Figure 4.3:-Hydrogeological map of the study area
61
Figure 4.4:-map showing recharge and discharge areas
62
Figure 4.5:-Cross-section along A-B indicating local and intermediate flow system
63
Figure 4.6:-Groundwater table contour map and flow direction
64
Figure 4.7:-Washing at Asasa eye spring
67
Figure 5.1:-Relation between laboratory measured EC and TDS
73
Figure 5.2:- Piper plot of all sources water sample.
75
v
Figure 5.3:- Cross-section from A-A’.Conceptual flow path from recharge to
discharge area.
79
Figure 5.4:-Wilcox diagram
82
Lists of Tables
Table 2.1:-Relative abundance & description of lithofacies
24
Table 3.1:- Long term arithmetic mean monthly depth of rainfall (mm)
of the seven stations in and the surrounding study area
30
Table 3.2:-Aerial mean depth of precipitation using Theissen polygon.
31
Table 3.3:-Aerial mean depth of precipitation using Isohyetal method
32
Table 3.4:-Meteorological stations in and around the study area
33
Table 3.5:- mean monthly maximum temperature of the five stations
in the study area( oC).
35
Table 3.6:- mean monthly minimum temperature of the five stations
in the study area ( oC).
35
Table 3.7:-mean monthly temperature of the five stations
in the study area ( oC)
35
Table 3.8:- Mean monthly relative humidity of stations in the study area.
36
Table 3.9:-Monthly average wind speed at 2m above ground surface in m/sec
37
Table 3.10:- Average monthly mean sunshine duration (in hours) in the study area.
38
Table 3.11:- Mean annual PET obtained from Penman method
41
Table 3.12;-Annual PET calculated by Thornthwaite Method
42
Table 3.13:- Suggested available water capacities for combinations
of soil texture and vegetation.(From Thornthwaite and Mather 1957.)
44
Table 3.14:-AET for clay loam soil with an available water capacity of 100mm
under shallow rooted crop cover with estimated rooting depth of 0.40m
(7.58% land cover of the area) (peas,beans)
47
Table 3.23:- Result of base flow separation
50
Table 4.1:- Common porosity values of volcanic formations.
53
Table 4.2:-Aquifer characteristics of rocks related to the study area
(WWDSE, 2004, in OWWDSE,2007)
55
vi
Table 4.3:-Vertical electrical sounding result at Hinja Burkitu BH_1
58
Table 4.4:-Vertical electrical sounding result at Hinja Burkitu BH_2.
59
Table 4.5:-Common ranges of permeability for water at normal temperature
(10 to 25°C) and values of transmissivity
60
Table 4.6:- Some cold springs observed in the study area.
66
Table 5.1:- pH ranges of different sources of water
71
Lists of Annexes
Annex 3.1:-Point Precipitation of meteorological stations in and around the study area93
Annex 3.2:- Long term arithmetic mean monthly depth of rainfall (mm)
of the seven stations in and the surrounding study area
94
Annex 3.3:- Meteorological stations in and around the study area
95
Annex 3.4:- Mean annual PET obtained from Penman method
96
Annex 3.5:-AET for fine sandy loam soil with an available water capacity of 300mm
under mature forest cover with estimated rooting
depth of 2.00m (8.0% land cover of the area)
97
Annex 3.6:-AET for clay loam soil with an available water capacity of 400mm
under mature forest cover with estimated rooting depth of 1.60m
97
(13.48% land cover of the area)
Annex 3.7:-AET for clay loam soil with an available water capacity of 250mm
under deep rooted crop cover with estimated rooting of 1.00m (8.77% land cover of the area)
depth(pasture grass, bushes, shrubs)
98
Annex 3.8:-AET for clay loam soil with an available water capacity of 100mm
under shallow rooted crop cover with estimated rooting depth of 0.40m
98
(7.58% land cover of the area) (peas, beans)
Annex 3.9:-AET for fine sandy loam soil with an available water capacity of 75mm
under shallow rooted crop cover with estimated rooting depth of 0.50m
(7.58% land cover of the area) peas, beans)
99
vii
Annex 3.10:-AET for fine sandy loam soil with an available water capacity of 150mm
Moderately rooted crop cover with estimated rooting depth of 1.00m (27.82%
land cover of the area)(wheat, barely, corn)
99
Annex 3.11:-AET for clay loam soil with an available water capacity of 200mm
moderately deep rooted crop cover with estimated rooting depth of 0.80m (20% land
cover of the area) wheat, barely, corn)…………………………………………………………100
Annex 3.12:- Melka wakena Hydroelectric Power Reservoir Water Evaporation
101
Annex 3.13:- Projected mean river discharge of Upper Wabe river
catchment (m3/s) (1976-2006)
102
Annex 4.1:- Some of the cold fracture type springs observed in the study area.
103
Annex 4.2:-Ground water sources inventory data
104
Annex 4.3:- Laboratory analysis results of collected sample.
Annex 4.4:- Laboratory analysis results of collected samples (secondary data)
Annex 4.5:- SAR values of Water samples.
105
106
107
viii
List of Acronyms
AET - Actual evapotranspiration
APWL -Accumulated potential water loss
EC - Electrical Conductivity
FAO - Food and Agricultural Organization.
GIS - Geographical Information System
GPS- Geographical Positioning System
ITCZ - Inter Tropical Convergence Zone
MCE -Metaferia Consulting Engineers
OWWDSE- Water Works Design and Supervision Enterprise
P - Precipitation
PET - Potential evapotranspiration
pH - Negative of the logarithm to the base ten of the hydrogen ion concentration.
RO - Runoff
SAR - Sodium Adsorption Ratio
Sm - Soil moisture
SMD - Soil moisture deficit
TARO – Total Available for Runoff
TDS - Total Dissolved Solids
UNESCO - United Nations Educational Scientific and Cultural Organization
UTM -Universal transverse mercator
WAPCOS - Water and Power Consultancy Service (India) Ltd
WHO - World Health Organization
WMO - World Meteorological Organization
WWDSE - Water Works Design and Supervision Enterprise
ix
CHAPTER ONE
INTRODUCTION
1.1) General
In recent decades it has become evident in many countries of the world that groundwater is one
of the most important natural resources. It has a number of essential advantages when
compared with surface water because it is naturally least direct contact with the activities
carried out on the surface. It is of higher quality, better protected from possible pollution. It is a
good alternative in areas with scarce surface water resource and less subject to seasonal and
perennial fluctuation. The subsurface storages have also advantages of being free from the
adverse effects like inundation of large surface area, loss of cultivable land, displacement of
local population, and substantial evaporation losses, and much more uniformly spread over
large region than surface water. No gigantic structures are needed to store groundwater. On the
other hand, Surface water is often in close contact with human activities and/or other natural
processes that take place on the earth’s surface; hence is vulnerable to pollution that makes it
unsuitable for drinking and other purposes before treatment.
To meet the increase demand of water due to rapid growth of population, urbanization and
industrialization especially in developing countries, it is very important to evaluate
groundwater resources quantitatively to make management strategies.
The research in Upper Wabe River Sub basin is therefore, contribute some points as an input
for water resource management of the area to provide sustainable and equitable supplies for
communities in and around the catchment.
Wabe Shebelle river basin is one of the seven trans-boundry rivers in Ethiopia with a total area
of about 205,410 km2. Upper Wabe River sub basin is part of the Wabe Shebelle river basin
and is found in two zones of Oromia National Regional State, in Arsi and Bale Zones and the
southwestern margins of the area also extend to adjoining areas of Sidama Zone in South
Nation, Nationalities and Peoples Regional State. The area is surrounded by Arsi-Bale massifs
1
and located in relatively high rainfall area where both perennial and intermittent rivers draining
the catchment are from these massifs.
The elevation of the Catchment increases from the central relatively low lying volcanic plains
to the surrounding mountains. The altitude ranges from 2000m.a.s.l to above 4000m.a.s.l.
The area hosts both urban and rural communities with high rate of population growth and
recently the supply of water for drinking and other domestic use is from unprotected family
and community based hand dug ponds, rivers(especially in rural areas) and ground water
sources(both from shallow and deep bore holes) and springs. The water supply for most
domestic use is from uncapped springs.
Rain fed seasonal crop cultivation is the principal activity in the area. The most common
cultivable crops in the area are wheat, barley, and other cereals such as sorghum, beans and
maize. At the western part of the proposed study area especially in Kofele and its surrounding
“Enset” plant growing is also common.
As a result of uncontrolled deforestation over the past decades & the soil erosion, the land
cover of the area has been heavily degraded. The densely forested areas of the high lands of the
area are now dramatically decreasing due to human activities. Areas formerly covered with
dense forests now changed to agricultural lands due to high population growth. Only tracers of
indigenous trees left in compounds of churches and at inaccessible areas. The deforestation of
these forests has negative effect on climate of the area which in turn affects surface and
subsurface water resources of the area. In order to manage the resources of surface and
subsurface water in a more sustainable manner the ecological balance of the area must be
managed properly.
1.2) Previous Works
•
Wabe Shebele River Basin Integrated Development Master Plan Study Project
Report (WWDSE, MCE and WABCOS, 2004)
The study provides preliminary evaluation of groundwater resources of the basin and produced
hydrogeological maps at 1:250,000 scale.
2
The main objectives of the Master plan study were to assess the groundwater resources of the
area by determining the hydrogeological conditions such as recharge and discharge conditions,
spatial distribution of different aquifers, hydraulic parameters and water quality of the aquifers
and by giving recommendations on the strategy of groundwater resources development. The
activities conducted to achieve the objectives include review of previous studies, inventory of
water points, and analysis of water quality, geological mapping and hydrometeorological
studies. This study is more regional and did not focus in particular on the Upper part of the
Wabe river sub basin.
•
Shanan-Dhungeta & Middle Wabe Dhare Sub-basins Groundwater Resource
Potential Evaluation Project (OWWDSE, 2006)
The study mainly focuses on evaluating groundwater potentials of the major sub-basins,
namely Shanan, Dhungeta, middle Wabe Shebele and Dhare to identify and assess areas of
interest for well field development. The investigation involves a multi-disciplinary approach
which comprises hydrometeorological studies, geological, structural and geomorphological
investigation, geophysical surveys, and hydrogeological investigations.
This study also focuses on water resources evaluation at the partial upper Wabe, Middle WabeDhare Sub-basins, and Ramis-Mojo-Erer-Daketa Western Jerer Sub-basins and the study is a
preliminary groundwater resource potential evaluation. So, the study in this particular
area(Upper Wabe Sub-Basin) amid at filling gaps and to provide detail geomorphological,
geological, hydrogeological and hydrogeochemical of the area by conducting thorough and
intensive field investigation and data analysis. The present study have been tried to incorporate
recent data to the existing ones and supplement the existing ones to provide useful information
for policy makers and general public to manage the resource on sustainable basis.
•
Carl-Gosta Wenner, 1973.A Master Plan for Water resources and supplies in
Chilalo Awraja. This study tried to investigate both surface water and groundwater potential
of the area giving emphasis on Katar river catchment.
•
Getaneh Assefa, M.A.J. Williams, D.J.Clark. 1982. Late Cenozoic history and
Archaeology of the Upper Wabe Shebele Basin, East-Central Ethiopian. Department of
Geology, Faculty of science, Addis Ababa University.
3
This study concentrated on Cenozoic history and Archaeology of the Upper Wabe Sheble basin
and detail investigation done on describing the paleo-lake and its lacustrine deposits with the
archaeology of the area.
• Gobena, H., Belayneh, M., Kebede, T., Tesfaye, S., Abraham, A. 1997. Geology of
the Dodola Area. Geological Survey of Ethiopia.
• Borehole site investigation and drilling completion reports for rural communities
water supply projects.
1.3) Objective of the study
General Objective
The general objective of the research is to describe and give detailed picture of the
hydrogeologic characteristics of the groundwater system concerning aquifer type, groundwater
recharge estimation and mechanism, the role of tectonics and structures in groundwater flows
and hydro chemical nature of water resources in the Upper Wabe river sub basin.
Specific Objective
To regroup aquifer types on different aquifer parameters and produce hydrogeological map.
•
To analyze spatial variation of the hydrochemistry of water samples and indicating the
hydrogeologic implications behind the hydrochemical variations.
•
To determine the regional and local groundwater flow systems in the catchement.
•
To delineate potential sites for future groundwater development & recommend potable
water alternative sources.
•
To quantify major hydrologic components.
•
To delineate recharge and discharge zones.
•
To suggest the possible measures that can be applied to safeguard water resources of
the area from natural and/or anthropogenic pollution sources.
•
To propose the future groundwater management system so that to keep the natural
balance of the environmental condition and sustainable integrated utilization.
4
1.4) Methodology
a) Office work
The office work includes literature review, collection and organization of the previous works
within and around the study area from all possible sources (reports, maps, borehole record
hand dug wells and springs, pumping test data ,water quality test data, VES data, hydrometeorological data, river discharge data), interpretation of 3-D global maps, topographic maps
& use of remote sensing and GIS techniques for mapping of land cover, land use, and
geological structures and related land features.
b) Field activity
•
The field work comprises identification and collection of relevant previous geological
and hydrogeological works, identification and mapping of the lithological units and geological
structures through field observation at valley and river cuts, quarry sites, exposed mountain
cliffs, dug wells and borehole logs using topographic maps, satellite imageries, 3-dimensional
digital elevation maps, and aerial photographs to produce the final report.
•
Collecting water samples for physico-chemical analysis from boreholes, dug wells, and
springs at representative sites and interpretation of the result.
• Identify preliminary recharge and discharge zones in the study area
• Describe mode of occurrences of the springs.
•
Geo-referencing distributions of meteorological and river discharge gauging stations, all
water points (boreholes, dug wells and springs) using GPS.
•
Observation of land cover and land use practice.
c) Laboratory Analysis of selected representative samples.
Important hydrogeochemical parameters of 12 collected water samples have been analyzed in
Water Works Design and Supervision Enterprise Laboratory.
d) Data interpretation, Finalizing and writing the thesis
The collected data (both primary and secondary) is analyzed using relevant softwares, global
maps together with topographic maps, air photos, hydrogeological and geological maps,
combined with primary data acquired from field investigation and laboratory analysis can be
integrated using GIS techniques to come up with sound and reasonable result.
5
CHAPTER TWO
DESCRIPTION OF THE STUDY AREA
2.1) Location, Arial extent and Accessibility
Geographically, the area is located between 6º45’15’’N and 7º 25’18” N latitude and
38º42’42’’E and 39º39’05’’E longitude, which covers an area of about 4489square kilometers
and representing approximately 2.19% of the total area of 205,410 square kilometers of the
Wabe Shebele River Basin. The area is bounded to the west and northwest by Rift Valley, and
to the south and southwest by Genale-Dawa basin. It is found at the upper part of Wabe
Shebelle basin at about 320 km south east of Addis Ababa (Figure1).
Wabe Shebelle River flows southeast from the mountains of the eastern rift shoulder across the
Gedeb plain into a deep canyon.
The major access road network from Addis Ababa to the project area is provided by partly
asphalted and partly graveled all-weather roads. Access to the area is possible in two
directions, one through the main Addis Ababa-Modjo-Shashemene asphalt road which is about
250km; and then on all gravel road for about 35 km to east to reach the project area. Along this
way after branching from Shashemenne to Kofele it gives access routes to investigate the
western parts of the study area. The other access to the proposed study area is through the
Addis Ababa-Adama-Asela asphalt road which is 175km and then from Asela to the site on the
115km gravel road to the site. Inter site mobilization is also possible through the existing all
weather gravel and dry weather roads. In general the area has relatively good road network
density but in some parts of the project area like the extreme parts of the high land areas where
the topography is rugged hardly accessible.
6
34
36
38
40
42
44
46
1600000
14
1400000
12
1200000
h
as
Aw
1000000
Wa
bi
Ziway/Abijata
Basin
800000
38.8
820000
si n
ba
38.9
Sh
eb
400000
800000
Bokoji
8
ele
B39
asi
n
39.1
0
200000
400000
600000
800000
39.2
39.3
6
Genale Basin
600000
810000
10
0
200000
400000
1000000
1200000
1400000
39.4
39.5
39.6
Meraro
Gimbite
Shashamane
Asassa
be
Wa
Riv
Adaba
770000
760000
0
20000
40000
60000
Figure 2.1:- Location Map of the study area
7
Towns
Springs
River Gauge stations
Rivers
All weather gravel roads
Catchment Area
Dinsho
Dodola
750000
39.9
er
Kofele
780000
39.8
Legends
4
790000
39.7
2.2) Population
The primary source of information for data on population numbers and distribution is the
respective ‘woredas’ in the study area. Accordingly, the total population of the catchment area
is estimated to be about 1,016,994. The majority of the population of the area concentrated on
the mountain hill sides, foot slopes and towns such as Adaba, Dodolo, Herero, Asasa, Kofele,
Meraro, Eddo, Robegerjeda, and Negele. Most of the eastern central part of the plain occupied
by state and small family holding farms and Melka Wakena hydroelectric power water
reservoir and has not experienced comparable population pressure as compared to the
mountain hill sides and foot slopes.
2.3) Climate
Climate is the function of the location latitude, altitude, angle of the sun, distance from oceans
or other water bodies, terrain and the like. The different combinations of these factors resulted
in the prevalence of diverse climatic conditions in Ethiopia.
National Atlas of Ethiopia (1981) divided the climatic zones of Ethiopia into five traditional
climatic zones;
-"Kur" (Alpine) from an altitude of 3000m and above mean sea level
-"Dega" (temperate) from 2300m to about 3000m
-"Weina Dega" (Sub tropical) from 1500 to about 2300m
-"Kolla" (Tropical) from 800m to about 1500m
-"Bereha" (Desert), less than 800m.
The most part of the study area falls under the “Dega”(temprate) climatic zone and the
peripheries of the mountain areas falls under the "Kur" (Alpine) climatic zone.
The climate of the Catchment area ranges from dry humid ecological zone in the relatively
lower parts of the plain and to humid in the peak highlands. The large proportion of the area
can be classified as dry humid. The mean daily temperature of the area varies between 120c
and 160c.
2.4) Physiography
The physiographic land features of the study area are formed by volcanic mountains and cones
on the peripheries which are characterized by rugged topography whereas the volcanic plains
8
in the central part characterized by flat land of insignificant elevation difference. These are
covered by thick piles of volcanic lava flows emanated from both central and fissural type of
volcanism which forms the Arsi- Bale Massifs surrounding the relatively low lying volcanic
plains. The mountain ranges slope towards the centre of the relatively low lying flat volcanic
plains of the research area from all directions. The area is located with in upper tip of Shebele
basin and it is bordered by the Genale drainage basin in the south and southwest and by ZiwayAbijata basin in the west and northwest. The Wabe River originates just east of Wetera Resa
village in South Nations, Nationalities and Peoples Regional State. The upper course of Wabe
Shebele (also known as Wabe) is bounded on either side by volcanic mountains, namely
Kurduro, Somkeru, Beranta, Wege Harena and Shewiso to the right, Mt. Kaka and Hunkolo to
the left. Mt. Kaka with elevation of 4157 m a.m.s.l is the highest at southwestern boundary of
the project area with the Ethiopian rift. The mountains to the left side (Kaka and Hunkolo)
separate Wabe Shebele from Ketar River, which flows to Ziway Lake; while those to the right
(Kurduro, Somkeru, Beranta, Wege Harena and Shewiso) separate it from upper reach of
Genale River Basin.
The volcanic mountains and cones to the right side of the upper Wabe Shebele (Kurduro,
Somkeru, Beranta, Wege Harena and Shewiso) are underlain by Tertiary volcanics of Pre-Rift
succession (Alaji basalt, alkali trachyte flows, and alkali trachyte and basalt flows); while those
to the left (Kaka, Hunkolo and Galema Mountain Range) are underlain by Post-Rift volcanic
succession including alkaline to peralkaline basalts and trachytes. The volcanic mountains are
formed both by central (e.g., Mt. Kaka) and fissural (e.g. Galema Mountain range) eruptions.
The upper part of the Wabe Shebele River, from Malka Wakana downstream up to the point
where volcanic rocks continue to occur predominantly, forms a gorge or canyon of more than
350m deep from the volcanic plateau around it. The valleys are generally broader maybe
because of tremendous mass wasting and slope wash from the sides.
The elevation of the area ranges from 2063m to 4157m a.s.l. and consists of elevated areas
such as Mt. Kaka (4157m), Mt. Hunkolo(3850m), and the majority of the northwestern,
western, southeastern and eastern water divide is above 2800m.a.s.l.The eastern water
9
divide(the Bale Mountains) are relatively continuous mountain ranges as compared to the Arsi
Mountains in the northwest.
820000
Meraro
Legend
A
Rivers
810000
Gimbite
Towns
Catchment area
800000
790000
Asassa
a
en
ak ir
W vo
lk a s e r
Me Re
Kofale
780000
Adaba
Dodola
770000
760000
750000
0
20000
B
40000 Meters
470000 480000 490000 500000 510000 520000 530000 540000 550000 560000 570000
Figure.2.2:- Elevation range of the study area
Figure 2.3:- Cross-section along A-B from Kaka Mt. to Bale Mts.
10
2.5) Land use and Land cover
Protecting groundwater, lakes, rivers, streams, and wetlands requires wise land use. Human
population growth and changes in land use increasingly impact aquatic environments.
Decreasing the amount of plant cover (particularly perennial vegetation) in watersheds reduces
the quantity of precipitation stored in vegetation and the amount of rainfall returned to the
atmosphere via evapotranspiration, which increases surface runoff and reduces water
infiltration. Vegetal cover increases infiltration as compared with barren soil because (i) it
retards surface flow giving the water additional time to enter the soil (ii) the root system make
the soil more pervious and (iii) the foliage shields the soil from rain drop impact and reduces
rain packing of surface soil. To maintain optimal flow regimes, land management practices that
improve the ability of watersheds to infiltrate and slowly release precipitation will reduce the
duration and intensity of flooding as well as low flow periods. Protection of groundwater
recharge areas, cropland soil conservation practices, such as minimum tillage, strip and contour
cropping, grassed water way, filter strips, water detention basins, and terraces reduce
precipitation runoff and increase water infiltration into the soil, thus promoting ground water
recharge.
The main land cover of the study area is dense mixed high forest, open wood land, dense
wood land, afro alpine vegetation and intensively cultivated lands. The eastern high lands are
covered by mixed forests in sloppy and inaccessible areas and the northwestern peak
mountains such as Kaka and Hunkolo mountain ranges are by Afro-alpine heath vegetation. In
the study area these forests of the high lands at higher risk. Areas formerly covered with dense
forests now changing to agricultural lands due to high population growth. Only tracers of these
indigenous trees left at some places and inaccessible areas. Adverse land practices aggravate
surficial erosion through deforestation and these could even have persistent effect on climate.
The low lying central flat plain is occupied by state and small family holding farmlands which
is intensively cultivated, grazing lands and Melka wakena Hydroelectric Power water dam and
11
hardly any tree vegetation is visible and virtually all the land being opened up for cropping
(wheat and barely) and/or grazing of livestock. The State farms occupied the vast land of
central part of the plain areas. The mountain hillsides and foot slopes are occupied by
residential areas and open wood lands. There are four State farms in the study area. These are
Hunte, Herero, Geredela and Gofer state farms. All of them are rain dependant (seasonal)
cultivation and the main agricultural activity is annual cereal crop production. Wheat and
barely are the major crops cultivated in the area.
38.8
38.9
39
39.1
39.2
820000
39.3
39.4
39.5
39.6
Meraro
7.4
Gimbite
810000
7.3
800000
7.2
790000
780000
7.1
Asassa
Kofale
Adaba
7
Dodola
770000
6.9
760000
750000
6.8
0
20000
40000 Meters
470000 480000 490000 500000 510000 520000 530000 540000 550000 560000 570000
Legend
Dense mixed high forest
Dense coniferous high forest
Disturbed high forest
Dense wood land
Open Wood land
Perennial crop cultivation
Afro Alpine heath vegitation
Moderately Cultivated
Intensively cultivated
Intensively cultivated (State farms)
Figure 2.4:-.
Land cover map
of the study area
Towns
Catchment area
12
2.6) Soil Classification
The soil is the uppermost layer of the earth's crust and is the product of complex accumulation
of unconsolidated mineral grains from the physical and chemical weathering of rock fragments
and the addition of organic material from vegetation.
Soil and land use conditions which control the rate of infiltration and down ward percolation of
the water falling on the surface of the soil have special importance. Infiltration capacity of a
soil depends on many factors such as soil type, moisture content, organic matter, vegetative
cover, season air entrapment, formation of subsurface seals etc. Climate, parent material,
vegetation, depth of water table or drainage, and physiographic features such as slope,
geomorphology are the main factors for soil formation erosion.
According to FAO soil classification there are four major soil types are found in the study area.
These are:
Luvisols: Occur on the steeper slopes of the eastern part of the study area such as on the Bale
mountain ranges and in the northern parts on mountain Kaka and Hunkolo, and also found
covering the southwestern peripheries of the area from where Wabe river emerges. They are
generally well drained, deep to very deep, fine to medium textured, clay loam and sandy loam
soils. Luvisols are widely distributed and covers 1589.63km2 or 35.41% of the studied area.
Cambisols: Cambisols are moderately developed soils characterized by slight or moderate
weathering of the parent material and by absence of appreciable quantities of accumulated
clay, organic matter, aluminium or iron compounds.
This soil type found covering wide area of the central part of the Gedeb-Asasa plain in the
catchment area. The soils are generally well drained, moderately deep to very deep, fine to
medium textured clay loam to sandy loam. It covers an area of 1383.40 km2 or 30.81% of the
studied area.
Vertisols: Vertisols are soils having 30%or more clay in all horizons to a depth of at least
50cm, which develops crack from the soil surface down ward. Vertisols are characterized by
13
their high clay content. Texturally it is clay loam and the proportion of clay fraction may reach
up to 60%. They are often dark coloured, hence common names such as 'black cotton soil' due
to the smectite clay mineralogy and they are very hard and crack when dry, sticky and plastic
when wet.
The Vertisols are found in the catchment area on slopes side of volcanic mountains of mt.Kaka
such as Shashe, Shire and around Dodola, Adaba and found surrounding the cambisols. This
soil type covers an area of 1095.10 km2 or 24.40% of the studied area.
Nitosols: They are clay loam to sandy loam. Physically they are porous, well drained, have a
stable structure and a high water storage capacity. Nitosols are among the most productive
soils of the tropics and they are intensively used for plantation crops and food production. They
have a high moisture storage capacity and a deep rooting volume. Nitosols occur on the gently
sloping to steep land covering parts of southwestern plains and high lands of Kofele area. This
soil type covers an area of 417.87 km2 or 9.31% of the studied area.
Lithosols: This soil type covers very small portions of the eastern part of the area. It covers an
area of 3 km2 or 0.07% of the studied area.
38.8
820000
38.9
39
39.1
39.2
39.3
Meraro
Legend
Cambisols
Lithosols
Luvisols
Nitosols
Vertisols
810000
39.4
39.5
39.6
7.4
Gimbite
7.3
Towns
Catchment area
800000
7.2
790000
7.1
Asassa
Kofale
780000
Adaba
7
Dodola
770000
6.9
760000
750000
6.8
0
20000
40000 Meters
470000 480000 490000 500000 510000 520000 530000 540000 550000 560000 570000
Figure 2.5:-Soil map of the study area
14
2.7) Drainage pattern
The drainage density and pattern mainly depend on climate, rock and soil formations,
topography and surface and sub-surface fracture intensities.
The relief configuration of the project area and the surrounding which are the result of past
geological history control the direction of the flow of the rivers. All the rivers in the area drain
to Wabe river. The drainage patterns of the tributaries are mainly parallel and the drainage
density of the catchment is relatively small which is about 0.38km/km2 and the western parts
are relatively denser than the eastern parts. Some of the tributaries of Upper Wabe river are
Asasa, Ukuma, Maribo,Leliso, and Totolamo, are perennial and all ends up in the Melka
Wakena Hydroelectric power water reservoir.
38.8
38.9
39
39.1
39.2
820000
39.3
39.4
39.5
39.6
Meraro
7.4
Legend
Gimbite
Rivers (Drainage)
810000
Towns
Catchment area
7.3
800000
7.2
790000
Asassa
Kofale
780000
7.1
Wa
be
Adaba
7
Dodola
770000
6.9
760000
750000
6.8
0
20000
40000 Meters
470000 480000 490000 500000 510000 520000 530000 540000 550000 560000 570000
Figure 2.6:-Drainage pattern of Upper Wabe River Sub Basin
15
2.8) Geology and Structure
2.8.1) Regional Geology
Ethiopia can be divided in to four major physiographic regions, widely known as the western
plateau, southeastern plateau, the main rift and the afar depression. The Ethiopian plateau is
underlain at depth by Precambrian rocks of the Afro-Arabian Shield. The Precambrian
basement is covered for the most part by glacial and marine sediments of Permian to Paleogene
period and Tertiary volcanic rocks with related sediments.
The Tertiary volcanism is started in the late Oligocene, and was dominantly characterized by
fissure-fed eruptions of plagioclase (tholeiitic) and transitional basaltic lavas with minor
ignimbrites. This formed the thick sequences of lava flows and interbedded ignimbritic sheets
that constitute the Ethiopian plateaus. In the middle of Miocene large central volcanoes of
transitional to peralkaline were formed. This volcanic province is divided into the northwest
and southeast plateaus which are bounded in either side by the Main Ethiopian Rift which is a
huge graben and is part of the great Africa continental rift and Afar depression. This volcanic
province is structurally affected by normal and step faults of various dimensions. They are
mainly along NNE-SSW and rarely along NE-SW, N-S and NW-SE directions (Mohr 1967
and Di paola 1972).
Cenozoic volcanism in Ethiopia is related to the final stage of Gondwanaland rifting that took
place between Late Eocene and the Early Miocene (Guiraud and Bellion, 1995), and is
generally associated with continued rifting at the margins and within the African-Arabian
plate. The Dead Sea-Red Sea-Gulf of Aden rifted during this time, and rifting in the East
African Rift has continued up to recent times. Deep seated Pan-African contact nappes are
thought to have been reactivated as detachment faults during Cenozoic extension in the East
African Rifts (Chorowicz, 2005). These rifting events may be related to the onset of
magmatism associated with the Afar plume.
The Geology of the South-Eastern escarpment of Ethiopia between 390 and 420 longitude East
(Juch, 1975) and some K-Ar age determinations by Kunz et al. (1975) are important works in
16
understanding the geological evolution of the volcanic rocks of the area. Juch (1975) divided
the Cenozoic volcanics of Southe-Eastern Ethiopia into:- (i) Lower Trap basalt, (ii) Main
Silicic Formation, (iii) Upper Trap basalt, (iv) Pliocene Silicics, and (v) Pliocene Main basalt.
However, Kazmin and Berhe (1978) reviewed in detail the Cenozoic volcanics rocks of SouthEastern Ethiopia which divided the Cenozoic volcanics into the following lithological units.
From oldest to youngest these as:- (i) Alaji Volcanics, (ii) Termaber Basalt, (iii) Arba Guracha
Silicics, (iv) Anchar Basalt, (v) Nazareth Group, (vi) Arba Gugu Basalt, (vii) Chilalo
Volcanics, and (viii) Wonji Group. According to Zanettin(1992), the Cenozoic Ethiopian
volcanic province can be divided into two main series. These are: (i) Trap (plateau) Series, and
(ii) Rift Series, however, in their review of the main petrological characteristics and
emplacement ages of volcanic rocks in Ethiopia, Yemane et al. (1999) divided the entire
sequence into Paleogene Lower Volcanic, the Neogene Upper Volcanic, and the Pleistocene
Post-Rift Volcanic Chronostratigraphic units.
The southeastern plateau volcanics are characteristically different from the Northwestern and
Southwestern plateau volcanics for the following features:1. Volcanism is less voluminous and started late between 25 and 20 Ma (Kunz et al.,
1975). In northwest and south-west plateau volcanism started as early as 45 Ma. In the
southern part of the western escarpment volcanic activity started about 60 to 40 Ma
(Ashangi Basalt (Zanettin 1992).
2. Silicic volcanics are scarce compared to the western plateau volcanics.
Volcanic activity in the southeastern plateau began with the extrusion of Alaji basalt about 25
Ma (Kunz et al., 1975). The eruption of the Alaji basalt is followed at about 15 to 10 Ma by a
central type alkaline volcanic activity (Termaber basalt), representing eruption of basalt with
acidic and mostly ignimbritic volcanic cover. The eruption of trap volcanics in south-eastern
Ethiopia plateau came to an end before the deposition of the Chorora sediments about 10 Ma
ago (Juch, 1975). The Chorora sediments (7 to 10 Ma), up to 60 m thick, separates the volcanic
sequences of the south-eastern escarpment from the volcanic rocks filling the rift (Kazmin and
Berhe, 1978).
17
Volcanism in the south-eastern plateau ceased by eruption of Ginir Volcanics and Quaternary
alkali basalts on the plateau.
Figure 2.7:-Regional Lithostratigraphy. Formational names are taken from Teferra et al.(1996),
Kazmin and Berhe (1978), Berhe (1987), Merla et al. (1973)
18
2.8.2) Local Geology
2.8.2.1) Cenozoic Rocks
The late Cenozoic history of the Upper Wabe Shebele Basin has been reconstructed from
sedimentary and archaeological data (Getaneh Asefa et al.,1982). Within the sedimentary
succession, there is a sequence of lake sediments (The Gedeb Formation) followed, in
ascending order, by pyroclastic and volcaniclastic deposits (The Adaba Formation),
distributary channel and high sinuosity flood plain deposits (the Lower Part of the Mio Goro
Formation) and finally by ephemeral low sinuosity flood plain deposits (the Upper Part of the
Mio Goro Formation).
The Cenozoic volcanism in the project area preserves a record of flood basalts and bimodal
basalt-rhyolite/trachyte-pyroclastics volcanism spanning from Tertiary to Quaternary. The
Tertiary volcanics are divided into Pre-Rift (trap) series and Rift-series. The Pre-Rift series
(Oligocene-Miocene) are represented by Alaji volcanics and Termaber Formation. The Alaji
volcanics are dominantly fissural basalt associated with trachyte in the upper portions of the
sequences and underlies the Nazareth group. The Termaber Formation is absent in the study
area. The Pre-Rift Series volcanics are separating from the Rift Series by deposition of the
Chorora Sediments.
The Cenozoic rocks in the area are divided into Tertiary volcanics and Quaternary volcanic and
cover the rocks from the oldest to the youngest described as follows;
2.8.2.2) Tertiary Volcanics
The Tertiary volcanics are divided into: - (i) Pre-Rift (trap) Series, and (ii) Rift-Series,
following the classification of Zanettin (1992) for the Cenozoic Volcanic Provinces of
Ethiopia.
Pre-Rift (trap) Series
The Pre-Rift series are represented by Alaji Volcanics in the area. This rock unit found
covering eastern parts of the study area around Dodola and Adaba under laying the Nazareth
Group.
19
The Alaji Volcanics are represented by two units: - Alaji Basalt and Trachyte. Alaji basalt is
referred to as Lower Trap Series (Juch, 1975), Lower stratoid basalt (Berhe et al., 1987) and
Lower aphyric to porphyritic basalt (Gobena et al., 1997). It is generally formed of aphyric to
locally porphyritic basalt measured in thickness up to 800 m. It unconformably overlies the
Mesozoic sedimentary successions. Geochemically, the Alaji basalt is transitional with mildly
alkaline to tholeiitic. For a given percentage of silica lie between those for alkaline and midoceanic ridge tholeiitic basalts, the Alaji basalts are characterized by moderate values of Al2O3
(c. 13-14%), TiO2 (2.5-3.1%), and K2O (< 0.4-1.1%) (Zanettin,1992). K-Ar age determinations
place the Alaji basalts between 28 and 25 Ma. (Kuntz et al., 1975).
The trachyte unit conformably overlies the Alaji basalts. It is exposed as subhorizontal lava
flows with occasional plugs. The rocks are essentially massive, fresh, medium to dark grey and
green. They are locally columnar jointed. Petrographic descriptions from Gobena et al. (1997)
revealed that the trachyte is composed of alkali feldspar (commonly sanadine), pyroxene
(aegirine and aegirine-augite), grey to yellowish brown glassy matrix and opaques (mainly
magnetite).
Post -Rift Series
The exposure of Rift Series volcanics in the area are restricted to the western part, which is
exposed along the south-eastern escarpment. They are unconformably overlies the Pre-Rift
(trap) series volcanics. The Chorora sediments (7 to 10 Ma), up to 60 m thick, separates the
Rift Series from the underlying Pre-Rift volcanics (Kazmin and Berhe, 1978).
The Rift Series volcanics are represented in the study area by the following units:1. Nazareth Group
2. Chilalo Volcanics
Nazareth Group (Nn)
The Nazareth Group covers large portions of the Gedeb-Asasa plain. The unit consists of a
succession of alkaline and per alkaline stratoid silcics, welded ignimbrite, pumice, ash and
rhyolite flows and domes with rare intercalations of basalt flows and lacustrine sediments,
which attains a maximum thickness of 250 m in the escarpment and tends to thin on the plain.
20
This unit is referred to as Dodola Ignimbrite (Berhe et al., 1987), which is made up of
predominantly of rhyolitic ignimbrites, trachytes and ash flow tuffs. Fluvio lacustrine
sediments are also found intercalated with the Dodola Ignimbrites. Generally it appears that the
formation of this unit is involved first sedimentation, followed by pyroclastic activity.
Figure 2.8:-columnar joints of ignimbrite cliff at the start of canyon down Melka Wakena
Reservoir.(Photo from eastern wall of the gorge)
Chilalo Volcanics (Nc)
This unit occurs covering the top parts of Mt. Kaka, Mt. Hunkolo and Galema Mountain
Ranges and overlies the Nazareth Group. It is divided into Lower part, consisting of
ignimbrites and trachytes and Upper part, consists of porphyritic alkaline basalt. Trachyte
basalt is the dominant unit and characteristics of this volcanic unit. The mountains and ridges
were formed along the center of eruption by outpouring of volcanic rocks with total thickness
of about 3000m near the center of eruption and the thickness gradually decreases towards west
and east overlying the plateaus trap series.
21
The permeability of this rock unit is characterized by degree of weathering, fracturing, and
jointing which also is a function of depth. The upper few tens of meters are affected intensively
and have high hydraulic conductivity. With depth since the effect of weathering, fracturing and
jointing decrease, it is not expected to have such structure below 400meters below ground
surface in the area that do not affected by large faults.
The volcanic mountains and cones to the right side of the Upper Wabe Shebele (Kurduro,
Somkeru, Beranta, Wege Harena and Shewiso) are underlain by Tertiary volcanics of Pre-Rift
succession (Alaji basalt, alkali trachyte flows, and alkali trachyte and basalt flows); while those
to the left (Kaka, Hunkolo and Galema Mountain Range) are underlain by Post-Rift volcanic
succession including alkaline to peralkaline basalts and trachytes. The volcanic mountains are
formed both by central (e.g., Mt. Kaka) and fissural (e.g., Galema Mountain range) eruptions.
Volcanic pipes and craters are also frequent in this geomorphic unit.
2.8.3) Quaternary Volcanics and Sediments
2.8.3.1) Quaternary Volcanics
Cenozoic volcanism in the area ceased by eruption of Quaternary Volcanics represented by
Ginir Volcanics and basalt, often scoriaceous, with minor cinder cones and vitric tuffs.
2.8.3.2) Quaternary Sediments
The Quaternary volcanic sequence is topped by the Plio-Pleistocene fluvio-lacustrine
sedimentary sequence. This Plio-Pleistocene fluvio-lacustrine sedimentary sequence comprises
three main formations (Getaneh Asefa et al.,1982). From oldest to youngest they are (i) the
Gedeb Formation, (ii) the Adaba Formation, and (iii) the Mio Goro Formation.
The Gedeb Formation
Towards the end of Tertiary, basaltic lavas were extruded immediately upstream of the present
Hako-Wabe confluence. The north-east flowing late Pliocene Wabe became impounded behind
this lava dam and a large lake came into existence. The microscopic diatom flora which lived
in Pliocene lake Gedeb shows that throughout much of its existence the lake was relatively
deep and fresh. The lake was persisted for some 300,000years. Capping the lake sediments is a
welded tuff or ignimbrite dating to 2.35 Myr. Beneath the oldest diatomites and partially
22
interstratified with them are basalts dated to between 2.71 and 2.51Myr, indicating that
volcanic activity persisted after the formation of the lava dam.
The Adaba Formation
The Adaba welded tuff, dated 2.35 Myr, overlies three thick and highly weathered volcanic ash
horizons. These pyroclastic deposits are related to the late Pliocene faulting uplift which
culminated in the formation of the Ethiopian Rift.
The formation lies unconformably over the Gedeb Formation in localities such as around
Melka Wakena. It is composed mainly of ash and ignimbrites as well as feldspathic and
pumiceous sands. Gravels, breccia and paleosol are interbedded with the formation. The
thickness of this formation is about 10m.Its age is late Pliocene.
The Mio Goro Formation
The Mio Goro Formation, which lies unconformably over the Gedeb Formation in some
localities forming the Gedeb sedimentary sequence. No contact has been established between
the Adaba and Mio Goro Formations. The latter consists mainly of sands, sandstones, and
mudstones interbedded with clays, gravels, conglomerates, tuffs ashes and paleosols. The
thickness of this formation which is Pleistocene in age is about 20m.
Six major lithofacies have been recognized in the Gedeb sequence, and each defined in terms
of litholgic features and sedimentary structures(Getaneh Asefa et al, 1982).
These lithofacies are :
-Gravels/Conglomerates,
-Sands/Sandstones,
-Siltstones/Claystones,
- Diatomites,
-Ashes/Tuffs, and
- Calcareous sandstone.
23
Table 2.1:-Relative abundance & Description of lithofacies ( Adapted from Late Cenozoic
History &Archaeology of the Upper Wabe Shebele Basin, East-Central Ethiopian (Getaneh
Asefa et al.,1982).
Estimated Abundance
Facies
Gedeb
Adaba
Mio Goro
Formation
Formation
Formation
Description
-consists of sandy ,muddy gravels &friable conglomerates. The main
clasts are Tuffs(welded tuff, crystal tuff, glassy tuff &lithic tuff) and
Gravel/Conglomerates
-
-
Common
pebbles of pumice, rhyolites & tracytes; clasts of basalt ,cinder &clays
&granules of basaltic glass(≈10mm maximum dimension)
-composed
of
fine-grained,
moderately
to
poorly
sorted
sands
&sandstones
-texturally classified as muddysand, gravellysand, & silty sand. Pebbles,
mostly pumice, are distributed irregularly throughout the facies. Siltstones
&silty clay-stones are irregularly intercalated within the lithofacies.
-generally massive to horizontally bedded.Laminations,chnnels(up to
Sands/Sandstones
Rare
Abundant
Abundant
250cm horizontal dimensions), cross-stratification, convolte beddings,
polygonal mudcracks are fairly frequent. Cross-beddings are mostly
planer &of small to medium scale.
-Constituents include potassium & plagioclase feldsparse, quartz, &
volcanic rock fragments. Micas &other heavy minerals found as minor
constituent..Presence of channels, convolute bedding, small scale bedding
with over turned foresets & soil horizons with disturbed bedding suggest
that the rocks of this lithofacies are low-sinuosity flood plain deposits.
-Texturally classified as clayey silt, gravely silt & silty clay. Horizontallybedded or massive with rare &finely –developed laminations. Soil
Siltstones/Claystones
Abundant
-
-
horizons are well developed. Bedding thickness ranges from 0.2 to 8.5cm.
- Constituents include potassium feldsparse, plagioclase, quartz & pumice
as the predominant detrital grains.
-Composed of soft &porous diatomites, together with silty-sandy &
gravelly diatomaceous clays.
Diatomites
Abundant
-
-
-Massive or finely laminated(show an aqueous deposit)
-Contain abundant & well preserved planktonic fossil diatoms that are
characteristic of an oligohaline lake. Air fall ash beds within this
lithofacies suggest concurrent volcanism.
-Range from fine-grained to lapilli-tuffs, & compositionally most are
Ashes and Tuffs
Rare
Abundant
Rare
vitric tuffs.
-Belongs to air- fall deposits.
Calcareous
Sandstones
-Shows parallel bedding, less massive laminated,& small scale cross-
-
Common
stratified beds. Framework grains include quartz, potassium feldsparse,
biotite,sub-rounded to rounded pumice & aphanitic volcanic rocks.
24
2.8.3.3) Structures
A series of E-W, NE and N-SSW trending lineaments traverse the area. E-W-trending
lineaments dominate. The general trends of lineaments appear to coincide roughly with the
general trend of drainage systems in the area.
The lineaments are distributed throughout the study area, but more concentrated in north, west
and southwest of the studied area. They vary in length from 2km to more than 24km and
shifted both the regional and local flow of rivers from NW_SE and NNW-SSE to almost
ENE_WSW and ESE-WNW as seen in the upper course of Wabe river.
38.8
38.9
39
39.1
39.2
820000
39.3
39.4
39.5
39.6
Meraro
7.4
They dominantly affected the
4000m
810000
Gimbite
Ncb
7.3
Nazareth group rocks mapped in
7.2
the Gedeb-Asasa plain and also
Nct
800000
28
00
m
26
00
m
traverses
790000
780000
Nn
760000
0
PNa2
20000
40000 Meters
PNa3
260
0m
6.8
the storage and movement of
Qb - Ginidhir (Pleatu) basalts
Ncb - Chilalo Volcanics (Basalts)
Nct - Chilalo Volcanics (Ignimbrites & trachytes)
Nn - Nazeret group (Stratoid silcics)
PNa3 - Alkali trachyte & basalt flow
PNa2 - Alkali trachyte flow
PNa1 - Alaji Basalts
Lineaments
Rivers
Elevation contours
Towns
Thus, these faults and fractures
seem to have great contribution in
Legend
Pre - Rift
Volcanic succession
groundwater in the area.
The Quaternary volcanics which
cover the lineaments are mainly
alkaline and per alkaline stratoid
silcics,welded ignimbrite, pumice,
ash and rhyolite flows and dome.
Catchment area
Figure 2.9:-Geological map of the study area & distribution of the main lineaments (adapted
from geological map of Dodola sheet(NB37-7)and geological map of Ethiopia compiled by
Mengesha Tefera et al,1996 and WWDSE,MCE,WAPCOS,2004)
25
and
6.9
470000 480000 490000 500000 510000 520000 530000 540000 550000 560000 570000
Post - Rift
Volcanic succession
ignimbrites
trachytes of the Chilalo volcanics.
7
3800m
26
00
m
PNa1
the
7.1
Adaba
Dodola
770000
750000
24
00
m
Asassa
Kofale
CHAPTER THREE
RECHARGE ESTIMATION
3.1) Introduction
Groundwater recharge can be defined as the entry into the saturated zone of water made
available at the water table surface together with the associated flow away from the water table
within the saturated zone (Freez & Cherry, 1979). Recharge of groundwater may occur
naturally from precipitation, rivers or lakes and/or from a whole range of man’s activities such
as irrigation and urbanization. Further, an important way of categorizing recharge is to
consider it as direct, localized or indirect. The first is defined as water that is in excess of soil
moisture deficits and evapotranspiration and which is added to the groundwater reservoir by
direct vertical percolation through the unsaturated zone. The second is an intermediate form of
recharge that results from percolation to the water table following surface or near-surface
movement and subsequent collection and ponding in low-lying areas and in fractured zones as
a result of small-scale topographic or geological variability. Indirect recharge is percolation to
the water table through the beds of rivers, lakes and canals.
In characterizing catchments or aquifers for protection, understanding how and where recharge
occurs is necessary for three principal reasons:
• The relationship between the amount of recharge and the amount of abstraction defines the
land area subject to or receiving the recharge that needs to be protected;
• The locations and processes of recharge and their relationship to potential sources of
pollution help to determine pollutant loads; and
• The relationship between the amount of recharge and the amount of abstraction helps to
define the susceptibility of the aquifer to the effects of excessive pumping.
26
The distinction between the last two is important. Thus, in relation to the objective of
groundwater protection, it may be often be more critical to identify locations, mechanisms and
speed of recharge rather than total volumes. General estimates of total recharge volumes are
needed to help define catchments and to estimate diffuse pollution loads. A greater degree of
effort is required to make estimates that are as precise and reliable as possible for groundwater
resources management. Recharge estimation can, however, be technically difficult and costly.
Recharge is governed by the intricate balance between several components of the hydrologic
cycle, each of which is a function of several controlling factors such as;
- Precipitation: which is a function of intensity, frequency, variability, spatial distribution. It is
the primary factor for the occurrence of ground water which is the input for both surface and
subsurface waters.
-Evapotranspirative losses: which is a function of temperature, wind, humidity and
phreatophites.
- Discharge losses: which is a function of interflow, springs, base flow, lateral flow and
artificial discharge.
-Catchment: which is a function of soil type, thickness, spatial distribution, topographic
features, and vegetation. Soil type and vegetation cover controls the amount of recharge to
the groundwater as well as ground water chemistry (Freeze and Cherry, 1979). Areas with
thick soil covers having deep rooted vegetation will promotes infiltration while areas with
bare lands and thin soil cover enhances more surficial erosion. Adverse land practices
aggravate surficial erosion through deforestation and these could even have persistent effect
on climate.
-Geology: which is a function of rock types, characteristics of fracture networks, occurrence of
dykes).
3.2) Precipitation
The quantity and the duration of rainfall vary from place to place due to the inter-tropical
convergence zone (ITCZ) and physiographic conditions. Air masses change their positions
with apparent movement of the overhead sun, north and south. For most of the areas the rainy
session starts in March and extends to October with the highest rain fall concentration in June,
27
July and August. On the average, the area gets a monthly mean rainfall of 85mm and an annual
mean rain fall of 924.4mm. However, there is a disparity from one area to another in the
amount of rain fall distribution. The central plain of the study area receives mean annual rain
fall of 683 - 800mm and the peripheries of high lands of the study area receive rainfall ranging
from 800 - 1184mm. It can be said that the foot hills of the mountain areas have adequate
rainfall both in amount and in seasonal distribution but the central plain of the area has low
precipitation.
The mean annual rain fall on the bases of 7 to 38 years of record of 7 rain fall stations that
contribute to the catchment is estimated to be 939.74mm.
The peak of rainfall is observed
M e an m onthly pre cipitationTre nd
in August. The water discharge
160
MeanmonthlyPPt(mm)
140
120
Mean
monthly
precipitation
100
80
at the outlet, starts to increase
in July, reaches its maximum in
August and its minimum in
60
40
December.
20
0
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
M onth
Figure 3.1:-Mean Monthly precipitation trend from stations in and around the catchment area.
The correlation between
Variation of precipitation w ith altitude
precipitation and altitude is
Precipitation(mm)
1600
1400
poorly(negatively) correlated
1200
(-0.54) in and around the
1000
catchment area due to
800
600
400
1600
y = 0.391x
R2 = -0.2938
1800
2000
2200
2400
2600
2800
3000
significant orographic effect.
3200
Altitude(m .a.s.l)
Figure 3.2:-Relation ship between surface elevation and depth of precipitation
28
The Arithmetic mean
Most of the hydrological and meteorological observation stations of the networks of Ethiopia
are characterized by uneven and very sparse density in their spatial distribution. Inaccessibility
of most of the areas of the country has been the main reason for such unsatisfactory level of
distribution of stations. Almost all major towns of the country are located on the highland areas
and the road networks connecting these towns and are aligned along basin divides and the
stations have been established using these roads. Also the stations are established on tributaries
and at the head catchments of the river basins. There are only very few roads that cross big
rivers as such only very few large catchments have been gauged and the rainfall and stream
flow gauging stations are located at the head catchments of the river basins. In the study area
similar condition has been reflected. Most of the river gauging stations installed on tributaries
of Wabe river and meteorological observation stations of the networks are located on the
highland areas (eg.Adaba, Dodola, Kofele, Meraro, Siltana & Kore).
The rain fall stations used in the calculation are usually those inside the catchment area, but
neighbouring gauges outside the boundary may be included if it is considered that the
measurements are representative of the near by parts of the catchment (Shaw, 1994).
Accordingly, for the arithmetic mean evaluation of precipitation in the study area neighbouring
gauging stations from Kore, Siltana, and Meraro are included.
The Arithmetic mean method of estimating aerial depth of precipitation over an area is reliable
if the rain gauge network is of uniform density, distributed over flat area, and the variation of
individual gauge records from the mean is not too large. However, meteorological stations in
the study area more concentrated near more elevated high lands than in plain areas except at
Asasa and relatively at Hunte. The distribution of rainfall in the study area also highly
influenced by orographic effect and therefore, the arithmetic mean method is not reliable. In
such areas Isohyetal method is the best method in estimation of aerial depth of precipitation.
The arithmetic mean aerial depth of precipitation is calculated using point rain fall data
obtained from seven meteorological stations with record period varying from 9 to38 years
29
found with in the study area. The result is obtained by dividing the sum of the rainfall amounts
recorded at all the rain gauge stations which are located within and near by the area under
consideration with the number of stations.
Based on this method the average depth of
precipitation of the area, P = 924.40mm.
Table 3.1:- Long term arithmetic mean monthly depth of rainfall (mm) of the seven
stations in and the surrounding study area.
Stations
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Annual
Adaba
24.43
36.48
56.00
81.28
58.04
79.57
190.82
164.37
73.23
50.39
6.01
14.09
834.71
Asasa
28.71
38.88
48.81
55.49
52.55
58.19
129.53
148.61
60.59
42.11
16.25
4.13
683.33
Kofele
39.20
34.90
153.00
150.80
132.20
134.60
114.90
134.60
114.90
126.60
18.90
29.20
1183.80
Hunte
18.38
28.89
40.44
71.93
44.61
66.38
181.74
185.01
72.86
33.37
8.70
6.55
756.86
Dodola
38.62
52.92
90.12
94.12
67.41
116.91
159.87
167.19
108.81
55.73
22.27
20.37
994.34
Meraro
32.07
34.60
62.00
103.57
87.40
82.37
179.03
183.93
91.67
45.43
26.80
11.83
940.70
Ardayita
55.81
16.71
58.85
63.39
78.43
157.33
142.47
131.88
89.63
18.43
27.04
12.80
852.77
Kore
37.19
63.89
100.24
138.65
132.17
100.91
164.56
178.62
136.67
81.99
24.02
23.51
1182.44
Siltana
23.60
35.92
81.25
121.49
112.78
70.11
103.08
104.53
121.54
80.58
19.16
16.61
890.66
Mean
33.11
38.13
76.75
97.86
85.07
96.26
151.78
155.42
96.66
59.40
18.79
15.45
924.40
The Thiessen polygon method
The Thiessen method for determining aerial rain fall is sound and objective, but it is dependant
on a good network of representative rain gauges and not good for mountainous areas (Shaw,
1994). Based on the available rainfall data taken from 17 stations (annex 3.1) analysis was
done to obtain the effective uniform depth of precipitation for the catchement (Fig.3.3).
There are variations of the precipitation depths in each of the stations and the geomorphology
of the central study area show a significant change with its surroundings in slope. The
minimum aerial depth of precipitation was recorded at Asasa and the maximum at Kofele
station.
30
38.6
38.7
38.8
38.9
39
39.1
39.2
39.3
39.4
39.5
39.6
7.4
Bekoji
830000
Legends
Meraro
Enclosed polygon
820000
7.3
Siltana
Catchment area
Meteorological stations
7.2
810000
7.1
Kore
800000
Assasa
790000
7
Dinsho
Kofele
780000
Ardayita
Hunte
6.9
Adaba
Dodola
770000
6.8
760000
6.7
750000
6.6
Arbegona
0
20000
40000 meters
460000 470000 480000 490000 500000 510000 520000 530000 540000 550000 560000 570000
Figure 3.3:- Aerial rain fall using the Thiessen polygon method.
Table 3.2:- Aerial mean depth of precipitation using Theissen polygon
Enclosed Area by
Polygon
2
Mean Annual
Annual Weighted Rainfall
Weighed
Rainfall
(P = An/AT)Pn
Station
(An in Km )
area (%)
(Pn in mm)
(in mm)
Adaba
776.00
17.29
834.71
144.29
Asasa
758.00
16.89
683.84
115.47
Kofele
499.00
11.12
1183.80
131.59
Hunte
407.00
9.07
758.86
68.8
Dodola
783.00
17.44
994.34
173.44
Meraro
38.20
0.85
940.70
8.01
Bekoji
51.20
1.14
1066.71
12.17
Kore
203.00
4.52
1182.44
53.47
Ardayita
771.00
17.18
852.76
146.46
Siltana
82.10
1.83
890.66
16.29
Dinsho
53.30
1.19
1346.84
15.99
Arbegona
67.20
1.50
894.58
13.39
Total
4489.00
100.00
899.37
31
The Isohyetal method
The Isohyetal method for calculating monthly or annual precipitation over a catchment takes
into consideration the topographical effects on rain fall distribution.
38.8
38.9
39
39.1
39.2
39.3
39.4
39.5
39.6
820000
7.4
1000m
m
Isohyetal lines
810000
Catchment area
7.3
800000
7.1
1050mm
50
10
1000mm
85
0m
m
mm
m
0m
95
7
100
0m
m
mm
00
11
900
m
m
770000
0m
0m
780000
75
790000
80
mm
7.2
760000
750000
6.9
6.8
0
20000
40000 Meters
470000 480000 490000 500000 510000 520000 530000 540000 550000 560000 570000
Figure 3.4:- Aerial mean depth of rain fall using the Isohyetal method
Table 3.3:- Aerial mean depth of precipitation using Isohyetal method
Area between
Weighed
Isohyets
Range
2
Average Value of
Weighted Rainfall
Isohyets
(P =[(Σ pnAn)/AT]
0.25
(Pn in mm)
700
(in mm)
1.78
area (%)
< 700
(An in Km )
11.4
700-750
204.6
4.56
725
33.04
750-800
394
8.78
775
68.02
800-850
441
9.82
825
81.05
850-900
682
15.19
875
132.94
900-950
669
14.90
925
137.85
950-1000
1129
25.15
975
245.23
1000-1050
450
10.02
1025
102.75
1050-1100
293.6
6.54
1075
70.31
1100-1150
163
3.63
1125
40.85
>1150
51.4
1.16
1150
13.34
Total
4489
100
927.26
32
Table 3.4:- Meteorological stations in and around the study area
No
Station
Recording
period
1976-2006
Location
Altitude
(UTM)
m.a.s.l
Mean monthly precipitation
(mm)
(m)
X
Y
544179
775610
2420
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Annual
24.43
36.48
56.00
81.28
58.04
79.57
190.82
164.37
73.23
50.39
6.01
14.09
834.71
1
Adaba
2
Asasa
1976-1983
534961
795870
2370
28.71
38.88
48.81
55.49
52.55
58.19
129.53
148.61
60.59
42.11
16.25
4.13
683.84
3
Kofele
1976-2006
477913
781123
2620
39.20
34.90
153.00
150.80
132.20
134.60
114.90
134.60
114.90
126.60
18.90
29.20
1183.80
4
Hunte
1980-2006
544176
779295
2380
18.38
28.89
40.44
71.93
44.61
66.38
181.74
185.01
72.86
33.37
8.70
6.55
758.86
5
Dodola
1988-2006
520250
771910
2620
38.62
52.92
90.12
94.12
67.41
116.91
159.87
167.19
108.81
55.73
22.27
20.37
994.34
6
Meraro
1968-2006
540459
823512
2975
32.067
34.6
62
103.57
87.4
82.367
179.033
183.933
91.667
45.4333
26.8
11.83
940.7
7
Bekoji
1976-1996
527676
832847
2810
34.77
55.48
91.02
115.01
112.85
111.47
180.13
193.52
84.45
54.45
18.89
14.69
1066.71
8
Gobessa
1980-2006
555245
843925
2500
51.34
46.63
108.88
174.74
128.58
84.18
150.49
200.93
138.97
126.10
57.77
43.45
1312.06
9
Kersa
1977-1997
496418
834681
2700
32.78
56.41
82.03
124.80
108.74
81.08
118.09
124.96
120.03
64.28
17.44
18.35
948.99
10
Kore
1977-1995
489055
797836
2500
37.19
63.89
100.24
138.65
132.17
100.91
164.56
178.62
136.67
81.99
24.02
23.51
1182.44
11
Ardayita
1981-1997
501936
781255
2900
55.81
16.71
58.85
63.39
78.43
157.33
142.47
131.88
89.63
18.43
27.04
12.80
852.76
12
Siltana
1977-2006
543073
818457
2960
23.60
35.92
81.25
121.49
112.78
70.11
103.08
104.53
121.54
80.58
19.16
16.61
890.66
13
Dinsho
1978-2006
584760
785009
2750
22.55
39.60
86.93
189.35
126.24
93.25
178.26
201.29
153.74
155.98
59.40
40.23
1346.84
14
Arbegona
1990-2006
468781
740734
2500
38.14
39.44
81.72
133.99
86.26
83.85
92.27
76.82
85.55
93.02
41.46
42.06
894.58
15
Wondogenet
1977-2005
454089
792329
1880
30.04
51.73
106.16
141.35
128.14
105.16
135.23
135.93
147.83
101.70
29.00
22.38
1134.64
16
Agarfa
1977-2006
590249
803443
2550
19.11
47.83
103.94
210.20
154.46
91.69
143.78
174.28
138.34
134.10
46.28
32.76
1296.75
17
Shashemenne
1977-2006
455933
796012
2080
23.60
35.92
81.25
121.49
112.78
70.11
103.08
104.53
121.54
80.58
19.16
16.61
890.66
33
3.3) Eastimation of Evapotranspiration (ET)
Evaporation from a vegetated land surface is normally a combination of direct evaporation
from a wet surface, and water consumption or transpiration by the vegetation. This combined
effect is called evapotranspiration. Climatological parameters that influence evapotranspiration
include radiation, air temperature, relative humidity, wind movement, soil moisture, and
vegetative type (different plants at different development stage and plant density transpire
water at different rates). A wind break reduces wind velocities and decreases the
evapotranspiration rate of the area directly beyond the barrier. Factors such as soil salinity, soil
water content, poor land fertility, limited application of fertilizers, the presence of hard or
impenetrable soil horizons, the absence of control of diseases and pests and poor soil
management may limit the crop development and reduce the evapotranspiration.
3.3.1.) Common hydrometeorological factors affecting evapotranspiration
3.3.1.1) Temperature
Temperature enhances evapotranspiration through making the environment hot and favors the
passage of liquid state of water to vapor state.
Temperature records in the study area are available for all meteorological stations except for
Asasa station. It is measured at five stations and the mean monthly temperature is computed as
the arithmetic average of the mean daily temperature of all the days in the month. The mean
maximum temperature (22.79 oC) of five stations is recorded in the month of February where
as the mean minimum temperature (4.52 oC) recorded is during the month of December.
Based on the records of the mean monthly maximum and mean minimum temperature data the
monthly average maximum, average minimum and average temperatures of the study area are
about 21.14 oC, 7.49 oC, and 14.32 oC respectively.
34
Table 3.5:- mean monthly maximum temperature of the five stations in the study area( oC).
No
1
2
3
4
5
6
Station
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Adaba
24.60
24.98
24.68
24.19
24.67
24.12
22.34
21.80
22.65
22.78
23.44
24.01
Dodola
22.69
22.84
22.95
21.95
22.89
21.05
17.67
18.04
18.48
18.71
19.47
19.81
Kofele
20.81
21.68
21.60
20.14
20.02
18.76
17.67
17.84
23.03
19.27
19.96
20.28
Meraro
18.77
19.18
18.61
18.09
18.42
18.39
16.31
16.34
16.69
16.41
17.39
17.93
Hunte
24.68
25.25
25.27
24.19
24.86
24.68
22.28
21.47
21.99
22.48
23.28
23.74
mean max.
21.96
22.26
22.13
21.31
21.58
20.85
19.08
19.03
20.20
20.05
20.80
21.18
Table 3.6:- mean monthly minimum temperature of the five stations in the study area ( oC).
No
Station
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
1
Adaba
3.33
4.21
5.42
6.40
6.38
6.28
7.53
7.23
6.55
5.79
3.81
2.47
2
Dodola
4.34
5.68
6.09
8.41
7.12
7.83
8.97
8.90
8.22
5.64
3.56
2.99
3
Kofele
6.18
6.94
7.85
8.78
8.60
8.73
9.07
8.82
8.25
7.51
5.70
5.61
4
Meraro
8.04
9.63
11.68
13.95
13.29
11.39
12.56
12.39
11.63
10.74
8.74
8.55
5
Hunte
4.03
4.74
6.91
8.70
8.47
8.44
9.45
9.25
8.02
6.63
4.04
2.99
6
mean min.
5.26
6.39
7.46
8.75
8.33
8.13
8.81
8.59
7.92
6.91
5.32
4.90
Table 3.7:-mean monthly temperature of the five stations in the study area ( oC).
No
1
2
3
4
5
6
Station
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Adaba
13.97
14.60
14.96
15.30
15.52
15.20
14.93
14.52
14.60
14.28
13.63
13.24
Dodola
13.52
14.26
14.52
15.18
15.01
14.44
13.32
13.47
13.35
12.18
11.52
11.40
Kofele
13.50
14.31
14.73
14.46
14.31
13.74
13.37
13.33
15.64
13.39
12.83
12.94
Meraro
13.40
14.41
15.14
16.02
15.85
14.89
14.44
14.36
14.16
13.57
13.06
13.24
Hunte
14.36
14.99
16.09
16.45
16.66
16.56
15.87
15.36
15.01
14.56
13.66
13.37
Average
13.75
14.51
15.09
15.48
15.47
14.97
14.39
14.21
14.55
13.60
12.94
12.84
35
28
24
16
12
8
4
Mean Max.T
Month
Mean Min.T
Dec
Nov
Oct
Sep
Aug
Jul
Jun
May
Apr
Mar
Feb
0
Jan
Mean Tem.(0c)
20
Average T.
Figure 3.5:-Mean Maximum, Minimum and Average monthly temperature
3.3.1.2) Relative Humidity
Relative humidity is the relative measure of the amount of moisture in the air to the amount
needed to saturate the air at the same temperature (Shaw, 1994). It varies from time to time,
depending on variation in rainfall and air temperature.
The relative humidity of almost all the stations is more than 50% except that of Hunte for the
months of January, February and March. The relative humidity at Hunte and Asasa stations is
less than the relative humidity of the remaining three stations. This is due to their relative
location and change in wind pattern.
Relative humidity records in the area show the mean monthly values of 67.01% with mean
minimum monthly of 56.34% in February and reaches maximum in August (79.84%).
Generally, the wet season has mean monthly relative humidity values of 74.94%.
There are five meteorological stations recording relative humidity in the study area.
Table 3.8:- Mean monthly relative humidity of stations in the study area.
Mean Monthly Relative Humidity (%)
No
Station name
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
1
Adaba
67.67
68.92
69.83
70.75
71.42
74.17
76.08
75.92
75.25
73.83
71.90
68.10
2
Asasa
50.67
53.16
52.25
59.12
55.82
60.49
72.26
74.96
69.36
60.40
55.03
52.74
3
Hunte
48.90
47.00
43.33
59.00
57.33
58.33
76.50
79.67
76.67
70.50
63.00
56.67
4
Meraro
62.33
56.83
58.33
68.52
66.57
72.57
77.38
82.38
78.00
73.00
64.29
61.57
5
Kofele
63.90
55.80
65.40
72.20
75.30
83.50
86.50
86.25
83.25
76.25
65.75
67.58
6
Mean
58.69
56.34
57.83
65.92
65.29
69.81
77.74
79.84
76.51
70.80
63.99
61.33
36
The lowest and maximum average monthly mean relative humidity was registered in the
months of February and August respectively.
Relative humidity (%)
Relative hum idity(%)
85
80
75
70
65
60
55
50
45
40
Jan Feb Mar Apr May Jun
Jul Aug Sep Oct Nov Dec
Relative humidity(%)
Figure 3.6:- Average monthly mean trend of relative humidity (%)
3.3.1.3) Wind speed
The rate of evaporation is influenced to some extent by air movement. The higher the wind
speed takes away the moisture in the air which facilitate evaporation if its movement is
turbulent than laminar. Wind speed varies with the height above the ground.
Table 3.9:-Monthly average wind speed at 2m above ground surface in m/sec.
Mean monthly wind speed (in m/sec)at 2meters a.g.l
Station
name
Adaba
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
1.05
0.70
0.60
0.95
0.75
0.30
1.20
0.95
0.80
1.05
1.50
1.40
Asasa
2.20
2.13
2.30
2.24
3.20
2.74
2.50
1.86
1.36
1.84
2.10
2.23
Hunte
1.55
1.55
1.53
1.90
1.83
1.48
1.27
0.97
1.10
1.33
1.75
1.70
Meraro
2.46
2.62
2.93
3.04
2.81
1.86
1.53
1.49
1.97
2.76
2.77
2.56
Kofele
1.30
1.38
1.30
1.40
1.18
1.43
1.48
1.48
1.21
1.39
1.50
1.48
Mean
1.71
1.68
1.73
1.91
1.95
1.56
1.60
1.35
1.29
1.67
1.92
1.87
The lowest and maximum average monthly mean wind speed was registered in the
months of September and May respectively.
37
From this figure we can under
stand that wind speed is very
high. The writer of this paper
m e a n m o n th ly w in d s p e e d ( m /s e c )
wind speed(m/sec)
2.40
observed the same case during his
2.20
field visit in mid of February.
2.00
This high wind speed facilitates
1.80
1.60
high moisture loss from soil. The
1.40
high AET&PET estimated in the
1.20
area were because of high wind
1.00
speed rather than temperature of
Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
the area.
wind speed(m/sec)
Figure 3.7:-Monthly average wind speed
3.3.1.4) Sunshine hours
It plays an important role for evapotranspiration & has a direct relationship with it. The area
attains its minimum and maximum sunshine hours during July and January respectively. The
lowest and maximum average monthly mean sunshine hours were registered in the months of
July and January respectively.
Table 3.10:- Average monthly mean sunshine duration (in hours) in the study area.
Mean Monthly Sun Shine Hours(Hrs/day)
No
Station name
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
1
Adaba
8.76
8.19
7.25
6.41
6.90
6.29
4.76
5.54
5.74
6.61
8.19
9.16
2
Asasa
8.89
7.38
7.12
6.36
6.88
5.62
4.63
5.37
5.81
6.82
8.15
8.71
Oct
Nov
Dec
3
Hunte
7.30
6.90
7.20
7.00
7.28
6.55
5.10
6.15
5.88
7.80
8.30
7.23
4
Meraro
8.43
7.82
7.94
6.72
7.48
6.42
4.63
5.26
5.79
7.10
8.67
9.27
5
Kofele
9.30
8.50
6.60
5.60
6.60
5.00
3.78
2.68
3.80
6.43
8.35
7.90
6
Mean
8.54
7.76
7.22
6.42
7.03
5.98
4.58
5.00
5.40
6.95
8.33
8.45
38
Mean sunshie hours
mean sunshine(in hrs)
10
9
8
7
6
5
4
3
2
Jan Feb Mar Apr May Jun
Jul Aug Sep Oct Nov Dec
mean sun shine hours
Figure 3.8:-Mean sunshine hrs of the study area
3.3.2) Estimation of evapotranspiration (ET)
3.3.2.1) Potential evapotranspiration(PET)
Potential evapotranspiration is defined as the evapotranspiration which would occur under
unrestricted availability of water from a vegetated surface (Shaw, 1994).
For calculating potential evapotranspiration various methods are used; two methods,
Penman modified and Thornthwaite approach empirical formulae are used in this study
based on the available input data of meteorological stations such as sunshine, temperature,
humidity and wind speed.
a) Penman Formula (Combination Method)
The method gives good estimates of PET if sufficient data available because it takes in to
account many meteorological variables but lack of relevant data such as long year temperature,
relative humidity and wind speed may hinder the accuracy of quantification of PET. It is
estimated on the basis of very limited hydrometeorological data and the available
meteorological stations have few years of incomplete data.
Table 3.11 summarizes the variables used PET estimate of the year.
⎛Δ⎞
⎜⎜ ⎟⎟ H T + E at
γ
PET = ⎝ ⎠
Δ
+1
γ
39
HT -The available heat, calculated from incoming (RI) & outgoing (Ro) radiation determined
from sunshine records, temperature and humidity using the formula: HT = 0.75RI − Ro ;
⎛n⎞
R I (1 − r ) = 0.75 Ra f a ⎜ ⎟
⎝N⎠
Ra – Solar radiation (fixed by latitude and season and is constant for a given latitude and
season, obtained from standard meteorological tables);
r- The reflective coefficient for incident radiation or albedo of the vegetation covers of the
catchment that depends on the nature of the surface.
⎛n⎞
- f a ⎜ ⎟ takes several forms based on latitude and for the study area latitudes south of 54 ½
⎝N⎠
0
n⎞
⎛n⎞ ⎛
N is taken as f a ⎜ ⎟ = ⎜ 0.16 + 0.62 ⎟ (Shaw, 1994)
N⎠
⎝N⎠ ⎝
n- Monthly mean sun shine hrs (from Meteorological record);
N – Daylight factor (Fixed by latitude and season and is constant for a given latitude and
season)
(
)
- R0 = σT 4 0.47 − 0.075 ed (0.17 + 0.83n / N ) ; R0 = outgoing radiation
σ T4 – The theoretical blackbody radiation at the temperature of the air (T in Kelvin scale) ;
σ -(Stefan –Boltzmann constant) = 5.67 x 10-8 Wm-2k-4,
ed – The saturated vapor pressure at dew point(mm of mercury), ed = ea (RH/100)
ea – The saturated vapor pressure at air temperature Ta,
RH– Relative Humidity in % - obtained from meteorological record. The Energy for
evaporation based on the air humidity and air temperature, the subscript t signifies inclusion
of transpiration effects.
u ⎞
⎛
E at = 0.35⎜1 + 2 ⎟(ea − ed ) ; ea-ed is saturation deficit,
⎝ 100 ⎠
U2 – Mean wind speed (miles/day) at 2m above the surface (from Meteorological record)
∆- The slope of the curve of saturated vapor pressure against temperature corresponding to
the air temperature (ea at Ta against Ta). ∆ = (ea-ed)/(Ta-Td)
-γ = hygrometric constant (0.27mmHg/0F), is the reflective coefficient for incident radiations
or the Albedo of the basin that depends on the nature of the surface. For the specific study area,
this is taken as 0.24.
40
Table 3.11:- Mean annual PET obtained from Penman method.
T
Month
0
( C)
T
n
N
(Kelvin)
(Hrs)
(Hrs)
n/N
HR
U2
ea
ed
(%)
(miles/d)
(mm/d)
(mm/d)
σΤa4
Ra
Eat
RI (1-r)
R0
Δ/γ
(mm/d)
(
mm/d)
(mm/d)
(mm/d)
fa(n/N)
(mm/d)
HT
PET
PET
(mm/d)
(mm/month)
Jan
13.75
286.75
8.54
11.80
0.72
58.69
91.82
11.80
6.93
1.94
13.25
3.27
6.05
2.76
0.61
13.18
3.22
3.24
100.39
Feb
14.51
287.51
7.76
11.90
0.65
56.34
90.21
12.38
6.97
1.65
14.20
3.60
6.01
2.57
0.56
13.32
3.32
3.42
95.84
Mar
15.09
288.09
7.22
12.00
0.60
57.83
92.90
12.80
7.40
1.70
14.90
3.64
5.96
2.39
0.53
13.43
3.45
3.52
109.23
Apr
15.48
288.48
6.42
12.20
0.53
65.92
102.56
13.12
8.65
1.74
15.08
3.17
5.50
2.05
0.49
13.50
3.42
3.33
99.84
May
15.47
288.47
7.03
12.30
0.57
65.29
104.71
13.11
8.56
1.73
14.70
3.26
5.67
2.18
0.51
13.50
3.37
3.33
103.20
Jun
14.97
287.97
5.98
12.40
0.48
69.81
83.77
12.78
8.92
1.69
14.45
2.48
4.97
1.87
0.46
13.41
3.05
2.84
85.14
Jul
14.39
287.39
4.58
12.30
0.37
77.74
85.92
11.98
9.31
1.64
14.58
1.74
4.27
1.53
0.39
13.30
2.68
2.32
72.01
Aug
14.21
287.21
5.00
12.30
0.41
79.84
72.49
12.18
9.72
1.62
14.80
1.48
4.57
1.60
0.41
13.27
2.89
2.35
72.93
Sep
14.55
287.55
5.40
12.10
0.45
76.51
69.27
12.39
9.48
1.66
14.83
1.72
4.86
1.73
0.44
13.33
3.10
2.58
77.43
Oct
13.60
286.60
6.95
12.00
0.58
70.80
89.68
11.62
8.23
1.57
14.40
2.25
5.61
2.18
0.52
13.15
3.36
2.93
90.81
Nov
12.94
285.94
8.33
11.90
0.70
63.99
103.10
11.21
7.17
1.50
13.48
2.87
6.01
2.63
0.59
13.03
3.26
3.10
93.06
Dec
12.84
285.84
8.45
11.80
0.72
61.33
100.42
11.05
6.78
1.49
12.95
3.00
5.87
2.74
0.60
13.02
3.01
3.01
93.20
Annual Evapotranspiration=1093.08mm
41
b) Thornthwaite Method
Thornwaite produced a formula for calculating PET based on temperature as index of energy
available for evapotranspiration with an adjustment being made for the latitude location and
number of daylight hours (Dunne and Leopold, 1978). This method ignored the effect of
vegetation index and maturity.
An estimate of the potential evapotranspiration, PET, calculated on a monthly basis, is given
by
the equation:
PET
m
_
⎛
⎜ 10 T
= 16 N m ⎜
I
⎜
⎝
a
m
⎞
⎟
⎟ mm
⎟
⎠
Where m is the months 1, 2, 3,…12, and N m
is the monthly adjustment factor related to
hours of daylight obtained by dividing the possible sunshine hours for the appropriate
_
latitude by 12, T m is the monthly mean temperature 0C(from meteorological stations in the
study area) (Table 3.12), I is the heat index for the year, given by:
1.5
⎛ _ ⎞
⎜T ⎟
I = ∑ im = ∑ ⎜ m ⎟
⎜ 5 ⎟
⎝ ⎠
for m =1,2,3,…12
and:
a = 6.7 x10−7 I 3 − 7.7 x10−5 I 2 + 1.8x10−2 I + 0.49
Table 3.12;-Annual PET calculated by Thornthwaite Method.
Months
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Tm ( C)
13.75
14.51
15.09
15.48
15.47
14.97
14.39
14.21
14.55
13.60
N
11.8
11.9
12.00
12.2
12.30
12.40
12.3
12.3
12.1
12
Nm
0.983
0.992
1.000
1.017
1.025
1.033
1.025
1.025
1.008
1.000
0.992 0.983
im
4.56
4.94
5.24
5.45
5.44
5.18
4.88
4.79
4.96
4.49
4.16
I
58.22
a
1.41
0
PETm(mm/yr)
Nov
Dec
12.94 12.84
11.9
11.8
4.12
52.84 57.52 61.28 64.61 65.05 62.59 58.74 57.71 58.68 52.92 48.94 47.97 688.86
42
The evaluated annual PET of the catchment area using Thornthwaite empirical formula is
688.86 mm/yr (Table 3.12). This value is extremely less than the value calculated by using
Penman formula (1093.08mm/yr). Thornthwaite empirical formula uses only temperature with
minor adjustment for the number of day light hours as an input to calculate evapotranspiration.
This significant difference shows that evapotranspiration in the study area is more affected by
wind rather than temperature. The Penman method gives reasonable estimate of PET because it
takes in to account many meteorological variables which govern the rate and magnitude of
evapotranspiration and therefore, the annual PET of the catchment obtained by this method is
used for further analysis. The area is rainfall deficit, i.e. the annual rainfall is less than the
evaporation.
3.3.3) Actual evapotranspiration(AET)
Actual evapotranspiration is used to describe the amount of water loss that occurs under field
condition. It is therefore, the amount of evaporation that occurs under a given climate and soil
moisture and is less than or equal to potential evapotranspiration.
Estimation of Actual Evapotranspiration (AET)
a)Turc method:- (1954, 1955, in Shaw,1994). According to Turc, precipitation and
temperature could be the dominant factors in evaporation. The empirical formula is:(mm/annum)
P
AET =
0 .9 +
P
2
[L (T )]2
Where P is the mean annual precipitation (mm),=927.26mm, T is the mean air
temperature of the area(0C)=14.320C and L (T) = 300 + 25T + 0.05T3. =804.83
Accordingly the estimated annual AET of the study area using Turc is 621.30mm/yr.
Soil water balance method (Thornthwaite and Mather, 1957: This approach strictly
follows the one outlined by Thornthwaite and Mather, 1957.
The values of soil moisture deficit and actual evapotranspiration vary with soil type and
vegetation (Shaw, 1994). Accordingly, the catchment area has been classified in to two major
groups of soil (clay loam and fine sandy loam) with four types of vegetation cover depending
on their root depths: Shallow rooted crops and vegetables (peas, beans carrot etc.), deep rooted
43
(grass, shrubs, and bushes), moderately deep rooted plants on intensively cultivated land which
include cereals such as wheat, barely and corns, and mature forest Table 3.13.
Table3.13:- Suggested available water capacities for combinations of soil texture and
vegetation.(From Thornthwaite and Mather 1957.)
Soil texture
Type of land
cover
Shallow rooted
Fine sandy
plants(peas
loam
beans etc.)
Clay loam
Moderately deep
Fine sandy
rooted plants
loam
(wheat, barely
Clay loam
corn)
Rooting
Available
depth
water
(m)
Capacity
0.50
75
340.27
7.58
0.40
100
340.28
7.58
1.00
150
1248.84
27.82
0.80
200
897.80
20.00
Area
Area
AET
(km2)
( %)
(mm)
Fine sandy
crops(grasses,
loam
1.00
150
224.45
5.00
clay loam
1.00
250
393.68
8.77
bushes and
Mature forest
830.57
62.96
865.85
240.88
880.45
176.09
865.85
43.29
924.46
81.08
927.26
74.18
124.99
19.27
7.334
5.438
2.884
0.310
359.12
8.00
clay loam
2.00
400
605.57
13.48
4410
98.24
927.26
878.66mm
79.00
1.76
AET=PET
4489
100
-
9.067
16.047
300
-
Surplus
61.22
2.00
-
AET(mm)
807.63
Fine sandy
Total
Reservoir
Weighted
(mm)
Deep rooted
shrubs)
Weighted
864.69
0.000
0.000
41.079
869.83
Descriptions and computation approaches of each of the main components that basically
influence the balance is presented in the following way:
•
P- The mean monthly precipitation values obtained from Isohyetal polygon
method is presented in raw 1 of the table.
•
PET- The mean potential evapotranspiration calculated by the Penman method is
listed in raw 2
•
P-PET-Changes in the balance of the precipitation and potential evapotranspirationof
each month is computed and presented in raw 3, following the computation,
44
•
APWL-The accumulated potential water loss which is obtained by adding the
negative values of P-PET of consecutive dry months are listed in raw 4. The
summation begins with the first month of dry season.
•
SM- The amount of water that will be retained by the soil (soil moisture) for
each month is calculated and listed in raw 5.
The soil moisture during the dry moths is obtained from the following formula:
⎡ (La m ) ⎤
S m = W exp ⎢−
⎥
⎣ W ⎦
Where, Sm: Soil moisture during the month m (mm)
Lam: Accumulated potential water loss at month m (mm).
W: Available water capacity of the root zone (mm)
For each wet months the soil moisture(SM) is obtained by adding the excess of rain of the
current month to the soil moisture of the month before. However, this sum may not exceed
the water capacity and excess is booked as moisture surplus.
•
AET- Actual evapotranspiration. Monthly actual evapotranspiration (AETm), The
relationship between AET & PET depends up on the soil moisture content. When the
soil is saturated or when there is abundant moisture in the soil, PET = AET (Shaw,
1994). On the other hand, when the vegetation is unable to abstract water from the soil,
then the actual evaporation becomes less than potential evapotranspiration. Therefore,
it is always less than or equal to potential evapotranspiration (PET).Thus, it is found as:
AET = PET
if Pm > PETm
Otherwise, AETm = Pm + Sm - 1 – Sm, , m stands for month, Where, Sm-1 and Sm are soil
moisture during the month m-1 and m respectively.
45
•
∆S-Change in soil moisture. The change in the soil moisture during the month is
obtained by deducting the soil moisture of the month under consideration from the soil
moisture of the preceding month. These values are entered in raw 6.
•
SMD- Soil moisture deficit. Monthly SMD is the difference between PETm & AETm
•
TARO-Total available for runoff. Based on the assumptions of Thornthwaite and
Mather, 1957, 50% of the surplus water that is available for run off in any month actually runs
off, the rest 50% of the surplus is detained in the subsoil, groundwater and channels of the
catchment and is available for runoff during the next month. The value is determined, starting
from the first month of the water surplus period. The first months surplus is the TARO for that
month, and from the surplus, 50% value is detained (D) and carried over to the next month of
TARO, and 50% is river discharged (RO). Therefore, the TARO of preceding month is given
by the surplus of that month (SM) plus the detention of last month (Dm – 1). These values are
listed in row 10.
Values of the major components of the soil water balance are presented in the following table
for shallow rooted crop cover(peas, beans,etc.) on clay loam soil with estimated rooting depth
of 0.40m and 7.58% land cover of the area with an available water capacity of 100mm and the
remaining indicated under annexes 3.5 to3.11.
46
Table 3.14:-AET for clay loam soil with an available water capacity of 100mm under shallow
rooted crop cover with estimated rooting depth of 0.40m (7.58% land cover of the
area)(peas,beans)
Parameter
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Total
P
33.20
38.24
76.96
98.13
85.31
96.53
152.20
155.85
96.93
59.57
18.84
15.49
927.26
PET
100.39
95.84
109.23
99.84
103.20
85.14
72.01
72.93
77.43
90.81
93.06
93.20
1093.08
P-PET
-67.19
-57.60
-32.27
-1.71
-17.89
11.39
80.19
82.92
19.50
-31.24
-74.22
-77.71
-165.82
-252.36
-309.96
-342.23
-343.97
-361.83
-31.24
-105.46
-183.17
8.02
4.51
3.26
3.21
2.68
100.00
73.17
34.83
16.01
APWL
SM
∆S
AET
SMD
S
TARO
RO
Detention
14.07
94.26
100.00
-8.16
-3.58
-1.27
-0.04
-0.54
11.39
80.19
5.67
0.00
-26.83
-38.33
-18.50
41.20
41.75
78.21
98.19
85.83
85.14
72.01
72.93
77.43
86.40
57.18
34.31
830.57
59.19
54.09
31.02
1.65
17.37
0.00
0.00
0.00
0.00
4.41
35.88
58.89
262.51
0.00
0.00
0.00
0.00
0.00
0.00
0.00
77.25
19.50
0.00
0.00
0.00
96.75
6.63
3.31
1.66
0.83
0.42
0.21
0.10
77.25
58.13
29.06
14.53
7.26
3.31
1.66
0.83
0.41
0.21
0.11
0.05
35.63
29.06
14.53
7.27
6.63
3.32
1.66
0.83
0.42
0.21
0.10
0.05
35.62
29.06
14.53
7.26
6.63
3.4) Ground Water Recharge Estimation
3.4.1) Recharge Estimation from soil moisture balance approach
According to Thornthwaite and Mather, 1957, assumptions, 50% of the surplus water available
has been considered as the amount that could be infiltrated to the groundwater and 50% as
surface runoff. Accordingly, the estimated recharge and runoff is found to be 20.55mm/yr.
3.4.2) Recharge Estimation from water balance method
Applying water balance equation for the area:
P = AET + RO + R + W +AETr x Ar/At ±∆S
Where P-precipitation,
AET-Actual evapotranspiration
RO-Runoff
R-Recharge
W-Withdrawal for different consummative use (assumed negligible)
AETr- Actual evapotranspiration from Melka Wakena Hydroelectric water reservoir
Ar- Area of Melka Wakena Hydroelectric water reservoir
47
At- Catchment total area
±∆S= Change in soil moisture obtained from soil moisture balance (negligible)
R = P - (AET + RO + W+AETr x Ar/At) ±∆S
= 927.26 - (864.69 + 20.55+19.27) = 22.75mm/yr.
The amount of water that evaporates from Melka Wakena Hydroelectric water reservoir is
calculated using meteorological data obtained at Hunte meteorological station as more
representative applying Penman aerodynamic and energy budget combined method (annex
3.12).
3.4.3) Recharge Estimation from Base Flow Separation
In estimating recharge for a given catchment from base flow the assumption is that the base
flow of a river is equal to the total groundwater recharge of the catchment upstream of the
discharge measuring site (Tenalem,1998) and the following assumptions are also included:
•
Surface water divide coincide with the groundwater divide and there is no inflow
or outflow of water from the catchment.
•
There is no loss of water below the river bed at the measuring site.
•
There is no (or negligible) diversion or addition of water in to the river.
Obtaining discharge data at ungauged sub catchment
There was a stream gauge on the main Wabe river outlet at which the catchment area is
delineated before the construction of Melka Wakena Hydroelectric power dam and it ceased its
operation since then. Therefore, the main Wabe rive does not have a stream gauge on the outlet
at which the catchment area is delineated. An operating gauge is located upstream near bridge
on the main river on the way from Asasa to Dodola (UTM=536816E & 775604N).The
bounding catchment area at this gauge is 1035km2. Therefore, drainage-area ratio method
between the drainage area of the gauged sub-catchment and ungauged sub catchment is
employed to estimate the discharge of the river at the delineated mouth of the river.
The estimation is made on considering the following conditions: similarity in topography,
climate patterns, soil characteristics, land-use and land cover.
48
The discharge data at the outlet delineated of Wabe River sub catchment is extrapolated from
the gauging station near Dodola at Bridge (UTM=536816E, 775604N) on the basis of drainage
area ratio as follows:Qoutlet /Qgauged = (A2/A1)
Where, A1 is the drainage area of the gauging station
A2 is total drainage area of the delineated catchment
Qgauged is stream flow in m3/sec of the gauged stream
Qoutlet is the discharge in m3/s at the mouth of delineated catchment area.
Projected mean river discharge of Upper Wabe river catchment (m3/s) from 1976-2006 at out
let of the delineated catchment(See annex 3.13).
The total extrapolated flow of the rivers is broadly broken down in to ground water run- off
(base flow) and surface water runoff, from which the surface water runoff values are taken to
equate the annual water budget of the catchment, and the hydrograph is further analyzed to
obtain the annual ground water recharge.
The hydrograph separation of the Upper Wabe River is performed by a time plot soft ware
developed by Gabriel Parodi which uses daily flow values and an attenuation coefficient that is
controlled by slope, land-use and land cover conditions of a water shed possessing a value in
the range of 0.9-0.995.
MelkaWakena Hydroelectric Power Reservoir floor assumed as compacted or water tight and
its area is deducted for recharge estimation from base flow and the total area is assumed for
runoff estimation.
49
90.0
80.0
70.0
Discharge(m3/s)
60.0
Measured
BF
50.0
40.0
30.0
20.0
10.0
0.0
1-Jan-80
10-Apr-80
20-Feb-80
30-May-80
27-Oct-80
7-Sep-80
19-Jul-80
16-Dec-80
Days
Figure 3.9:- Base flow Separation of Upper Wabe River using Time- plot
After extrapolation using attenuation coefficient of 0.995.
Table 3.23:- Result of base flow separation.
Average flow in the months (m3/s)
m3/s
Averge
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Measured
6.514
7.863
17.378
23.148
26.052
45.033
68.617
60.730
40.463
37.531
13.161
7.566
29.504
BF
5.834
6.295
13.735
17.584
22.074
30.431
35.055
39.429
36.856
34.305
11.637
6.834
21.672
RO
0.680
1.568
3.642
5.563
3.978
14.602
33.561
21.301
3.607
3.225
1.524
0.731
7.832
Accordingly, from the flow separation method the calculated recharge over the area is
154.98mm/yr.
Recharge estimated by base flow separation greatly higher than recharge estimated by soil
moisture and water balance methods. This may be attributed to groundwater inflow from near
by catchment or limitations (error introduced) during extrapolation of river discharge from
gauged station to ungauged station or due to other limitations during estimation of recharge
from soil moisture surplus. The average recharge in eastern (high peaks) and Central Ethiopian
high lands is from150-250mm (Tenalem et al., 2008). Therefore, recharge estimated by base
flow separation is reasonable and is taken as the groundwater recharge of the area.
50
CHAPTER FOUR
HYDROGEOLOGY
4.1) General
The subsurface geological formations may be considered as “ware house” for storing water
that comes from sources located on the land surface. Besides the origin, movement and
chemical constitution of groundwater is controlled by the type of lithology, distribution,
thickness and structure of hydrogeological units through which it moves. Man, through
socioeconomic activities have also potential power to alter the natural groundwater flow
systems and its quality. Moreover, the stresses due to tectonism and weathering conditions
govern the hydrogeochemical characteristics of earth materials. Therefore, acquiring
knowledge about the existing aquifer materials, their spatial distribution and their hydraulic
properties is a necessity.
The nature and distribution of aquifers, aquitards and aquicludes in geological system are
controlled by lithology, stratigraphy, and structure of geologic deposits and formations. Faults,
fractures and dykes may play a very important role in groundwater recharge and flow in
volcanic terrains. However, the effects of these structures on the volcanic rock permeability
depend on their distribution, orientation and density.
4.2) Hydrogeologic units and groundwater movement
Groundwater occurrence and movement in volcanic rocks is mainly governed by the type of
porosity and permeability formed during and after the rock formation. Thus, the type of rock
mineralogy, texture and structure (both primary and secondary) as well as its interconnection
with each other are important features for the occurrence of exploitable groundwater resource
in volcanic rocks.
The following important features govern the flow and storage of groundwater in volcanic
rocks; these are:
51
•
Vertical permeability due to primary and secondary fractures;
•
Horizontal permeability due to horizons containing openings due to lava flow and
gas expansion during solidification;
•
Occurrence of impervious horizons and dikes.
Fractured and porous volcanic rocks do not always serve for groundwater
circulation. In this regard the main controlling factors are:
•
Type, frequency and distribution of the fractures,
•
Degree of fracture and pore interconnection,
•
Thickness of the lava flow,
•
Occurrence of the cementing material and their hydraulic characteristics,
•
Constitution of the soil cover, the depth of the lava flow (at depth, volcanic rocks may
have low permeability due to the pressure exerted by the overlying units).
Porosities in rock formation have the capacity to enhance the occurrence and movement of
groundwater depending on the specific condition of the nature of that specific porosity.
Porosities in volcanic rocks could be categorized in to primary and secondary. Primary
porosities are made of original small and large scale structures contemporaneous to the rock
formation; and include: vesicles, degassing cavities, flow contacts or interflow spaces, lava
tubes or tunnels, clinker or rubble layers, tree moulds, shrinkage cracks or columnar joints.
Secondary porosities are those due to weathering discontinuities, tectonic fractures or faults,
inter-trappean beds, weathering zones, buried paleo-soils, etc. Weathering and tectonic
fractures form secondary porosities suitable for groundwater storage and movement provided
that the fractures are not filled with massive fault breccias and/or further weathering processes
do not cause the formation of impervious clay layers.
The whole terrain of the study area is covered by extrusive volcanic rocks mainly porphyritic
alkaline basalts, trachi-basalts, rhyolites, trachytes, welded tuffs (ignimbrites), succession of
alkaline and per alkaline stratoid silcics ignimbrites and unwelded tuffs topped by the PlioPleistocene fluvio-lacustrine sedimentary sequence named the Gedeb sequence (Getaneh Asefa
et al.,1982). These rocks are subjected to weathering, fracturing, jointing and faulting at
varying degrees.
52
Groundwater circulation and storage in these volcanic rocks depend on the type of porosity and
permeability formed during and after the rock formation. The primary and/or secondary
porosity developed in the different rock formations is different according to their genesis, the
weathering and tectonic conditions they were subjected to. Rocks possessing a primary
porosity may not necessarily give rise to primary permeability unless the primary porosities are
interconnected. Later connection of primary porosities by means of weathering or fracturing
may result in secondary permeability.
Table 4.1:- Common porosity values of volcanic formations. The ranges are only
indicative.Values are derived from a wide literature review (Kovalevsky et al 2004).
Material
Basaltic flows
Basaltic formations
Total
Drainable
porosity
Porosity
(%)
(%)
0.8-20
0.1-8
5-40
2-8
Remarks
Dense to highly vacuolar
Increases
with content
of
scoria
and
pyroclasts
Basalt sheets (traps)
4-10
<1-2
Several flows, no pyroclasts, moderately old
Loose pyroclasts
25-40
5-10
Fresh lapilli and blocks
Ash fall
25-40
1-5
Relatively fresh
Rhyolites
0.1-30
0.5-5
Dense to vacuolar
Rhyolitic ignimbrites
15-70
0.5-10
Dense to poorly welded tuff
Rhyolitic interflows
30-70
1-5
Breccia at the bottom of a lava flow
Volcanic soils
40-60
<1-5
Variable
The volcanics, especially of rhyolites, ignimbrites, trachytes, ash flow tuffs and basalts
extending over large parts of the central part of the study area, can transmit or store water
depending on several factors such as extent of weathering, fracture pattern and suitability of
geomorphology and other factors. The volcanic rocks that outcrop in the central parts of the
study area vary from massive to fractured type. It is fresh to slightly weathered. Aquifers can
be made up of weathered and fractured volcanic rocks such as ignimbrites, scoriaceous basalts,
rhyolites and trachytes. Volcanic deposits originated from different sources have varying aerial
extent and composition giving rise to a complex geological setup. As a result of this
complexity, the extremely variable transmissivity values in volcanic rocks. A big variation
53
among well yields can therefore be attributed to this heterogeneity in the transmissivity of the
water bearing geological formations.
4.2.1) Aquifer types and their Yield
The volcanic rocks in the study area are generally categorized under moderate permeability
aquifers. According to borehole yield inventory data the volcanic rocks of the Nazareth Group
(Nn) productivity is better than the other volcanic groups of the area. The Nazareth Group
covers large portions of the Gedeb-Asasa plain and this unit consists of a succession of alkaline
and per alkaline stratoid silcics, welded tuff (ignimbrite), pumice, ash and rhyolite flows and
domes with intercalations of basalt flows and lacustrine sediments.
The yield of bore holes inventoried was not given the available data to estimate the anticipated
maximum and minimum well discharges. However, few of the observations available reveal
that the discharge of the wells may not exceed 5l/sec.unless it is possible to strike fracture
zones serving as the conduit of groundwater. For example, the yield of Dodola town well is
about 4l/sec.for a depth of 140m below ground level and that of Herero State farm is about
1.4l/sec.for a depth of about 66mb.g.l.Therefore, by appropriately locating and fully
penetrating the aquifers it may be possible to increase the yield of wells.
Recently two bore holes were drilled at Berisa village about 7km from Dodola town on the
way to Adaba-Robe and both of them found to be dry. The deeper well is 185m b.g.l and only
little amount of water is struck (1L/sec) from 40 to 50meters b.g.l.
The volcanic rocks in the study area are categorized under moderate permeability aquifers
(Tesfaye Chernet,1993).In hydrogeological study of the Nazareth sheet(Getahun Kebede,1987)
the geologic units are categorized into four permeability groups, namely high (<100 m/day and
mean value of 28.3); moderate (<30 m/day and mean of 4.5); low (<1 m/day and mean of 0.2);
and very low (<0.1 m/day). Accordingly, the project area falls in the second category.
According to Wabe Sheble master plan study, lithological units of the area have been
categorized into the following aquifer systems based on the productivity, aerial extent and
aquifer types. Accordingly, the Nazareth Group has categorized as high productivity with
yields of 0.1to 6l/sec.and mean discharge of 2.5l/sec. This also shows that the amount of water
54
we can obtain from a single well is relatively small. Ground water table (static water table) in
the area varies from the ground surface to as deep as 137metres below groundwater surface.
Table4.2:-Aquifer characteristics of rocks related to the study area (WWDSE,2004, in
OWWDSE,2007)
Aerial
Type of
extent
aquifer
Qb
Localized
Nc+Nn
PNa1+PNa2
Aquifer
Yield
Specific capacity
(l/sec)
(l/sec/m
Range
Mean
Fractured
-
-
Extensive
Fractured
0.1-6.0
2.5
Extensive
Fractured
Range
0.030.21
0.0431.9
Spring discharge=0.1-5l/sec.
Transmissivity
Mean
Range
Mean
0.12
-
64.5
10.9
Productivity
of aquifer
Moderate
High
low
From my conclusion, three different aquifer formations are outcropped in the study area;
fractured and weathered ignimbrite, (welded tuff), weathered and fractured trachytes and
rhyolites, weathered basalts and scoriaceous basalts.
Fractured and weathered ignimbrite (welded tuff)
From bore holes yield inventory of this volcanic group the yield varies from 0.5-7 l/sec.
Generally, the yield of bore holes increases to the central plain area. Semira Kolba and Temela
springs emerge through these fractured and weathered ignimbritic units. The degree of
weathering and fracturing increases from west to east in the central part of the area. The
ignimbrite (welded tuff) is exposed widely in the western, central and south western parts of
the study area. Weathered, fractured and jointed ignimbrite (welded tuff) formations are
outcropped at a variable depth from 3m–70m around Asasa town and south of Asasa up to
Melka wakena area. According to Davis (1966) welded tuff has medium to low primary
porosity and very low permeability. Thus the water circulation and storage capacity of welded
tuff depends on the secondary porosity and permeability developed through fracturing and
weathering processes. In the study area, the degree of weathering and fracturing of these rocks
varies from place to place.
55
In most places the welded tuffs are fresh to slightly weathered. In areas of western parts
(eg.Kofele area) as well as along most of Totolamo and upper Wabe river banks it is deeply
weathered and covered by soils having different thickness. In some localities the welded tuff is
massive, slightly weathered and fractures are scarce or absent. Thus, the secondary processes
have very small contribution in the overall water circulation and storage capacity of welded
tuff. On the contrary, there are places where block fractures divided the massive welded tuff
into rectangular blocks. Mostly these fractures are open to a considerable depth and transmit
large quantities of water. Therefore, in most localities welded tuff developed good secondary
permeability largely from open fractures and to some extent from weathering zone. When there
is high degree of fracturing and weathering, welded tuffs have the capacity to hold water and
become a productive aquifer.
Highly weathered and fractured ignimbritic columnar joints found exposed at some places
forming large cliffs fore example, at the out let of Melka Wakena Reservoir (see figure2.8,
section 2.8.2.2). Semira Kolba spring emerges through this rock at the start of gorge or canyon
down Melka Wakena Reservoir.
Figure 4.1:-Semira Kolba spring through fractured ignimbritic rock
down Melka Wakena reservoir
56
Weathered and fractured trachytes and rhyolites
Trachytic domes have steeper slopes, hence there is thin or no soil formation. Therefore, the
water that precipitated on the trachytic domes of Mt. Kaka, Mt. Hunkolo, Mt.Kurduro and Mt.
Semkeru are mostly lost as runoff rather than vertical infiltration. This rock unit is slightly to
moderately weathered and intersected by fractures. The occurrence of major tectonic
displacement and deep weathering zone in trachytic lava flows strongly changes the hydraulic
characteristics of the rock. On the other hand, minor fractures have local permeability effect.
However, an intensively weathered and fractured trachytic lava flow under favorable
conditions develops not only water transmitting but also water holding properties.
The Chilalo trachytes show variable permeability from one locality to another but in most
localities this trachyte is massive and the permeability is very much reduced. So the Chilalo
trachytes are generally considered to have low permeability. The rhyolitic and trachytic lava
flows are mostly considered as impervious rocks. The water storage and transmitting capacity
is thus largely dependent upon secondary porosity and permeability. Moreover, weathering and
fracturing locally increases the porosity of the rhyolitic lava flows. The secondary porosity in
rhyolite is due to weathering and associated fractures. Thus, the weathering fractures and
weathering zones significantly modify the limited primary porosity and permeability of
rhyolitic lava flows. Rhyolitic lava flows are found dominantly along the slopes and foothills
of Kaka mountain ridge. On the other hand, the rhyolitic lava flows outcrop in eastern parts of
Bale mountain ridges is slightly weathered and less fractured. Consequently, there is poor soil
development particularly on the slope and top parts of the ridge. Rock fragments are
dominantly covering this part. Relatively shallow soil profile constitutes the gentle slope and
foothills of the ridges. Therefore, in some places where the rhyolitic lava flows are intensively
weathered and highly fractured, infiltrated water through fractures feed the aquifers that lie on
flat-laying areas. Where there are massive and slightly weathered parts of this rock unit, most
of the precipitated water is readily lost as runoff.
These are the list productive aquifers. These rocks generally have low to very low
permeabilities and porosities. They do not contain groundwater in appreciable quantities in the
57
area. However locally, these rocks can be permeable due to weathering and the presence of
fractures.
Weathered basalts and scoraceous basalts
From borehole yield inventory of this volcanic group the yield varies from 0.5-15 l/sec and the
most productive unit in the area especially, the central plain of the area. The high discharge
such as Asasa and Hinja Burkitu springs which emerge through this fracture of volcanic units
are few to mention.
From two bore hole logs drilled in Gedeb Asasa plain at UTM 537855E, 791521N,elevation
2408m.a.s.l & UTM537742E, 798987N,elevation 2455m.a.s.l, the main aquifer of the two
bore holes are the weathered and fractured basalt(see geophysical data below) The lithologic
logs of these wells also confirm this fact. The depth of these bore holes are 72m&216m
respectively. In the first bore hole drilled upstream of Hinja Burkitu spring eye(fig.4.2) water
struck at 12.5m b.g.l and the static water level rose at 5m.b.g.l indicating semi-confined
condition and its yield is estimated to be 15liters/sec. where as in the second well water struck
at depth of 131m & the static water level is112m b.g.l. Its discharge is estimated to be
7liters/sec. There is a gradual increase in aquifer yield towards the center of the plain from all
direction of steeper slopes. Therefore, depending on the degree of weathering and the resulting
weathering zones the basaltic rocks show difference in water infiltration properties and water
yielding capacity. The fractured variety is the most permeable and productive aquifer in Gedeb
Asasa plain. The recharge area for the springs is also from Kaka and Hunkolo mountains in the
west, northwest and from Bale mountains chains in the east.
Table 4.3:-Vertical electrical sounding result at Hinja Burkitu BH-1
1) VES 1: Coordinate (UTM Zone 37) Easting 536304m, Northing 796705m, Altitude 2405m;
No
Layer Resistivity
Layer thickness
Depth
(Ωm)
( m)
(m)
Possible lithologic description
1
4.35
2
28.6
19.5
19.76
Weathered volcanic(tuffs)
3
355
91.3
111
Slightly fractured basalt
4
154
116
227
Fractured basalt (Water bearing?)
5
12.5
0.264
0.264
-
Top soil
-
58
Highly fractured basalt (Water
Table 4.4:-Vertical electrical sounding result at Hinja Burkitu BH-2.
2) VES 2: Coordinate (UTM Zone 37) Easting 537753m, Northing 798992m, Altitude 2460m;
No
Layer Resistivity (Ωm)
Layer thickness(m)
Depth (m)
Possible lithologic description
1
7.9
2.3
2.3
Top soil
2
16
8.4
10.7
Weathered volcanic(tuffs)
3
605.2
84.5
95.2
Slightly fractured basalt
4
111
49
144.2
Fractured basalt (Water bearing?)
5
91.4
-
-
Fractured basalt (Water bearing?)
Source:(OWWDSE,2008 Hinja Burkitu Field Report)
Lithology
Description
0m
Black clay soil
4m
8m
Fractured and weathered scoraceous basalt
Highly fractured basalt
14m
Highly weathered and fractured scoria
22m
Highly weathered and fractured basalt
34m
weathered basalt
48m
Highly weathered and fractured basalt
60m
62m
64m
weatherd tuff
Highly fractured basalt
Scoria
68m
Massive basalt
72m
59
Figure 4.2:-Lithologocal
log of Hinja Burkitu BH-1
Aquifer Transimissivity
Transimissivity is a measure of the hydraulic capacity of the aquifer i.e. its ability to transport
groundwater. In the water points inventory data transmissivity of water wells have not recorded
and also pumping test data not available. According to groundwater resource potential
evaluation project of Shanan-Dhungeta & Middle Wabe Dhare Sub-basins (OWWDSE, 2006),
the average transimissivity value for Arsi and Bale volcanic rocks (Nct+Nn) estimated to be
2.93 m2/day.
Comments
Permeability
(m/day)
Transmissivity
(m2/day)
0.01 - 20
2 – 100
Basaltic traps
0.001 - 10
1 – 50
Loose pyroclastics
0.1 - 50
10 - 500
Young
Ash falls
0.01 - 0.1
0.5 - 5
Relatively fresh
Material
Basaltic
formations
Several flows with
pyroclasts
Several flows, no
pyroclasts,
moderately old
Phonolites
0.1 - 20
20 - 1500
Effect of major
fissures
Phonolite
Ignimbrites
10-6 - 0.01
0.1 - 10
Welded to fractured
Trachy-Syenites
0.01 - 0.1
1-5
Rhyolites
0.01 - 0.1
0.1 - 10
Rhyolite
Ignimbrites
10 - 0.01
0.02 – 0.04
Alluvium &
terraces
1 - 10
2 - 200
-4
Table 4.5:-Common ranges of
permeability for water at normal
temperatures (10 to 25°C) and
values of Transmissivity
(UNESCO, 2004).
Poorly sorted,
derived from
Volcanics
(Source: Ground Water Studies, International Guide for Hydrogeological Investigations, edited
by-V.S.Kovalevsky, G.P Kruseman, K.R Rushton).
60
Figure 4.3:- Hydrogeological map of the study area
4.3) Ground water flow Conceptual Model
Recharge -Discharge conditions
Recharge area is defined as that portion of drainage basin in which the net saturated flow of
groundwater is directed away from the water table and discharge area is that portion of the
drainage basin in which the net saturated flow of groundwater is directed towards the water
table (Freeze and Cherry, 1979).
Springs and base flows in rivers manifest groundwater discharge. In the upper part of the Wabe
Shebele River Basin, a number of springs emerge from the volcanic rocks. The groundwater
discharge, in the project area, coincides with the direction of the surface water flow direction.
WAPCOS (1990) applied the base flow separation method to conclude that all ground waters
in the upper Wabe Shebele catchment are discharged to the river.
Water that enters the flow system in a given recharge area may be discharged in the nearest
topographic low or it may be transmitted to the regional discharge area in the bottom of the
major valley.
61
Waters that are discharged from areas of relatively flat topographic set-up, especially those
high yielding and perennial springs (Asasa, Hinja Burkitu, Semira Kolba, Temela, and Robe
Gerjeda springs), the flow system is considered to be intermediate for the fact that the areas
where the springs are considered to be emanating are not relatively far from topographic highs
and their water type is Ca-Na-HCO3.
Intermittent and low yielding springs and dug wells are considered to be crossed by either local
or intermediate flow systems for they are recharged from nearby areas that susceptible to
surfacial phenomena. The majority of these water sources are concentrated in topographic
highs (eg.most intermittent springs on Kofele high lands).
Figure 4.4:-Map showing recharge and discharge areas
62
Figure 4.5:-Cross-section along A-B indicating local and intermediate flow system based on
geochemical and geological evidences
From the above schematic section Gebecho spring is discharged at the hillside of Honkolo
mountain through Chilalo volanics( lower part, consisting of ignimbrites and trachytes and
upper part, consists of porphyritic alkaline basalt) with Ca-HCO3 type water and Robe Gerjeda
spring down the plain area near Melka Wakena Water Reservoir discharged with Ca-Na-MgHCO3 water type through Nazareth volcanic succession of alkaline and per alkaline stratoid
silcics, welded ignimbrite indicating intermediate groundwater flow system(fig 4.5).
Groundwater Level and Flow
Water level measurements are used to estimate the general direction of groundwater flow,
location of recharge and discharge areas and connection between aquifers and subsurface
systems. The groundwater flow direction is estimated by using existing water point data of
boreholes and dug wells (annex 4.2).
63
Figure 4.6:Groundwater
table contour
map and flow
direction
Groundwater Discharge
A spring is a location where groundwater is discharged naturally from the rock or soil forming
a superficial flow. The discharge of springs in humid regions usually fluctuates with the rate of
precipitation during the year. The springs are predominantly controlled by structural
lows/depressions, fractures and lithologic contacts.
The flow rate from a spring may depend on ground water recharge conditions, seasonal
discharge, and the water demands of vegetation. According to the duration of flow, three
categories of springs are distinguished:
1) perennial springs with a continuous flow,
2) periodic springs with periodically changing flow rates not associated with rain fall or
seasonal effects but may be caused by variations in evaporation, by atmospheric pressure
changes, by tides affecting confined aquifers, and by natural siphons acting in under ground
storage basins.
3) intermittent springs with a flow which is interrupted a certain time during the year e.g.
during the dry season.
64
The largest numbers of springs are characterized by very low discharges ranging from <1 to 2
liters per second. However, there are few springs with quite considerable discharges. Four high
discharge springs emerging through basalt found in the central plain area: The Asasa spring in
the Asasa town, the Hinja Burkitu spring & Temela spring near Geredella state Farm camp
some 2km & 5km west of Melka Wakena Hydroelectric Power reservoir respectively and
Semira Kolba spring emerging at deep valley cut at approximately 0.5km from Melka Wakena
Hydroelectric Power turbine.
Population of Asasa, rural communities settled left and right along the way from Asasa to
Dodola town, including Dodola town and its surrounding rural communities get water supply
for domestic use from Asasa spring. Rural communities of four Peasant Associations
surrounding Hinja Burkitu currently get water supply for domestic use from Hinja Burkitu
spring. Due to their high discharge Asasa and Hinja Burkitu springs are among the water
contributing water bodies for Melka Wakena Hydroelectric Power reservoir forming streams.
During field trip(22nd,Feb,2009) trial have been made to estimate the discharge of Asasa &
Hinja springs using floating method down the stream and the discharge of these springs are
estimated to be 24120m3/day and 14688m3/day respectively. A number of springs ooze out
from eastern parts of Bale mountain ranges and west, northwest and northen parts of Arsi high
lands with discharges varying from less than 0.5liter/sec to 15liter/sec. Most of the Seepage/or
low discharge springs are emerging through unconsolidated tuffs at hill sides and slope breaks
from western parts of Kofele high lands have discharges less than 1liter/sec.This is an
indication of shallow ground table and local flow.
Most of the geomorphologic features are generated by structures such as faults, joints,
beddings and fractures. Thus, one can understand that the springs are controlled by structures.
All springs in the study area are cold and emerge from the basaltic plateau. Most of them
emanate through fractures and are fracture springs and discharge ground water to streams in
the study area.
65
Table 4.6:- Some of the cold fracture type springs observed in the study area.
Site description
No
Location
Ele. (m)
2645
Estimated
Yield
(l/s)
Type of spring
254
1
Fracture Spring
Site Name
Zone
Woreda
Balo
Bale
Adaba
557806
778498
2
Boro
Bale
Dodola
530098
772773
2442
332
0.8
Fracture Spring
3
Arsi
Asasa
529142
806704
2691
306
0.5
Fracture Spring
4
Sirko
Robe
gerjeda
Arsi
Asasa
547511
793107
2295
444
3
Fracture Spring
5
Boricho
Arsi
Kofele
481290
788661
2668
57
<1
Fracture Spring
6
Soboro
Bale
Adaba
550088
776575
2468
488
1
Fracture Spring
7
Semira
Arsi
Asasa
551221
800796
2311
289
>15
Fracture Spring
8
Kaka
Arsi
Asasa
514135
807345
3280
92
20
Fracture Spring
9
Arsi
Asasa
534474
808353
2850
206
6
Fracture Spring
10
Gebecho
Hnja
Burkitu
Arsi
Asasa
538009
791636
2404
290
>100
Fracture Spring
11
Asasa
Arsi
Asasa
521822
785702
180
>100
Fracture Spring
1
Northing
(m)
EC
(µs/cm)
Easting
(m)
Sanitation situation of the springs
Among with the provision of water supplies, the safe and efficient disposal of human waste is
one of the measurements of environmental sanitation.
Most of the populations in rural and urban areas do not have access to safe and reliable
sanitation facilities. Majority of households do not have sufficient understanding of hygienic
practices regarding food, water and personal hygiene. People residing in the rural area defecate
openly on land or at the banks of water bodies, which will find its way into natural
watercourses. Consequently, both surface and ground waters in these areas will be subjected to
faecal pollution leading to prevalence of a wide variety of water borne diseases.
The major problems in the rural settings are the absence of at least dry pit latrines, the washing
of clothes and taking of baths right in the river water or at the spring eyes and the wide spread
littering of animal dung which in one way or the other way contributes to contamination of the
water supply sources. Women and children particularly girls are the main water collectors for
the family and have regular contact with contaminated water and therefore, they are the
segment of the population most vulnerable to water born diseases.
66
Figure 4.7:-Washing at Asasa eye spring.
The writer of this paper observed the sanitation situation of Asasa spring during his field visit.
The situation is most serious even though the chemical analysis result currently shows no sign
of pollution. The use of sanitary latrines is very limited in the town. As this spring eye is
located in the center of the town at relatively lower slope it is subjected to faecal pollution
leading to prevalence of a wide variety of water borne diseases. Domestic solid wastes and
animal dung are dumped around this spring. Though, part of this spring capped and taken to
the town, the communities are unwillingness to be charged for water at public water points and
go to this near by spring over flow to wash clothes, taking baths and watering of animals at the
eye of this spring because the area is not protected.
Hydrogeological Data Gaps
Generally, few hydrogeological investigations were carried out in the area. The following data
gaps and deficiencies are identified during review of the previous studies:
•
The relationship of geomorphological setting and surface drainage distributions to
groundwater occurrence and movement is not well known,
67
•
Most of the previous studies have given due consideration to the water supply of the required
demand; and hence the boreholes did not fully penetrate the aquifers. Failure of drilling to the
desired depth to fully penetrate aquifers has been observed.
•
Pump test results available are few, partially penetrating, wells could not provide full
information about the hydrogeological characteristics of the aquifers. Moreover, in most of the
cases pump tests have been limited by the maximum capacity of the pump used but not the well
capacities; and all tests are single well tests not designed for aquifer testing,
•
Data on water points (boreholes, springs and hand dug wells) are incomplete,
•
Lack of groundwater level monitoring for the existing boreholes and springs,
•
Absence of water quality data records for some of the boreholes and springs,
•
The majority of the well completion reports have no necessary information (site description,
construction events, drilling events, water struck depth, development, casing, etc.)
•
To date, there is no comprehensive groundwater potential study that take into
consideration the interactions of fractures/faults, lithology,geomorphology, etc.to characterize
the hydrogeological properties of the volcanic aquifers prevalent in the area.
68
CHAPTER FIVE
HYDROGEOCHEMISTRY
5.1) General
Though water is commonly thought of as simply H2O, literally thousands of other substances are
dissolved in water in the environment. Most of these substances occur naturally, and many are
present in water in only small quantities. The term “water chemistry” (or water quality) refers to
the quantities of these various substances (commonly called solutes) that are present in a
particular water sample, making up its chemical composition. The water chemistry of a ground
water sample can be thought of as a chemical signature that reflects the sum total of all physical
processes and chemical reactions that affected the water from the time it began as dilute rainfall,
infiltrated the soil above the water table, passed into the aquifer (ground-water recharge), and
traveled, sometimes over great distances and depth, to the point of sample collection or discharge
from the aquifer. Water chemistry also differs depending on the source of water, the degree to
which it has been evaporated, the types of rock and mineral it has encountered, and the time it
has been in contact with reactive minerals. Water acquires very small quantities of some solutes
from dust and gases when it falls through the atmosphere as precipitation, but water typically
acquires the majority of its solutes once it reaches the land surface. Solutes that already present
in the water increase in concentration because of the processes of evaporation and transpiration
processes that, for the most part, remove water while leaving the solutes behind. As water
infiltrates through the soil zone, it also tends to dissolve carbon dioxide (CO2) gas that exists in
the soil in large quantities (relative to the atmosphere) because of biological activity. When CO2
dissolves in water in the soil zone, a weak acid is formed. This acid promotes the dissolution of
minerals that are present in the soil and rocks, which releases solutes to the water and causes
their concentrations to increase. Because of these processes, water in the soil zone can acquire
the bulk of its chemistry before it reaches the water table.
69
In order investigate the water chemistry of the area the following technical approach is used.
Technical approach
Representative samples were collected from areas where samples have not been taken before
especially from hill sides of mountains and measurements of major, minor and trace elements
for 12 water samples (9springs, 1borehole, 2hand dug wells) were analyzed in laboratory of
Water Works Design and Supervision Enterprise Water Quality Laboratory. Results the
physio-chemical analyses including data from previous work are given in annexes 4.2 and 4.3.
•
Secondary data from different sources (boreholes, springs and hand dug wells) were
collected from different organizations.
•
The results are checked for error by ion balance technique, and data up to error of 10%
were taken for interpretation and error greater than 10% discarded and not used in the
interpretation.
5.1.1) pH, EC and TDS
pH (hydrogen ion activity)
The pH is defined as the negative of the logarithm to the base ten of the hydrogen ion
concentration. The pH scale runs from 0 to 14 (i.e. Very acidic to very alkaline) with pH = 7
representing a neutral condition (H+ = OH-) at 25oC. It is an important variable in water quality
assessment as it influences many biological and chemical processes within a water body and all
processes associated with water supply and treatment. At a given temperature, pH (or the
hydrogen ion activity) indicates the intensity of the acidic or basic character of a solution and is
controlled by the dissolved chemical compounds and biochemical processes in the solution.
Although pH usually has no direct impact on consumers, it is one of the most important
operational water quality parameters. A guideline value pH range of 6.5–8.5 was established
for pH for drinking water (WHO International Standards for Drinking-water,1984).
Most ground waters found in the study area have pH values ranging from about 6 to 8.6 except
in two samples one from spring and one from shallow well(12m.b.g.l) at western water divide
around Kofele area located at (GPS location 525559mE, 792266mN and elevation2700m a.s.l.
and 477711mE, 784665mN and at elevation 2674m.a.s.l.) respectively. This slightly lower pH
value of some waters below the WHO guide lines are attributed to rain water source. The other
70
reason is probably that these samples are collected from the vegetated area and chemical and
biochemical decomposition of vegetative residues by activity of microorganisms induce CO2
which in turn decreasing the pH of the water then by increasing the acidity of the soil with
which the water is in contact and ooze out through or decrease in groundwater burial(shallow
depth) and aquifer permeability facilitates mixing of shallow groundwater with meteoric (rain)
water containing considerable CO2 because infiltrating meteoric(rain) water introduces HCO3
to the groundwater.
The general equation for organic matter oxidation can be written as:
CH2O + O2(g) t CO2(aq) + H2O (Oxidation)
(Organic matter)
CO2(g) + H2O tH2CO3(aq)
H2CO3t (HCO3)- + H+
There is no systematic increase or decrease of pH value of water samples from recharge to
discharge area or vise versa in the study area.
Table 5.1:- pH ranges of different sources of water.
Source
Minimum
Maximum
Average
Spring
5.9
8.08
6.99
Bore hole
7.03
8.53
7.78
Hand Dug Well
6.18
8.17
7.18
Electrical conductivity (EC) & Total dissolved solids (TDS)
Conductivity, or specific conductance, is a measure of the ability of water to conduct an
electric current. It is sensitive to variations in dissolved solids mostly mineral salts. The
degrees to which these dissociate into ions, the amount of electrical charge on each ion, ion
mobility and the temperature of the solution all have an influence on conductivity.
Conductivity is expressed as microsiemens per centimetre (μS/cm) or micromho/cm(μmho/cm)
at a specified temperature, usually 25 degrees celsius and, for a given water body, is related to
the concentrations of total dissolved solids(TDS) and major ions. In general, the larger the
71
value of specific conductance the greater the concentration of dissolved solids in the water
sample. Total dissolved solids, TDS (in mg/l) may be obtained by multiplying the conductance
by a factor (A) which is commonly between 0.55 and 0.75. This can be expressed as TDS =
EA (where E is conductivity in μs/cm and TDS is total dissolved solids in mg/l. The
multiplication factor is close to 0.62 for waters of the study area (i.e TDS = 0.62EC, with a
correlation coefficient r = 0.99)
The conductivity of most freshwaters ranges from 10 to 1,000 μS/cm but may exceed 1000
μS/cm, especially in polluted waters, or those receiving large quantities of land run-off. In
addition to being a rough indicator of mineral content when other methods cannot easily be
used, conductivity can be measured to establish a pollution zone, e.g. around an effluent
discharge, or the extent of influence of run-off waters. It is usually measured in situ with a
conductivity meter, and may be continuously measured and recorded. Such continuous
measurements are particularly useful in rivers for the management of temporal variations in
TDS and major ions.
In the research area, the water from springs generally shows EC that ranges from 57 704μs/cm. The highest value is measured near Adaba town GPS location (539803 mE,
774411mN, elevation 2520m) which is 704 μs/cm and the lowest value is measured on the
western water divide between rift lakes and research area on the way from Kofele to Kore town
at GPS location (481290mE, 788661mN, elevation 2700m) which is 57 μs/cm and the over all
average of EC value from all springs in the research area is about 312.25 μs/cm). Springs
which are mostly found near the recharge areas have relatively lower values of EC than springs
down the recharge area depicting soluble ions as water moves from recharge to discharge
areas.
For bore holes EC values range from240 μs/cm to 636 μs/cm and the over all average EC
values of bore holes in the study area is about 470 μs/cm.
The depth of these hand dug (shallow wells) ranges from 3m to 36m and the relative higher
value of EC is attributed to weathering and leaching of upper humic soil and rocks and short
period of time to join the shallow groundwater table.
The highest EC value in hand dug wells is measured at Gedisa(854μS/cm) and the lowest is
measured from hand dug well in Serofta Tulu village (321μS/cm)at GPS location(504520mE,
757706mN, elevation 2432m) & 476749 mE, 744850 mN, elevation 2590m ) respectively.
72
Generally, EC increases with decreasing altitude ie, EC increases from recharge area to
discharge area from measurement in spring samples. The TDS of the groundwater in the area
varies from 62mg/l to 640mg/l.The least value (62 mg/l)is measured in spring water sampled at
the foot hills of Kaka mountain from Kaka spring at altitude 3280m, location 514935mE,
807345mN, and the highest value is measured in hand dug well of Gedisa (558mg/l).
Generally, EC increases with increasing TDS and most of the plot of data set fit a straight-line
EC(microS/cm)
regression closely with correlation coefficient(r= 0.99).
700
Figure 5.1:-Relation
600
between laboratory
500
measured EC and
400
TDS from all water
300
y = 0.5952x + 12.649
R2 = 0.9778
200
samples in the study
area.
100
0
0
200
400
600
800
1000
1200
TDS(m g/l)
5.2) Graphical presentation, Classification and interpretation of Analytical
Results of laboratory measured parameters
As described in the geology of the area, the area is totally covered by volcanic rocks of
dominantly basaltic origin. The volcanic rocks are mainly the Nazareth group type which is
made up of predominantly of rhyolitic ignimbrites, trachytes and ash flow tuffs. The
composition of the majority of the highland volcanic plateau is silicate minerals of mostly
plagioclase feldspars of the albite and anorthite group and pyroxene composition. These
minerals are rich in Ca, Mg and Na. Hydrolysis, decomposition and/or leaching of these
silicate minerals enriches the water in the highlands by Ca, Mg, and Na cations. These rocks
are dominantly affected by fracturing and weathering. Ground water recharge takes place
rapidly through fractured basalts and flow paths are short and the waters from these areas are
characterised by low TDS and Ca-Mg-HCO3 type waters (Seifu, 2005). In the study area most
73
water types obtained from laboratory analyses at/or near recharge area have low TDS and most
of them are Ca-Na-HCO3 type evolving down the flow path to dominantly Na-Ca-HCO3 water.
Piper tri-linear diagram
The diagram displays the relative concentrations of the major cations and anions on two
separate lower triangular fields and a central diamond-shaped field. All the three fields have
scales reading in 100 parts. The percentage reacting values of the cations and anions are plotted
as a single point at the lower left and right triangles respectively. These are projected upwards
parallel to the sides of plot where the points from the two tri-linear plots are projected as
percentage of milli-equivalent to the central diamond-shaped (rhombus field) to show the
overall chemical character of the water. Thus, the identification of trends and classification of
waters according to their chemical characteristics is possible. Piper tri-linear diagram is
convenient for depicting the effect of mixing two waters from different sources.
Reliability check is also very important before using the chemistry of water samples for
interpretation. From the study area 40 water samples have been used for interpretation of water
chemistry of the area. Twenty (20) water samples are less than 5% of ionic balance and the
remaining twenty (20) water samples are between 5% and 10% of reliability check. Water
NO
3
samples greater than 10% are not used in the interpretation.
plot of all sources
Mg
SO
4+
Cl
+
+
Ca
Figure5.2:- Piper
Legend
Samples from Borehole
Samples from hand dug wells
CO
3+
K
Mg
4
SO
+
Na
HC
O3
Samples from Springs
Ca
Cl + NO3
74
of water samples.
From the above water sample clustering on the diagram the following water types are
identified:
-Ca-HCO3 type water
-Ca-Mg-HCO3 type water
-Ca-Na-HCO3 type water
-Na-Ca-HCO3 type water
-Ca-Na-Cl-NO3 type water
The piper plot shows that most of water samples are of calcium- sodium-bicarbonate type.
5.3) Major ion evolutions and their Controlling Factors
The composition of groundwater along the flow path is primarily dependant up on chemistry
(chemical composition) of the starting water (precipitation), climate, types and relative
solubility of the minerals available in the rock, topography/physiography and physical aspects
of the hydrogeologic system. These factors together combine to create diversified water types
that change in compositional character spatially and temporally as precipitation infiltrates the
soil zone, moves down a topographically-defined flow path and interacts with the minerals
derived primarily from the underlying bed rock.
5.3.1) Calcium, Magnesium and Sodium
The cationic composition of groundwater is related to the type of volcanic rock. In ground
water, only seven solutes make up nearly 95 percent of all water solutes (Runnells, 1993).
These solutes are calcium (Ca), magnesium (Mg), sodium (Na), potassium (K), chloride (Cl),
sulfate (SO4), and bicarbonate (HCO3). Although many sources and reactions influence the
concentrations of these solutes, the predominant sources of these solutes to ground water in the
Upper Wabe River Sub-Basin is derived from the hydrolysis, dissolution of silicate minerals,
such as plagioclase feldspars and pyroxene group, and ion exchange reactions whereby sodium
is released to the water in exchange for calcium or magnesium.
Weathering reactions: Dissolution and precipitation of mineral phases determine the
contribution to and removal of ionic species in solution. In these cases primary minerals
75
become dissolved or altered and secondary minerals may be produced. An example of
weathering reaction is the incongruent dissolution of aluminum-silicates. At neutral pH,
aluminum remains in the solid and the process is represented in a general way as:
Aluminum-silicate + H+pMem++ Aluminum mineral + H4SiO4 Where, Mem+ refers to metals
such as Na, K, Ca and Mg.
The main aquifers dominating the catchment area are volcanic rocks of basaltic origin. These
rocks consists of silicate minerals of feldspars(Na-feldspar, Albite, and Ca-feldspar, Anorthite)
and micas, silicate minerals such as Olivine (Mg,Fe)SiO4 and Pyroxene (freeze,1979).
In the volcanic aquifer system of the Upper Wabe river sub-basin patterns in the water
chemistry of ground water have helped refine important concepts about the ground-water flow
system, including sources of water, directions of flow, and travel times.
i) Hydrolysis of silicate minerals: One of the factors that control the evolution of ions is
consequently depletion of calcium ion, and enrichment by sodium ion as the hydrolysis process
procceeds along the ground water flow path. Among the rock-forming minerals involved in
hydrolysis, plagioclase is perhaps the most common and important. Plagioclase comprises a
solid-solution between Na and Ca end-members.
The Ca end-members are preferentially weathered relative to Na and therefore the Na/Ca ratio
in plagioclase increases as the reaction proceeds. Dissolution of the two end-members,
anorthite and albite, which produces kaolinite plus cations, can be written separately as
follows:
Anorthite to Kaolinite : CaAl2Si2O8 + 2CO2 +3H2Ot Al2Si2O5 (OH)4 + Ca
+2
+ 2HCO3-
Albite to Kaolinite: 2NaAlSi3O8+2CO2+3H2Ot Al2Si2O5(OH)4 + 2Na+ + 2HCO3-+4SiO4
Clinoproxene, namely Ca-Mg aluminosilicate, can be dissolved incongruently according to
the following idealized stoichiometry:
[CaMg0.7Al0.6Si1.7]O6+3.4CO2+4HO2=0.3Al2Si2O5(OH)4=Ca2++0.7Mg2++1.1H4SiO4+
3.4HCO3In the study area in terms of an ion, the dominant is HCO3-, and it shows an increasing trend
from recharge area to discharge area with similar trend of Ca2+ and Na+ ions and HCO3
concentration of the samples in the area is also attributed to hydrolysis of these silicate
minerals and as a result of dissolved CO2 in the soil zone from atmosphere and unsaturated
zone.
76
HCO3 concentration of the samples in the area as a result of dissolved CO2 in the soil zone
from atmosphere and unsaturated zone can be expressed by the following reaction
(Freeze,1979):
CO2+H2O ⇌ H2CO3(aq)
H2CO3 ⇌ H++HCO3ii) Cation Exchange:- It is a reaction which involves a replacement of one chemical for
another one at the solid surface. Cation exchange process plays an important role in controlling
the chemical composition of groundwater. The relative abundance of Ca2+ and Na+ cations in
the ground water is due to cation exchange (Hem,1985). The cation exchange capacity is
determined by the clay content, type of clay minerals present, and the organic matter content.
For Cation evolution in ground water along the flow path, calcium and magnesium in the water
are exchanged for sodium that is adsorbed to aquifer solids such as clay minerals, resulting in
higher sodium concentrations. The cation exchange reactions along the groundwater flow paths
can be represented as:
M2+(aq) +2Na-X2 ⇌2Na+(aq)+M- X 2,
where, M2+ is a divalent cation such as Ca2+, Mg2+, and X is the exchange substrate, such as
clay mineral or aquifer solid and substrate (aq) refers to cations in aqueous solution.
In the study area ground water near recharge area is represented by water dominant in calciumsodium-bicarbonate (Ca-Na-HCO3) with lesser amounts of magnesium. Pure magnesium type
water or calcium-magnesium water is rare probably due to absence of dark-colored
ferromagnesian minerals such as olivine, pyroxenes, amphiboles and the dark-colored micas as
might be the sampling area mostly covered by rocks composed of calcium (Ca)-rich, anorthite,
and sodium (Na)-rich, albite plagioclase feldspar or might be attributed to less number of
sampling taken at the peak of mountains due to inaccessibility at peak mountains. As the
groundwater flows away from the source of recharge towards west from Bale and to the east
from Arsi mountains, Na-Ca-HO3 water increases at the contact of E-W slopes at center of
Gedeb Plain. This mirrors that calcium and magnesium ions are exchanged for sodium ions
attached to aquifer solids with some anomalies in which unexpected increasing trend in the
calcium ion concentration down the discharging zone or presence of Na-Ca-HO3 water type in
77
recharge areas which may attributed to the effect of the geo-media in that some of the minerals
forming rhyolites and trachytes (such as plagioclase feldspar and amphiboles) bear significant
amount of sodium or these rocks may found intercalating with paleosols containing significant
amount of sodium bearing clay minerals.
Groundwater in the study site generally begins as a Ca-HCO3 type in the recharge areas and
chemically evolves along the flow paths as a function of the lithologies encountered.
The chemical evolution path from recharge to discharge area is from Ca-rich rainwater in to
Ca-Na-HCO3 to Na-Ca-HCO3-type waters. The water type differences are the result of the
rocks contacted during ground water circulation.
Ca - HCO3
Ca -Mg - HCO3
Ca - Na - HCO3
Na -Ca - HCO3
Accordingly, based on cation composition four water types are identified. These are:
1) Ca-HCO3
2) Ca-Mg-HCO3 type water
3) Ca-Na-HCO3 water.
4) Na-Ca-HCO3 type water.
The other two Na-Ca-Cl-NO3 and Ca-Na-NO3-HCO3 are not natural but attributed to
anthropogenic effect.
Ca-Na-HCO3 is the dominant water type followed by Na-Ca-HCO3 in the study area. Water
groups represented by Ca-Mg-HCO3 and Ca-Mg-Na-HCO3 are often weakly mineralized
waters circulating with in the basaltic and scoraceous aquifers at a relatively shallower depth
(Tenalem et al.,2008). Those represented by Ca-Na-HCO3, Na-Ca-HCO3 and Ca-HCO3 are
draining the fractured acidic and intermediate volcanic rocks such as rhayolites, ignimbrites,
tuff, trachyte and have a more dilute chemistry. These waters represent groundwaters which
are either in recharge area at the early stages of geochemical evolution or rapidly circulating
groundwaters which have no under gone significant water-rock interactions ( Seifu et al.2005).
78
Generally, the water samples from the study area are characterized by low TDS (average
TDS=256.1mg/l) concentrations indicating the rock-water interaction (residence time) is short
and the resistance of volcanic rocks to weathering.
Figure 5.3:- Cross-section from A-A’.Conceptual flow path from recharge to discharge area
from Soboro ridge(Bale Mt.) to Melka Wakena Hydroelectric power station camp Bore hole.
5.3.2) HCO3-, SO4=, Cl-, F-, and NO3In areas of non-carbonate rocks, HCO3 originate entirely from the atmosphere and soil CO2 and
hydrolysis of silicate minerals. The whole study area is covered by volcanic rocks and due to
absence of non-carbonate rocks, bicarbonate ion is the product of atmospheric and soil CO2
and hydrolysis of silicate minerals. Therefore, bicarbonate water type is the predominant in the
study area except in three samples which are two of them Cl- from shallow hand dug well at
Serofta Tulu and near Kokosa town and one is NO3- from Sheneka shallow well at Kofele high
land some 4km on the way from Kofele to Kore. NO3- is expected to be the influence of
anthropogenic effect. The nitrate concentration in groundwater and surface water is normally
low but can reach high levels as a result of leaching or runoff from agricultural land due to the
use of inorganic nitrate fertilizers or contamination from human or animal wastes as a
consequence of the oxidation of ammonia and similar sources.
The over all average concentration of NO3 is greater in hand dug wells than in bore holes and
springs.(HDW=16.64,BH=15.52,SP=13.59).This can be attributed to the ease access of
leachates through soil layers, weathered part of rocks and shallow ground water table. In cases
79
of extreme pollution, concentrations may reach 200 mg/l. The World Health Organization
(WHO) recommended maximum limit for NO3 in drinking water is 50 mg /1.
Volcanic formations do not usually contain significant quantities of soluble materials such as
halides or sulphates. Though there is no SO4=type water, the concentration of SO4= is present in
water samples. This is the result of dissolution of salts from paleo-lake deposits of lacustrine
sediments as described in geology of the area under section 2.8.3.2. The source of the SO4= in
the aquifers with low SO4= (< 5 mg/l) could be the rainfall. It is noticeable that, the average
relative concentration of SO4= is higher in bore holes than in springs and hand dug wells.
Maximum chloride (Cl-) ion (47.7mg/l) is registered in water sample from shallow hand dug
well at Keta (GPS location 537038mE, 77164350mN, elevation 2560m and the minimum
value (2.98mg/l) is registered in water sample from Bucha Roye spring (GPS
location547476mE, 7666752mN, elevation 2620m. As chloride is frequently associated with
sewage, it is may be incorporated into water from faecal contamination discharged in water
bodies.
Concerning F-, relatively high concentration is measured in water samples from Tedecha
Bulura and Berisa bore hole (2mg/l) and 1.84mg/l in water samples from Hinja Burkitu spring
which is above the limit of the WHO guide lines for F-concentration (1.5mg/l).This ion is
probably evolved from weathering and leaching of volcanic rocks of the Nazareth Group
which is made up of predominantly of rhyolitic ignimbrites, trachytes, and ash flow tuffs
topped by fluvio-lacustrine sediments. Generally, water samples from volcanic rocks of the
Nazareth group relatively show higher F-concentration than volcanic rocks of the other groups
in the study area with exception of some anomalies.
5.4) Water Quality Criteria
With the advent of industrialization and increasing populations, the range of requirements for
water has increased together with greater demands for higher quality water.
Therefore, the study of water quality has great significance as it plays an important role in
assuring a good quality of water for different purposes such as for domestic, livestock supply,
irrigation and industry, etc.
80
Drinking water standards
The main water quality indicators are physical and chemical constituents of water. These
constituents are highly influenced as a function of geological formation and human
interferences.
Quality of groundwater samples are evaluated using World Health Organization (WHO)
guidelines. Elevated concentrations of a number of constituents beyond this guideline can
cause problems for water use. According to these standards all analyzed water samples from
the research area fit for drinking except in water sample from Hinja Burkitu(SSP-12) in which
slightly higher value of fluoride content is measured (1.84mg/l)which slightly more than the
recommended value of WHO(1.5mg/l).
Agricultural Water Quality
The sodium adsorption ratio (SAR) is used to evaluate the suitability of water for irrigation.
The ratio estimates the degree to which sodium will be adsorbed by the soil. High values of
SAR that is sodium in the irrigation water may replace the calcium and magnesium ions in the
soil, potentially causing damage to the soil structure. The SAR for irrigation water is defined
as:
SAR
=
Na
Ca + Mg
2
in meq/l
The classification of
terms of suitability is
below.
SAR
< 10
10-18
18-26
> 26
SAR in
tabulated
Class
Excellent
Good
Fair
Poor
In all water samples analyzed (boreholes, springs, and hand dug wells) the SAR value is less
than 10meq/l which is very suitable for irrigation purpose (annex 4.5).
In the spring waters analyzed in the study area, the sodium adsorption ratio(SAR) ranges from
0.37meq/l at Kaka spring(PSP-7) up to 2.18meq/l at Kokosa town spring(SSP-14) while in the
bore hole waters it ranges from0.76meq/l at Wabe Burkitu (BH-12) upto 4.09meq/l at
Terdecha Bulura(BH-2).In hand dug well waters, the sodium adsorption ratio(SAR) ranges
from0.44meq/l at Serofta Tulu (HDW-2) upto 2.6meq/l at Gedisa(PHDW-1).
81
Figure 5.4:- Wilcox Diagram
From Wilcox diagram most
water samples are grouped
under medium salinity and
low sodium (alkali) hazard.
Most samples from springs
have low sodium (alkali)&
salinity hazard than water
samples from borehole and
hand dug wells.
Hardness
The hardness of natural waters depends mainly on the presence of dissolved calcium and
magnesium salts. The total content of these salts is known as general hardness, which can be
further divided into carbonate hardness (determined by concentrations of calcium and
magnesium hydrocarbonates), and non-carbonate hardness (determined by calcium and
magnesium salts of strong acids). Hydrocarbonates are transformed during the boiling of water
into carbonates, which usually precipitate. Therefore, carbonate hardness is also known as
temporary or removed, whereas the hardness remaining in the water after boiling is called
constant.
Hardness (H) is given by the formula:
H = 2.5Ca + 4.1Mg, Where, Ca & Mg are given in mg/l.
A guideline value of 500 mg/litre (as calcium carbonate) was established for hardness, based
on taste and household use considerations by WHO. Accordingly, total hardness in water
samples of the study area as of CaCO3 is between 20 mg/l & 304 mg/l and this value is below
the WHO standard and the ground water of the study area considered as soft (annexes 4.3 &
4.4).
82
CHAPTER SIX
CONCLUSION AND RECOMENDATION
6.1) Conclusion
•
Land degradation is a basic problem in the study area due to pressure of human and
animal population growth. Areas formerly covered with dense forests now changing to
agricultural lands due to high population growth. Adverse land practices aggravate land
erosion through deforestation and these could even have persistent effect on climate of
the area.
•
Aerial depth of precipitation was estimated using arithmetic, Theissen polygon and
isohyetal methods. The value obtained by Theissen polygon method is slightly less than
the value obtained by arithmetic and isohyetal methods. The Isohyetal method is used for
further analyses because the method considers the topographic variation which is the
characteristics of the area and has significant effect on rain fall distribution.
•
Potential evapotranspiration (PET) is estimated by penman combination and
Thornthwaite methods and the obtained results are 1093.80mm/yr and 688.86mm/yr
respectively. The evaluated annual PET of the catchment area using Thornthwaite
empirical formula is extremely less than the value calculated by using Penman formula.
Thornthwaite empirical formula uses only temperature with minor adjustments for the
number of day light hours as an input to calculate evapotranspiration. This significant
difference shows that evapotranspiration in the study area is more affected by wind
rather than temperature and the Penman combination method gives reasonable estimate
of PET because it takes in to account many meteorological variables.
•
The amount of PET is higher than that of the rainfall through eight months of a year
except in rainy months of June, July, August and September. Actual evapotranspiration
(AET) is quantified by empirical formula of Turc and soil water balance developed by
Thornthwaite and Mather,1957. The methods gave 617.44mm/yr and 850.56mm/yr
respectively. The results obtained with these two methods vary greatly. The result
obtained by Turc method under estimates the actual values and is not reliable as
83
compared to soil water balance because this method takes into account only precipitation
and temperature as inputs. The available soil moisture, field capacity of the soils and root
depths of the vegetal cover controls the rate of actual evapotranspiration from an area
however, these controls are not incorporated in this empirical formula. Therefore, soil
water balance method gives reasonable value because it takes many variables as inputs
such as PET, precipitation, the effects of land use land cover types, soil types and soil
moisture deficit and also the parameters used to calculate PET from penman combination
methods affect AET.
•
Though the study area found at higher altitudes it gets lower annual rain fall because of
its location on lee ward side from both “kiremt” and “Belg’rain fall which is the result of
northward movement of wind from Atlantic Ocean in the southwest and Indian Ocean
from southeast respectively. Potential evapotranspiration is greater than mean annual
precipitation and moisture surplus. Moisture surplus is very minimal. Actual
evapotranspiration is also very close to annual precipitation. This is an implication of
less available moisture for recharge in the area.
•
Groundwater recharge from precipitation (direct groundwater recharge) was estimated
using the base flow separation, water balance, and soil moisture balance methods and the
obtained results are 154.98 mm/yr, 22.75mm/yr, and 20.55mm/yr respectively. In this
case the direct groundwater recharge estimated by base flow separation higher than the
groundwater recharge estimated by soil moisture and water balance method. All the
methods have their own limitations. The water balance method is also not free of the
shortcomings mentioned above because runoff and AET used as an input for calculation
of water balance is those obtained from soil moisture water balance. However, the
presence of high discharge springs such as Asasa, Hinja Burkitu and Semira kolba and
from discharge of bore holes recently drilled in the area, it can be assumed that the area
probably receive higher values of recharge from nearby basin because the available soil
moisture very minimal and actual evapotranspiration is near to precipitation which
resulted in underestimated recharge but this needs further detail investigation to know
the sources of recharge using different available techniques.
84
•
Groundwater discharge from the aquifer system of the study area is mostly through
withdrawal from springs, hand dug wells and boreholes.
•
Analysis of the hydrochemistry of different water sources show that generally four types
of water are identified based on their major cation composition; Ca-HCO3/ Ca-MgHCO3, Ca-Na-HCO3 and Na-Ca-HCO3 water types. The dominant water type is Ca-NaHCO3 type followed by Na-Ca-HCO3. The chemical evolution path from recharge to
discharge area is from Ca-HCO3 rich rainwater in to Ca-Na-HCO3 to Na-Ca-HCO3-type
waters. The process of dissolution, cation exchange and hydrolysis primarily control the
formation of water types that evolve between the Ca-HCO3 and the Na-Ca-HCO3 type
waters. Cation exchange process is mainly responsible for the formation of Na-CaHCO3.
Ca - HCO3
•
Ca-Mg-HCO3
Ca -Na -HCO3
Na - Ca -HCO3
Naturally, based on anion composition all water types are HCO3 type waters whereas the
Cl and NO3 water types are not due to natural evolution but attributed to anthropogenic
effect.
•
Most of the water types collected from all sources is Ca-Na-HCO3 followed by Na-CaHCO3 water type.
•
Most ground waters found in the study area have pH values ranging from about 6 to 8.6
except in two samples one from spring and one from shallow well.
•
Water from springs generally shows EC that ranges from 57 - 704μs/cm. Springs which
are mostly found near the recharge areas have relatively lower values of EC than springs
down the recharge area depicting soluble ions as water moves from recharge to discharge
areas.
•
For bore holes EC values range from240 μs/cm to 636 μs/cm and the over all average EC
values of bore holes in the study area is about 470 μs/cm and the highest EC value in
hand dug wells is about 854μS/cm.
85
•
The whole study area is covered by volcanic rocks and due to absence of non-carbonate
rocks, bicarbonate ion is attributed to atmospheric and soil CO2 and hydrolysis of silicate
minerals.
•
Generally, the water samples are characterized by low TDS (average TDS=256.1mg/l)
concentration indicating the rock-water interaction (residence time) is short and the
resistance of volcanic rocks to weathering.
•
The over all average concentration of NO3 is greater in hand dug wells than in bore holes
and springs. This can be attributed to the ease access of leachates through soil layers,
weathered part of rocks and shallow ground water table.
•
Although there is no SO4=type water, the concentration of SO4= is present in water
samples. This is probably the result of dissolution of salts from paleo lake deposits of
lacustrine sediments as described in geology of the area. The average relative
concentration of SO4= is higher in bore holes than in spring and hand dug well.
•
Maximum chloride (Cl-)ion (47.7mg/l) is registered in water sample from shallow hand
dug well at Keta (GPS location 537038mE, 77164350mN, elevation 2560m and the
minimum value(2.98mg/l) is registered in water sample from Bucha Roye spring(GPS
location547476mE, 7666752mN, elevation 2620m. As chloride is frequently associated
with sewage, it is may be incorporated into water from faecal contamination discharged
in water bodies.
•
Concerning F-, relatively high concentration is measured in water samples from Tedecha
Bulura and Berisa bore hole (2mg/l) and 1.84mg/l in water samples from Hinja Burkitu
spring which is above the limit of the WHO guide lines for F-concentration (1.5mg/l).
•
Concerning drinking water quality, all water samples fit for drinking except in water
sample from Hinja Burkitu spring which has relatively higher value of fluoride content
(1.84mg/l)which slightly more than the recommended value of WHO(1.5mg/l).
•
Most of the water samples from all sources (boreholes, springs, and hand dug wells)
from agricultural point of view were also analyzed and found that the sodium adsorption
ratio (SAR) is below 10 indicating that the water meets the required standard of quality.
86
6.2) Recommendations
•
An active management strategy aimed at effective integrated environmental
improvement by all concerned bodies through extensive soil conservation, reforestation,
balanced and planned growth of the human and animal population in order to improve
the distribution of water through the entire catchment.
•
In volcanic high lands of Ethiopia, less hydrological and hydrogeological studies have
been carried out including in this study area and so, detail investigation needs as they are
the headwater sources for most of Ethiopian River Basins.
•
There is no comprehensive groundwater potential study that take into consideration the
interactions of fractures/faults, lithology, geomorphology, etc. to characterize the
hydrogeological properties of the volcanic aquifers prevalent in the area hence, detail
geological and structural controls on groundwater movement should be conducted in the
study area.
•
The sources of high discharge springs at the central Gedeb-Asasa plain and groundwatersurface water interaction in the catchment needs further detail investigation using
hydrogeochemical and isotope techniques.
•
High yield springs emerging from hillsides at plain areas should be developed for
community utilization before commencing drilling of deep wells because they are not
well developed in the area for the needy communities.
•
The majority of the well drilled in the study area have no observation pipes that facilitate
water level fluctuations and depth to water level measurement and their completion reports
have no necessary information (site description such as geographic location, elevation,
construction events, drilling events, water struck depth, well development methods, casing
arrangement, capacity of pump equipment used for pumping test, pumping type and their
well analyzed results such as storage coefficient, transmissivity, hydraulic conductivity,
with raw pumping test data attached, Water quality analysis data etc.) Therefore,
hydrological and hydrogeological data obtained from developmental activities should be
organized and archived in a proper manner so that data can be available for further
valuable studies and research.
87
•
Reliable meteorological data on daily basis are suggested to be measured, recorded on
regular bases and should be archived properly by the concerned organizations to enhance
the management and evaluation of both surface and groundwater resources.
•
Awareness should be created among the rural communities for safe and efficient disposal
of human and animal waste to protect surface and subsurface water from pollution so
that to reduce water borne diseases and to provide the communities with clean and
adequate potable water.
•
Spring capping is recommended to protect the water supply points from pollution by
animals and humans.
•
Periodical sampling of wells and springs for groundwater quality monitoring is
recommended.
88
References
Berhe, S.M.,1978. Geological Map of Nazareth Sheet (1:250,000). Ethiopian Institute of
Geological Surveys, Ministry of Mines, Energy and Water Resources.
Chorowicz, J., 2005. The East African Rift system. Journal of African Earth Sciences 43, 379-410.
Davis N.S., De Wiest J.M.,1966. Hydrogeology. John Wiley and Sons, Inc., New York.
Di Pola, G.M., 1972. The Ethiopian Rift Valley (between 71000’ and 8040’ lat. North). Bulletin
of Volcanology.
Dunne, T. and Leopold B.L.,1978. Water in environmental planning. Freeman, San
Fransisco, 815pp
Fetter, C.W., 1994. Applied hydrogeology. Third edition, Prentice-Hall, New Jersy, 695PP
Freeze, R. A., and Cherry, J. A., 1979. Groundwater. Prentice Hall, Englewood Cliffs, New
Jersy, 616pp.
Getahun Kebede.,1985. Hydrogeology of the Nazareth Sheet NC37-15. Ethiopian Institute of
Geological Surveys.
Getaneh Assefa, M.A.J.Williams, D.J.Clark.,1982.Late Cenozoic History and Archaeology of
the Upper Wabe Shebele Basin, East-Central Ethiopia. Department of Geology, Faculty of
science,Addis Ababa University.
Gobena, H., Belayneh, M., Kebede, T., Tesfaye, S., Abraham, A., 1997. Geology of the
Dodola area. Geological Survey of Ethiopia.
Guiraud, R., Bellion, Y., 1995. Late Carboniferous to recent Geodynamic evolution of the
West Gondwanian, Cratonic, Tethyan margins.
89
Hem J.D., 1985. Study and interpretation of chemical characteristics of natural water.Third
edition, United States Geological survey water supply paper 2254, United States government
printing office, Washington, 264pp.
Herczeg, A., 2001. Can major ion chemistry be used to estimate groundwater residence time in
basaltic aquifer? 9th International Symposium, Water-Rock Interaction. Balkema, Rotterdam.
Houghton D.D. ,1985. Handbook of applied meteorology. John Wiley & Sons, 1461pp.
Huaming, G, Yanxin, W., 2004.Hydrological processes in shallow quaternary aquifers from
the northern part of Datnong Basin. University of Geosciences, China.
Jayarami P.,1996. A text book of hydrology. Laximi publications, New Delhi, 530pp
Juch., 1975. The Geology of the South-Eastern escarpment of Ethiopia between 390 and 420
longitude East.
Kazmin, V., Berhe, S.M.,1978. Geology and Development of the Nazareth area, Northern
Ethiopian Rift. Ethiopian Institute of Geological Surveys, Note No. 100, 26pp.
Kebede, G.,1985. Hydrogeology of the Nazareth Sheet NC37-15. EIGS, Addis Ababa,
Ethiopia
Kunz, K., Kreuzer, H., Muller, P., 1975. Potassium - Argon age determinations of the Trap
basalts of the southern part of the Afar rift. Schweizerbartsche, Stuttgart.
Kovalevsky, V.S., Kruseman, G.P, Rushton, K.R,.(2004). Ground Water Studies, International
Guide for Hydrogeological Investigations.
Merla G., Abbate E., Azzaaroli A., Bruni P., Sagri M. and Tacconi P., 1973. Geological map of
Ethiopia and Somalia, 1:2,000,000 ). Consiglio Nationale delle Richerche, Italy.
Ministry of Water Resources., 1999. Ethiopian Water Resources Management Policy. Addis
Ababa, Ethiopia.
90
Ministry of Water Resources., 2001. National Water Supply and Sanitation Master plan Status
Report. Addis Ababa, Ethiopia.
Mohr P. A., 1967. The Ethiopian Rift System Bulletin. Geophysical Observatory, Addis Ababa
University.
National Atlas of Ethiopia., 1981.
OWWDSE., 2006. Shanan-Dhungeta & Middle Wabe-Dhare Sub-basins Groundwater
Resource Potential Evaluation Project and Ramis-Mojo-Erer-Daketa Western Jerer Sub-basins
Groundwater Resource Potential Evaluation.
Shaw E.M., 1994. Hydrology in practice. 2nd edition, Chapman and Hall, New York, 539pp.
Seifu Kebede, and Travi, Y., 2002. Comments to an article entitled “ A geochemical survey of
spring water from the main Ethiopian rift valley, southern Ethiopia: implications for well head
protection’’ by Mckenzie et al, Hydrogeology Journal (2001) 9:265.272
Seifu Kebede, Yves Travi, Tamiru Alemayehu and Tenalem Ayenew, 2005.
Groundwater recharge, Circulation and geochemical evolution in the source region of the
Blue Nile River. Ethiopia, ELSEVIER, Science Direct, Applied Geochemistry
20(2005)1658-1676.
Tenalem Ayenew, 1998. The hydrological system of the Lake District basin, central main
Ethiopian rift. Ph.D. Thesis, ITC Publication, 259pp.
Tenalem Ayenew and Tamiru Alemayhu., 2001. Principle of hydrogeology. Department of
Geology and Geophysics, Addis Ababa University, 125pp.
Tenalem Ayenew., 2005. Major ion Composition of the groundwater and surface water
systems and their geological and geochemical controls in the Ethiopian Volcanic terrain.
Journal of Science, SINET: Ethiopian Journal of Science,28(2):171-188.
91
Tenalem Ayenew, Stephan W., Molla Demile., 2008. Hydrogeological Framework and
occurrence of groundwater in the Ethiopian aquifers. Journal of African Earth
Sciences,52(2008) 97-113.
.
Thornthwaite, C.W. and Mather, J.R., 1957. Instructions, manual and tables for computing
potential evapotranspiration and the water balance. Publication 10, 185-311. Centeron, New
Jersy.
United Nations Development Programme and Food and Agriculture Organization., 1984. Field
Document 3, Geomorphology and Soils Assistance to land use planning, and the
accompanying resource maps. Report prepared for the Government of Ethiopia, Addis Ababa
UNESCO., 2004. World Water Assessment Program, National Water Development Report for
Ethiopia. Addis Ababa.
Wenner, C.G., 1973. A master plan for water resources and supplies in the Chilalo
Awraja.CADU Publication 89, Swedish International Development Agency, Stockholm.
World
Health
Organization.,
(1984).
Guidelines
for
Drinking
Water
Quality
Recommendations. Vol.1, Geneva.
WWDSE, MCE and WAPCOS., 2004. Wabe Shebelle River Basin Integrated Development
Master Plan Study Project, Phase II: Data Collection, Site Investigation Survey and Analysis,
Volume VII-Water Resources; Part 3 Hydrogeology, MoWR; Addis Ababa, Ethiopia.
Yemane, T., WoldeGabriel, G., Tesfaye, S., Berhe, S.M., Durary, S., Ebinger, C., Kelley, S.,
1999. Temporal and Geochemical characteristics of Tertiary Volcanic Rocks and tectonic
history in the southern Main Ethiopian Rift and the adjacent volcanic fields.
Zanettin, B., 1992. Evolution of the Ethiopian Volcanic Province. Atti Della Accademia
Nazionale Dei Lincei, Serie IX-Volume I-Fascicolo 6, 155-181.
92
Annexes
Annex 3.1:-Point Precipitation of meteorological stations in and around the study area.
Altitude
Recording
Location (UTM)
No
Station
period
X
1
Adaba
1976-2006
544179
Mean monthly precipitation (mm)
m.a.s.l
Y
(m)
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Annual
775610
2420
24.43
36.48
56.00
81.28
58.04
79.57
190.82
164.37
73.23
50.39
6.01
14.09
834.71
2
Asasa
1976-1983
534961
795870
2370
28.71
38.88
48.81
55.49
52.55
58.19
129.53
148.61
60.59
42.11
16.25
4.13
683.84
3
Kofele
1976-2006
477913
781123
2620
39.20
34.90
153.00
150.80
132.20
134.60
114.90
134.60
114.90
126.60
18.90
29.20
1183.80
4
Hunte
1980-2006
544176
779295
2380
18.38
28.89
40.44
71.93
44.61
66.38
181.74
185.01
72.86
33.37
8.70
6.55
758.86
5
Dodola
1988-2006
520250
771910
2620
38.62
52.92
90.12
94.12
67.41
116.91
159.87
167.19
108.81
55.73
22.27
20.37
994.34
6
Meraro
1968-2006
540459
823512
2975
32.07
34.60
62.00
103.57
87.40
82.37
179.03
183.93
91.67
45.43
26.80
11.83
940.70
7
Bekoji
1976-1996
527676
832847
2810
34.77
55.48
91.02
115.01
112.85
111.47
180.13
193.52
84.45
54.45
18.89
14.69
1066.71
8
Gobessa
1980-2006
555245
843925
2500
51.34
46.63
108.88
174.74
128.58
84.18
150.49
200.93
138.97
126.10
57.77
43.45
1312.06
9
Kersa
1977-1997
496418
834681
2700
32.78
56.41
82.03
124.80
108.74
81.08
118.09
124.96
120.03
64.28
17.44
18.35
948.99
10
Kore
1977-1995
489055
797836
2500
37.19
63.89
100.24
138.65
132.17
100.91
164.56
178.62
136.67
81.99
24.02
23.51
1182.44
11
Ardayita
1981-1997
501936
781255
2900
55.81
16.71
58.85
63.39
78.43
157.33
142.47
131.88
89.63
18.43
27.04
12.80
852.76
12
Siltana
1977-2006
543073
818457
2960
23.60
35.92
81.25
121.49
112.78
70.11
103.08
104.53
121.54
80.58
19.16
16.61
890.66
13
Dinsho
1978-2006
584760
785009
2750
22.55
39.60
86.93
189.35
126.24
93.25
178.26
201.29
153.74
155.98
59.40
40.23
1346.84
14
Arbegona
1990-2006
468781
740734
2500
38.14
39.44
81.72
133.99
86.26
83.85
92.27
76.82
85.55
93.02
41.46
42.06
894.58
15
Wondogenet
1977-2005
454089
792329
1880
30.04
51.73
106.16
141.35
128.14
105.16
135.23
135.93
147.83
101.70
29.00
22.38
1134.64
16
Agarfa
1977-2006
590249
803443
2550
19.11
47.83
103.94
210.20
154.46
91.69
143.78
174.28
138.34
134.10
46.28
32.76
1296.75
17
Shashemenne
1977-2006
455933
796012
2080
Mean
23.60
35.92
81.25
121.49
112.78
70.11
103.08
104.53
121.54
80.58
19.16
16.61
890.66
32.37
42.13
84.27
123.04
101.39
93.36
145.14
153.59
109.43
79.11
26.97
21.74
1012.55
93
Annex 3.2:- Long term arithmetic mean monthly depth of rainfall (mm) of the seven stations in
and the surrounding study area.
Stations
Jan
Feb
Mar
Apr
May
Adaba
24.43
36.48
56.00
81.28
58.04
Asasa
28.71
38.88
48.81
55.49
52.55
Kofele
39.20
34.90 153.00 150.80 132.20 134.60 114.90 134.60 114.90 126.60 18.90
29.20 1183.80
Hunte
18.38
28.89
40.44
71.93
44.61
33.37
8.70
6.55
Dodola
38.62
52.92
90.12
94.12
67.41 116.91 159.87 167.19 108.81 55.73
22.27
20.37 994.34
Meraro
32.07
34.60
62.00 103.57 87.40
Ardayita 55.81
16.71
58.85
63.39
Jun
Jul
Aug
Sep
Oct
Nov
79.57 190.82 164.37 73.23
50.39
6.01
14.09 834.71
58.19 129.53 148.61 60.59
42.11
16.25
4.13
66.38 181.74 185.01 72.86
Dec
Annual
683.33
756.86
82.37 179.03 183.93 91.67
45.43
26.80
11.83 940.70
78.43 157.33 142.47 131.88 89.63
18.43
27.04
12.80 852.77
Kore
37.19
63.89 100.24 138.65 132.17 100.91 164.56 178.62 136.67 81.99
24.02
23.51 1182.44
Siltana
23.60
35.92
19.16
16.61 890.66
Mean
33.11
38.13
81.25 121.49 112.78 70.11 103.08 104.53 121.54 80.58
76.75
97.86
85.07
96.26 151.78 155.42
94
96.66
59.40
18.79
15.45 924.40
Annex 3.3:- Meteorological stations in and around the study area
No
Station
Recording
Location (UTM)
period
X
Y
Altitude
m.a.s.l
(m)
Mean monthly precipitation (mm)
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Annual
1
Adaba
1976-2006
544179
775610
2420
24.43
36.48
56.00
81.28
58.04
79.57
190.82
164.37
73.23
50.39
6.01
14.09
834.71
2
Asasa
1976-1983
534961
795870
2370
28.71
38.88
48.81
55.49
52.55
58.19
129.53
148.61
60.59
42.11
16.25
4.13
683.84
3
Kofele
1976-2006
477913
781123
2620
39.20
34.90
153.00
150.80
132.20
134.60
114.90
134.60
114.90
126.60
18.90
29.20
1183.80
4
Hunte
1980-2006
544176
779295
2380
18.38
28.89
40.44
71.93
44.61
66.38
181.74
185.01
72.86
33.37
8.70
6.55
758.86
5
Dodola
1988-2006
520250
771910
2620
38.62
52.92
90.12
94.12
67.41
116.91
159.87
167.19
108.81
55.73
22.27
20.37
994.34
6
Meraro
1968-2006
540459
823512
2975
32.067
34.6
62
103.57
87.4
82.367
179.033
183.933
91.667
45.4333
26.8
11.83
940.7
7
Bekoji
1976-1996
527676
832847
2810
34.77
55.48
91.02
115.01
112.85
111.47
180.13
193.52
84.45
54.45
18.89
14.69
1066.71
8
Gobessa
1980-2006
555245
843925
2500
51.34
46.63
108.88
174.74
128.58
84.18
150.49
200.93
138.97
126.10
57.77
43.45
1312.06
9
Kersa
1977-1997
496418
834681
2700
32.78
56.41
82.03
124.80
108.74
81.08
118.09
124.96
120.03
64.28
17.44
18.35
948.99
10
Kore
1977-1995
489055
797836
2500
37.19
63.89
100.24
138.65
132.17
100.91
164.56
178.62
136.67
81.99
24.02
23.51
1182.44
11
Ardayita
1981-1997
501936
781255
2900
55.81
16.71
58.85
63.39
78.43
157.33
142.47
131.88
89.63
18.43
27.04
12.80
852.76
12
Siltana
1977-2006
543073
818457
2960
23.60
35.92
81.25
121.49
112.78
70.11
103.08
104.53
121.54
80.58
19.16
16.61
890.66
13
Dinsho
1978-2006
584760
785009
2750
22.55
39.60
86.93
189.35
126.24
93.25
178.26
201.29
153.74
155.98
59.40
40.23
1346.84
14
Arbegona
1990-2006
468781
740734
2500
38.14
39.44
81.72
133.99
86.26
83.85
92.27
76.82
85.55
93.02
41.46
42.06
894.58
15
Wondogenet
1977-2005
454089
792329
1880
30.04
51.73
106.16
141.35
128.14
105.16
135.23
135.93
147.83
101.70
29.00
22.38
1134.64
16
Agarfa
1977-2006
590249
803443
2550
19.11
47.83
103.94
210.20
154.46
91.69
143.78
174.28
138.34
134.10
46.28
32.76
1296.75
17
Shashemenne
1977-2006
455933
796012
2080
23.60
35.92
81.25
121.49
112.78
70.11
103.08
104.53
121.54
80.58
19.16
16.61
890.66
95
Month
T
T
n
0
(Kelvin)
(Hrs)
(Hrs)
( C)
N
HR
U2
ea
ed
n/N
(%)
(miles/d)
(mm/d)
(mm/d)
σΤa4
Ra
Eat
RI (1-r)
R0
Δ/γ
(mm/d)
(mm/d)
(mm/d)
(mm/d)
fa(n/N)
(mm/d)
HT
PET
PET
(mm/d)
(mm/month)
Jan
13.75
286.75
8.54
11.80
0.72
58.69
91.82
11.80
6.93
1.94
13.25
3.27
6.05
2.76
0.61
13.18
3.22
3.24
100.39
Feb
14.51
287.51
7.76
11.90
0.65
56.34
90.21
12.38
6.97
1.65
14.20
3.60
6.01
2.57
0.56
13.32
3.32
3.42
95.84
Mar
15.09
288.09
7.22
12.00
0.60
57.83
92.90
12.80
7.40
1.70
14.90
3.64
5.96
2.39
0.53
13.43
3.45
3.52
109.23
Apr
15.48
288.48
6.42
12.20
0.53
65.92
102.56
13.12
8.65
1.74
15.08
3.17
5.50
2.05
0.49
13.50
3.42
3.33
99.84
May
15.47
288.47
7.03
12.30
0.57
65.29
104.71
13.11
8.56
1.73
14.70
3.26
5.67
2.18
0.51
13.50
3.37
3.33
103.20
Jun
14.97
287.97
5.98
12.40
0.48
69.81
83.77
12.78
8.92
1.69
14.45
2.48
4.97
1.87
0.46
13.41
3.05
2.84
85.14
Jul
14.39
287.39
4.58
12.30
0.37
77.74
85.92
11.98
9.31
1.64
14.58
1.74
4.27
1.53
0.39
13.30
2.68
2.32
72.01
Aug
14.21
287.21
5.00
12.30
0.41
79.84
72.49
12.18
9.72
1.62
14.80
1.48
4.57
1.60
0.41
13.27
2.89
2.35
72.93
Sep
14.55
287.55
5.40
12.10
0.45
76.51
69.27
12.39
9.48
1.66
14.83
1.72
4.86
1.73
0.44
13.33
3.10
2.58
77.43
Oct
13.60
286.60
6.95
12.00
0.58
70.80
89.68
11.62
8.23
1.57
14.40
2.25
5.61
2.18
0.52
13.15
3.36
2.93
90.81
Nov
12.94
285.94
8.33
11.90
0.70
63.99
103.10
11.21
7.17
1.50
13.48
2.87
6.01
2.63
0.59
13.03
3.26
3.10
93.06
Dec
12.84
285.84
8.45
11.80
0.72
61.33
100.42
11.05
6.78
1.49
12.95
3.00
5.87
2.74
0.60
13.02
3.01
3.01
93.20
Annual Evapotranspiration(PET)=1093.08mm
Annex 3.4:- Mean annual PET obtained from Penman method
96
Annex 3.5:-AET for fine sandy loam soil with an available water capacity of 300mm under
mature forest cover with estimated rooting depth of 2.00m (8.0% land cover of the area)
Parameter
Jan
P
33.20
PET
P-PET
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
96.53
152.20
155.85
96.93
59.57
Nov
Total
38.24
76.96
98.13
85.31
100.39
95.84
109.23
99.84
103.20
85.14
72.01
72.93
77.43
90.81
-67.19
-57.60
-32.27
-1.71
-17.89
11.39
80.19
82.92
19.50
-31.24
-252.36
-309.96
-342.23
-343.97
-361.83
-31.24
-105.46
-183.17
130.22
-33.68
107.47
-22.75
96.51
-10.96
96.16
-0.35
90.59
-5.57
102.29
11.70
182.48
80.19
265.40
82.92
284.90
19.50
270.33
-14.57
211.08
-59.25
162.91
-47.08
33.20
60.99
87.92
98.49
90.87
85.14
72.01
72.93
77.43
74.14
78.09
63.67
894.87
S
67.19
0.00
34.85
0.00
21.31
0.00
1.35
0.00
12.33
0.00
85.14
0.00
72.01
0.00
72.93
0.00
77.43
0.00
16.67
0.00
14.97
0.00
29.53
0.00
198.21
0.00
TARO
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
RO
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
Detention
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
APWL
SM
∆S
AET
SMD
18.84
Dec
15.49
927.26
93.06
93.20
1093.08
-74.22
-77.71
-165.82
Annex 3.6:-AET for clay loam soil with an available water capacity of 400mm under
mature forest cover with estimated rooting depth of 1.60m (13.48% land cover of the area)
Parameter
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Total
P
33.20
38.24
76.96
98.13
85.31
96.53
152.20
155.85
96.93
59.57
18.84
15.49
927.26
PET
100.39
95.84
109.23
99.84
103.20
85.14
72.01
72.93
77.43
90.81
93.06
93.20
1093.08
P-PET
-67.19
-57.60
-32.27
-1.71
-17.89
11.39
80.19
82.92
19.50
-31.24
-74.22
-77.71
-165.82
APWL
-252.36
-309.96
-342.23
-343.97
-361.83
-31.24
-105.46
-183.17
129.36
106.76
95.87
95.32
89.81
101.20
181.39
264.31
283.81
270.33
211.08
162.91
-33.56
-22.60
-10.89
-0.55
-5.51
11.39
80.19
82.92
19.50
-13.49
-59.25
-48.17
0.00
66.76
60.83
87.85
98.69
90.82
85.14
72.01
72.93
77.43
73.05
78.09
63.66
927.26
S
33.63
0.00
35.01
0.00
21.38
0.00
1.15
0.00
12.38
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
17.76
0.00
14.97
0.00
29.54
0.00
165.82
0.00
TARO
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
RO
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
Detention
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
SM
∆S
AET
SMD
97
Annex 3.7:-AET for clay loam soil with an available water capacity of 250mm under deep
rooted crop cover with estimated rooting depth of 1.00m (8.77% land cover of the
area)(pasture grass,bushes,shrubs)
Parameter
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Total
P
33.20
38.24
76.96
98.13
85.31
96.53
152.20
155.85
96.93
59.57
18.84
15.49
927.26
PET
100.39
95.84
109.23
99.84
103.20
85.14
72.01
72.93
77.43
90.81
93.06
93.20
1093.08
P-PET
-67.19
-57.60
-32.27
-1.71
-17.89
11.39
80.19
82.92
19.50
-31.24
-74.22
-77.71
-165.82
-183.17
-183.17
-183.17
-183.17
-183.17
-183.17
-183.17
-183.17
-183.17
-183.17
-183.17
-183.17
-183.17
91.11
72.36
63.60
63.15
58.80
70.19
150.38
233.31
250.00
220.63
163.96
120.15
-29.04
-18.75
-8.76
-0.44
-4.35
11.39
80.19
82.92
16.69
-29.37
-56.67
-43.81
0.00
62.25
56.98
85.73
98.57
89.66
85.14
72.01
72.93
77.43
88.94
75.51
59.30
924.46
93.95
38.86
23.50
1.27
13.54
0.00
0.00
0.00
0.00
1.87
17.55
33.90
224.43
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
2.81
0.00
0.00
0.00
2.81
0.18
0.09
0.06
0.03
0.015
0.00
0.00
0.00
2.81
1.41
0.71
0.36
0.09
0.05
0.00
0.00
0.00
0.00
0.00
0.00
1.41
0.71
0.36
0.18
0.09
0.05
0.00
0.00
0.00
0.00
0.00
0.00
1.41
0.71
0.36
0.18
APWL
SM
∆S
AET
SMD
S
TARO
RO
Detention
Annex 3.8:-AET for fine sandy loam soil with an available water capacity of 150mm deep
rooted crop cover with estimated rooting depth of 1.00m (5.0% land cover of the area)(
pasture grass,bushes,shrubs)
Parameter
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Total
P
33.20
38.24
76.96
98.13
85.31
96.53
152.20
155.85
96.93
59.57
18.84
15.49
927.26
PET
100.39
95.84
109.23
99.84
103.20
85.14
72.01
72.93
77.43
90.81
93.06
93.20
1093.08
P-PET
-67.19
-57.60
-32.27
-1.71
-17.89
11.39
80.19
82.92
19.50
-31.24
-74.22
-77.71
-165.82
-252.36
-309.96
-342.23
-343.97
-361.83
-31.24
-105.46
-183.17
27.89
19.00
15.32
15.14
13.44
24.83
105.02
150.00
150.00
121.80
74.26
44.23
-16.35
-8.89
-3.68
-0.18
-1.70
11.39
80.19
44.98
0.00
-28.20
-47.54
-30.03
0.00
APWL
SM
∆S
AET
SMD
S
TARO
RO
Detention
49.55
43.17
80.64
98.31
87.01
85.14
72.01
72.93
77.43
87.77
66.38
45.52
865.85
50.84
52.67
28.59
1.53
16.19
0.00
0.00
0.00
0.00
3.04
26.68
47.68
227.23
0.00
0.00
0.00
0.00
0.00
0.00
0.00
37.95
19.50
0.00
0.00
0.00
57.45
3.59
1.80
0.90
0.45
0.23
0.115
0.0575
37.95
57.45
28.72
14.36
7.18
1.80
0.90
0.45
0.23
0.115
0.0575
0.0288
18.98
28.72
14.36
7.18
3.59
1.80
0.90
0.45
0.23
0.015
0.0575
0.0288
9.49
28.72
14.36
7.18
3.59
98
Annex 3.9:-AET for fine sandy loam soil with an available water capacity of 75mm under
shallow rooted crop cover with estimated rooting depth of 0.50m (7.58% land cover of the
area)(peas,beans)
Parameter
Jan
P
Feb
33.20
Mar
Apr
May
38.24
76.96
98.13
85.31
Jun
Jul
Aug
Sep
96.53
152.20
155.85
96.93
Oct
59.57
Nov
18.84
Dec
Total
15.49
927.26
PET
100.39
95.84
109.23
99.84
103.20
85.14
72.01
72.93
77.43
90.81
93.06
93.20
1093.08
P-PET
-67.19
-57.60
-32.27
-1.71
-17.89
11.39
80.19
82.92
19.50
-31.24
-74.22
-77.71
-165.82
-252.36
2.66
-309.96
1.24
-342.23
0.80
-343.97
0.79
-361.83
0.62
12.01
75.00
75.00
75.00
-31.24
49.45
-105.46
18.38
-183.17
6.70
APWL
SM
∆S
-4.04
-1.42
-0.44
-0.02
11.39
62.99
0.00
0.00
-25.55
-31.07
-11.68
AET
37.24
39.66
77.40
0-.01
98.14
85.48
85.14
72.01
72.93
77.43
85.12
49.91
27.18
807.63
SMD
63.15
56.18
31.83
1.70
17.72
0.00
0.00
0.00
0.00
5.69
43.15
66.02
285.45
S
0.00
0.00
0.00
0.00
0.00
0.00
17.20
82.92
19.50
0.00
0.00
0.00
119.62
TARO
4.80
2.40
1.20
0.60
0.30
0.15
17.20
91.52
65.26
32.63
16.32
8.16
RO
2.40
1.20
0.60
0.30
0.15
0.075
8.60
45.76
32.63
16.31
8.16
4.80
Detention
2.40
1.20
0.60
0.30
0.15
0.075
8.60
45.76
32.63
16.32
8.16
4.80
Annex 3.10:-AET for fine sandy loam soil with an available water capacity of 150mm
moderately deep rooted crop cover with estimated rooting depth of 1.00m (27.82% land
cover of the area)(wheat,barely,corn)
Parameter
Jan
P
33.20
PET
P-PET
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
96.53
152.20
155.85
96.93
59.57
Nov
Total
38.24
76.96
98.13
85.31
100.39
95.84
109.23
99.84
103.20
85.14
72.01
72.93
77.43
90.81
-67.19
-57.60
-32.27
-1.71
-17.89
11.39
80.19
82.92
19.50
-31.24
SM
-252.36
28.26
-309.96
19.25
-342.23
15.52
-343.97
15.41
-361.83
13.68
25.07
105.26
150.00
150.00
-31.24
121.80
-105.46
74.26
-183.17
44.83
∆S
-16.57
-9.01
-3.73
-0.11
-1.73
11.39
80.19
44.74
0.00
-28.20
-47.54
-29.43
49.55
47.13
80.64
98.31
87.01
85.14
72.01
72.93
77.43
87.77
66.38
45.52
869.81
S
50.84
0.00
48.71
0.00
28.59
0.00
1.53
0.00
16.19
0.00
0.00
0.00
0.00
0.00
0.00
38.18
0.00
19.50
3.04
0.00
26.68
0.00
47.68
0.00
223.27
57.68
TARO
2.41
1.20
0.60
0.30
0.15
0.075
0.0375
38.18
38.59
19.29
9.65
4.82
RO
1.21
0.60
0.30
0.15
0.075
0.0375
0.0180
19.09
19.29
9.65
4.83
2.41
Detention
1.20
0.60
0.30
0.15
0.075
0.0375
0.0180
19.09
19.30
9.65
4.82
2.41
APWL
AET
SMD
99
18.84
Dec
15.49
927.26
93.06
93.20
1093.08
-74.22
-77.71
-165.82
Annex 3.11:-AET for clay loam soil with an available water capacity of 200mm
moderately deep rooted crop cover with estimated rooting depth of 0.80m (20% land cover
of the area)( wheat,barely,corn)
Parameter
P
Jan
Feb
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
Total
33.20
38.24
76.96
98.13
85.31
96.53
152.20
155.85
96.93
59.57
18.84
15.49
927.26
PET
100.39
95.84
109.23
99.84
103.20
85.14
72.01
72.93
77.43
90.81
93.06
93.20
1093.08
P-PET
-67.19
-57.60
-32.27
-1.71
-17.89
11.39
80.19
82.92
19.50
-31.24
-74.22
-77.71
-165.82
SM
-252.36
57.20
-309.96
42.88
-342.23
36.49
-343.97
36.29
-361.83
33.19
44.58
124.77
200.00
200.00
-31.24
171.07
-105.46
118.04
-183.17
80.84
∆S
-23.64
-14.32
-6.39
-0.02
-3.10
11.39
80.19
75.23
0.00
-28.93
-53.03
-37.20
56.63
42.46
36.13
35.82
32.76
85.14
72.01
72.93
77.43
171.07
118.04
80.04
880.45
43.76
0.00
53.38
0.00
73.10
0.00
64.02
0.00
70.44
0.00
0.00
0.00
0.00
0.00
0.00
7.69
0.00
19.50
-80.26
0.00
-24.98
0.00
13.16
0.00
212.63
27.19
TARO
1.46
0.73
0.36
0.18
0.09
0.05
0.02
7.69
23.35
11.67
5.84
2.92
RO
0.73
0.37
0.18
0.09
0.05
0.02
0.01
3.85
11.67
5.83
2.92
1.46
Detention
0.73
0.36
0.18
0.09
0.05
0.02
0.01
3.84
11.67
5.84
2.92
1.46
APWL
AET
SMD
S
100
T
(kelvin)
N
(Hrs)
N
(Hrs)
H
(%)
U1
(m/s)
U2
(miles/d)
ea
ed
(mm/d)
(mm/d)
Ra
(mm/d)
Ea
(mm/d)
RI (1-r)
(mm/d)
R0
(mm/d)
Jan
14.36
287.36
7.30
11.80
0.62
48.90
1.55
81.77
11.80
5.77
1.63
13.25
0.94
6.84
3.11
0.54
13.77
3.73
2.67
Feb
14.99
287.99
6.90
11.90
0.58
47.00
1.55
84.61
12.38
5.82
1.69
14.20
1.00
7.01
2.96
0.52
13.90
4.04
2.91
84.48
Mar
16.09
289.09
7.20
12.00
0.60
43.33
1.53
112.64
12.80
5.55
1.79
14.90
1.31
7.53
3.12
0.53
14.01
4.41
3.30
102.30
n/N
H
Eo
mm/day
Eo
mm/mo
nth
Month
Δ/γ
fa
(n/N)
sTa4
(mm/d)
T
( C)
0
82.76
Apr
16.45
289.45
7.00
12.20
0.57
59.00
1.90
100.32
13.12
7.74
1.83
15.08
0.89
7.39
2.68
0.52
14.05
4.71
3.36
100.77
May
16.66
289.66
7.28
12.30
0.59
57.33
1.83
63.58
13.11
7.52
1.86
14.70
0.69
7.36
2.79
0.53
14.10
4.56
3.21
99.55
Jun
16.56
289.56
6.55
12.40
0.53
58.33
1.48
69.95
12.78
7.45
1.84
14.45
0.72
6.69
2.54
0.49
14.07
4.15
2.94
88.22
Jul
15.87
288.87
5.10
12.30
0.41
76.50
1.27
43.95
11.98
9.16
1.77
14.58
0.43
5.78
1.90
0.42
13.98
3.87
2.63
81.56
Aug
15.36
288.36
6.15
12.30
0.50
79.67
0.97
56.44
12.18
9.70
1.73
14.80
0.47
6.61
2.15
0.47
13.96
4.46
3.00
92.95
Sep
15.01
288.01
5.88
12.10
0.49
76.67
1.10
92.46
12.39
9.50
1.69
14.83
0.65
6.50
2.11
0.46
13.90
4.39
3.00
89.95
Oct
14.56
287.56
7.80
12.00
0.65
70.50
1.33
108.04
11.62
8.19
1.67
14.40
0.78
7.70
2.89
0.56
13.94
4.81
3.30
102.45
Nov
13.66
286.66
8.30
11.90
0.70
63.00
1.75
88.91
11.21
7.06
1.58
13.48
0.77
7.59
3.15
0.59
13.49
4.44
3.02
90.49
Dec
13.37
286.37
7.23
11.80
0.61
56.67
1.70
83.23
11.05
6.26
1.53
12.95
0.82
6.64
2.94
0.54
13.46
3.71
2.57
79.56
1095.1
mm
Anuual
Annex 3.12:- Melka wakena Hydroelectric Power Reservoir Water Evaporation
101
Annex 3.13:- Projected mean river discharge of Upper Wabe river catchment (m3/s) (1976-2006)
Year
Jan
1976
7.38
1977
16.27
1978
6.19
1979
10.07
1980
6.60
1981
5.57
1982
7.71
1983
13.53
1984
6.10
1985
6.78
1986
6.06
1987
6.82
1988
8.42
1989
6.97
1990
10.55
1991
6.83
1992
9.36
1993
8.93
1994
6.73
1995
8.93
1996
16.74
1997
11.34
1998
20.74
1999
7.27
2000
6.08
2001
7.30
2002
9.43
2003
9.14
2004
7.12
2005
10.39
2006
6.20
Mean
8.95
Feb
5.44
8.64
10.21
6.49
5.81
7.28
36.01
8.05
9.62
7.65
9.17
9.79
7.45
42.06
8.75
6.61
13.96
7.47
6.55
6.55
7.62
10.75
7.40
6.77
12.35
7.93
9.17
7.78
7.32
6.94
6.94
10.15
Mar
Apr
May
Jun
Jul
Aug
Sep
Oct
Nov
Dec
6.46
7.75
12.20
15.05
27.05
62.89
57.03
13.61
19.39
7.91
8.93
18.30
15.24
28.63
73.99
81.46
100.19
46.23
26.30
8.64
24.34
13.01
21.36
17.49
37.10
83.71
68.22
53.65
10.38
10.21
16.42
21.49
10.77
15.90
23.41
32.32
57.00
24.90
8.54
6.49
7.50
12.36
21.89
17.50
73.40
55.57
38.81
23.95
7.37
5.81
15.81
42.26
9.04
7.19
21.68
80.62
80.32
23.65
9.66
7.28
7.62
12.24
15.92
18.49
51.92
99.65
75.07
54.34
42.38
36.01
18.48
60.36
106.24
25.62
25.36
100.91
138.07
96.20
37.73
8.05
5.94
6.16
10.03
34.19
66.97
69.94
47.27
10.64
12.25
9.62
7.12
17.16
10.03
40.89
59.20
83.81
86.66
28.15
9.70
7.65
7.75
22.29
24.79
37.96
46.87
71.77
77.21
38.14
8.45
9.17
23.02
31.15
64.49
49.52
26.64
80.99
62.05
57.23
22.23
9.79
7.24
13.40
11.77
25.25
54.76
121.23
88.73
60.38
11.86
7.45
7.82
14.83
10.87
10.43
18.40
46.17
56.87
31.46
12.32
42.06
33.44
54.32
16.86
16.16
19.86
63.57
52.62
26.90
8.63
8.75
16.08
13.37
14.14
8.40
10.41
38.69
57.63
15.20
7.24
6.61
13.46
40.39
27.47
36.66
62.10
71.66
91.94
42.28
17.29
13.96
13.08
15.76
57.68
56.93
83.94
79.48
51.76
58.24
19.80
7.47
8.70
8.53
25.87
56.57
86.54
91.77
48.94
13.87
19.80
6.55
8.70
15.76
25.87
73.98
26.09
35.54
60.64
18.13
6.82
6.55
16.63
35.33
42.44
73.98
50.70
104.66
61.22
18.13
9.53
7.62
6.79
19.98
13.57
29.92
47.75
51.25
33.30
48.63
51.74
10.75
15.31
21.28
51.15
36.60
47.40
44.33
89.34
89.77
19.03
7.40
9.27
8.34
9.56
11.24
33.11
65.46
55.53
88.61
10.24
6.77
5.10
11.20
12.52
9.42
25.40
79.80
63.29
68.86
23.63
12.35
10.66
13.05
22.13
50.05
66.36
38.79
51.86
42.10
20.00
7.93
11.90
8.08
9.84
13.70
23.67
67.56
48.83
13.44
8.09
9.17
11.90
12.83
8.20
14.67
23.64
33.14
62.70
50.89
8.14
7.78
7.70
21.76
12.59
23.09
50.14
64.97
70.38
79.22
9.69
7.32
8.27
23.35
91.71
14.38
40.94
71.99
79.72
20.69
7.34
6.94
13.37
24.98
24.31
37.56
97.98
102.73
66.34
70.74
24.65
6.94
12.09
20.68
26.15
29.27
45.25
70.21
67.08
42.85
16.46
10.23
102
Annex 4.1:- Some of the cold fracture type springs observed in the study area.
Site description
No
Location
Ele.
(m)
Estimated
Yield
(l/s)
Site Name
Zone
Woreda
1
Balo
Bale
Adaba
557806
778498
2645
254
1
2
Boro
Bale
Dodola
530098
772773
2442
332
0.8
3
Sirko
Arsi
Asasa
529142
806704
2691
306
0.5
4
Robe gerjeda
Arsi
Asasa
547511
793107
2295
444
3
5
Boricho
Arsi
Kofele
481290
788661
2668
57
<1
6
Soboro
Bale
Adaba
550088
776575
2468
488
1
7
Semira
Arsi
Asasa
551221
800796
2311
289
>15
8
Kaka
Arsi
Asasa
514135
807345
3280
92
20
9
Gebecho
Arsi
Asasa
534474
808353
2850
206
6
10
Hnja Burkitu
Arsi
Asasa
538009
791636
2404
290
>100
11
Asasa
Arsi
Asasa
521822
785702
180
>100
103
Northing
(m)
EC
(µs/cm)
Easting
(m)
Type of
spring
Fracture
Spring
Fracture
Spring
Fracture
Spring
Fracture
Spring
Fracture
Spring
Fracture
Spring
Fracture
Spring
Fracture
Spring
Fracture
Spring
Fracture
Spring
Fracture
Spring
No
Source
Zone
Wereda
Locality
E
N
Elevation
(m)
SWL
(m)
1
BH
Arsi
Gedeb
Ardayita
509383
784701
2620
137
Groundwater
Elevation
(m)
2483
2
BH
Arsi
Gedeb
Ardayita
509614
784647
2600
118
2482
3
BH
Bale
Dodola
Herero St. Farm
530306
775215
2360
30
2330
4
BH
Bale
Dodola
Herero Town
536057
773363
2540
35
2505
5
BH
Arsi
Gedeb
Huruba walkite
515612
778588
2550
32
2518
6
BH
Arsi
Gedeb
Huruba Hunta
520370
780283
2530
20
2510
7
BH
Arsi
Gedeb
Obolto
514899
784553
2580
90
2490
8
BH
Bale
Adaba
Wesha tullu
553113
777597
2610
15
2595
9
BH
Bale
Adaba
Wesha tulu-1
552963
778752
2530
19.9
2510.1
10
BH
Bale
Adaba
Hunte state Farm
547851
784680
2550
22.5
2527.5
11
BH
Arsi
Adaba
Melka wakena
549000
792450
2530
23.6
2506.4
12
BH
SNNP
Sidama
Arbegona
468912
737750
2593
6
2587
13
BH
SNNP
Awassa
Hogiso#1
465388
757681
2725
12
2713
14
BH
SNNP
Awassa
Hogiso#2
465662
757539
2726
18
2708
15
BH
Bale
Kokosa
Hogiso#3
465486
757451
2728
15
2713
16
BH
Bale
Adaba
Alola Hunte#1
542333
782903
2350
10
2340
17
BH
Bale
Adaba
Geredilo#1
555259
798746
2414
2
2412
18
BH
Bale
Adaba
Geredilo#2
555094
798091
2436
5
2431
19
BH
Bale
Adaba
Geredilo#3
555964
797981
2438
7
2431
20
BH
Bale
Adaba
Geredilo#4
554349
798609
2421
7
2414
21
BH
Bale
Adaba
Haro Hunte#1
538420
778404
2366
30
2336
22
BH
Bale
Adaba
Washe#2
551542
777388
2470
10
2460
23
BH
Bale
Dodola
Baka#1
536814
779218
2350
15
2335
24
BH
Bale
Dodola
Deneba#2
519947
772125
2520
12
2508
25
BH
Bale
Dodola
Chere#1
517237
776781
2410
43
2367
26
BH
Bale
Dodola
Gedera#1
527610
779212
2370
32
2338
27
BH
Bale
Dodola
Herero#1
536188
773460
2420
15
2405
28
BH
Bale
Dodola
Alwaso
520969
778064
2400
30
2370
29
HDW
Bale
Adaba
Lajo Birbirsa
563448
806672
2515
11
2504
30
HDW
Bale
Adaba
Fonsho
558996
803884
2530
24
2506
31
HDW
Arsi
Gedeb
Ela No 3
500625
788725
3.75
2726.25
3.65
2766.35
17
2723
21
2709
29
2511
1
2589
32
HDW
Arsi
Gedeb
Ela 1&2
500584
788764
33
HDW
Bale
Dodola
Negele Metema 2
493472
757748
34
HDW
Bale
Dodola
Negele Metema 3
493557
757769
35
HDW
Bale
Dodola
Herero Town 1
535775
773002
36
HDW
Bale
Dodola
Burachele 1
533391
770935
2730
2770
2740
2730
2540
2590
37
HDW
Bale
Kokossa
Kokosa town
476749
744850
2590
5
2585
38
HDW
Bale
Dodola
Keta 2
537038
771643
2570
1.5
2568.5
Annex 4.2:-Ground water sources inventory data
104
Location
UTM
(m)
EC
Na
K
Ca,
Mg.
Fe
Mn
Cl
NO3
HCO3
SO4
NO2
F
Hardness
asCaCO3
mg/l
12
2.9
36.12
5.610
trace
0.020
5.15
4.28
176.78
0.29
nill
0.6
113.4
0.12
26.5
0.8
34.44
7.140
trace
0.050
10.3
33.40
148.59
15.8
nill
0.94
115.5
202
0.09
34
2.2
26.88
5.100
trace
0.050
17.51
11.29
128.1
19.99
nill
0.75
88.2
42
290
0.18
31
2.3
53.76
14.280
trace
0.070
14.7
0.35
286.9
0.38
nill
1.29
193.2
5.9
320
42
0.08
6.4
3.9
4.20
2.040
trace
0.050
3.09
1.96
35.87
0.48
nill
0.53
18.9
550088
6.8
190
320
0.31
18.5
1.9
69.72
13.770
0.01
0.020
23.69
27.11
238.27
5.14
nill
0.97
231
800796
551221
6.97
62
190
0.09
23
6.1
32.76
8.160
trace
0.020
8.24
4.00
169.09
7.52
nill
0.6
115.5
spring
807345
514135
6.39
136
62
0.11
5.4
1.7
10.08
3.570
0.09
0.050
3.09
7.26
56.36
1.43
nill
0.19
39.9
Arsi
spring
808353
534474
6.24
558
136
0.13
9.3
1.8
26.04
5.100
0.02
0.020
7.21
11.42
107.6
1.14
nill
0.48
86.1
Gedisa
Bale
HDW
774450
522748
7.19
84
558
0.34
91
12.3
83.16
6.120
trace
0.020
26.78
10.30
538.02
1.57
nill
1.24
233.1
11
Sheneka
Arsi
HDW
784665
477711
5.22
204
84
0.1
8.9
7.2
10.08
2.550
trace
0.070
4.12
32.51
30.74
0.57
nill
0.41
35.7
12
Hasano
Arsi
BH
775636
500024
7.03
311
204
0.11
36
8.7
25.2
7.140
trace
0.100
3.09
1.00
194.71
0.76
nill
0.94
92.4
In mg/l
μS/cm
S.No.
Locality
name
Zone
source
N
E
pH
1
Balo
Bale
spring
778498
557806
6.38
218
166
2
Boro
Bale
spring
772773
530098
6.27
202
218
3
Sirko
Arsi
spring
806704
529142
6.89
290
4
Robe
gerjeda
Arsi
spring
793107
547511
7.14
5
Boricho
Arsi
spring
788661
481290
6
Soboro
Bale
spring
776575
7
Semira
Arsi
spring
8
Kaka
Arsi
9
Gebecho
10
TDS
NH4
Annex 4.3:- Laboratory analysis results of collected samples
105
Location
UTM
(m)
EC
In mg/l
μS/c
Hardness
asCaCO3
mg/l
BH-2
Locality
Name
Tedacha
Bulura
Arsi
792266
525559
636
406
8.53
0.22
100
12.7
36.8
5.35
0
0.02
Cl
32.7
7
8.75
272.8
110.12
0.01
2
114
BH-3
Debara
Arsi
790405
523396
331
213
8.08
0.38
25
11.7
31.2
7.78
0
0
9.9
0.75
178.1
1.05
0.1
0.5
110
BH-4
Ardu
Arsi
786568
521909
240
152
8.22
0.13
20
11.4
20.8
5.84
0.03
0
7.9
0
127.6
0.79
0
0
BH-5
Serafta state F
Bale
770950
501973
384
260
7.83
0.06
25
18
44
5.84
0
0
16.9
8.5
193.2
0.26
0.02
0.8
134
BH-6
Dodola Town
Bale
772034
519834
540
321
7.94
0.025
55
11.6
60
3.4
0.02
0.02
18.9
10.5
276.4
5.8
0.05
1.24
164
BH-7
Herero St. F
Bale
775215
530306
608
367
7.66
0.06
30
20.5
80
12.16
0.02
0
30.8
5
319.4
1.05
0.02
0.15
250
BH-8
Bale
773363
536057
539
344
8.07
0.04
30
11
68.8
9.24
0
0
282
0.26
0.02
0.45
210
Arsi
778588
515612
416
280
7.63
0.15
35
11.2
46.4
5.8
0
0
0
233.5
4.22
0
0.45
140
BH-10
Huruba Hunta
Arsi
780283
520370
385
226
7.58
0.59
30
10.7
43.2
6.81
0
0
12.9
12.9
1
11.9
2
5
BH-9
Herero Twn
Huruba
walkite
3.25
212
2.64
0.8
136
BH-14
Berisa
Bale
772443
525855
553
355
7.93
0.1
60
15.1
64.8
4.38
0
0
24.8
3.5
287.2
19.52
0
0.01
5
2
180
Source
Zone
N
E
m
TDS
pH
Na
NH4
K
Ca
Mg
Fe
Total
Mn
NO3
HCO3
SO4
NO2
F
BH-12
Wabe Birkitu
Bale
780070
528375
558
329
8.08
0.04
25
16
68
9.24
0
0
20.9
8.5
246.4
4.75
0.04
1.1
208
BH-13
Hunte state F
Bale
784680
547851
423
244
8.1
0
30
6
53.6
4.86
0
0
6.95
1.5
213.74
0.26
0.04
0
154
BH-14
Melka wakena
Arsi
792450
549000
0.12
55.3
7.7
41.7
9.7
0.12
0
42.5
9.3
219.6
28.6
0.03
0.64
BH-3
Dabara
Arsi
804068
515228
112
69
7.75
0.025
8
6.3
8.8
3.4
0.02
0.02
5.96
1.5
53.7
1.58
0.02
0
36
SSP-11
Ejakelo
Bale
766515
519622
357
269
7.88
0.06
27.5
4
42.4
3.89
0.02
0
11.9
16.2
166.4
2.11
0
122
SSP-12
HinjaBurkitu
Arsi
791636
538009
290
194
7.8
0.38
30
13.4
30.4
3.4
0.02
0
11.9
3.5
155.7
0.53
0.02
0.01
5
SSP-13
Assasa
Arsi
785702
521822
180
117
7.83
0.03
10
8.2
16
5.35
0.02
0.02
3.97
1.3
99.3
0.79
SSP-14
Kokosa Town
Bale
745050
477417
615
400
6
0.13
45
38.75
32
9.74
0
0.02
78.45
81
42.94
SSP-15
Wesha Tulu
Bale
777265
552593
326
205
7.98
0.025
15
7.8
41.6
5.84
0.02
0
12.9
8.75
155.7
SSP-16
Bale
793616
553836
480
329
8.02
0.025
30
4.7
62.4
6.81
0
0
20.9
10.5
222
SSP-17
Kinsho
Ejersa
Mudemtu
Bale
774411
539803
704
392
7.81
0.06
25
6.5
112.8
5.35
0
0
27.8
35.25
SSP-18
Bucha Rouye
Bale
766752
547476
80
48
7.87
0.025
6
4
8
0.97
0
0
2.98
7
SSP-19
Ashelecho
Bale
770110
524796
513
334
8.08
0.04
25
6.5
70.4
8.76
0
0
14.8
5
HD-6
Serofita tulu
Bale
757706
504520
321
210
7.46
0.025
10
8.1
33.6
3.89
0
0
33.8
35.25
67.1
HD-7
Kokosa town
Bale
744850
476749
97
66
6.18
0.025
13
8.6
4.8
1.95
0.02
0
9.9
3.25
42.94
HD-8
Keta
Bale
771643
537038
837
450
7.88
0.13
45
5.4
110.4
11.67
0.03
0.02
47.7
22.5
383.6
5.01
HD-9
Haro Hunte
Bale
779156
539464
472
290
7.89
0
40
10.9
52.8
8.76
0.02
0
7.94
1.25
281.82
0.26
0.04
0.5
168
HD-10
Halola Hunte
Bale
782535
543104
490
283
8.17
0.025
20
10.3
80
2.43
0.02
0
13.9
7
237.4
0.53
0.015
0.96
210
Annex 4.4:- Laboratory analysis results of collected samples from different source
106
1.84
90
0.15
62
36
0.02
0.011
5
0
120
0.26
0.02
0.5
128
2.9
0.05
1.24
184
327.4
1.53
0.05
0.45
304
34.9
0.53
0.02
0
24
0.05
0.2
0.79
0.055
0.15
0.53
0.01
0
20
0.055
1.1
234
258
0.53
100
Annex 4.5:- SAR values of Water samples
S. No
Locality
Zone
E
N
Source
SAR
(meq/l)
1
Dabara
Arsi
251528
804068
spring
0.58
2
Ejakelo
Bale
519622
766515
spring
1.09
3
Hinja
Arsi
538009
791636
spring
1.38
4
Assasa
Arsi
521822
785702
spring
0.55
5
Kokosa Town
Bale
477417
745050
spring
1.79
6
Wesha Tulu
Bale
552593
777265
spring
0.58
7
Kinsho
Bale
477417
745050
spring
0.97
8
Ejersa Mudemtu
Bale
552593
777265
spring
0.63
9
Bucha Rouye
Bale
547476
766752
spring
0.53
10
Achelecho
Bale
552593
770110
spring
0.75
11
Balo
Bale
557806
778498
spring
0.49
12
Boro
Bale
530098
772773
spring
1.08
13
Sirko
Arsi
529142
806704
spring
1.58
14
Robe gerjeda
Arsi
547511
793107
spring
0.97
15
Boricho
Arsi
481290
788661
spring
0.64
16
Soboro
Bale
550088
776575
spring
0.53
17
Semira
Arsi
551221
800796
spring
0.93
18
Kaka
Arsi
514135
807345
spring
0.37
19
Gebecho
Arsi
534474
808353
spring
0.44
20
Tedacha Bulura
Arsi
525559
792266
spring
4.09
21
Debara
Arsi
523396
790405
BH
1.04
22
Ardu
Arsi
521909
786568
BH
1.00
23
Serafta state farm
Bale
501973
770950
BH
0.94
24
Dodola Town
Bale
519834
772034
BH
1.87
25
Herero St. Farm
Bale
530306
775215
BH
0.83
26
Herero Town
Bale
536057
773363
BH
0.90
27
Huruba walkite
Arsi
515612
778588
BH
1.29
28
Huruba Hunta
Arsi
520370
780283
BH
1.12
29
Berisa
Bale
525855
772443
BH
1.95
30
Wabe Birkitu
Bale
528375
780070
BH
0.76
31
Hunte state Farm
Bale
547851
784680
BH
1.06
32
Melka wakena
Arsi
549000
792450
BH
2.01
33
Hasano
Arsi
500024
775636
BH
1.63
34
Gedisa
Bale
522748
774450
HDW
2.60
35
Sheneka
Arsi
477711
784665
HDW
0.65
36
Halola Hunte
Bale
543104
782535
HDW
0.60
37
Serofita tulu
Bale
504520
757706
HDW
0.44
38
Kokosa town
Bale
476749
744850
HDW
1.27
39
Keta
Bale
537038
771643
HDW
1.09
40
Haro Hunte
Bale
539464
779156
HDW
1.35
107