LOW FLOW CALCULATION TO MAINTAIN
ECOLOGICAL BALANCE IN STREAMS
A.Deniz ÖZDEMİR
Meteorology Eng.-Env.Science (MSc) - Hydrology Section, [email protected],
Dr. Ömer KARACA
Meteorology Eng.-Climatologist - Hydrology Section, [email protected],
M. Kemal ERKUŞ
Meteorology Eng.-Msc - Hydrology Section, [email protected],
General Directorate of State Hydraulic Works, Planning and Investigation Departmant,
Yücetepe/ANKARA
ABSTRACT
Water infrastructures in river bed led to some impact on water quantity, water
quality and also water ecosystem. So, the determination of necessary low flow becomes an important problem in order to protect the water ecosystem in downstream.
In hygrology low flow calculations are made for determination of water amount
which is released from hydropower plant to downstream, domestic use, irrigation,
water pollution studies and determination of the required amount of water for sustainability of aquatic life.
In this study we aimed to determine the minimum discharges which is released
from a Hydroelectric Power Plant (HPP) for sustaining the ecological balance in a
stream. For this purpose, various methods in the literature were examined and some
methods using discharge data were selected. After the selection of methods, minimum discharge values from selected gauging stations were calculated for each
method and the results are compared to each other.
Key Words: River basin management, ecosystem, low flow.
INTRODUCTION
Environmental topics are getting more and more popular all over the world. In
this context, not only the place and the quantity of the water but also the quality and
protection of environment has been considered. Nowadays, a new concept that says
use of water resources but save them is being accepted in management of river cathcment. This concept may be named “wise use”.
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BASIN WATER MANAGEMENT
To make use of water resources, some kind of water infrastructure are conctructed on streams. These constructions have some negative effects on stream ecosystem. These structures and water usage change flow regime in the stream. That is why
in hydrological studies of water related projects, it is important to calculate the
minimum water amount (environmental flow) to be related to downstream.
WHAT IS ENVIRONMENTAL FLOW?
Environmental flow is defined as the flow that is necessary to ensure the existence of habitats in a river. Environmental flows may comprise elements from the full
range of flow conditions which describe long term average flows, variability of flows
including low flows and irregular flooding events [EFG, 1999].
The weirs and hydroelectric power plants which will be constructed on the
streams can be shown schematically as follows (Figure 1). Water at the weir is carried
to a certain level and then energy is produced by dropping that water to HPP by
means of penstock in the projects. In that case, the environmental flow amount which
is essential for sustainability of aquatic life in the dry section of the river becomes
important. On the other hand, this water decreases the project rentability because it
cannot be used in energy production.
Figure 1. Hydroelectric power plant shown as schematically.
Now there are not enough regulation/law which depend on the biological requirements of the streams. There are a lot of methods to calculate the minimum (environmental) flow that requires for water habitats. Each method takes different properties of water habitat into account. That is why these methods depend on different
assumption. These methods is described briefly and given below.
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INTERNATIONAL CONGRESS ON RIVER BASIN MANAGEMENT
CALCULATION METHODS OF ENVIRONMENTAL FLOW
As mentioned above, there are a lot of methods. Different methods are used in
different countries. There are fair main group [Pyrce, 2004]: There are
a)
b)
c)
d)
Hydrological methods
Hydroulic rate methods
Habitat simulation methods
Holistic methods
Hydrological methods use daily or monthly flow data to give information about
environmental flow. In addition, this method can be used to calculate a minimum
flow in the streams with/without gauging station. Hydrological method can also be
applied easyly in the planning stage of the water resources development project.
Hydroulic, habitat and holistic methods give more detailed knowledge. But they
also need more time, data, effort and detailed information about catchment.
A) HYDROLOGICAL METHODS
♦ Environmental flow is calculated by using daily and monthly measurement
values.
♦ The results may not be very precise but they can be obtained in short time.
♦ This method is accepted to be convenient for planning stage of a water related projects.
♦ The most widdy used method is the Tennant (or modified Tennant) method.
♦ The second most widely used method include various flow duration exceedance percentiles (e.g. Q95, Q75) or single low flow indices (e.g. 7Q10,
7Q2).
B) HYDRAULIC RATING METHOD
♦ This method uses changes in hydraulic variables (such as river stage or wetted perimeter) to assess the habitat factors known or assumed to be limiting
to target biota, thus a treshold value of the selected hydraulic parameter will
sustain biota/ecosystem integrity.
C) HABITAT RATING METHOD
♦ These methods attemp to assess environmental flow requirements on the basis of detailed analyses of the suitability of instream physical habitat under
different flow discharges using integrated hydrological, hydraulic and biological response data.
♦ Flow is typically modelled using data on flow depth, channel slope, crosssection shape, etc. collected at multiple cross-section within a study reach.
♦ The results usually take form of habitat-discharge curves to predict optimum flows as environmental flow requirements.
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BASIN WATER MANAGEMENT
D) HOLISTIC METHOD
♦ The requirements of the complete ecosystem are integrated and considered
(including the river channel, source areas, riparian zone, floodplain, etc.) .
♦ The naturel regime of the river is the fundamental guide and must be incorporated into the modified flow regimes.
♦ Critical flow criteria are identified for some or all major components of the
riverine ecosystem.
♦ The basis for most approaches is a systematic construction of a modified
flow regime on a month by month and element by element basis which defines features of the flow regime to achieve particular ecological, geomorphological, water quality, social or other objectives of the modified system.
♦ Advanced holistic methods rountinely utilize several of the tools found in
hydrologic, hydraulic and habitat rating methods.
HYDROGICALLY BASED INSTREAM FLOW METHODS
Instream flows have been defined as the minimum flows required to protect and
maintain aquatic resources in streams and rivers (Tennant,1976; Reiser at al., 1989).
As instream flows are related to minimum river flows, low flow indices are often
used as instream methods.
The increasing demand for river water conflicts with the environmental needs
for sustaining flows during drought and low flow periods, leads to competition between water taking and instream flow needs (Caisse and El-jabi, 2003).There are a lot
of methods to balance between the environmental needs and water usage. Some of
these methods are given below [Pyrce, 2004].
1) TENNANT (MONTANA) METHOD
Tennant (or Montana) method (1976) is the most common method applied
worlwide and has been used by at least 25 countries (Tharme, 2003). It’s appeal is in
its simplicity ease of use. Tennant method uses a percentage of the mean annual flow
(MAF) for two different six month periods to define conditions of flow related to
fishery, wildlife, recreation and environmental resources (Table 1). Tennant (1976)
used original headings of “recomended base flow regimens Oct-Mar. and Apr-Sept”.
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INTERNATIONAL CONGRESS ON RIVER BASIN MANAGEMENT
Narrative description of general condition of flow
Flushing or maximum
Optimum range
Outstanding
Excellent
Good
Fair or degrading
Poor or minimum
Severe degradation
Recommended flow
regimens
(% of MAF)
October to March
200%
60-100%
40%
30%
20%
10%
10%
<10%
Recommended flow
regimens
(% of MAF)
April to September
200%
60-100%
60%
50%
40%
30%
10%
<10%
Table 1. Tennant (Montana) method (1976).
The percentage of mean annual flow is assumed to roughly describe aquatic
habitat conditions. For example, 10 % of the mean annual flow offer “poor” habitat
conditions, 30 % is “fair” and 40 % or more is “good” [Pyrce, 2004].
2) 7Q10 METHOD
This method is the second most widely used hydrogical environmental flow
method (Tharme, 2003). This method can be interpereted as the 7-day low flow with
a 10-year return using daily discharge data. This method has been used for various
purposes in various countries. Tese can be classified rughly as follows.
♦ Protection or regulation of water quality from wastewater discharges or
waste load allocations.
♦ Habitat protection during drought conditions.
♦ Criteria for aquatic life.
The original use of the 7Q10 flow is related to stream water quality standarts to
regulate pollution, however the uses have expanded to include and serve many other
interests. The U.S. Fish and Wildlife Service (1981) argued that the 7Q10 flow had
been misused in the past as a minimum flow for protection of the aquatic community. Caisse and Al-jabi (1995) warned that the use of the 7Q10 flow could significantly underestimate instream flow. The State of Massachussets (2004) stated that the
7Q10 flow statistic is sometimes claimed to represent an adequate streamflow for
maintaining a healty ecosystem, when in fact, much higher streamflow levels are
required [Pyrce, 2004].
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BASIN WATER MANAGEMENT
3) OTHER 7Q LOW FLOWS
Along with the 7Q10 flow there are a variety of other 7Q flows that have been
used or are currently in use. These are 7Q1, 7Q2, 7Q5, 7Q20 and 7Q25. Ontario Ministry of Natural Resources (MNR) and Ontario Ministry of Environment (MOE) make
use of 7Q20, 7Q2 flows. The MNR (1994, 2002) use the 7Q20 and 7Q2 flows as a
measure of habitat or ecosystem maintenance. The MOE (2000) use this indices as a
design or limiting condition for wastewater discharges and water taking. For continious and non-continious point source discharges the 7Q20 used as a basic design
flow (Stainton, 2004). The 7Q5 and 7Q25 flows are used South Dacota Department of
Environment and Natural resources (1998) with regard to critical low flows for low
and high quality fishery waters, respectively. The annual 7-day low flow (7Q1; or
MAM7, the mean annual 7-day average minimum flow) is used as an alternative
index in the United Kingdom for water abstraction licensing (Smakhtin and Touluse,
1998), [Pyrce, 2004].
4) FLOW DURATION INDICES
A floe duration curve is one of the most informative means of displaying the
complete range of river discharges, from low flows to flood events (Smakhtin, 2001).
Using average daily discharges data, flow duration curves are cumulative frequency
disrtibutions that show the percent of time that a specified discharge is equaled or
exceeded during a period of interest. Smakhtin (2001) indicated that the “design” low
flow range of a flow duration curve is the 70 % - 99 % range or the Q70, to Q99 range.
The Q95 and Q90 flows are most often used as low flow indices in the government
literature and academic sources . Q75, Q84, Q96, Q97, Q98 and Q99 flows are occasionally noticed in the literature as well. Monthly median flows during summer
month is another common flow duration index [Pyrce, 2004].
5) WETTED PERIMETER
This method assumes that, there is a direct relation between the wetted perimeter in a riffle and fish habitat in streams (Parker and Armstrong, 2001). An index of
fish food availability is used in this method. It is assumed that by maximzing the
wetted perimeter of riffles, enough fish food and habitat will be available to support
a healty aquatic communitiy in the river [LFI, 2002].
6) AQUATIC BASE FLOW METHOD
This method was devloped as an interim policy for minimum streamflows in
New England by the US Fish and Wildlife Service. The August median flow is suggested as a minimum summer flow value because it represents the most severe natu-
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INTERNATIONAL CONGRESS ON RIVER BASIN MANAGEMENT
rally occouring condition that a stream community would experience. For ungauged
streams, where the August median flow is unknown, a summer default value of 0,5
cubic feet per second per square mile of dreinage area (cfsm) is suggested based on a
survey of minimally impacted reference streams and rivers in New England. Because
higher flows are needed at other times of the year for spawning, migration and other
biological needs, USFWS’s minimum flow recommendation for fall is 1,0 cfsm and
for winter and spring is 4,0 cfsm. Where a minimum of 25 years of data are available,
monthly median flows are suggested [LFI, 2002].
7) INSTREAM FLOW INCREMENTAL METHODOLOGY (IFIM)
IFIM was developed by the USGS as a way to quantify and manage the effects of
streamflow on fish. First, atarget fish species or several key species are indentified.
Next, a study of the types of habitat occupied by each life stage of each species is
conducted in the river. Then, a relationship between different streamflows and the
availability of each type of habitat is determined. Minimum flow recommendations
are developed by weighing the amount of habitat available for each species and life
stage at low flows. Some indices used for various purposes are given below (Table 2),
[Pyrce, 2004].
Table 2. Hydrological based instream flow methods.
Study
Reiser et al. (1989)
Purpose
Instream flow methods most
often used in North America
Indices Used
1. IFIM
2. Tennant method (1976)
3. Wetted Perimeter
4. Aquatic Base Flow
Method
5. 7Q10 flow
Karim et al. (1995)
Instream flow methods used in
1. Tennant method
Australia
2. Flow duration (Q95, Q90)
3. Constant yield
Caissie and El-Jabi
To compare hydrologically based 1. Tennant method
(1995)
instream flow methods in Atlantic 2. 25% of the MAF
Canada
3. Monthly Q50
4. Aquatic Base Flow
method
5. Q90 flow
6. 7Q10 flow
Yulianti and Burn (1998) To examine links between cli1. Seasonal 7-day low flow
mate warming and low stream2. Seasonal 25% of mean
flow in the Canadian Prairies
flow
3. Seasonal Q80
4. Monthly Q50
5. Monthly Q90
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BASIN WATER MANAGEMENT
COMPARISON OF VARIOUS INSTREAM FLOW METHODS
There a lot of HPP projects in various basins of Turkey. In these projects, it is
important to know what amount water will come to dam or weir. Therefore, water
supply calculations which are made for this purpose are vital part of the HPP projects. At this point, we face another problem that is what amount water should be
released from dam or weir to downstream in order to protect and maintain aquatic
life in stream.
This study was carried out to determine the environmental flow amount for
some streams in Turkey. Table 3 illustrates the information for selected gauging stations.
Table 3. Gauging station information.
Station
Period
Owner
Station Name
Catchment
No
Drainage Ar. Elevation
(km2)
(m)
3226,60
467
93,60
815
116
735
1988
-
2000
EİE
Banaz Çayı-Dörtdeğirmenler
Büyük Menderes
8-83
1979
-
2004
DSİ
Dargaz Deresi-Langüme
Batı Akdeniz
902
1985
-
2001
EİE
Köprüçay-Beşkonak
Antalya
1942,40
919
1985
-
2001
EİE
Köprüçay-Bolasan
Antalya
1538,40
435
1233
1963
-
2003
EİE
Aladağ Çayı-Karaköy
Sakarya
1984,80
512
1073,40
286
71,50
634
1237
1961
-
2004
EİE
Mudurnu Çayı-Dokurcan
Sakarya
13-32
1981
-
2004
DSİ
Aksu Deresi-Çiftkese
Batı Karadeniz
1339
1981
-
2004
EİE
Aksu Deresi-Dereevi
Batı Karadeniz
105,20
837
14-15
1963
-
2001
EİE
Çobanlı deresi roski
Yeşilırmak
725,50
1307
1418
1964
-
2004
EİE
Yeşilırmak Nehri-Gömeleönü
Yeşilırmak
1608,00
820
1422
1969
-
2001
EİE
Kelkit Çayı-Çiçekbükü
Yeşilırmak
1714,00
1350
14-126
1969
-
2001
DSİ
Cevizlik Deresi-erenkaya
Yeşilırmak
21,96
1500
18-12
1971
-
2004
DSİ
Körkün Çayı-Kamışlı
Seyhan
1065,00
1109
18-27
1971
-
2001
DSİ
Ecemiş Çayı-Elekgülü Köprüsü
Seyhan
1833,00
1550
20-58
1985
-
2003
DSİ
Tahta Suyu-Cambazköy
Ceyhan
24,38
1000
21-1
1975
-
2004
DSİ
Karasu-Kırkgöze
Fırat
233,20
1830
2202
1967
-
2003
EİE
Kara Dere-Değirmencik Köyü
Doğu Karadeniz
637,50
78
2218
1961
-
2004
EİE
İyidere-Şimdirli
Doğu Karadeniz
834,90
307
22-57
1979
-
2004
DSİ
Ögene Deresi-Alçakköprü
Doğu Karadeniz
242,64
650
22-61
1980
-
2003
DSİ
Altın Dere-Ortaköy
Doğu Karadeniz
260,97
450
2325
1974
-
2004
EİE
Oltu Suyu-Aşağıkumlu
Çoruh
1762,00
1129
2329
1982
-
2004
EİE
Oltu Suyu-Coşkunlar
Çoruh
3538,80
1004
24-18
1969
-
2000
DSİ
Oragaz Çayı-Çayağzı
Aras
502,00
1790
24-80
1969
-
2000
DSİ
Posof Çayı-Posof
Aras
510,10
1350
2505
1972
-
2001
EİE
Bendimahi-Göndürme
Van
1373,40
1915
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INTERNATIONAL CONGRESS ON RIVER BASIN MANAGEMENT
Table 4. Result of the calculation according to Tennat, Flow Duration and 7Q methods.
Station No
Mean An.
Flow
(m3/s)
10%
735
Ağu.83
902
919
1233
1237
13-32
1339
14-15
1418
1422
14-126
18.Ara
18-27
20-58
21.Oca
2202
2218
22-57
22-61
2325
2329
24-18
24-80
2505
5,36
1
75,91
22,44
13,52
8,02
2,13
3,96
6,35
18,16
7,99
0,35
7,86
3,59
1,17
2,63
10,96
28,07
5,24
4,46
7,01
16,39
5,52
5,61
9,38
0,425
0,132
7,112
2,304
1,365
0,885
0,156
0,305
0,354
1,242
0,372
0,021
0,426
0,198
0,082
0,11
0,615
1,593
0,216
0,177
0,322
0,625
0,166
0,297
0,55
Station No
Mean An.
Flow
(m3/s)
735
Ağu.83
902
919
1233
1237
13-32
1339
14-15
1418
1422
14-126
18.Ara
18-27
20-58
21.Oca
2202
2218
22-57
22-61
2325
2329
24-18
24-80
2505
5,36
1
75,91
22,44
13,52
8,02
2,13
3,96
6,35
18,16
7,99
0,35
7,86
3,59
1,17
2,63
10,96
28,07
5,24
4,46
7,01
16,39
5,52
5,61
9,38
Tennant Method (m3/s)
October-March
April-September
20%
30%
10%
30%
0,85
0,264
14,224
4,608
2,73
1,769
0,312
0,611
0,707
2,484
0,745
0,041
0,852
0,396
0,164
0,221
1,23
3,187
0,432
0,355
0,644
1,25
0,333
0,594
1,101
1,275
0,395
21,336
6,912
4,094
2,654
0,468
0,916
1,061
3,725
1,117
0,062
1,278
0,594
0,246
0,331
1,844
4,78
0,648
0,532
0,966
1,875
0,499
0,891
1,651
0,508
0,033
6,348
1,552
1,287
0,801
0,215
0,385
0,891
2,385
1,189
0,047
0,875
0,407
0,117
0,383
1,555
4,063
0,769
0,659
0,987
2,461
0,89
0,758
1,194
Flow Duration Metodhods (m3/s)
1,525
0,1
19,045
4,656
3,862
2,404
0,645
1,154
2,673
7,155
3,567
0,141
2,624
1,22
0,35
1,149
4,666
12,189
2,308
1,978
2,962
7,382
2,67
2,274
3,583
40%
2,033
0,133
25,394
6,208
5,149
3,205
0,86
1,539
3,563
9,54
4,756
0,189
3,499
1,626
0,467
1,532
6,221
16,253
3,078
2,638
3,949
9,843
3,56
3,032
4,777
7Q Methods (m3/s)
Q75
Q90
Q95
7Q2
7Q10
7Q20
2,65
0,018
38,6
1,77
1,41
3
0,62
1,333
2
4,22
1,57
0,11
2,7
1,69
0,393
0,98
3,74
12,5
1,74
1,515
2,77
5,8
1,3
2,944
5,2
2,18
0,004
33,9
0,4
0,36
2,15
0,38
0,92
1,4
3,27
0,71
0,06
1,8
1,18
0,26
0,64
2,54
10
1,201
1,09
1,25
3,74
1,05
2,49
4,32
1,97
0,001
32
0,144
0,104
1,64
0,32
0,78
1,1
2,86
0,41
0,034
1
0,8
0,255
0,57
2,04
8,88
0,9
0,868
0,57
2,47
0,8
2,23
3,6
2,1
0,001
32,886
0,082
0,213
2,076
0,319
0,804
1,17
2,824
0,474
0,063
1,511
1,058
0,249
0,628
1,973
9,012
0,878
0,804
0,768
2,7
0,94
2,774
4,296
1,6
0
27,551
0,001
0,01
1,001
0,139
0,312
0,49
1,996
0,134
0,008
0,419
0,338
0,142
0,267
1,237
6,402
0,308
0,468
0,089
0,998
0,415
1,809
2,748
1,4
0
25,528
0
0,003
0,731
0,098
0,208
0,337
1,718
0,078
0,004
0,241
0,207
0,112
0,185
1,012
5,525
0,197
0,371
0,035
0,65
0,292
1,505
2,266
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BASIN WATER MANAGEMENT
Firstly, a number of station were selected and then missing data were obtained
by regression analysis. After that, the Tennat, Q75, Q90, Q95 and 7Q2, 7Q10, 7Q20
methods were used for calculating low flow of these stations (Table 4).
According to the Tennant method, 10 %, 20 %, and 30 % of the flows in OctoberMarch period are lower than the flows calculated by using flow duration method
while 10 %, 30 % and 40 % of the flows in April-September period are higher than the
flows calculated by using the flow duration method.
In summer time some streams and rivers have no or very little water in their
bed. In this situation according to the Tennat method, 10 %, 20 % and 30 % of the
flows in October-March period are higher than the flows calculated by using 7Q
methods. For the other kind of stream and river, calculated flows are lover generally
while 10 %, 30 % and 40 % of the flows in April-September period are higher the
flows calculated by using 7Q methods.
In general, the results of Q75 and Q90 are higher than the result of Q7 methods.
Secondly the results of Q95 are higher than these of 7Q10 and 7Q20.
The results are high according to flow duration method and 7Q methods due to
having high average and low variable coefficient of the gauging station numbered
902 existent in a karstik region. During the observation period, 7Q methods gives
very low results at the drying gauging stations number 8-83, 919, 1233 and at the
gauging station number 14-126 where the momentary flow decreases to 1 lt.
CONCLUSION
As a result, there are different methods for the environmental flow calculation in
different countries. Application of some methods are easy but some of them are very
difficult due to providing specific inputs.
Regime of a river, include some parameters such as seasonal feeding of the
stream, base flow, drying of the stream bed, geological properties of the catchment
etc. is important, because of using of the hydrological flow methods which are calculated based on the flow. These properties must be taken into cansideration for the
determination of the methods to be applied.
As a result of these studies, we assume that Tennat method can be used for the
karstik region and drying or low flow stream and 7Q10 method can be used for the
other streams. However, these methods must be determined by using more data
providing from the different gauging stations.
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INTERNATIONAL CONGRESS ON RIVER BASIN MANAGEMENT
Moreover, in order to determine the environmental flow by taking into consideration the stream ecosystem together with projects rentability, the specialist should
create a group work facility for which method will be used at which conditions. In
our developing country “wise use” principle must be based on the relationship between economy and ecology during the planning of the projects.
REFERENCES
1. DSI Flow Yearbooks, Ankara, Turkey. (in Turkish)
2. EIE Flow Yearbooks, Ankara, Turkey. (in Turkish)
3. Environmantal Flow Guidelines, Environment Act, Australia, 1999.
4. Low Flow Inventory, www.state.ma.us.
5. Pyrce, R., Hydrological Low Flow Indices and Their Uses, Watershed Science Centre,
Trent
University, Canada, 2004.