Processes of salinization and strategies
to cope with this in irrigation in
Santiago del Estero
MSc thesis TUDelft by
Maria Alcaraz Boscà
Supervisors:
Dr.ir. Maurits Ertsen
Prof.dr.ir. Nick van der Giesen
PROCESSES OF SALINIZATION AND STRATEGIES TO COPE WITH THIS IN IRRIGATION IN
SANTIAGO DEL ESTERO
Abstract
In arid and semi-arid regions, salinity is one of the most important
problems that affect the irrigation systems. This is the case of Rio Dulce irrigation
system. The Proyecto Rio Dulce is an irrigation system located in the province of
Santiago del Estero in Argentina. This system is characterized by having irrigated
and non-irrigated fields where salinization occurs due to capillary rise.
In this thesis, the evolution of salinity on the fields has been studied at
small and large scale with different models: the WASIM model (small scale) and
the SALTMOD model (large scale). Also, several possibilities of soil reclamation
in the area have been considered in order to know how the system behaves
given different options of drainage and irrigation.
Finally, it can be concluded that reclamation of the soil is feasible with the
installation of a drainage system and the application of irrigation in the (until now)
non-irrigated fields.
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Table of contents
ABSTRACT ...............................................................................................1
TABLE OF CONTENTS.............................................................................3
1
INTRODUCTION.................................................................................5
1.1
Irrigation systems & salinity problems ............................................................. 5
1.2
Rio Dulce Irrigation Project ................................................................................ 6
1.2.1
Brief description of the irrigation system ........................................................... 6
1.2.2
Data related to salinity in the current PRD........................................................ 7
1.3
2
Structure of this thesis ....................................................................................... 9
THE MODELS ..................................................................................10
2.1
The WASIM model ............................................................................................. 10
2.2
The SALTMOD model........................................................................................ 11
3
STUDY OF SALINITY WITH WASIM ...............................................14
3.1
Evolution of salinity at small scale .................................................................. 14
3.1.1 Reclamation experiment executed in the Pilot Area of Drainage and Soil
Reclamation at INTA-EEASE .............................................................................. 14
3.1.2 Using the WASIM model to study and model reclamation experiment
at Lote 6............................................................................................................... 16
3.1.2.1 Study of required inputs............................................................................ 16
3.1.2.2 Calibration ................................................................................................ 19
3.1.2.3 Salinity ...................................................................................................... 23
3.1.3
3.2
Possibilities of soil reclamation in PRD.......................................................... 28
3.2.1
4
Conclusions..................................................................................................... 27
Conclusions..................................................................................................... 32
STUDY OF SALINITY WITH SALTMOD..........................................33
4.1
Study of salinity in the PRD at larger scale .................................................... 33
4.1.1
Calibration ....................................................................................................... 33
4.1.2
Different scenarios .......................................................................................... 37
4.1.3
Inputs variables ............................................................................................... 39
4.1.4
Conclusions..................................................................................................... 39
4.2
Possibilities of soil reclamation in PRD.......................................................... 40
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4.2.1
Drainage.......................................................................................................... 40
4.2.2
Drainage & Irrigation ....................................................................................... 42
4.2.3
Conclusions..................................................................................................... 43
5
CONCLUSIONS................................................................................44
6
BIBLIOGRAPHY...............................................................................48
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1 Introduction
1.1 Irrigation systems & salinity problems
In those irrigation systems that are located in arid or semi-arid areas, the
salinization of the soil is a common problem. When the soils of these regions are
irrigated, the natural conditions of the area change completely and percolation
increases considerably. Generally, this amount of water percolating in the soil is
higher than the drainage capacity of the soil and the groundwater table level
becomes shallow. If this water is used by the plants to transpire by means of
capillary rise, then the salinity of the soil will increase since the water that is
transpirated leaves behind the salts it contained.
If irrigated and non-irrigated fields can be distinguished in the irrigation
scheme, the salinization will take place in the non-irrigated fields. This is because
in these fields there is not percolation from the surface that counteracts the effect
of capillary rise. This is the case of the irrigation system that will be studied in this
work, The Proyecto Dulce Irrigation System where those fields that are nonirrigated have serious problems with salinity. As an example, in the graphs shown
below it can be seen the salinity levels in different fields: two irrigated and two
non-irrigated. In the non-irrigated fields, salinity reaches very high levels whereas
in the irrigated fields the salinity values are suitable for agricultural use.
In the irrigated field the salinity values increase during the dry season
(May-October) because of capillary rise. Later, when the growing season starts
salinity levels decrease thanks to rainfall and irrigation that remove salts from the
topsoil. On the contrary, in the non-irrigated fields the salinity levels increase in
the wet season (November-April) because of the growing of natural vegetation
like plants.
Graph 1 Salinity levels in irrigated and non-irrigated fields in PRD
Prieto D. & Angueira C. 1994. La dinámica de la salinidad en los suelos bajo riego y su
relación con el manejo del agua.
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1.2 Rio Dulce Irrigation Project
1.2.1 Brief description of the irrigation system
The Rio Dulce Irrigation Project (PRD) is one of the most important
irrigation systems in Argentina. It is located in the province of Santiago del Estero
which is one of the poorest of the country.
Santiago del Estero is placed in the north of Argentina between the south
latitudes 30º29’ and 25º38’ and west longitudes 61º40’and 65º11’. This is a
semiarid area with a mean annual temperature of 21.5 ºC. Winters are dry and
summers are wet and hot. The annual precipitation ranges from 500 to 800
mm/year while the potential evapotranspiration ranges from 1300 to 1600
mm/year, so the annual water balance is negative in all the areas.
The Rio Dulce irrigation
project is located in the middle part
of the Rio Dulce basin which covers
the provinces of Tucumán, Santiago
del Estero and Córdoba. It is divided
in five different zones according to
historical reasons and the different
modernization of each one.
Figure 1: Political map of Argentina
The maximum irrigable area of the
PRD is about 120,000 ha but its
gross command area is about
350,000 ha. However, the irrigated
area has never reached 120,000 ha,
so water has never been scarce. In
the system it can be distinguished
irrigated and non-irrigated fields.
Also, the shape of the parcels varies
from a few hectares to more than
100 hectares per field.
In the next table the distribution and size of the parcels is shown:
Size of parcels
0-5
5-10
10-25
25-50
50-100
100-500
500-1000
Total/Mean
HOLDINGS
Number
2648
1741
1621
510
196
92
2
6810
%
39
26
24
7
3
1
0
100
Table 1: Holding size distribution in PRD
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Figure 2 Holding size distribution in PRD
1.2.2 Data related to salinity in the current PRD
In Daniel Prieto’s thesis: ‘Modernization and the evolution of irrigation
practices in the Rio Dulce irrigation project’ there is data about salinity levels,
irrigation and rainfall in some areas (cases) of the irrigation system. In the next
tables, this data is shown.
SALINITY LEVELS (dS/m)
N
IRRIGATED FIELD
NON-IRRIGATED FIELD
CASE 1
8
1.6
7.7
CASE 3
5
1
16.7
CASE 4
8
1
25.9
N= number of sampled cases
Table 2 Average soil salinity of irrigated and not irrigated parcels in three sampled areas.
Salt concentrations of the soil are expressed in ECe, the electric conductivity of an extract
of saturated soil paste.
Also, for Case 1 data of salinity of irrigation water and groundwater are
available. It can be noticed that ground water salinity is 12 times higher than
irrigation water salinity. This fact has favored secondary salinization in the area.
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In this area, the water table remains at around 2 meters depth at present, where
it stabilizes due to evapotranspiration.
IRRIGATION WATER
WATER TABLE
0.8
9.4
EC
(dS/m 25 C)
Table 3 Salinity of irrigation and groundwater in Case 1
January
February
March
April
May
June
July
August
September
October
November
December
Total
irrigation (m)
In the next numbers, data of irrigation and rainfall in several areas of the
irrigation system are shown.
1
0
1
1
1
1
1
1
1
1
1
1
0.9
1
0
1
1
0
0
0
0
0
1
1
1
0
1
0
0
1
0
0
0
0
1
1
1
0
1
0
1
1
1
1
1
1
1
1
0
1
1
1
1
1
1
1
1
0
0
0
1
0.586
0.862
0.908
0.803
Official
Schedule
Case 1
Case 2
Case 3
Case 4
1= irrigation turn; 0= no irrigation turn
Table 4 Official irrigation schedule and real irrigation schedules at different areas of PRD
EFFECTIVE PRECIPITATION
January
February
March
April
May
June
July
August
September
October
November
December
Total
precipitation
99
97
70
11
18
9
6
0
5
50
35
76
20002001
104
155
84
72
5
9
2
0
1
30
79
39
20012002
109
90
115
46
15
0
0
3
63
36
79
100
476
580
656
95-96
96-97
98-99
99-2000
80
93
45
27
77
1
0
0
11
12
35
41
80
91
81
10
15
1
0
1
31
32
46
16
120
54
100
14
3
26
0
1
1
43
29
61
422
404
452
Table 5 Effective precipitation for different growing seasons, mm
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Mean
98.7
96.7
82.5
30
22.2
7.7
1.3
0.8
18.7
33.8
50.5
55.5
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January
February
March
April
May
June
July
August
September
October
November
December
Potential ET
(mm/day)
Finally, the potential evapotranspiration obtained for each month.
6.39
5.38
4.27
3.24
2.19
1.67
2.29
3.4
4.65
5.66
6.15
6.66
Table 6 Potential evapotranspiration
1.3 Structure of this thesis
As it is said before, the Rio Dulce irrigation system has serious problems
with salinity. In order to find a solution to this problem, this thesis has been
focused in these main parts:
Firstly, to study the salinity process in the fields: which is the behavior of
the salts in the fields within the years (small scale) and also, over the years (large
scale).
Lately, consider the different possibilities of soil reclamation in the area.
The objective of this point is not to find the perfect solution to the salinity problem
but to observe how the system behaves in front of different actions.
Therefore, simulation models are helpful to develop and evaluate drainage
and irrigation strategies once calibrated using experimental data. In this case two
different models have been used:
•
The WASIM model used to study salinity at small scale.
•
The SALTMOD model utilized to study salinity at large scale.
Between these models there are several differences as it is explained
later. The most important one is the time-step used to carry out the water and
salt balances: SALTMOD does a seasonal water balance while WASIM has a
daily time-step.
The reasons why two models are used are: from one hand, to compare
both models and check how different the results are and by the other hand, to
evaluate the applicability of these models for the Rio Dulce Irrigation System.
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2 The models
In this section, the two models that have been used in this thesis are
explained with more detail.
2.1 The WASIM model
A general description of the WASIM model as given in Meenakshi
Hirekhan et al. (2006) is:
HR Wallingford and Cranfield University, UK jointly developed the Water
Simulation model, WaSim (Hess et al., 2000).
WaSim is a one-dimensional, daily, soil water balance model that requires
daily reference evapotranspiration and rainfall data. Reference ET is used in
WaSim to estimate the changes in the soil water content taking into account
inputs of rainfall and irrigation including canal seepage wherever relevant. Daily
surface runoff due to rainfall is estimated using US SCS curve number technique
(USDA, 1969).
For the redistribution of soil water, the upper boundary is the soil surface
and the lower boundary is an impermeable layer. With WASIM only a one
layered soil can be simulated. The soil is divided in five different compartments
where water can be stored:
•
Compartment 0: The surface layer (0 - 0.15m)
•
Compartment 1: The active root zone (0.15m - root depth)
•
Compartment 2: The unsaturated compartment below the root-zone
(root depth – water table)
•
Compartment 3: The saturated compartment above drain-depth
(water table – drain depth)
•
Compartment 4: The saturated compartment below drain-depth
(drain depth – impermeable layer)
The boundary between the second and third depth increments changes as
the roots grow. Before plant roots reach 0.15 m, the second depth increment has
zero thickness. Similarly, the boundary between the third and fourth depth
increments fluctuates with the water table depth.
Soil water moves downward from one depth interval to the next only when
the soil water content of the upper depth interval exceeds field capacity. In this
case, the rate of drainage is a function of the amount of excess water. If the
volume water fraction is between the field capacity and saturation then the
drainage released from the compartment is calculated from an equation
proposed by Raes and van Aelst (1985). The deep percolation forms the input
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into a subroutine, which predicts the depth to the water table that includes the
impact of a field drainage system.
Figure 3 Overview of the soil water balance
WaSim utilizes the mass balance of salt in a one dimensional profile with
the same depth intervals (increments) used for the water balance model.
Respective salt concentrations of the various inputs and outputs are multiplied
with water contents to arrive at the salt contents of soil and drainage water.
On the basis of water and salt balances, WaSim predicts surface runoff,
evapotranspiration (modified for the crop cover and soil water status), mid span
water table, drain outflow, soil water content, soil salinity and drainage water
quality.
The WaSim is user friendly with clear instructions on the operation of the
model provided in the manual written by Hess et al. (2000).
It seems that Wasim is a simple tool to evaluate and design drainage
systems. Nevertheless, as this model is a recent incorporation to the literature it
will be appropriated that some evaluations were done in order to validate it.
2.2
The SALTMOD model
SALTMOD is a computer program for the prediction of salinity of soil
moisture, ground water and drainage water developed by R.J. Oosterbaan and
Isabel Pedroso de Lima at ILRI (International Institute for Land Reclamation and
Improvement). With the SALTMOD model it is possible to study the evolution of
salinity, depth of water table and the drain discharge.
The general principles of SALTMOD and the assumptions based on which
the model was developed as given in Oosterbaan (2000) are:
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Seasonal approach
The model is based on seasonal water balances of agricultural lands. Four
seasons in one year can be distinguished. The number of seasons can be
chosen between a minimum of one and a maximum of four. The duration of each
season is given in number of months. Seasonal time step is considered in the
computation method depending upon the specific situation of the study site.
Hydrological data
The model uses seasonal water balance components as input data. These
are related to the surface hydrology (e.g. rainfall, evaporation, irrigation, use of
drain and well water for irrigation, runoff) and the aquifer hydrology (e.g. upward
seepage, natural drainage, groundwater pumping). The other water balance
components (e.g. downward percolation, upward capillary rise, subsurface
drainage) are predicted as output.
Soil strata
SALTMOD accepts four different reservoirs namely (i) surface reservoir
above the soil surface, (ii) upper shallow soil reservoir or root zone, (iii) an
intermediate soil reservoir or transition zone and (iv) deep reservoir or aquifer. If
a horizontal subsurface drainage system is present, this must be placed in the
transition zone, which is then divided into two parts: an upper transition zone
(above drain level) and a lower transition zone (below drain level).
Water balances
The water balances are calculated for each reservoir separately. The
excess water leaving one reservoir is converted into incoming water for the next
reservoir. The three soil reservoirs can be assigned different thicknesses and
storage coefficients, to be given as input data. The depth to water table,
calculated from the water balances, is assumed to be the same for the whole
area.
Salt balances
The salt balances are calculated for each reservoir separately. They are
based on their water balances, using the salt concentrations of the incoming and
outgoing water. The initial salt concentrations of water in the different soil
reservoirs, in the irrigation water and in the incoming groundwater from the deep
aquifer are required as input to the model. Salt concentration of outgoing water
(either from one reservoir into the other or by subsurface drainage) is computed
on the basis of salt balances, using different leaching or salt mixing efficiencies.
Output data
The output of SALTMOD is given for each season of any year during any
number of years, as specified with the input data. The output data comprise
hydrological and salinity aspects. The data are filled in the form of tables that can
be inspected directly or further analysed with spreadsheet programs. The
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program offers the possibility to develop a multitude of relations between varied
input data, resulting outputs and time.
According to Oosterbaan (1998), SALTMOD has the following advantages
in front of daily water balances programs:
•
It is very difficult to collect daily data,
•
The model is designed to make long-term simulations,
•
Because of high variability in daily data, long-term simulations are
more reliable than short-term simulations.
Another advantage of SALTMOD is that the data needed to run the model
is easily available or it can be measured with relative ease.
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3 Study of salinity with WASIM
As it is said before, the aims of this thesis are from one hand, to study the
salinity evolution in the irrigation system and by the other hand, consider different
possibilities of soil reclamation. In this section, these objectives have been
studied with the WASIM model.
First of all, a study of salinity at small scale is done by reproducing a
reclamation experiment carried out in the irrigation system. Here, the calibration
of the model has been done. Later, several possibilities of soil reclamation have
been considered.
3.1 Evolution of salinity at small scale
3.1.1 Reclamation experiment executed in the Pilot Area of
Drainage and Soil Reclamation at INTA-EEASE
In Rio Dulce Irrigation System, a Pilot Area of Drainage and Soil
Reclamation of San Isidro at INTA-EEASE was created with the aim of
recovering its saline soils. In this area, several experiments were carried out in
order to study salinity and its properties.
Picture 1 Main entrance to San Isidro Pilot
Area of drainage
Source: MW Ertsen
Picture 2 San Isidro Pilot Area
Source: MW Ertsen
One of this experiments was executed in field 6 (2.25 ha) of the mentioned
pilot area. The area has artificial drainage by means of two pipe-drains, 150 m
apart at a mean depth of 190 cm. Six irrigations of 150 mm each one were
applied with a mean frequency of 17 days. This experiment was done in winter,
beginning in May of 1991, which is the dry season in the area.
Soils in the area are classified as Ustorthent Tipico, they are formed
mostly from loessical silt with scarce development with their profiles being
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characterized by A-AC-C horizons. In the first 25 cm there is a moderate content
of organic matter, even thought it decreases with depth.
The soils in the area are
saline-sodic. Salinity and also
cationic exchange capacity vary all
over the area. Initial average salinity
of the soil is 15.8 dS/m and the initial
average cationic exchange capacity
is 26.3, with Ca as the most common
cation adsorbed. Vegetation is scant
and it is composed mostly by small
brushes.
Figure 4 Pilot Area of Drainage and Soil
Reclamation at INTA-EEASE
Salinity levels were measured in thirteen plots and at several depths
during the experiment. It was seen that thanks to irrigations, soil reclamation was
effective. Even thought, since the 4th irrigation, resalinization occurred at the
center of the field due to a high water table level. Here, the water table level rose
the first meter below surface because the drainage system was not able to
maintain water table level below 1.4 m depth. The evolution of salinity with the
different irrigations at the center of the field (plot D4) is shown in the following
graph.
Salinity levels in area D4
40
35
30
Initial
1st irrigation
dS/m
25
2nd irrigation
20
3rd irrigation
4th irrigation
15
5th irrigation
6th irrigation
10
5
0
20
40
60
80
100
120
180
depth, cm
Graph 2 Evolution of measured salinity levels at the centre of the field, plot D4
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3.1.2 Using the WASIM model to study and model reclamation
experiment at Lote 6
Firstly all the inputs needed by WASIM have been studied: those that are
known and those that must be calibrated. Later, the calibration of the unknown
parameters has been carried out.
3.1.2.1 Study of required inputs
Soil data
The type of soil must be defined, therefore in WASIM is necessary to
introduce the next values:
•
Percentage water at saturation, θsat
•
Percentage water at field capacity, θFC
•
Percentage water at permanent wilting point, θWP
•
Drainage constant, τ
•
Leaching efficiency
•
Saturated hydraulic conductivity, Ksat
•
Curve Number
In the WASIM database it is possible to select different soils types with
default values that can be changed and saved. The soil in Lote 6 has been
classified as Ustorthent Típico, this soil is closely similar to those defined in the
WASIM database as Silt Loam and Sandy Loam.
Some soil properties of Ustorthent Típico are known from the experiment
called: ‘Determinación de propiedades físicas en el Área Piloto de San Javier,
provincia de Santiago del Estero’, (Physics properties determination in the Pilot
Area of San Javier, in Santiago del Estero). In this experiment, the bulk density,
characteristic hydraulic curve and infiltration rate were determined.
The percentage of water at field capacity and the percentage of water at
permanent wilting point can be estimated from the characteristic hydraulic curve.
According to Smeda & Rycroft (1983) field capacity is reached after a watering of
the soil with a moisture content corresponding to values of pF between 2.2 and
2.5. In the same way, the wilting point occurs when the suction has risen to -15
atm (pF = 4.2).
pF
A
AC
4.2
0.13
0.12
4
0.14
0.13
3.8
0.15
0.14
Characteristic Hydraulic Curve
3.6
3.3
3
2.8
2.5
0.17
0.11
0.22
0.24
0.28
0.16
0.09
0.2
0.23
0.26
2.3
0.31
0.29
2
0.37
0.35
Table 7 Characteristic curve for the soil Ustorthent Tipico
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1.8
0.4
0.4
1.6
0.66
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Also, in the experiment called ‘Ensayo de microlavado de sales’, soil was
taken from a plot in Lote 6 in San Isidro to be analyzed according to its physical
and chemical properties. Saturation percentage of the soil was calculated. The
percentage of water at saturation can be approximated by the porosity of the soil
calculated with the values of saturation percentage and bulk density.
HORIZON
Ustorthent típico
A
AC
C1
C2
C3
Saturation
Percentage
Bulk
Density
Porosity
0.41
0.34
0.33
0.32
0.31
1.33
1.37
1.38
1.33
1.33
0.5453
0.4658
0.4554
0.4256
0.4123
Mean
0.4582
Table 8 Values of the saturation percentage, bulk density (kg/m3) and porosity for
Ustorthent Tipico.
Taking this into account, the adopted soil parameters values are:
•
Percentage water at saturation, θsat = 0.458
•
Percentage water at field capacity, θFC = 0.26
•
Percentage water at permanent wilting point, θWP = 0.12
Other soil parameters need to be determined by running the model. The
default values that WASIM database gives for those soils that are the closest to
soil in Lote 6 are shown in table 9. These default data were used as starting point
for modeling the experiment. The saturated hydraulic conductivity, leaching and
drainage constant have to be calibrated. The curve number is not important in
this case, because it is related with surface runoff that doesn’t take place in Lote
6 experiment.
Sandy loam
Silt loam
Ksat
(m/day)
0.624
0.163
τ
0.37
0.17
Curve
Number
67
81
Leaching
90%
90%
Table 9 Default data for the WASIM model
Crops
The next step in WASIM is to introduce the surface conditions (type of
crops). With WASIM is possible to distinguish between crop cover, mulch and
bare soil. Some information is required about the type of crop: cover, root depth,
ponding… and also, some transpiration factors like the fraction of total available
water (θFC - θWP) that is easily available, expressed as p.
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Figure 5 Crop data entry form (Wasim)
In Lote 6, vegetation is scarce and it is formed by small plants and
brushes, there is not a specific crop. To simulate this situation in WASIM a
hypothetical crop has been created with a constant depth of 25 cm which is the
depth where the amount of organic matter is important. This theoretical crop is
covering 100 % of the area. The P factor is unknown and it must be calibrated.
Drainage
In WASIM, drainage from compartment to compartment is calculated from:
q = τ ·(θ − θ FC )·(e (θ −θ FC ) − 1) /(e (θ SAT −θ FC ) − 1) * 1000mm / m
In which
•
q
drainage from compartment to compartment,
•
τ
drainage constant,
•
θ
volume water fraction,
•
θFC
volume water fraction at field capacity
•
θPWP volume water fraction at permanent wilting point
Volume water fractions at field capacity and at permanent wilting point are
already known. The drainage constant is the only parameter to calibrate.
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SANTIAGO DEL ESTERO
Climate data & irrigation
Also, to run the model, data about potential evapotranspiration, gross
rainfall and irrigation are needed. Potential evapotranspiration is obtained from
Lieveld (2005), and irrigation data is known from the experiment. Rainfall is not
considered during all the simulation since the experiment was done in winter, the
dry season.
Initial salinity values and water content
Finally, is necessary to introduce some data about the initial water
content, initial water table depth below surface and initial salinity in each of the
compartments. The initial water content and the water table depth below surface
have to be calibrated. The initial salinity is known.
3.1.2.2 Calibration
As it has said before, some parameters need to be calibrated to reproduce
the experiment as it was done.
•
Drainage constant, t
•
Initial water content & depth
•
P, fraction of easily available water
•
Leaching efficiency
•
Saturated hydraulic conductivity, Ksat
At the same time, these parameters are influencing diverse processes. In
this section, all these processes are studied in order to calibrate all the
parameters related with them.
•
Drainage
•
Water table depth
•
Capillary rise
•
Actual ET
•
Salinity
Drainage
Drainage constant ‘τ’ is the only parameter related with drainage that
remains unknown. To study its influence on the drainage, several trials have
been done with different values of this parameter: τ = 0.55, τ = 0.37, τ = 0.17.
Default values are taken for the other parameters. In the same way, the
initial water table level is taken at 3 meters depth. Initial water content is studied
at field capacity and at permanent wilting point for all the compartments.
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SANTIAGO DEL ESTERO
In the next graphs it can be seen that in the evolution of the water table
depth, the influence of the drainage constant is insignificant compared to the
others parameters like the initial water content. Therefore, to simulate Lote 6
experiment, the default database WASIM value was used as drainage constant.
Water table level
Initial conditions at PWP
0
01
/0
5/
19
08
91
/0
5/
19
15
91
/0
5/
19
22
91
/0
5/
19
29
91
/0
5/
19
05
91
/0
6/
19
12
91
/0
6/
19
19
91
/0
6/
19
26
91
/0
6/
19
03
91
/0
7/
19
10
91
/0
7/
19
17
91
/0
7/
19
24
91
/0
7/
19
31
91
/0
7/
19
91
01
/0
5/
19
08
91
/0
5/
19
15
91
/0
5/
19
22
91
/0
5/
19
29
91
/0
5/
19
05
91
/0
6/
19
12
91
/0
6/
19
19
91
/0
6/
19
26
91
/0
6/
19
03
91
/0
7/
19
10
91
/0
7/
19
17
91
/0
7/
19
24
91
/0
7/
19
31
91
/0
7/
19
91
Water table level
Initial conditions at FC
0
0.5
0.5
1
1
1.5
1.5
2
2
2.5
2.5
3
3
3.5
3.5
t = 0.55
t = 0.37
t=0.17
Graph 3 Evolution of water table level with
initial conditions at FC and different values
of τ
t=0.55
t=0.37
t=0.17
Graph 4 Evolution of water table level with
initial conditions at PWP and different values
of τ
Water table depth
The parameters related with the water table depth are the initial water
content of the different compartments of the soil and the initial water table depth.
As the experiment was carried out in the dry season, it can be assumed
that the water content in the top-soil compartment was at permanent wilting point.
Similarly, in the unsaturated zone the water content is supposed to be at field
capacity because in this compartment water is not extracted due to transpiration
of plants and this is the point where no drainage takes place. Taking these
arguments into account, initial water content in the root zone is presumed to be
between field capacity and permanent wilting point. An average value between
permanent wilting point and field capacity is chosen.
The next step is to select the initial water table depth. Several tries have
been done with different initial water table depths. To select the correct one, it
must be taken into account that in the experiment resalinization took place after
the 4th irrigation because the water table raised to 1.4 m depth. Moreover, in the
5th and 6th irrigations the water table depth reached the first meter.
The results from graph 5 show that the initial water table depth should
remain between 3.5 and 4 meters.
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Water table depth
0
0.5
1
Initial water table
depth 3 m
m
1.5
2
Initial water table
depth 3.5 m
2.5
3
Initial water table
depth 4 m
3.5
4
31/07/91
24/07/91
17/07/91
10/07/91
03/07/91
26/06/91
19/06/91
12/06/91
05/06/91
29/05/91
22/05/91
15/05/91
08/05/91
01/05/91
4.5
Graph 5 Evolution of the water table depth with different initial water table levels. Default
values are used for those values that remain unknown
Capillary rise & actual evapotranspiration
Actual evapotranspiration depends on the amount of water that is
available in the upper layer of the soil (root zone) and on the capillary rise.
WASIM takes water from the saturated zone when no water is available in the
upper part of the soil.
In the experiment resalinization took place since 4th irrigation, due to
capillary rise. So, if capillary rise occurred is because not enough water was
easily available in the root zone.
P is the fraction of total available water that is easily available. The higher
the p factor is, the larger the amount of easily available water is. In graph 6 it is
shown the actual ET for different values of p, for the first 47 days of the
experiment (starting at 1st of May). The saturated hydraulic conductivity is taken
equal to 0.6 m/day (default value in WASIM). The water table level is low enough
to avoid capillary rise, so the effect of p can be studied without any interference.
With values of p higher than p = 0.3, plants transpire at its potential rate.
With a value of p equal to zero, this is not possible. There is not enough easily
available water stored in the root zone between field capacity and permanent
wilting point. As capillary rise only happens when there is no easily available
water in the root zone, p must be equal to zero.
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Actual ET
2.5
2
mm
1.5
1
0.5
0
1
3
5
7
9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45
days
p=0
p=0.3
p=1
Graph 6 Actual ET for different values of p factor
Once the value of p has been set, the next step is to study capillary rise
carefully. In WASIM, capillary rise depends of the water table level, root depth
and saturated hydraulic conductivity.
In the graphs below, capillary rise is plotted for different values of
saturated hydraulic conductivity. In the left one, actual ET for different values of
saturated hydraulic conductivity is calculated with WASIM for a ‘p’ value equal to
zero. In this case, the water table depth is shallow enough to allow capillary rise.
The right graph shows the amount of capillary rise that these situations produce.
With a lower value of Ksat, capillary rise is higher as is actual ET.
The range of values of saturated hydraulic conductivity for the kind of soil
in Lote 6 fluctuates from 0.2 to 0.6 m/day (Wasim database).
Actual ET
C a p i l l a r y R i se
0.5
2.5
0.45
0.4
2
1.5
mm/day
mm/day
0.35
1
0.3
0.25
0.2
0.15
0.1
0.5
0.05
0
0
1
4
7
10
13
16
19
ET k=0.6
22
25
28
ET k=0.3
31
34
37
40
43
46
ET k=20
Graph 7 Actual ET for different values of
saturated hydraulic conductivity.
1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45
Capillary Rise, k=0.6
Capillary Rise, k=0.3
Capillary Rise, k=0
Graph 8 Capillary rise for different values of
saturated hydraulic conductivity.
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SANTIAGO DEL ESTERO
Summary
By now, most of the parameters have been calibrated or they have been
placed in an accurate interval of values. Only the leaching efficiency remains
completely unknown. This parameter will be calibrated with the study of salinity.
Drainage
constant
Unsaturated hydraulic
conductivity; Ksat
Initial Water Table
level
Fraction of easily
available water; p
τ = 0.37
0.2 – 0.6 m/day
3.5 – 4m
0
Table 10 Summary of calibrated parameters
3.1.2.3 Salinity
The evolution of salinity can be studied in more detail. The parameters
and processes that are connected with salinity are the next:
-
Leaching
-
Capillary rise Æ Saturated hydraulic conductivity
-
Water table level Æ Initial water table level
In order to find those values of each parameter that best represent the
salinity behavior in the experiment, several trials have been done with different
values of leaching efficiency, unsaturated hydraulic conductivity (capillary rise)
and initial water table depth.
Leaching
Different values of the leaching efficiency have been used in the WASIM
model for an initial water table level of 4 meters and a saturated hydraulic
conductivity of 0.6 meters per day. The results are shown in graph 9.
In the upper graph the evolution of salinity in the top-soil is shown. The
values that the WASIM model presents are quite different from the measured
ones. Moreover, these values do not change with the leaching efficiency.
In the middle graph, the evolution of salinity in the root zone is shown. In
this case, the salinity values vary with the leaching efficiency. Moreover, for some
leaching efficiency values (70 – 80 %) the evolution of salinity that WASIM gives
is approximate to the measured one in the experiment. However, there is no
resalinization. This is because the water table level is too deep and capillary rise
can not take place. So, a shallower initial water table depth has to be chosen.
Page 23 of 49
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SANTIAGO DEL ESTERO
In the lower graph, it can be seen how salinity behaves in the unsaturated
zone. In this case, although salinity values change with the leaching efficiency,
they do not reproduce the real values at all.
Top soil
3.5
3
10-1 dS/m
2.5
Leaching 90%
2
Experiment
1.5
Leaching 80%
Leaching 70%
1
Leaching 25%
0.5
0
Initial
16/05/1991
01/06/1991
17/06/1991
03/07/1991
19/07/1991
31/07/1991
Root-zone
5
4.5
4
10-1 dS/m
3.5
Leaching 90%
3
Leaching 80%
Leaching 70%
2.5
Leaching 25%
Experiment
2
1.5
1
0.5
0
Initial
16/05/1991
01/06/1991
17/06/1991
03/07/1991
19/07/1991
31/07/1991
Unsaturated zone
4
3.5
10-1 dS/m
3
Leaching 90%
Leaching 80%
Leaching 70%
2.5
2
Leaching 25%
Experiment
1.5
1
0.5
0
Initial
16/05/1991
01/06/1991
17/06/1991
03/07/1991
19/07/1991
31/07/1991
Graph 9: Salinity values after each irrigation in the top-soil, root zone and unsaturated
zone, respectively, for different leaching efficiencies values.
Page 24 of 49
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SANTIAGO DEL ESTERO
Saturated hydraulic conductivity:
In the graph 10 the evolution of salinity with different saturated hydraulic
conductivity is shown. Only when resalinization takes place there are important
differences between them.
Top soil
3.5
3
10-1 dS/m
2.5
K = 0.6 m/day
K = 0.2 m/day
Experiment
2
1.5
1
0.5
0
Initial
16/05/1991
01/06/1991
17/06/1991
03/07/1991
19/07/1991
31/07/1991
Root-zone
3.5
3
10-1 dS/m
2.5
2
K = 0.6 m/day
1.5
K = 0.2 m/day
Experiment
1
0.5
0
Initial
16/05/1991
01/06/1991
17/06/1991
03/07/1991
19/07/1991
31/07/1991
Unsaturated zone
4
3.5
3
10-1 dS/m
2.5
K = 0.6 m/day
2
K = 0.2 m/day
Experiment
1.5
1
0.5
0
Initial
16/05/1991
01/06/1991
17/06/1991
03/07/1991
19/07/1991
31/07/1991
Graph 10 Salinity values after irrigate in the top-soil, root zone and unsaturated
zone, respectively, for different saturated hydraulic conductivities.
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SANTIAGO DEL ESTERO
Initial water table depth:
An initial water table depth of 3.5 meters is too high because with an
unsaturated hydraulic conductivity of 0.2 m/day, there is ponding since the 6th
irrigation (see graph 12). Therefore, an initial water table depth of 3.7 meters is
chosen.
Top soil
3.5
3
10-1 dS/m
2.5
Initial watertable depth 3.5 m
2
Experiment
Initial watertable depth 3.7 m
Initial watertable depth 4 m
1.5
1
0.5
0
Initial
16/05/1991
01/06/1991
17/06/1991
03/07/1991
19/07/1991
31/07/1991
Root-zone
3.5
3
2.5
10-1 dS/m
Initial watertable depth 3.5 m
2
Initial watertable depth 3.7 m
Initial watertable depth 4 m
1.5
Experiment
1
0.5
0
Initial
16/05/1991
01/06/1991
17/06/1991 03/07/1991
19/07/1991
31/07/1991
Unsaturated zone
4
3.5
3
10-1 dS/m
2.5
Initial watertable depth 3.5 m
Initial watertable depth 3.7 m
2
Initial watertable depth 4 m
Experiment
1.5
1
0.5
0
Initial
16/05/1991
01/06/1991
17/06/1991 03/07/1991
19/07/1991
31/07/1991
Graph 11 Salinity values after irrigate in the top-soil, root zone and unsaturated
zone, respectively, for different initial water table depths
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SANTIAGO DEL ESTERO
Water table depth
0
0.5
1
10- 1 dS/m
1.5
Initial watertable depth 3.5 m
2
Initial watertable depth 3.7 m
2.5
Initial watertable depth 4 m
3
3.5
4
4.5
Graph 12 Evolution of water table level with different initial water table depths for an
unsaturated hydraulic conductivity of 0.2 m/day
Finally, the calibrated values in this section are:
Unsaturated hydraulic
conductivity; Ksat
Initial Water Table
level
Leaching
efficiency
0.2 m/day
3.7 m
70 %
Table 11 Calibrated values
3.1.3 Conclusions
From the results obtained in the experiment and the simulation done in
WASIM it can be concluded that the model is able to simulate the evolution of the
water table depth and the salinity in the root-zone, with leaching of salinity and
resalinization since the 4th irrigation. On the contrary, the values of salinity that
WASIM offers for the topsoil and the unsaturated compartment don’t reproduce
those that were measured in the experiment. In general, the model is a good
approximation of the system and it can help to understand how it works.
Taking into account the experiment results and the outcomes of the
model, the soil reclamation is possible in the irrigation system. The WASIM
model is a useful tool to find the optimal irrigation schedule and drainage system
that allow reclamation of the soil.
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SANTIAGO DEL ESTERO
3.2 Possibilities of soil reclamation in PRD
In this section different options to remove salts from the soils of Proyecto
Rio Dulce irrigation system have been considered. The main purpose of this
section is not to find a definite solution for salinity in the Rio Dulce irrigation
system but to test how the system behaves in front of different actions.
First of all, the effects that install a drainage system have in the irrigation
scheme have been studied. Therefore, different drain depths (1.8, 2 & 2.2 m) and
different spacing between drains (100, 150 & 200 m) have been tested. These
values have been selected since are
similar to those used in the Lote 6
reclamation experiment. Also, how
salinity behaves at the non-irrigated
fields when an irrigation schedule is
applied has been studied. The irrigation
schedules considered have been: the
official irrigation schedule and actual
irrigation in Case 3 (paragraph 1.2.2,
table 6). All these scenarios have been
tested with the WASIM model.
Picture 3 Actual drain in Rio Dulce
Irrigation System
As it is well known, some inputs are required in the WASIM model. Those
parameters calibrated in the section 3.1 for the experiment in ‘Lote 6’ have been
used. Likewise, the data of Case 3 related to salinity has been used in all the
scenarios (paragraph 1.2.2, table 4).
Inputs WASIM
Soil data for the WASIM model
Soil type
Saturation (%)
Field capacity (%)
Permanent wilting point (%)
Drainage coefficient(mm/day)
Hydraulic conductivity (m/day)
Curve number for
runoff calculations
Leaching efficiency (%)
Sandy loam
45.8
26
12
0.37
0.2
67
70
Initial salinity CASE 3
Non-irrigated field (ds/m)
Irrigated field (ds/m)
16.7
1
Table 12 Soil data and initial salinity content for the WASIM model calibrated in section 3.1
Page 28 of 49
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SANTIAGO DEL ESTERO
Also, daily inputs of climatic data are required: information about potential
ET is available at Lieveld (2005). Daily rainfall data is available for the year 2005
(http://www.wunderground.com/global/stations/87129.html), while monthly data is
known for a period of 20 years (INTA-EEASE). Using as a pattern the events in
the year 2005, a daily rainfall series has been created.
In the next paragraphs the evolution of salinity, the water table depth and
the drain flow according to WASIM results with different scenarios are explained.
3.2.1 Salinity
In this part, the evolution of salinity is studied through three different
scenarios of irrigation and with a drainage system installed:
•
Drainage without irrigation at the non-irrigated field
•
Irrigation case 3 at the non-irrigated field.
•
Official schedule at the non-irrigated field
After a simulation with WASIM for the three scenarios studied, with
different drain depths and spacing between them, the values of salinity reached
are shown in the table 13. Those values that are suitable for agricultural use are
remarked in the table. It can be seen how only when the fields are irrigated it is
possible to achieve suitable values. Salinity decreases with a deeper drain and a
smaller spacing
No irrigation
Case 3
Official Schedule
drain depth 2.2 m
s=100 s=150 s=200
m
m
m
25.8
25.9
26.1
1.2
2.1
4.5
0
0.6
2.2
drain depth 2 m
s=100 s=150 s=200
m
m
m
25.4
25.4
25.4
1.8
3.5
6.5
0.3
1.2
3.2
drain depth 1.8 m
s=100 s=150 s=200
m
m
m
25.1
25.1
25.1
3.6
6.1
9.4
0.7
2
4.4
Table 13 Salinity values (ECe) after 20 years of simulation for Case 3
But not only is important that the system could reach suitable salinity
values for agricultural use. But also, the time needed to reach these values is
important. This time is shown in the table 14. In most of the cases, the time
needed for the system to reclamate the soil is too high to be economically viable.
Case 3
Official Schedule
drain depth 2.2 m
s=100 s=150 s=200
m
m
m
8
14
5
9
12
drain depth 2 m
s=100 s=150 s=200
m
m
m
11
20
8
9
15
drain depth 1.8 m
s=100 s=150 s=200
m
m
m
20
10
17
-
Table 14 Years needed by the system to reach suitable salinity values
In the graph 13 the evolution of salinity in the top soil and the root zone of
the three scenarios studied are shown (for a depth drain of 2.2 meters and a
distance between drains of 100 meters).
Page 29 of 49
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In all the scenarios, it can be noticed salinity changes in the soil not only
along the years but also during the year (in a monthly scale). This gives away the
importance that the moment when the measures are done has. Moreover, the
fact that inputs are not constant during the simulation contributes to obtain more
realistic outcomes.
In the case where no irrigation is applied, salinity levels don not decrease
during the simulation. The WASIM model confirms the fact that the installation of
drains without an increase of the leaching is useless. In addition, the top soil
shows a large variation in the salinity content between the dry and the wet
season.
Ece Root zone
d=2.2 m & s=100 m
non-irrigation
35
30
30
25
25
20
20
Jan-03
Jan-03
Jan-03
Jan-01
Jan-01
Jan-01
Jan-95
Jan-93
Jan-95
Jan-93
Jan-95
Jan-93
Jan-91
Jan-89
Jan-87
Jan-85
Jan-81
Jan-03
Jan-01
0
Jan-99
5
0
Jan-97
10
5
Jan-83
15
10
Jan-95
Jan-99
20
15
Jan-93
Jan-99
25
20
Jan-91
Jan-99
30
25
ds/m
35
30
Jan-89
Jan-91
Ece Root-zone
d=2.2 m & s=100 m
Official Schedule
35
Jan-87
Jan-89
Jan-87
Jan-85
Jan-81
Jan-03
Jan-01
Jan-99
Jan-97
Jan-95
Jan-93
Jan-91
Jan-89
Jan-87
0
Jan-85
5
0
Jan-83
10
5
Jan-83
15
10
Jan-85
Jan-97
20
15
Jan-83
Jan-97
25
20
Jan-81
Jan-97
30
25
ds/m
35
30
Ece Top-Soil
d=2.2 m & s=100 m
Official Schedule
ds/m
Jan-91
Ece Root zone
d=2.2 m & s=100 m
Case 3
35
Jan-81
ds/m
Ece Top-Soil
d=2.2 m & s=100 m
Case 3
Jan-89
Jan-81
Jan-03
Jan-01
Jan-99
Jan-97
Jan-95
Jan-93
Jan-91
Jan-89
Jan-87
0
Jan-85
5
0
Jan-83
10
5
Jan-87
15
10
Jan-85
15
Jan-83
ds/m
35
Jan-81
ds/m
Ece Top-Soil
d=2.2 m & s=100 m
non-irrigation
Graph 13 evolution of salinity levels at non-irrigated fields in Case 3 without
irrigation (a), with irrigation case 3(b) and with the official irrigation schedule (c).
Page 30 of 49
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Looking at the cases where irrigation is applied, it can be seen how
salinity levels decrease along the simulation reaching in some years suitable
salinity values for agricultural use.
In general, the topsoil compartment suffers more dramatic variations in its
salinity during the year compared to the root zone compartment. This is because
the top soil compartment is directly exposed to the climatic events. In the
irrigated scenarios, the evolution of the root zone and the topsoil compartments
is similar whereas in the case where no irrigation is applied the behavior of both
compartments is totally different.
If irrigation is considered without a drainage system, then the salinity
levels continue being high due to capillary rise during the dry seasons. The
results of the simulation can be seen in the graph 14. As a conclusion it can be
said that the application of irrigation without the appropriate drainage is useless.
Ece root zone
no drainage
14
20
18
16
14
12
10
8
6
4
2
0
12
ds/m
10
8
6
4
2
Ja
n03
Ja
n01
Ja
n99
Ja
n97
Ja
n95
Ja
n93
Ja
n91
Ja
n89
Ja
n87
Ja
n85
Ja
n83
Ja
n81
Jan-03
Jan-01
Jan-99
Jan-97
Jan-95
Jan-93
Jan-91
Jan-89
Jan-87
Jan-85
Jan-83
0
Jan-81
ds/m
Ece Top-Soil
no drainage
Graph 14 evolution of salinity levels at non-irrigated fields in Case 3 without drainage
3.2.2 Water table level
In the next table the average water table levels registered during the
simulation for each case is given. Also, graph 15 represents the evolution of the
water table level for the three cases studied, specifically for a drain depth of 2.2
m and a spacing of 100 meters.
Official schedule
Case 3
No irrigation
drain depth 1.8 m
s=100 s=150 s=200
m
m
m
-1.46
-1.37
-1.27
-1.54
-1.49
-1.43
-2.39
-2.39
-2.39
drain depth 2 m
s=100 s=150 s=200
m
m
m
-1.56
-1.46
-1.35
-1.61
-1.55
-1.49
-2.41
-2.41
-2.41
drain depth 2.2 m
s=100 s=150 s=200
m
m
m
-1.66
-1.54
-1.41
-1.69
-1.60
-1.53
-2.42
-2.42
-2.41
Table 15 Water table depth, in m, after 20 years of simulation for Case 3
When the field is irrigated, the water table level increases considerably
because the drainage system can not drain the entire overflow. It is possible that
bigger drains work properly, but this has not been tested in this thesis.
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If the two irrigation schedules are compared, it can be seen that with the
official irrigation schedule the water table depth is lightly shallower than with the
case 3 irrigation schedule. When no irrigation is applied, the water table level
remains around 2.5 meters depth where the drainage system is not effective.
Although the water table level remains around 1.5 meters depth during all
the simulation, the water table levels vary every year because the inputs are
variable.
Water table depth
d=2.2m & s=100m
0
-0.5
m
-1
-1.5
-2
-2.5
Case 3
Official Schedule
Jan-04
Jan-03
Jan-02
Jan-01
Jan-00
Jan-99
Jan-98
Jan-97
Jan-96
Jan-95
Jan-94
Jan-93
Jan-92
Jan-91
Jan-90
Jan-89
Jan-88
Jan-87
Jan-86
Jan-85
Jan-84
Jan-83
Jan-82
Jan-81
-3
Non-irrigated
Graph 15 Water table levels for a drain depth of 2.2 m and a spacing of 100 m
3.2.3 Drain flow
If the total amount of flow that the drains evacuate during all the simulation
is compared, it can be seen that the drain flow with the official irrigation schedule
is higher than the drain flow with the case 3 irrigation schedule. This indicates
that WASIM takes into account the total amount of irrigation and also, the
distribution of this irrigation along the year. The amount of irrigation per year is
similar in both cases but not the distribution of this irrigation ((paragraph 1.2.2,
table 6).
Official
Schedule
Case 3
drain depth 2 m
s=100 s=150 s=200
m
m
m
drain depth 2.2 m
s=100 s=150 s=200
m
m
m
drain depth 1.8 m
s=100 s=150 s=200
m
m
m
2512.5
1784.5
4194.3
3166.1
2218.7
2546.6
1942.8
1408.3
3342.5
1941
1265
3912.7
2729.5
1756
1781.3
1264.6
866.9
2799.2
Table 16 Drain flow after 20 years of simulation for Case 3
3.2.4 Conclusions
It is possible to reach suitable salinity levels for agriculture whit the
installation of a drainage system and the irrigation of the fields. Nevertheless, the
schedule and the total amount of irrigation have to be studied in more detail. In
the same way, the drainage system has to be tested more exhaustively.
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4 Study of salinity with SALTMOD
Like in the last section, here the main purposes are to study the salinity
process and the possibilities of soil reclamation of the fields in Rio Dulce
Irrigation System. In this case, the model used is SALTMOD.
In the first part, the objective is to study the evolution of salinity in the PRD
over a larger period of several years. This is done with two purposes: first, to see
if SALTMOD can simulate the salinity levels measured on the fields and second
to study these measures.
In the second part, several possibilities of soil reclamation have been
considered.
4.1 Study of salinity in the PRD at larger scale
Most of the data that is needed to study salinity from the Rio Dulce
irrigation system are already known (see paragraph 1.2.2). Nevertheless, some
parameters remain unknown and must be calibrated. Once this calibration has
been done, different scenarios have been studied.
4.1.1 Calibration
To carry out the calibration of the unknown parameters, CASE 1 has been
used as it is the one with more data available. Starting with low salinity values,
the objective is to reach after several years the measured values in CASE 1 in
Río Dulce Irrigation System.
The Rio Dulce irrigation system is characterized by having irrigated and
non-irrigated fields within the command area. Within SALTMOD two fields were
simulated: one irrigated and one non-irrigated. To implement a SALTMOD input
file, it is necessary to define information about hydrological data, irrigation
schedules, soil properties, area, initial salinity values… As the model calculates a
seasonal water balance, the values of irrigation and hydrological data have to be
introduced per season. In this case, there are two seasons: one dry (from May to
October) and another wet (November to April). Initial salinity values used in
SALTMOD are expressed as the EC of the soil moisture when saturated under
field conditions.
Data related to salinity, irrigation schedules and hydrological data is
obtained from Prieto (2006) as it can be seen in paragraph 1.2.2. As it is known,
from several experiments carried out at the Pilot Areas of San Isidro and San
Javier, located in Rio Dulce irrigation system, some properties of the soil are
known. These values were already used for the WASIM model. Also, with the
WASIM model some parameters that are required in SALTMOD, like leaching
efficiency, were calibrated. Their values were used also in SALTMOD. All these
values are shown in table 17. As the values of precipitation are effective
precipitation, runoff is supposed to be equal to zero.
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Summary of input parameters of SALTMOD
1. Soil properties
Fraction of irrigation or rain water stored in root zone
UNKNOWN
Total porosity of root zone
0.54
Total porosity of transition zone
0.46
Total porosity of aquifer (assumed)
0.46
UNKNOWN
Critical depth for capillary rise
Leaching efficiency of root zone
70 %
Leaching efficiency of transition zone
70 %
Leaching efficiency of aquifer
70 %
2. Water balance components
Irrigation in the season 1
0.195
Irrigation in the season 2
0.391
Rainfall in the season 1
0.414
Rainfall in the season 2
0.085
Evapotranspiration in the season 1
0.9627
Evapotranspiration in the season 2
0.5958
Incoming groundwater flow through aquifer during the season (assumed)
Outgoing groundwater flow through aquifer during the season
0
UNKNOWN
0
Surface runoff in the season
3. Drainage criteria and system parameters
0.5
Root zone thickness (m)
No drains
Depth of subsurface drains (m)
Thickness of transition zone between root zone and aquifer (m)
4
Thickness of aquifer, assumed (m)
4
4. Initial and boundary conditions
Depth of the water table in the beginning of the season (m)
4
Initial salt concentration of soil moisture in root zone at field saturation (dS/m)
4
Initial salt concentration of the soil moisture in transition zone (dS/m)
4
Average salt concentration of incoming groundwater (dS/m)
9.6
Average salt concentration of incoming Irrigation water (dS/m)
0.8
Table 17 Summary of inputs parameters for Saltmod
m: meter; d: days; dS: deci-siemens.
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Finally, those parameters that have to be calibrated are:
•
Fs is the storage fraction of the surface water resources,
representing the moisture holding capacity.
•
Go is the outgoing flow through the aquifer
•
Dc is the critical depth for capillary rise
Note that SALTMOD uses as salinity measure the EC of the soil moisture
when saturated under field conditions, while the data we have from PRD is ECe,
the electric conductivity of an extract of saturated soil paste. Approximately, it
can be assumed that EC= 2 ECe.
To calibrate the values of the outgoing flow through the aquifer and the
critical depth for capillary rise, several trials have been done with different values
of these parameters in order to find that combination which makes that the water
table remains at 2 meters depth (measured level, see paragraph 1.2.2) and the
salinity levels reach the measured values for Case 1 (paragraph 1.2.2).
The Fs values used in this case are:
•
Fs irrigated field, wet season: 0.5
•
Fs irrigated field, dry season: 0.5
•
Fs non-irrigated field, wet season: 0.5
•
Fs non-irrigated field, dry season: 1
The results for the end of the simulation (40 years) are shown in the next
table. It can be seen that it is possible to obtain the measured water table depth
with different combinations of Go and dc. Nevertheless, the salinity value for the
non-irrigated field is closer to the measured value with an outgoing flow through
the aquifer of 0.05 m per season. So, if Go is fixed to 0.05 m per season, then
the critical depth has to be 2.5 m.
Water table depth (m)
dc=2.2
dc=2.3
dc=2.4
dc=2.5
Go=0
m/s
Go=0.05
m/s
Go=0.1
m/s
-1.59
-1.65
-1.72
-1.79
-1.77
-1.85
-1.92
-2
-1.95
-2.04
-2.12
-2.21
Salinity irrigated field
(dS/m)
Go=0 Go=0.05 Go=0.1
m/s
m/s
m/s
Salinity non-irrigated field
(dS/m)
Go=0
Go=0.05 Go=0.1
m/s
m/s
m/s
3.3
3.3
3.31
3.31
80.9
81
81.1
81.2
2.45
2.45
2.45
2.45
1.98
1.98
1.99
1.99
18.3
18.4
18.2
18
1.75
1.75
1.76
1.78
Table 18 depths of water table and salinity values (ds/m) for different combinations of Go
and dc.
To conclude, the
fraction of the surface
capacity and is different
combination of values of
last parameter unknown is Fs. This is the storage
water resources, representing the moisture holding
on each season and on each field. To find the best
Fs, several trials have been done with the assumption
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that no percolation occurs at the non-irrigated field during the dry season. So,
FsNIrrig&dry is equal to one.
First of all, the Fs value for the non-irrigated field at the wet season has
been calibrated. The evolution of salinity can be seen in graph 16. Finally, a
value of Fs= 0.5 is chosen, since is the one with which closest salinity values to
the measured ones are obtained.
Salinity non-irrigated land
for different values of Fs
(non-irrigated land and wet season)
35
b
c
Fs I&wet
0.5
0.5
0.5
Fs NI&wet
Fs I&dry
Fs NI&dry
0.5
0.5
1
0.6
0.5
1
0.4
0.5
1
30
25
20
ds/m
a
Fs=0.5
Fs=0.6
Fs=0.4
15
10
5
Table 19 Fs values
40
35
30
25
20
15
10
5
0
0
years
Graph 16 Evolution of salinity
The next step is to calibrate other Fs parameters. Several combinations
were made. Of all the studied combinations, C was chosen.
b
c
d
Fs I&wet
0.5
0.5
0.4
0.4
Fs NI&wet
Fs I&dry
Fs NI&dry
0.5
0.5
1
0.5
0.6
1
0.5
0.6
1
0.5
0.6
1
Table 20 Fs values
40
35
30
25
ds/m
a
Salinity non-irrigated land
for different values of Fs
(irrigated land)
a
b
20
c
d
15
10
5
40
38
36
34
32
30
28
26
24
22
20
18
16
14
12
8
10
6
4
2
0
0
years
Graph 17 Evolution of salinity
It is possible to reach the measured values in the irrigation system with the
SALTMOD model. However, it can be noticed that salinity at the non-irrigated
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field increases along the years and is not possible to reach a stable situation.
This is because rainfall like the rest of inputs is a constant rate along the years.
So, if leaching of salts due to percolation and resalinization due to capillary rise
are constant, then salinity increases year by year. As an assumption it is
considered that in a real case after 20 years a stable situation would be reached.
So, to choose the best combination the values reached at 20 years of simulation
will be considered.
4.1.2 Different scenarios
In this section, different scenarios are considered in order to know how
salinity behaves with different irrigation schedules. The procedure is the same
that in the last section: starting with low salinity values, study how salinity
evolutions. The parameters calibrated in Case 1 have been used.
Official
Schedule
Case 1
Case 2
Case 3
Case 4
Season
1
(m)
Season
2
(m)
Total
irrigation
(m)
0.409
0.491
0.9
0.195
0.323
0.389
0.402
0.391
0.539
0.519
0.402
0.586
0.862
0.908
0.804
Table 21 different irrigation schedules at different areas of PRD
In all the scenarios studied salinity has an increasing trend in the nonirrigated fields. This is because all the inputs are invariable during all the
simulation. Secondary salinization occurs because there is capillary rise. If
salinization in the dry season is larger than the leaching of salts due to
percolation in the wet season, salinity level increases. As here this is constant
along the years, salinity has a rising tendency.
It can be seen as the larger the amount of irrigation is, the higher salinity
values are. This is because SALTMOD does a seasonal water balance and does
not take into account the distribution of this irrigation along the season and how it
affects percolation and capillary rise respectively. As irrigation increases, the
water table depth is shallower and consequently, capillary rise increases.
As an example if Case 3 and Official Schedule are compared the total
amount of irrigation per year is similar but the distribution of this irrigation is
completely different in each case (see table 6, paragraph 1.2.2). Nevertheless,
the salinity values reached are quite similar. In the same way, the values reached
in Case 3 are closer to the measured ones (see table 6, paragraph 1.2.2). The
salinity values in Case 4 that SALTMOD gives as an output are different to the
measured ones.
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Salinity Non-irrigated land
40
35
30
ds/m
25
Official Schedule
Case 1
Case 2
Case 3
Case 4
20
15
10
5
19
20
17
18
15
16
13
14
11
12
9
10
7
8
5
6
4
2
3
1
0
0
years
Graph 18 Evolution of salinity (EC of soil moisture when saturated under field
conditions) at non-irrigated fields for a running of 20 years
Water table depth
19
20
17
18
15
16
14
13
12
11
9
10
7
8
6
4
5
3
2
1
0
0
-0.5
-1
-1.5
Official Schedule
Case 1
Case 2
Case 3
Case 4
m
-2
-2.5
-3
-3.5
-4
-4.5
years
Graph 19 Evolution of water table depth
Salinity Irrigated Field
4
3.5
ds/m
3
Official Schedule
Case 1
Case 2
Case 3
Case 4
2.5
2
1.5
20
19
18
17
15
16
14
13
12
11
9
10
8
7
6
4
5
3
1
2
0
1
years
Graph 20 Evolution of salinity (EC of soil moisture when saturated
under field conditions) at irrigated field
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4.1.3 Inputs variables
By now, all the calculations made with SALTMOD have used constant
inputs. But SALTMOD also offers the possibility of variable inputs. In this section,
Case 1 is studied with rainfall variable data (Monthly values from INTA-EEASE).
In the graph 21 the evolution of salinity in Case 1 with constant inputs and
with variable inputs is shown. Although salinity has an increasing trend, in this
case it can be observed how this tendency is irregular. The increase or decrease
of salts in the soil is variable each year, the opposite of the previous cases.
At the same time, it can be seen a stabilization of the salinity levels in the
last years of the simulation. Nevertheless, the order of magnitude of salinity
levels for both simulations is the same.
Salinity non-irrigated field
14
12
ds/m
10
8
Input constante
6
Input variable
4
2
18
15
12
9
6
3
0
0
years
Graph 21 Salinity at the non-irrigated field
4.1.4 Conclusions
With SALTMOD is possible to predict how the evolution of salinity in the
next years is going to be, but the outputs that SALTMOD presents can diverge
considerably with reality. In the simulated cases, after 40 years of simulation
salinity continues increasing year by year. This is because all the inputs are
invariable during all the simulation.
The larger the amount of irrigation is, the higher the salinity values are.
This is because SALTMOD does a seasonal water balance and does not take
into account the distribution of this irrigation along the season and how it affects
to percolation and capillary rise respectively.
With the SALTMOD model is possible to simulate at the same time an
irrigated and a non-irrigated field which is really suitable for PRD.
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4.2 Possibilities of soil reclamation in PRD
In this section different options to remove salts from the soils of Proyecto
Rio Dulce irrigation system have been considered with SALTMOD. Like in the
part 3.2, the main purpose of this section is to test how the system behaves in
front of different actions but not to find a definite solution for salinity.
First of all, the effects that install a drainage system have in the irrigation
scheme have been studied. Therefore, different drain depths (1.8, 2 & 2.2 m) and
different spacing between drains (100, 150 & 200 m) have been tested for
different cases. Also, how salinity behaves at the non-irrigated fields when an
irrigation schedule is applied has been studied (with the drainage system
included). Like in the section 3.2, the irrigation schedules considered have been:
the official irrigation schedule and actual irrigation in Case 3 (paragraph 1.2.2,
table 6).
Note that with SALTMOD two fields are simulated: one irrigated and the
other one not irrigated.
4.2.1 Drainage
Taking as initial inputs the measured values of salinity and the parameter
values calibrated in Case 1, several drain depths and separations between them
have been considered for different scenarios (Case 1, Case 3 and Case 4) and
for two different irrigation schedules (for the irrigated field): those that in fact are
applied and the official irrigation schedule. In the next numbers, the salinity
values after 20 years of simulation with SALTMOD are shown.
Distance
between drains
100
150
200
Case 1
Critical depth (m)
Case 3
Critical depth (m)
Case 4
Critical depth (m)
2.2
2
1.8
2.2
2
1.8
2.2
2
1.8
2.12
4.25
6.01
13.7
16.7
18.8
61.4
63.9
64.3
5.27
15.4
33.7
34.4
48.6
48.6
25.6
25.6
25.6
6.27
17.3
37.2
47.3
65.4
66.8
76.7
77.7
78.4
Table 22 Salinity values (EC of soil moisture when saturated under field conditions) after
20 years of simulation for Case 1, Case 3 and Case 4
Distance
between drains
100
150
200
Case 1
Critical depth (m)
Case 3
Critical depth (m)
Case 4
Critical depth (m)
2.2
2
1.8
2.2
2
1.8
2.2
2
1.8
3.92
10.9
22.1
22.5
30.6
32.4
43.2
45.2
46.1
5.38
15.4
35.4
35.8
48.3
50.2
60.8
62.6
63.7
6.89
20.8
49.3
49.5
66.7
68.6
79.2
81
82.1
Table 23 Salinity values (EC of soil moisture when saturated under field conditions) after
20 years of simulation for Case 1, Case 3 and Case 4 with the official irrigation schedule
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In the tables 22 & 23, those values of salinity that are suitable for
agricultural use are bold. As can be seen, only with a critical depth of 2.2 m it is
possible to obtain such values. If the results obtained with the official irrigation
schedule and those achieved with the existents irrigation schedules are
compared, it can be seen that they are quite similar for cases 3 and 4, but they
are completely different in case 1. This is because the amounts of water irrigated
in cases 3 and 4 are closer to the official schedule than in case 1 where the real
irrigation is lower.
Not only it is important to know that suitable salinity values can be
reached, but also it is essential to know the time that the system needs to reach
these values. In this case, it takes a considerable number of years to reach lower
levels of salinity.
Case 1
Distance
between drains
100
150
200
Case 3
Case 4
Real
Official
Real
Official
Real
Official
6
9
13
8
-
15
-
15
-
17
-
18
-
Table 24 number of years necessaries to reach suitable salinity values for agricultural use.
In graph 22 is given the evolution of salinity at the non-irrigated field for
two different cases (Case 1 & Case 3). It can be seen how salinity follows a
decreasing trend. Every year, resalinization takes place during the dry seasons in
a constant rate. In the graph 22b it can be seen different behaviors of the salinity
levels depending on the distance between drains. With a spacing of 100 meters
reclamation of the soil is possible. Although salinity levels stop increasing with a
spacing of 200 meters, reclamation is not feasible.
Salinity non-irrigated land
CASE 3
Salinity non-irrigated land
CASE 1
18
15
12
s=100 m
18
15
12
9
6
3
0
s=150 m
9
s=100 m
s=200 m
6
s=150 m
40
35
30
25
20
15
10
5
0
3
s=200 m
ds /m
dc=2.2 m
18
16
14
12
10
8
6
4
2
0
0
d s /m
dc=2.2 m
years
years
Graph 22 Evolution of salinity at non-irrigated fields Case 1 and Case 3 for a drain depth of
2.2 m and different separations between the drains
Salinization only stops increasing when drains are able to maintain the
water table level deep enough to avoid capillary rise. Moreover, reclamation of
the soil only takes place when the salts of the upper parts of the soil are leached
and drained out of the system.
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In the next table, water table depths for each case are shown:
Distance
between drains
100
150
200
Case 1
Critical depth (m)
Case 3
Critical depth (m)
Case 4
Critical depth (m)
2.2
2
1.8
2.2
2
1.8
2.2
2
1.8
2.18
2.14
2.12
2.05
2.04
2.03
2.01
2.01
2.01
2.14
2.14
2.12
1.98
2.04
2.03
1.85
2.01
2.01
2.16
2.11
2.06
2.01
1.98
1.96
1.91
1.9
1.9
Table 25 Water table depths, in m, after 20 years of simulation for Case 1, Case 3
and Case 4
4.2.2 Drainage & Irrigation
Once several possibilities of drainage have been studied to solve salinity
problems, in this part the effects of drainage and irrigation at non-irrigated fields
have been considered. Starting from the known situation in case 3 a watering
has been applied at the non-irrigated field in order to understand how salinity
behaves.
In the following numbers, the results from a simulation with SALTMOD
model during 20 years can be seen. In this simulation, the same irrigation
schedule is applied to both fields (the initial salinity levels are shown in table 4,
paragraph 1.2.2). At the same time, a drainage system has been considered and
different drain depths and distance between them have been tested.
Distance
between drains
100
150
200
Case 3
Critical depth (m)
Official schedule
Critical depth (m)
2.2
2
1.8
2.2
2
1.8
4.59
4.94
5.96
5.18
5.55
5.55
6.02
6.38
7.8
4.52
5.13
5.89
5.17
5.91
6.73
6.02
6.89
7.8
Table 26 Salinity values (EC of soil moisture when saturated under field conditions) after
20 years of simulation for Case 3
Case 3
Critical depth (m)
Distance
between drains
100
150
200
Official schedule
Critical depth (m)
2.2
2
1.8
2.2
2
1.8
4
4
5
4
5
7
6
7
10
4
4
5
4
5
7
6
6
10
Table 27 number of year necessaries to reach suitable salinity values for agricultural use
in each case: 4 ECe (dS/m)
After 20 years of simulation in SALTMOD, agricultural suitable salinity
levels have been reached in all the cases. Here, not only it is important to know
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that such values are achieved but also, how many years the system needs to
reach these values. Diverse combinations allow obtaining the search values in
four years which is reasonable.
Reclamation of the soil is faster when irrigation is applied in the nonirrigated field.
Salinity non-irrigated land
CASE 3
Salinity non-irrigated land
CASE 3
dc=2.2 m
dc=2.2 m
40
35
35
30
30
years
18
15
12
18
15
12
9
0
6
5
0
3
s=100 m
10
5
0
s=150 m
15
9
s=100 m
10
s=200 m
6
s=150 m
15
25
20
3
s=200 m
20
0
d s /m
25
ds /m
40
years
Graph 23 evolution of salinity levels at non-irrigated fields in Case 3 with irrigation (a) and
without irrigation (b). Salinity measured as EC of soil moisture when saturated under field
conditions.
4.2.3 Conclusions
From the outcomes achieved some points can be concluded:
If a drainage system is installed in the area without irrigation, reclamation
is possible with deeper drains and less separation between them. But, the time
needed to reach this situation is not viable. However, if irrigation is also applied,
the drains depth can be shallower and the distance between them larger.
Moreover, reclamation can be achieved in less time.
Picture 4 Irrigated field in the PRD
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5 Conclusions
Once all the calculations have been done, the next conclusions can be
obtained:
From one hand, in the thesis the evolution of salinity has been studied at
small scale with the WASIM model and at larger scale with the SALTMOD model.
About this part, the following questions can be come up:
•
Are the results similar to the measured ones?
•
Can the studied models simulate what it is happening in reality?
In the first section of this part, the Lote 6 experiment was reproduced with
the WASIM model. At the same time, some unknown parameters were
calibrated.
Picture 5 Canal in the PRD
MW Ertsen
Looking at the outcomes of the program, it can be noticed that WASIM
program was able to reproduce not only the evolution of the water table level
during the experiment but also, the salinity levels in the root zone. Nevertheless,
the salinity outcomes that WASIM presented at the top soil and in the
unsaturated zone were completely different from the measured ones. In the top
soil compartment, the WASIM outcomes were independent of the different
parameter values that had to be calibrated. Moreover, the outcomes were always
closer to each other and different from those that were measured in the
experiment. In the unsaturated zone, although salinity values varied depending
on the different values of the parameters, these were different to those measured
in the experiment.
To conclude it can be said that the model is a good approximation of the
system and it can help us to understand how the system is going to evolution.
In the second part of the thesis, the SALTMOD model has been used to
study salinity at larger scale. Two fields have been simulated: one irrigated and
the other one without irrigation. Starting with lower salinity values in both fields,
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the main purpose of this simulation was to know how salinity behaves along the
years.
According to the SALTMOD outcomes, salinity levels in the irrigated fields
were low while salinity increased in the non irrigated field. In this case, salinity
had an invariable increasing trend during the time simulated. This is because the
inputs were constant each time-step. So salinization was constant each year.
Obviously, this does not happen in real fields.
Finally, it can be concluded that although SALTMOD helps to understand
how salinity is going to evolution along the years, the values it gives and the
behavior during the simulation are not reliable if constant inputs are used. By the
other hand, if a simulation with variable rainfall data is done, it can be seen how
salinity in the non irrigated field does not have a constant increasing trend. In this
case, salinity changes depending on the inputs and the outcomes are more
realistic.
By the other hand, in this thesis several possibilities of soil reclamation in
the PRD irrigation system have been considered. Like in the first part, SALTMOD
and WASIM have been used. Regarding this part, what should be answered is:
•
Is it possible to improve the salinity levels in the non irrigated
fields?
•
Are the models outcomes trustworthy?
Picture 6 Salinity in PRD
According to the models is possible to reach suitable salinity levels for
agriculture whit the installation of a drainage system and the irrigation of the
fields. Nevertheless, the time and requisites needed are different in each model.
SALTMOD outcomes are more favorable than WASIM ones. As an example, in
the next table it is shown the time needed by the system to reach suitable salinity
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values in each of the cases. The reclamation of the soil is more effective with
deep drains and less space between them.
Case 3
Distance
between drains
100
150
200
SALTMOD
Critical depth (m)
WASIM
Critical depth (m)
2.2
2
1.8
2.2
2
1.8
4
4
5
4
5
7
6
7
10
8
14
-
11
20
-
20
-
Table 28 Time (in years) needed by the system to reach suitable
salinity values in each case
Since WASIM carries out a daily water balance, more inputs are required
and also, the outcomes are more precise. On the contrary, SALTMOD offers the
possible evolution of the system in general trends but not the certain values that
will be reached in the future. WASIM shows more variation within and over the
years than SALTMOD (see graph 24).
Ece Top-Soil
d=2.2 m & s=100 m
Case 3
Salinity non-irrigated land
CASE 3
dc=2.2 m
40
35
35
30
30
15
Jan-03
Jan-01
Jan-99
Jan-97
Jan-95
Jan-93
Jan-91
18
Jan-89
years
15
12
9
6
3
0
0
5
0
Jan-87
10
5
Jan-85
s=100 m
10
20
Jan-83
s=150 m
15
Jan-81
d s /m
s=200 m
20
ds/m
25
25
Graph 24: Evolution of salinity for case 3, according to SALTMOD (a) and WASIM (b)
Note that SALTMOD uses as salinity measure the EC of the soil moisture
when saturated under field conditions, while WASIM uses the ECe, the electric
conductivity of an extract of saturated soil paste. Approximately, it can be
assumed that EC= 2 ECe.
In the literature it can be found some advantages of SALTMOD in front of
daily water balances programs (Oosterbaan, 1998):
•
It is very difficult to collect daily data,
•
The model is designed to make long-term simulations,
•
Because of high variability in daily data, long-term simulations are
more reliable than short-term simulations.
Despite of these statements, it can be said that if daily data is available
then the results that a daily water balance program like WASIM offers are more
reliable than those from seasonal water balance program like SALTMOD.
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About the models
In general, both models are useful to study salinity. Moreover, both of
them present advantages and disadvantages.
•
Both models are for free.
•
If variable inputs are used in SALTMOD, they must be introduced in
the model by means of the program interface and not with an input
file. This is an inconvenient, since it is a slow process and mistakes
are likely to occur.
•
The WASIM interface is simple and user friendly.
•
With WASIM is not possible that the water table level remains
below the drains level. Moreover, initial salinity values must be
lower than 12 ds/m. That means that if there is a case (like this
one) where there are higher values, they must be scaled in order to
use WASIM model. Furthermore, seepage and water pumped from
wells are constant outputs of the system during the simulation.
About the Rio Dulce Irrigation System
The Rio Dulce Irrigation System has important problems with salinity:
those fields that are not irrigated present high salinity levels. This is due to
capillary rise: salts that used to stay in the groundwater level ascend to the upper
part of the soil. The shallower the water table level is, the higher the capillary
rise is.
These problems can be solved if a drainage system is installed and
irrigation applied in the non-irrigated fields. The installation of a drainage system
without an extra irrigation is useless. Besides, if irrigation is applied in the nonirrigated fields without a drainage system, salinity do not decrease.
It should be studied in the future:
•
The amount of irrigation that is necessary to reclaim the soils
without wasting too much water and without resalinization.
•
The drainage needed and the optimum relationship between
spacing and depth of the drain.
•
The possibility of farming when the salinity levels are low.
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6 Bibliography
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