This study used the SWAT model to simulate runoff and soil loss in the 1491 km2 Sarrath river catchment in Tunisia to identify high erosion risk areas. The model performed well in calibration and validation. Simulation results showed high correlation between runoff and sediment yield. Only 10% of the catchment area had high to very high erosion rates, contributing 70% of sediment. The highest erosion sub-catchments were identified to prioritize conservation measures. The SWAT model proved effective for assessing spatial variability of erosion risk across the catchment and aiding management planning.

1. Risk assessment of water erosion by the
SWAT model: sarrath catchment case study
MOSBAHI Manel, BENABDALLAH Sihem
Centre de Recherches et de Technologies des Eaux, BP. 273 Soliman, 8020 Tunisie
sihem.benabdallah@certe.rnrt.tn
ABSTRACT
CONCLUSION
RESULTS & DISCUSSIONS
METHODS
REFERENCES
INTRODUCTION
WORKSHOP & BROKERAGE EVENTS
Addressing Sustainable water management:
Wastewater Legislation and Technologies
Soil erosion is one of the most serious problems in Mediterranean
regions. In Tunisia, 20% of the total land area is affected by erosion [2].
Quantification of soil loss is one of the greatest challenges in natural
resources and environmental planning. Hydrological models are
becoming increasingly popular in predicting soil loss .With the intention of
planning catchments with high erosion risk, it is needed to identify area
that are the source of higher amount of sediment.
The purpose of this study is to
predict water and sediment for
identifying the most degraded sub-
catchments in the Sarrath catchment
in order to adopt the appropriate
management intervention.
The Sarrath river catchment is located in the North West
of Tunisia, it is part of the large Tunisian Medjerda
catchment. The total drainage area is 1491 km2.
• Elevation ranges from 573 to 1350 m;
• Mean annual rainfall is 409 mm;
• Major land covers are corn (31 %), range brushes (27 %) and forest (18 %);
• Dominant soil types are calcareous brown soils (32 %), complex soils (30 %) and
undeveloped soils (29 %).
STUDY AREA
SWAT model
SWAT
Outputs:
Runoff,
sediment,
nutriment.
Climatic data
Digital Elevation
Model (DEM)
Landuse map
Soil map
Spatial data
HRU
SWAT (Soil and Water Assessment
Tool) is a distributed hydrological
model developed to predict the
impact of land management
practices on water, sediment,
and agricultural chemical yields in
large ungauged watersheds [1], [3].
SWAT model interface required a
number of data layers including a
Digital Elevation Model (DEM), a
soil map and a land use map.
* Sarrath catchment discretization
resulted into the definition of 27
sub-catchments.
* Based on the formation of
unique combinations of slope, land
use and soil type, the
subcatchments
were divided into 273 HRUs.
HRU
Soil map
Landuse map
DEM
Calibration and validation* Runoff calibration (1990-1995)
NSE= 0,87
R2= 0,89
PBIAS= -1,2%
* Runoff validation (2001-2008)
NSE= 0,81
R2= 0,89
PBIAS= -11,3%
Simulation results of monthly calibration and validation indicated good efficiency
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Rainfall(mm)
Discharge(m3/s)
Rainfall Observed Simulated
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Rainfall(mm)
Discharge(m3/s)
Rainfall Observed Simulated
Correlation: Runoff / Sediment
* Simulation of sediment yield was carried out for the period
2001-2008.
To assess the ability of the model in predicting sediment loads,
a comparison between simulated runoff and simulated
sediment yield was attempted:
Good correlation between runoff and sediment yield.
The highest monthly erosion rate (2,48 t/ha) occured in
January 2003, when the peak rate of runoff reached 12,87
m3/s.
y = 10,10x + 11,33
R² = 0,702
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sediment(kg/s)
Discharge (m3/s)
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Sedimentyield(t/ha)
Runoff(m3/s)
Runoff
Sediment yield
Spatial Variability of Erosion
Sub-catchments SC2, SC4, SC5 et SC27, which cover
10 % of the total area, contributed with the maximum
amount of runoff and sediment loss, about 70% of the
total sediment production.
The rest of the catchment which represent 90% of the
total area contributed only with 29.1 % of the total soil
loss.
- SC2, SC4, SC5, et SC27 high erosion risk
- SC3, SC7 et SC8 tolerable erosion risk
- Others low erosion risk
The estimated annual sediment yield is 5,05
t/ha/an.
Spatial distribution of soil erosion in the Sarrath
catchment is:
• 82,1 % : low erosion risk (0-5 t/ha)
• 7,9 % : tolerable erosion risk (5-10 t/ha)
• 3.0 % : high erosion risk (10-20 t/ha)
• 7.0 % : very high erosion risk (>20 t/ha)
0
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<5 5-10 10-20 20-25 >25
Area(%)
Soil erosion (t/ha)
Identification of areas with high erosion risk
[1] Arnold, J.G., Srinivasan, R., Muttiah, R.S., and Williams, J.R. 1998. «Large area hydrologic modelling and assessment. Part I: Model development» Journal of the American
Water Resources Association 34 (1), 73–89.
[2] Direction Générale des Aménagements et Conservation des Terres Agricoles (DG/ACTA) (1993) Stratégie nationale pour la conservation des eaux et des sols. Publication of
the General Directory for Water and Soil Conservation. Tunisian Ministry of Agriculture and Hydraulic Resources
[3] Neitsch SL, Arnold JG, Kiniry JR, Srinivasan R, Williams JR (2002a) Soil and water assessment tool, theoretical documentation. Blackland Research Center, USDA-ARS,
Temple
ACKNOWLEDGMENTS
The authors wish to express their gratitude to the Soil and Water Land Agriculture Authority for supplying land use database.
Water erosion is a major problem for semi-arid countries, due to the complex interplay of many factors such as climate, land use,
topography and human activities. In Tunisia 20% of soils are threatened by erosion. Thus, the quantification of soil loss is one of
the major challenges in natural resource and environmental planning. Hydrological models are suitable and to a greater extent
effective to predict soil loss.
This work aims to apply the SWAT model to simulate runoff and soil loss and to identify areas of sediment inputs in the catchment
of Wadi Sarrath (1491 km2), located in the northwest of Tunisia, in order to define intervention priorities and to target anti-erosion
activities.
Simulation results show that a good correlation between runoff and sediment, made in the high-risk erosion areas corresponding
to sub basins where runoff is quite important. Based on the rate of erosion and the prescribed annual permissible limit of soil loss,
only 10% of the total catchment area suffers from a high to very high erosion rates and contributes with 70% of sediment
production rates during the simulation period (2001-2008). However, the rest of the basin, which is under low to tolerable erosion
risk, contributes by only 30% to total soil loss. It is therefore necessary and strategic to classify the critical regions and to give
them priority for treatment with the appropriate water and soil conservation measures.
To conclude, SWAT model was found as effective tool to achieve a simple and fast way, a general diagnosis of the potential risk of
water erosion throughout the Sarrath catchment.
Presenter: Sihem Benabdallah; sihem.benabdallah@planet.tn
The spatial distribution of sediment yield across the catchment indicated that areas with high erosion risk are dominated by
cultivated land and uncovered land.
Finally, the study demonstrates that the SWAT model provides a useful tool to predict surface runoff generation patterns and soil
erosion hazard over semi-arid areas and can successfully be used for prioritization of the critical sub-catchments in order to
develop multi-year management plan to reduce runoff and sediment loss in the Sarrath catchment.