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Comparison of SWAT and GeoWEPP model in predicting the impact of stone bunds on runoff and erosion processes in the Northern Ethiopian Highlands

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Soil degradation is a major issue in the Ethiopian highlands which are most suitable for agriculture and, therefore, support a major part of human population and livestock. Heavy rainstorms during the rainy season in summer create soil erosion and runoff processes which affect soil fertility and food security. In the last years programs for soil conservation and afforestation were initiated by the Ethiopian government to reduce erosion risk, retain water in the landscape and improve crop yields. The study was done in two adjacent watersheds in the Northwestern highlands of Ethiopia. One of the watersheds is developed by soil and water conservation structures (stone bunds) in 2011 and the other one is without soil and water conservation structures. Spatial distribution of soil textures and other soil properties were determined in the field and in the laboratory and a soil map was derived. A land use map was evaluated based on satellite images and ground truth data. A Digital Elevation Model of the watershed was developed based on conventional terrestrial surveying using a total station. At the outlet of the watersheds weirs with cameras were installed to measure surface runoff. During each event runoff samples were collected and sediment concentration was analyzed. The objective of this study is 1) to assess the impact of stone bunds on runoff and erosion processes by using simulation models, and 2) to compare the performance of two soil erosion models in predicting the measurements. The selected erosion models were the Soil and Water Assessment Tool (SWAT) and the Geospatial Interface to the Water Erosion Prediction Project (GeoWEPP). The simulation models were calibrated/verified for the 2011-2013 periods and validated with 2014-2015 data. Results of this comparison will be presented.
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University of Natural Resources and Life Sciences, Vienna
Department of Water, Atmosphere and Environment
.
Introduction
In the Ethiopian Highlands deforestation increased the vulnerability of the soils
due to rainfall driven soil erosion (Addis et at. 2016; Nyssen et al. 2000). To
tackle this soil erosion problem soil and water conservation strategies are
considered were constructed.
Two neighboring subwatersheds, Abakaloye and Ayaye, were selected to
investigate the impact of soil and water conservation structures on soil erosion
processes. Soil and water conservation structures were applied in the Ayaye
subwatershed while Abakaloye subwatershed was used as a reference without
soil and water conservation structures. Soil and Water Assessment Tool (SWAT)
(Arnold et al., 1998)and GeoWEPP (Renschler, 2003) models were used.
Materials and Methods
Abakaloye and Ayaye subwatersheds are located in Gumara-Maksegnit watershed in
the Lake Tana basin, Ethiopia.
located between 12°2524’’ and 12°2554’’ latitude and between 37°3456’’ and
37°3538’’ longitude (Figure 1). Altitude ranges from 1998 to 2150 m asl.
Mean annual rainfall is 1170 mm of which more than 90 %of the rainfall occurs
during the rainy season (June to August).
The average monthly maximum and minimum temperatures are 28.5 oC and 13.3
oC respectively.
Comparison of SWAT and GeoWEPP model in predicting the impact of stone
bunds on runoff and erosion processes in the Northern Ethiopian Highlands
N.D. Melaku1,2*, J.Flagler5, C.S. Renschler5, H. Holzmann1, C. Zucca3, F. Ziadat4, S. Strohmeier3 H. K. Addis2, W.
Bayu and A. Klik1
(1) University of Natural Resources and Life Sciences, Vienna, Austria (2) Amhara Agricultural Research Institute,
Gondar Agricultural Research center, Gondar, Ethiopia (nigus.melaku@students.boku.ac.at), (3) International
Center for Agricultural Research in the Dry Areas (ICARDA), Amman, Jordan, (4) Food and Agricultural
Organization of the United Nations (FAO), Rome, Italy, (5) University at Buffalo, NY, USA
For the Abakaloye and Ayaye subwatersheds, separate SWAT and GeoWEPP
project were set for daily runoff and sediment yield. Sequential Uncertainty
Fitting-2 (SUFI-2), a SWAT-CUP2012 program was used to optimize the
parameters of the SWAT using daily observed runoff and sediment yield data.
DEM, climate data, landuse map, soil map and management data were used as
input files. Runoff and sediment yield were monitored at the outlets of both
subwatersheds (Fig. 4).
Fig.i1 Map of the study area
Results
Table 1 Daily runoff
Fig. 6 Observed and simulated daily runoff for calibration (a) and validation (b) period at the
outlet of Abakaloye subwatershed
Fig. 3 Slope classes, soil map, land use classes and sub basins
Fig. 2 Treated Ayaye subwatershed with field experiments
Fig. 4 Runoff and sediment monitoring
Conclusions
ArcSWAT and GeoWEPP were used to simulate runoff and sediment yield from
Ayaye and Abakaloye subwatersheds. Both ArcSWAT and GeoWEPP provided
satisfactory prediction for runoff while sediment prediction was low in both models.
Acknowlegements
The Austrian Development Agency (ADA), International Center for Agricultural Research in Dry Areas
(ICARDA) and Organization for Economic Co-operation and Development (OECD)
Fig. 7 comparison of observed and SWAT simulated daily sediment yield
Table 1. Model comparison
Subwatersheds
Sediment yield (t/ha)
Model
Simulated
Observed
ArcSWAT
Ayaye
21.8
33.5
40.7
68.7
49.3
Abakaloye
62.6
GeoWEPP
Ayaye
49.3
Abakaloye
62.6
1
Fig. 8 offsite (left) and Onsite (right) soil loss at both Subwatershed
References
Addis H K, Strohmeier S, Ziadat F, Melaku ND, Klik A. 2016. Modeling streamflow and
sediment using SWAT in the Ethiopian Highlands. Int J Agric&BiolEng, 2016; 9(5): 51-66.
Arnold JG, Srinivasan R, Muttiah RS, Williams JR., 1998. Large area hydrologic modeling and
assessment part I:Model development. Journal of the American Water
ResourcesAssociation, 1998;34(1): 7389.
Renschler, C. S. 2003. Designing geo-spatial interfaces to scale process models: The GeoWEPP
approach.Hydrol. Proc.17(5): 1005-1017.
Fig. 9 comparison of observed and WEPP simulated daily runoff and
daily sediment yield
Calibration
y = 0.687x + 9.990
R² = 0.57
NSE = 0.39
Validation
y = 0.871x + 8.402
R² = 0.698
NSE = 0.22
Observed sediment (t ha-1)
Simulated sediment (t ha-1)
Calibration
y = 0.794x + 10.70
R² = 0.63
NSE = 0.43
Validation
y = 0.703x + 7.135
R² = 0.542
NSE = 0.31
0
15
30
45
60
75
90
015 30 45 60 75 90
Observed sediment (t ha-1)
Simulated Sediment yield (t ha-1)
Subwatersheds
Calibration
Validation
R2
NSE
R2
NSE
Ayaye (Treated)
0.78
0.67
0.51
0.57
Abakaloye (Untreated)
0.71
0.58
0.59
0.52
1
Fig.5 Observed and simulated daily runoff for calibration (a) and validation (b) period at the outlet of
Ayaye subwatershed
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Full-text available
Soil erosion valuation at a spatial scale is crucial for assessing natural resource quality in a farming country like Ethiopia. The study’s goal was to determine the rate of soil erosion in the Megech-Dirma catchment in Northwest Ethiopia using the Revised Universal Soil Loss Equation model aggregation with Geographic Information System and Remote Sensing. Sediment yield and transport were also estimated using sediment delivery ratio. Revised Universal Soil Loss Equation model data inputs included precipitation data for the R value, soil data for the K value, land cover data from satellite images for the C and P value, and topographical data from a Digital Elevation Model for the LS component. It was completed using the ArcGIS 10.4 software. The mean annual soil loss is 110.60 t ha−1 yr−1. Each year, a total of 8499.74 t ha−1 yr−1 of soil eroded and on average resulting in 1605.30 t/km2/yr, sediment material has been transported to the stream channels and deposited with a sediment delivery ratio of 1.87. The strength of soil erosion in the area is divided into six categories. The erosion rate classes were 46.38 percent (0–12 t ha−1 yr−1) low, 13.63 percent (12–20 ha−1 yr−1) moderate, 9.22 percent (20–35 ha−1 yr−1) high, 12.30 percent (35–50 ha−1 yr−1) very high, 7.20 percent (50 up to 100 ha−1 yr−1) severe, and 11.27 percent (>100 ha−1 yr−1) very severe erosion. According to erosion severity, 46.38 percent of the watershed is at risk of low erosion, while 11.27 percent is at risk of extremely severe erosion. The north and northeastern sections of the watershed have a moderate to extremely severe erosion risk due to steep slopes, high rainfall, and weak conservation measures. The severely eroded parts of the plateau and steep portions are proposed to be covered by plantation, stone bund, and check dam constructions.
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