Abstract:
Soil erosion is a worldwide problem that causes land degradation and long-lasting
challenge in Ethiopia. Sediment yield of a watershed is influenced by the watershed
characteristics (LULC, soil type and slope), climatic conditions and surface runoff. Middle
Awata watershed is one of the Ethiopian watersheds which is susceptible for the problem of
soil erosion in the river system. The sediment yield rate of the watershed and the spatial
distribution of sediment yield at the subbasin level are necessary in order to create
alternative watershed management methods. This study aimed at modelling of sediment yield
and assessing management practices using Soil and Water Assessment Tool (SWAT) model
in Middle Awata Watershed. Spatial data and weather data were major inputs for the model.
The land use land cover map was created using ERDAS image classification of satellite
images. The model simulation was done by dividing the watershed of total area 1912Km2
into 37 sub basins and 294 Hydrologic Response Units (HRUs) for the period of 31years
(1988-2018). The model performance evaluation was carried out using Sequential
Uncertainty Fitting (SUFI-2) algorithm in SWAT-Calibration Uncertainty Programs
(SWAT-CUP) on monthly basis. The model was calibrated for the period of 18 years (1988-
2005) including the first two years as warm-up period and validated for 8 years (2006-2013)
for both stream flow and sediment yield by using more sensitive parameters. The model
performance was evaluated by coefficient of determination (R2
), Nash Satcliff Efficiency
(NSE), Root Mean Square Error Standard Deviation Ratio (RSR) and Percent bias (PBIAS)
to be quit acceptable for both stream flow and sediment yield estimation in the watershed. It
has resulted in (R2=0.76, NSE=0.75, RSR=0.51 and PBIAS= 5.6%) for calibration and
(R2=0.75, NSE=0.74, RSR=0.5 and PBIAS=2.7%) for validation of stream flow. Whereas
(R2=0.69, ENS=0.66, RSR=0.58 and PBIAS=3.7%) for calibration and (R2=0.67,
ENS=0.65, RSR=0.61 and PBIAS=5.6%) for validation of sediment yield. The simulated
annual average sediment yield was 34.543×103
ton/yr at outlet of the watershed. The
determination of spatial distribution of sediment yield was identified the first 12 upstream
sub watersheds to be soil erosion prone area. Evaluation of the selected BMPs (Filter strip,
Contouring, Grassed waterway and Terracing) concluded that application of parallel
terracing is the most recommended soil erosion reduction option with about 47.05% of
reduction for almost all the critical sub basins of the watershed.
