Modeling runoff, sediment yield and conservation measures using SWAT in Ajima catchment, Upper Blue Nile Basin, Ethiopia

Runoff induced erosion and thereby sediment yield present significant economic and environmental challenge. However, there are very limited research available addressing surface runoff and sediment yield in Ajima catchment located in the Upper Blue Nile basin in Ethiopia. This study aims to model runoff, sediment yield and conservation measures in Ajima catchment using the SWAT hydrological model. The essential model parameters for surface runoff and sediment yield were identified using model sensitivity analysis. Monthly streamflow and sediment yield data for the period 1990-2013 obtained from MoWE were used to calibrate and validate the model. The model's performance was assessed using statistical measures such as the coefficient of determination (R²) and Nash-Sutcliffe Efficiency (NSE), percent bias (PBIAS) and Kling-Gupta Efficiency (KGE). Sediment yield reduction strategy was proposed in the model by changing topographic, physiographic, and land use characteristics to recommend soil and conservation practices suitable for the area which are established as scenarios.The results showed that the Soil Evaporation Compensation Factor (ESCO) for streamflow and the USLE management parameter (USLE-MP) for sediment yield were parameters most sensitive to the model. During the model's calibration and validation, the model showed significant correlations between computed and observed data for streamflow (R² = 0.69 and 0.77; NSE = 0.76 and 0.86; PBIAS = 23.85 and - 11.84; KGE = 0.73 and 0.83) and sediment yield (R² = 0.84 and 0.87; NSE = 0.76 and 0.79; PBIAS = 34.88 and 26.00; KGE = 0.55 and 0.71), respectively. The catchment's annual estimated average runoff is approximately 748 mm, and average yearly sediment yield is 112.70 tons/ha. Terracing decreased sediment yield from 112.70 tons/ha to 48.10 tons/ha, and residue management decreased it to 107.17 tons/ha. Therefore, the most successful conservation technique is found to be terracing, which lowers sediment production by over 57%. This study provides valuable information that is helpful for ongoing efforts to regulate erosion and sedimentation in the study region. We suggest practices of implementing terracing for areas highly prone to erosion. In regions with moderate to low erosion risk, techniques such as contour farming, strip cropping, and residue management are recommended as cost-effective strategies to reduce sediment production and soil degradation.