Assessment of the Impact of Land Use Changes and Conservation Practices on Soil Loss and Sediment Yield using GeoWEPP Model; A Case Study of Mwogo Sub-catchment, Rwanda
MUGIRANEZA, Honorine Cyuzuzo
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Surface water bodies are essential resources that provide water for different activities. However, Soil erosion and runoff generated as a result of human activities contribute to considerable amount of sediment yield that is deposited in receiving water bodies. Sediment deposited in surface water bodies threatens the reservoir capacities which impacts negatively on their useful lifespan. Nyabarongo reservoir catchment in Rwanda has continued to experience increased mining and intensive agricultural activities that are carried out on the steep slopes of the mountainous areas. This study therefore assessed the impact of land use changes and conservation practices on soil loss and sediment yield using GeoWEPP model in Mwogo sub-catchment of Nyabarongo reservoir catchment. Sensitivity analysis, calibration and validation of the model was undertaken. The coefficient of determination (R2), Nash-Sutcliffe efficiency (NSE) and percent bias (PBIAS) were used to evaluate the GeoWEPP model performance. The calibrated and validated model was used to simulate runoff depth, soil loss and sediment yield over a period of 20 years. To assess effectiveness of conservation practices and land use changes in reducing soil loss and sediment yield, no tillage management, grassland and forestland Scenarios were adopted for all agricultural land use as well as on 25% and 50% of most critical hillslopes. It was observed that runoff was sensitive to effective hydraulic conductivity only while sediment yield was sensitive to critical shear, effective hydraulic conductivity, rill and interrill erodibility. The model evaluation showed a satisfactory performance with R2 of 0.75, NSE of 0.65 and PBIAS of 3.75 during calibration, and 0.88, 0.52 and 7.05 for validation respectively. The average annual runoff depth, soil loss and sediment yield were predicted as 418.4 mm, 194.6 ton/ha and 25.4 ton/ha, respectively under the Base Scenario. Compared to the Base Scenario, the simulated results under no tillage, grassland and forestland Scenarios showed a significant reduction in runoff depth, soil loss and sediment yield. Therefore, this study concluded that adopting a no tillage management and converting most critical hillslopes to grassland and forestland have an impact on reducing runoff depth, soil loss and sediment yield of Mwogo sub-catchment. This would further lead to reduction of sediment load into the reservoir. The results from this study are very useful to water resource managers in making informed decisions for sustainable catchment management.