4. Conclusion and recommendations
Runoff and suspended sediment concentration (SSC) were monitored at the outlets of three main sub-watersheds of Koga catchment, North-Western Ethiopian highlands during 2016-2017. The WATEM/SEDEM model was calibrated to quantify the effect of SWC measures on soil erosion and sediment yield. WATEM/SEDEM generally underestimated the effect of SWC measures in controlling soil erosion and sediment yield in all sub-watersheds. Soil erosion and sediment yield values were highly variable across study sub-watersheds. This is most likely due to the effect of SWC measures as well as biophysical characteristics such as topography, land use and soil types. The integrated use of bund structures, contour cultivation, strip cropping and grass strips had the highest impact on controlling soil erosion and sediment yield. Integration of bunds with one or more agronomic and vegetative measures shows a higher effect on soil erosion and sediment yield in the study sub-watersheds. This emphasises that the use of bund structures alone is not sufficient as conservation strategy to control runoff and soil erosion in NW Ethiopian highlands. This study demonstrates that the harmful effects of soil erosion remain significant in Koga catchment and the Koga reservoir at the outlet of Koga river is highly affected by sedimentation. The calibration of WATEM/SEDEM at sub-watershed level has provided good model performance for simulated erosion and sediment yields. Therefore, WATEM/SEDEM adequately represents the underlying erosion and sedimentary processes and can be further used to evaluate the impacts of land use change and existing or new SWC scenarios. The model output results emphasises the importance of integrated use of conservation strategies to reduce soil erosion and sediment delivery. Using a spatially explicit modelling approach, as done in this study, increases insight in the spatially explicit effect of various measures on erosion rates, as opposed to measurement of sediment yield at the outlet of a catchment.