Measuring and modelling the impacts of soil and water conservation measures on soil erosion and sediment yield in North-Western Ethiopian highlands

Abstract

Although large-scale implementation of SWC measures has been used to reduce soil loss and sedimentation in Ethiopian highlands, no method exists to evaluate how implementation of such measures affect erosion and sedimentary processes. In this study we measured and simulated the impacts of various SWC measures on soil loss and sediment yield using spatially distributed WATEM/SEDEM model calibrated at three sub-watersheds. The methods used comprised of field sampling and monitoring to characterize erosion and sediment yields and GIS analysis to calculate various model input parameters. The measurement and model simulation result showed all SWC scenarios reduced soil erosion and sediment yield and bund structures have reduced erosion by more than 57 to 65%. The integrated use of bund structures, contour cultivation, strip cropping and grass strips (scenario IV), sediment yield was reduced from 44.5 to 8.6 t ha-1 y-1, 30.7 to 5.3 t ha-1 y-1 and 36.6 to 6.3 t ha-1 y-1 in the upper, middle and lower part of Koga catchment respectively. Bund structures and grass strips had the highest specific contribution in controlling soil erosion and sediment yield in both study sub-watersheds. The measured and simulated erosion and sediment yield values were relatively lower at the middle of Koga for scenario I (present-day situation). This might be due to the lower transport capacity and lower sediment connectivity as a result of larger coverage of bunds and subordinate conservation measures such as: traditional diches and diversion channels in Debreyakob. This emphasises the importance of integrated use of conservation strategy to reduce soil erosion and sediment delivery. The calibration of WATEM/SEDEM at sub-watershed level has provided good model performance for measured and simulated erosion and sediment yields. Therefore, WATEM/SEDEM representing the underlying erosion and sedimentary processes can further be used to evaluate the impacts of land use and existing or new SWC scenarios.