Investigation of runoff and sediment transport processes in the hillslope headwaters of northern Ethiopia

Abstract

Soil erosion and sediment transport pose significant threats to soil fertility, agricultural productivity, and reservoir sustainability in the Ethiopian highlands. This study investigated rainfall-runoff and sediment transport processes on the steep hillslope of the Midmar watershed in northern Ethiopia. Field measurements of discharge and suspended sediment were conducted during the 2018 and 2019 rainy seasons at several subcatchments. The Hydrologic Engineering Center-Hydrologic Modeling System (HEC-HMS) was used to simulate the hydrological and sediment responses. The model utilized the soil conservation service (SCS) curve number method for runoff volume, Clarks unit hydrograph for peak discharge, kinematic wave routing for flow routing, and the modified universal soil loss equation (MUSLE) for sediment yield estimations. The model was calibrated via 8 events (NSE/PVE: 0.87/–4.49% at Maytsedo; 0.7/ + 0.12% at Midmar) and validated via 4 events (NSE/PVE: 0.80/–9.9% at Maytsedo; 0.8/ + 0.14% at Midmar). To quantify spatial runoff dynamics and identify critical erosion hotspots across the subcatchments, the estimated average peak discharges were 8.2 m 3 /s at Maytsedo and 43.9 m 3 /s at Midmar. The localized severity of soil degradation was evident from the substantial sediment yield, with average simulated suspended sediment concentrations of 29.4 g/l, 17.7 g/l, 32.4 g/l, and 21.6 g/l for Maytsedo, Mariamshewito, Embatabeb, and Embagesesew microcatchments, respectively. The results demonstrate that the HEC-HMS model is a robust and effective tool for predicting runoff and sediment yield in steep hillslope, data-scarce watersheds. Despite some overestimation of late-stage sediment concentrations, the model provided valuable insights for prioritizing soil and water conservation intervention sites to mitigate land degradation and reservoir sedimentation.