Event-scale runoff and sediment responses to soil and water conservation measures in semi-arid catchments

Erratic rainfall conditions combined with the sloping terrains and anthropogenic factors causes severe soil erosion rates in the semi-arid highlands of Northern Ethiopia. High soil erosion rates are known to cause on-site and off-site socioeconomic and ecological disturbances. Reservoir sedimentation is one of the well-known negative off-site effects of soil erosion. Single rainfall events contribute disproportionally to the annual sediment yield in semi-arid catchments. Event based hydrological analysis helps to understand the suspended sediment and discharge contribution of single events. We applied physically based hydrological model, OpenLISEM in small, paired catchments in the Northern Ethiopian highlands. The objectives of the study were a) Identify model sensitive parameters in the paired catchments and b) Evaluate the effects of soil and water conservation measures on sediment yield reduction at an event scale. Initial soil moisture content was found to be the most sensitive parameter affecting both peak and total discharges. The suspended sediment yield at the outlets of both catchments was also sensitive to changes in Manning’s roughness coefficient. OpenLISEM performed satisfactorily (NSE > 0.5) for most of the calibrated events. The model fails to adequately capture peak discharge and discharge hydrograph in events characterized by multiple peaks and smaller precipitation amounts. OpenLISEM successfully simulated the total suspended sediment yield of an event. Suspended sediment yield was found to be influenced by a combination of hydrological and sedimentological model parameters. The combined implementation of stone bunds on hillslopes and check dams across channels resulted in a simulated sediment yield reduction of 36-74 % at the outlets of the paired catchments. The model simulation results provide valuable insight for the implementation of different soil and water conservation management practices to reduce catchment sediment yield and reservoir sedimentation, contributing to the long-term sustainability of reservoir-based irrigation schemes in the semi-arid highlands of northern Ethiopia.