Modeling discharge and sediment concentrations after landscape interventions in a humid monsoon climate: The Anjeni watershed in the highlands of Ethiopia

Abstract

Increasing population and intensification of agriculture increase erosion rates and often result in severe land degradation and sedimentation of reservoirs. Finding effective management practices to counteract the increasing sediment load is becoming increasingly urgent especially in the Ethiopian highlands where the construction of the hydroelectric Grand Renaissance Dam on the Blue Nile is underway. In this paper, we examine the results of nine years of a watershed experiment in which discharge and sediment losses were observed in the 113 ha Anjeni watershed of the Blue Nile Basin. The study period encompasses conditions before, during, and after the installation of graded Fanya-Juu (“throw uphill” bunds) soil and water conservation practices (SWCP) which had the ultimate goal of creating terraces. We use a saturation-excess runoff model named the Parameter Efficient Distributed (PED) model as a mathematical construct to relate rainfall with discharge and sediment losses at the outlet. The PED model is based on landscape units in which the excess rainfall becomes direct runoff or infiltrates based on topographic position or hardpan characteristics. Deviations in this rainfall-discharge-sediment loss relationship are ascribed to the changes in infiltration characteristics caused by SWCPs on the hillslopes. With this technique we found that in the Anjeni basin the Fanya-Juu SWCP's are only effective in increasing the infiltration and thereby reducing the direct runoff and sediment concentrations in the first 5 years. At the end of the 9 year observation period the direct runoff and sediment concentrations were barely reduced compared to the levels before SWCP were installed. In addition, we found that the model structure based on landscape units was able to represent the varying runoff and erosion processes during the nine years well by varying mainly the portion of degraded land (and thereby representing the effectiveness of the Fanya-Juus to reduce runoff by increasing infiltration).