An efficient semi-distributed hillslope erosion model for the sub humid Ethiopian Highlands

Abstract. During the last two decades, saturated excess runoff has become accepted as the main source for overland flow in humid regions. Erosion modeling has generally not kept up with this new reality and predictions are often not based on landscape topographic position, which is a main variable in saturation excess runoff. In addition, predicting sediment loss in Africa has been hampered by using models that have been developed in western countries and do not perform as well in the monsoon climate prevailing in most of the continent. The objective of this paper is to develop a simple erosion model that can be used in the Ethiopian highlands in Africa. We base our sediment prediction on a simple distributed saturated excess hydrology model that predicts surface runoff from severely degraded lands and from bottom lands that become saturated during the rainy season and estimates interflow and base flow from the remaining portions of the landscape. By developing an equation that relates surface runoff to sediment concentration generated from runoff source areas, assuming that base flow and interflow are sediment free, we were able to predict daily sediment concentrations from the Anjeni Watershed and Blue Nile Basin with a Nash Sutcliffe efficiency ranging from 0.64 to 0.77 using only two calibrated sediment parameters. Anjeni is a 113 ha watershed in the 17.4 million ha Blue Nile Basin in the Ethiopian Highlands. The daily flows were predicted with Nash Sutcliffe efficiency values ranging from 0.80 to 0.93 if degraded areas were assumed the major sediment source areas and covered 14% of the Anjeni watershed and 20% of the Blue Nile basin. The analysis suggests that identifying the runoff source areas and predicting the surface runoff correctly is an important step in predicting the sediment concentration.