Spatial and temporal patterns of soil erosion in the semi-humid Ethiopian highlands: A case study of Debre Mawi watershed

Abstract

The effectiveness of water management interventions is hampered by the lack of knowledge about the spatial distribution of runoff and associated soil loss. A study was conducted in the 95-ha Debre Mawi watershed in the Upper Blue Nile basin to understand where and when runoff and erosion takes place on the landscape. During the rainy phase of the 2010 and 2011 monsoons, storm runoff and sediment concentrations were measured from five sub-watersheds. In addition, perched groundwater tables, infiltration rates, and rill erosion from agricultural fields were measured. The results show that saturation excess runoff was the main runoff mechanism because the median infiltration rate was only exceeded 3∈% of the time. Early during the rainy period, runoff produced from upslope shallow soils infiltrated downslope and did not reach the outlet. At the end of July, the bottomlands became saturated, and the runoff coefficient at the outlet became greater than upslope areas. Sediment concentrations were greater in the beginning of the rainy monsoon phase when the rill network had developed on the plowed land and it becomes lowest at the end of rainy phase when rill formation stopped. At all times, the sediment concentration at the outlet was greater than upslope because both runoff losses were greater in the saturated bottomlands and loose unstructured soil was available from newly forming gullies. This research indicates that watershed management interventions to control erosion should be implemented in areas which produce the most runoff such as those shallow upland soils and bottomlands near the river that become saturated by the end of the rainy phase. In addition, for proper planning and management, runoff and erosion models should capture these dynamics.