Identification of Erosion Hotspot Area using GIS and MCE Technique for Koga Watershed in the Upper Blue Nile Basin, Ethiopia

Soil erosion is a serious threat in Ethiopian highlands. Continuous land degradation resulted in loss of fertile top soil leading to low agricultural productivity. In addition, excessive soil erosion from Koga Watershed in upper catchment to an artificial reservoir (Koga Dam reservoir) is substantially reducing its service life. Community participatory based effective watershed management strategies may have tremendous potential to reduce soil erosion. However, it is not practical to implement management interventions in the entire basin. This study aims to identify and map erosion hotspot areas in Koga Watershed to assist local government decision towards implementing watershed management strategies. Multi Criteria Evaluation (MCE) technique was integrated with Geographic Information System (GIS). For these analysis four major factors: Topography, soil, land use and potential location of gullies were considered. Each of these was processed and analyzed for its potential contribution to erosion on a pixel by pixel basis. The factors were weighted using pair-wise comparison matrix and weights were combined using Weighted Overlay Tool of ArcGIS Spatial Analyst Toolbox to obtain the final erosion hotspot map. The results found that 2% (440 ha) to be highly sensitive, 43% (9,460 ha) to be moderately sensitive, 16% (3,520 ha) to be marginally sensitive and 32% (7,040 ha) currently not sensitive. The remaining 7% of the watershed area (22,000 ha) was constraint to erosion. The lowland area near the dam was found to be found most sensitive for erosion and sedimentation.