Erosion hotspot identification in the sub-humid Ethiopian highlands

Abstract

Soil degradation and associated soil erosion is a daunting socio-economic challenge for smallholder farmers that cause environmental harm in the developing regions of the world. This is especially true in the Ethiopian highlands where soil degradation and soil erosion significantly impact the productivity of land and water. In this study, GIS-based classic RUSLE model supported with multi-criteria evaluation (MCE) analysis was used to map erosion hotspot areas in the sub-humid Ethiopian highlands. RUSLE model results indicated that annual average soil loss in the study area reaches up to 288 t ha−1. In the majority (83%) of the study area predicted soil loss ranges between 0 and 40 t ha−1, whereas 13% of the study area has soil loss of 40–100 t ha−1 and the remaining 4% is in excess of 100 t ha−1. Seventy-two percent of upper Gilgel Abbay watershed was found moderately sensitive to erosion. The classic RUSLE predicted lower erosion rates mainly in the bottomlands because does not consider soil erosion by gullies. On the other hand, the MCE analysis integrated with field survey indicated that the potential location of gullies is the saturated bottomlands. The locations of bottomland gullies also coincided with important hydrologic parameters such as higher soil wetness and stream power index values. The implication is that in addition to RUSLE criteria such as gullies, topographic wetness index (TWI) and stream power index (SPI) should be considered in erosion risk mapping studies in the sub-humid Ethiopian highlands, where soil saturation plays an important role in the formation and expansion of gullies.