Spatial Erosion Hazard Assessment for Conservation Planning in the Gibe-III Dam Catchment, Southwest Ethiopia

Abstract

Soil erosion is one of the biggest global environmental problems resulting in both on-site and off-site effects. Sedimentation as off-site effect is considered to be critical in reservoirs and water bodies, both in reducing capacity and loss of value. Soil erosion hazard was assessed using adapted USLE (Universal Soil Loss Equation) model to spatially identify and prioritize areas for conservation planning in Gibe –III dam catchment, southwest Ethiopia. Rainfall, soil map, a 30x30 m DEM, and satellite images were used to determine the USLE variables. Individual Geographic Information System (GIS) files (thematic layers) were built for each USLE factor and these layers were spatially overlaid and combined by a cell by cell-grid modeling procedure to predict the mean annual soil loss and develop erosion hazard intensity map of the study area. The mean annual soil loss rate ranges from 0 - 51.57 tons ha-1yr1 with an estimated average of 7.47 tons ha-1yr-1. Based on the level of soil erosion rates, the study area was classified into five priority categories for conservation interventions. The analysis showed that 53% of the study area suffers from a moderate to very high erosion risk (>6.25 tons ha-1yr-1). These areas are located around the steeper slope banks of Gojeb and Gibe Rivers. However, Woreda level prioritization indicate that Menjiwo, Merka Gena, Loma Bosa, Esara Tocha, Gimbo, and Kindo Koysha Woredas were the largest sediment producer Woredas; together they cover 43% of the study area but they produce 53% of the total annual soil loss. Moreover, the total potential soil movement in the study area was estimated 9,700,823 tons yr-1 from 1,298,402 ha. Thus, any intervention designed to reduce the sediment load reaching to Gibe –III dam should start from these prioritized Woredas.