Sediment Yield Estimation and Identifying the Soil Erosion Prone Areas in Koysha Dam Watershed of Omo-Gibe Basin

Abstract

Sedimentation is huge problems that have threatened many reservoirs in Ethiopia. Koysha dam watershed is conditioned densely populated with intensive traditional agricultural practice and low soil and water conservation practice in left side. The left side of watershed is gentle slope terrain topography.  This study has been conducted to estimate mean annual sediment yield of watershed, to identify and prioritize the most sensitive sub-watersheds with the help of Arc SWAT 2012 for planning reservoir sedimentation mitigating strategies at the watershed level. Based on a digital elevation model the catchment was divided in to 23 sub-basins using the dam axis as the main outlet. The current LULC map was downloaded from satellite. The pre-processing and both unsupervised and supervised classification was conducted in ERDAS Imagine 2015. By overlaying land use, soil and slope maps, sub-watersheds were further divided in to 241 HRUs. Arc SWAT Model was calibrated and validated using SUFI-2 SWAT-CUP optimization algorithms for stream flow rate and sediment yield data observed at dam axis, which transposed from other gauging stations. The model performance was evaluated by using both stream flow and sediment yield data. The study has revealed that Koysha dam residual watershed has mean annual sediment yield of 7.22 t/ha/year. Out of the 23 sub-watersheds, seven sub-basins produce above average sediment yields ranging from 8.79 -56.70 t/ha/yr, while the others yield below the average value. Out of the 23 sub-basins two sub-basins were prioritized for immediate implementation of watershed management interventions.  The maximum sediment outflow of these two sub-basin are 26.33 and 56.7 t/ha/year and are characterized dominantly by cultivated land with average land slope of 31.45% and 28.74% respectively and with the dominant soil type of Humic Nitisols. The soil erosion was sensitive to cultivation land use and terrain steepness.

Keywords: Koysha dam watershed, Sediment yield, SWAT, Prioritization

DOI: 10.7176/CER/14-2-01

Publication date: April 30^th 2022