Soil Erosion and Sediment Export in Berta Watershed Using GIS and USLE Model in Central Rift Valley Basin, Ethiopia: Implications for Sustainable Natural Resources Management

Abstract

Soil erosion and sedimentation of lake due to runoff is a major problem in Southern Ethiopia. Berta watershed is one of the watersheds in Rift valley Basin, contributing runoff to small Abaya (Tuffa) lake. However, no studies have been conducted on soil erosion and sediment yield from the entire watershed. Therefore, an updated Universal Soil Loss Model and Geographic Information System have been used to start the current study, which will estimate the rate of soil erosion  and sediment output. Factors such as rainfall, land use, soil texture, and slope information were collected from different data sources. Long year monthly rainfall data have been collected from meteorological stations; slope of the watershed have been generated from Digital Elevation Model; land use and soil types have been collected from Food and Agricultural Organization adapted to Ethiopian condition. The ratio between the gross erosion and sediment delivery has been used to calculate the sediment yields. The result showed the watershed average annual soil losses ranged from 0 to 878.7 ton ha-1 yr-1 with average soil loss of 27 ton ha-1 yr-1. Of this, 9.6% of the sub-watershed (SWS3) areas have been identified as very high, 66.8% as high(SWS4&5), 12.6% as moderate (SWS1), and 11% as low (SWS2) soil erosion risk classes. From the total 81,864 ton/year gross soil erosion, 16,472.03 ton yr-1 sediment yield has been estimated at the watershed outlet. The very high, high, moderate, and low soil erosion risk classes should be given the conservation priority of first, second, third, and fourth-order, respectively. The three sub-watershed classes (SWS3, SWS4, and SWS5) soil loss value higher than the minimum and maximum tolerable soil loss to Ethiopian condition (2 –18 ton/ha/year). Grasslands are more prone to soil erosion, followed by croplands and mixed plantations. Free grazing by animals in grasslands, lack of conservation measures in steeper topography of watershed, ignorance of periodic maintenance of engineering measures by farmers, up and down cultivation methods in croplands, contributed to high soil erosion. Most of the watershed areas are more prone to erosion and classified under high soil erosion risk classes. The Geographic Information System tools with the Universal Soil Loss model is effective in mapping the spatial distribution of soil erosion from Berta watershed. Therefore, the sub-watershed areas more prone to erosion should require urgent soil and water conservation measures. Strategies such as numbers of cattle units have to be grazed, and improved cattle varieties have to be designed on grazing lands. Finally, based on the severity of soil erosion, appropriate soil and water conservation practices should be widely implemented based on slope, soil types, and land use types.