Geomorphological Instigated Runoff Water and Soil Erosion of Ashebeka Watershed, Digalu Tijo District, Arsi Zone, Oromia, Ethiopia

摘要

Watershed is a landform connected by ridges that descend into lower elevations and small valleys, thus carries rainwater into soil, rivulets and streams then flowing into large rivers. The soil in Ashebeka watershed is being eroded and the accusing finger is to geomorphological agents. This study is aimed to estimate geomorphological caused surface runoff water and soil erosion of Ashebeka watershed. Ashebeka watershed in Digalu Tijo district was purposively selected since it is providing drinking water for Asella town and one of rift valley lake called Zuway. It is also a source of water for small scale irrigation and domestic uses of communities in watershed. Clustering Ashebeka watershed in to five milli-watershed 22 householders from each of them, those are110 in total, were randomly selected for interviews. Data estimated by using universal soil loss algorism, and collected from field observation and householders’ interview were analyzed by using descriptive statistics. Majority 70.1% of Ashebeka watershed was covered by farmland. The watershed was producing 31,175.1 m3/hr of excess runoff water from this 74.9% were produced by farmland. The estimated average annual erosion rate per unit area of Ashebeka watershed were high in settlements (3.0776 ton/ha/year) and farmland (1.0664ton/ha/year). The estimated annual soil loss from overall area of the watershed were 9,521.1 tonnes/year. From this 84.8% of sediments, affecting Asella town drinking water reservoir, were produced by farmland. The adverse effects of surface runoff water on road and other infrastructure, crop and sedimentation at reservoir during rainy season, and decrease in river discharge and livestock production in dry season were significantly high. This adverse effects of surface runoff water and rate of soil loss have been increasing due to lack of proper land management practices. As a result, construction of physical and biological soil and water conservation structure in the watershed is convenient.