Dynamics and hotspots of soil erosion and management scenarios of the Central Rift Valley of Ethiopia

Most of the lowland in the central rift valley of Ethiopia is arid or semiarid and in degradation,with frequent occurrence of droughts.Soil erosion by water during the rainy season is a serious problem in the region,leading to declining agricultural production,decreased food security,and a sedimentation risk for water bodies.However,there has been no systematic study of this problem or of possible management solutions.To meet this need,we analyzed soil erosion rates from 1973 to 2006, identified erosion hotspots,and proposed possible soil conservation scenarios.We assessed the soil loss dynamicity using the universal soil-loss equation and geographical information system software, considering the land use change at the following three periods:1973,1985 and 2006.We characterized the watershed in terms of the erosion severity,topography,and land use to identify hotspots and proposed,modeled,and evaluated various watershed management scenarios to mitigate the problem.Soil erosion increased markedly from 1973 to 2006,with annual rates of 31,38,and 56 t ha"1 in 1973,1985,and 2006,respectively,as a result of vegetation degradation and particularly the conversion of thousands of hectares of forest or woodland into cropland.The observed soil erosion rates are far from the tolerable rate of soil loss of the country and hence require urgent soil conservation interventions,especially in the hotspot areas.We proposed eight scenarios for reducing soil losses and evaluated their effectiveness.Rehabilitating degraded land（using exclosures and planted vegetation） and installing stone erosion-control structures（stone bund） in cropland reduced the total soil loss by 12.6%and 63.8%,respectively.Treating hotspot areas with annual soil loss of more than 20 t ha;by integrated management（erosion-control structures and exclosures） was the most effective approach,reducing soil loss by 87.8%.