Environmental Implications of Soil Erosion and Sediment Yield in Lake Hawassa Watershed, South-central Ethiopia

Abstract Background: Assessing soil erosion, sediment yield and sediment retention capacity of watersheds is one of the under researched areas in watersheds of developing countries like Lake Hawassa watershed. The study examined soil erosion, sediment yield and sediment retention and their environmental implications in Lake Hawassa watershed. The quantification and mapping was carried out using Integrated Valuation of Ecosystem Services and Tradeoffs (InVEST) model. Data such as Land Use Land Cover (LULC), Digital Elevation Model (DEM), rainfall, soil, and management practice were used as input parameters.Results: The empirical analysis confirmed that the watershed has a total soil loss of about 5.27 Mt annually. The mean annual erosion rate from the watershed was estimated to be 37 t ha-1 yr-1. The estimated erosion rate was greater than the maximum tolerable erosion limit in Ethiopia (2-18 t ha-1 yr-1). The total amount of sediment which was exported to the nearby streams and lakes in the watershed was estimated to be 1.6 t ha-1 yr-1. The water bodies receive a total of 226,690.3 t of sediment annually. Although higher soil loss and sediment export per unit of area were estimated from the highest slope gradients, greater contributions to the total soil loss and sediment export were computed from slopes with 5-30% gradients. In terms of LULC, the highest contribution to the total soil loss was computed from cultivated land while the highest rate of soil loss per hectare was observed from bare land. Due to the existing vegetative cover, a total of 18.65 Mt (130.7 t ha-1 yr-1) of sediment was retained. Vegetation-covered LULCs such as forest, woodland, shrub land, and agroforestry revealed the highest sediment retention capacity. As a result of the increasing soil erosion and sediment yield in the watershed, a drying of a small lake and the rise in the water level of Lake Hawassa were identified. Conclusion: Most of the soil loss and sediment yield were contributed by small part of the watershed. Thus, the results underscore the urgent need for targeted soil and water conservation measures of various types to ensure sustainability of the watershed resources.