Soil erosion and sediment transport are quite complex processes as they depend on physical, biological, mechanical, and chemical processes within a particular catchment. Different soil erosion and sediment transport models have been developed while taking into account site-specific conditions. In Ethiopia, there is no commonly adopted soil erosion and sediment transport model. This may be because of limited land use management and hydro-climatic data. Therefore, it is highly essential to better explain engaged physical processes and means of accounting for site-specific conditions, for soil loss and sediment yield estimation. The objective of this Ph.D. dissertation is to propose regional equations of sediment yield estimation. The specific tasks of this study include data preparation, review, and analysis; regionalizing and improving the MUSLE; deriving a soil loss equation for sediment yield estimation; improving the accuracy of a model, checking the performance of the original SWAT+ model; modifying the SWAT+ model for sediment yield estimation, and proposing a method for deriving and solving an accurate sediment rating equation. The main reason behind regionalizing the MUSLE was the effect of the topographic factor of the MUSLE on soil erosion and sediment yield is not clear. Except for the coefficient, soil erodibility, cover, and conservation practice factors of the MUSLE, an individual effect of the exponent and topographic factor of the MUSLE on soil erosion and sediment yield can be seen by applying the model at different watersheds. Therefore, to regionalize the MUSLE we estimate the best exponent and topographic factor of the MUSLE under the hydro-climatic conditions of Ethiopia. For the sake of the evaluation procedure, the main factors of the MUSLE that directly affect the soil erosion process, such as the cover, conservation practice, soil erodibility, and topographic factors, are estimated based on past experiences from the literature and comparative approaches, whereas the parameters that do not directly affect the erosion process or that have no direct physical meaning (i.e., coefficient a and exponent b) are estimated through calibration. The main reason behind improving the MUSLE was our need to change the input data requirement of the MUSLE for the calculation of its runoff factor for possible application in data-scarce areas. Basically, the MUSLE was developed for a small agricultural watershed, where the extent of erosion is from sheet to rill erosion, but we cannot exactly tell whether it considers gully erosion or not. The underlying physical assumption to improve the MUSLE is that the amount of potential energy of runoff is proportional to the shear stress for sediment transport from a slope field and the kinetic energy of the runoff at the bottom of the slope field for gully formation. As the MUSLE or improved MUSLE does not consider sediment deposition, deriving Soil Loss Equations for Sediment Yield Estimation based on the improved MUSLE becomes important to explain sediment transport and deposition processes. To derive the Soil Loss Equations for Sediment Yield Estimation, we mainly considered physical concepts such as shear force, energy, and work done. To modify the SWAT+ model for sediment yield estimation, we replace the best exponent and the best equation of the topographic factor of the MUSLE in the source code, and also we replace the improved MUSLE or the first revised version of the Soil Loss Equation for Sediment Yield Estimation in place of the MUSLE in the source code. The Soil Loss Equation for Sediment Yield Estimation and its revised versions showed the best performance over the regionalized MUSLE or improved MUSLE. All types of the modified SWAT+ models showed better performance than the original SWAT+ model. This dissertation also presents an iterative approach for deriving and solving an accurate sediment rating equation.    «

Soil erosion and sediment transport are quite complex processes as they depend on physical, biological, mechanical, and chemical processes within a particular catchment. Different soil erosion and sediment transport models have been developed while taking into account site-specific conditions. In Ethiopia, there is no commonly adopted soil erosion and sediment transport model. This may be because of limited land use management and hydro-climatic data. Therefore, it is highly essential to better...    »