Estimating overland flow soil transport capacity and surface erosion rate using unit stream power

Abstract

The unit stream power was used to develop a physically-based model to estimate overland flow soil transport capacities and corresponding surface erosion rates in the study. This model considers the impact that physical factors have on computed overland soil transport capacities including varying hydraulic conditions and a maximum sediment concentration limit. This new soil transport capacity relationship, which is developed in this study, was incorporated in the CASC2D-SED watershed model. The revised CASC2D-SED model was applied to eight basins in three different watersheds to validate the overland flow soil transport capacity relationship and provide a practical tool for water and soil conservation management. In past studies of event-based overland flow soil erosion, both empirical and physically-based approaches have been used to define soil transport capacity relationships and corresponding erosion rates. However, these previous relationships only provide a description of the factors that influence transport capacity based on site-specific data sets that considered only a limited range of conditions. In contrast, the overland soil transport capacity relationship developed as part of this study provides a more complete description of the factors that determine transport capacity. A correlation analysis was used to determine the significant factors that impact transport capacity. The correlation analysis results show that unit stream power is the most dominant factor for soil transport. A bounded regression formula was used to reflect the limits that transported sediment concentrations cannot be less than zero or greater than a maximum value. The coefficients used in this new model were determined from nonlinear regressions using published experiment measurements. A one-dimensional overland flow diffusive wave model was used in conjunction with the new soil transport capacity equation to simulate field conditions and validate the model. The computed results were in good agreement with laboratory and field plot data. To further validate the performance of the new overland soil transport equation and to provide a practical tool for watershed-scale application, the new equation was incorporated into a revised CASC2D-SED watershed model. Unlike the Kilinc-Richardson equation originally used in CASC2D-SED, the crop management factor (C) of Universal Soil Loss Equation is not required as a model input because unit stream power and other physical factors in the transport capacity equation can reflect the effects of dynamic flow resistance. Eight basins in three different watersheds were used to validate the revised CASC2D-SED model. Sensitivity analyses show that overland roughness using the Manning coefficient is more sensitive for sediment yield calculations than it is for hydrology simulations. By comparing with observed data, the variation range of predicted results is considered acceptable using the revised model.