The Water Erosion Prediction Project: Erosion Processes

The erosion process on hillslopes is conceptualized in the USDA Water Erosion Prediction Project (WEPP) as being divided into rill and interrill processes. Interrill erosion is described as a process of soil detachment by raindrop impact and sediment-delivery to adjacent rill flow areas. Rill erosion is described as a rate process proportional to the flow's ability to detach sediment, sediment transport capacity, and the existing sediment load in the flow. Rill detachment occurs when hydraulic shear stress exceeds the critical shear stress of the soil and when sediment load in the rill is less than transport capacity. Net deposition occurs when sediment load is greater than transport capacity. Runoff peak rate, effective runoff duration, and effective rainfall intensity are the only hydrologic variables required for erosion calculations. This paper presents, with a certain detail, the basic concepts and governing equations used in WEPP to predict soil erosion on hillslopes. The physical nature of the parameters used in the erosion equations and their Implications on erosion predictions are also briefly described