Seasonal Change of WEPP Erodibility Parameters for Two Fallow Plots on a Palouse Silt Loam

Abstract. In cold regions, frozen soil has a significant influence on runoff and water erosion. In the U.S. Inland Pacific Northwest, major erosion events typically occur during winter as frozen soil thaws and exhibits low cohesion. Previous applications of the WEPP (Water Erosion Prediction Project) model to a continuous bare tilled fallow (CBF) runoff plot at the Palouse Conservation Field Station (PCFS) in southeastern Washington State showed that WEPP reproduced the occurrence of the large erosion events, but the amount of sediment yield was either under- or overestimated. The inability of WEPP to reproduce the magnitude of field-observed erosion events at the PCFS suggests the need for an examination of the dynamic changes in soil erosion properties and for improving the representation of such dynamics. The objective of this study was to evaluate the seasonal changes of rill erosion parameters for two CBF runoff plots at the PCFS. Field-observed runoff and erosion events during 1984-1990 were used to estimate WEPP hydraulic and erosion parameters, including soil effective hydraulic conductivity, critical shear stress (τc), and rill erodibility (Kr). The parameters for each event were best-fitted using WEPP single-event simulations to reproduce the observed runoff and sediment yield on both plots. The results suggest that the adjustment factors for τc and Kr of frozen and thawing soils in the WEPP model could be modified to better describe the changes of these parameters in winter.