Sediment particle sorting on hillslope profiles in the WEPP model

The USDA-ARS Water Erosion Prediction Project is a major effort to improve estimates of soil detachment, transport, and deposition on agricultural hil lslopes, as well as to estimate the amount and size distribution of the sed iment leaving the field. The WEPP hillslope model computes both detachment and deposition on a total sediment load basis, though the model also estima tes information on sediment particle sizes. This article describes the math ematical equations that predict the sediment particle sorting in WEPP for b oth interrill and rill areas on a hill slope, and presents a discussion on the advantages and disadvantages of the techniques. It also examines the am ount of predicted particle sorting as affected by various model input param eters. Soil type, random roughness, rainfall intensity, slope length, slope gmdierrr, and slope shape were all found to affect the predicted size dist ribution of sediment leaving a hillslope. Comparison of model results to me asured data show that the technique described in this article represents th e trends in sediment particle sorting observed in field experiments, with r (2) values between observed and predicted size fractions ranging from 0.44 to 0.97.