The USDA-ARS Water Erosion Prediction Project is a major effort to improve estimates of soil detachment, transport, and deposition on agricultural hillslopes, as well as to estimate the amount and size distribution of the sediment leaving the field. The WEPP hillslope model computes both detachment and deposition on a total sediment load basis, though the model also estimates information on sediment particle sizes. This article describes the mathematical equations that predict the sediment particle sorting in WEPP for both interrill and rill areas on a hillslope, and presents a discussion on the advantages and disadvantages of the techniques. It also examines the amount of predicted particle sorting as affected by various model input parameters. Soil type, random roughness, rainfall intensity, slope length, slope gradient, and slope shape were all found to affect the predicted size distribution of sediment leaving a hillslope. Comparison of model results to measured data show that the technique described in this article represents the trends in sediment particle sorting observed in field experiments, with r{sup}2 values between observed and predicted size fractions ranging from 0.44 to 0.97.
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