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 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 2 values between observed and predicted size fractions 
ranging from 0.44 to 0.97.