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Title: SEDIMENT PARTICLE SORTING ON HILLSLOPE PROFILES IN THE WEPP MODEL

Authors
 Flanagan, Dennis Nearing, Mark

 Submitted to: Transactions of the ASAE Publication Type: Peer Reviewed Journal Publication Acceptance Date: March 6, 2000 Publication Date: N/A Interpretive Summary: When soil erodes, it breaks into pieces of different sizes. This sediment is composed of small particles (clays and silts) and larger particles (sands and aggregates, which are made up of sand, silt, clay, and organic matter). The larger particles are much harder for runoff water to move, so any factor on a field that slows down the runoff (decrease in slope, higher infiltration, more residue cover, etc.) can cause them to drop out of the flow to the soil surface. This sediment deposition often results in a much larger fraction of fine particles reaching the end of a field. These fine sediment particles can have important off-site effects, as they are usually more chemically-active, and can carry nutrients and pesticides with them. The equations described in this paper describe a way to mathematically predict the different sizes of sediment that reach the end of a field. The results of tests of these equations with data from real field experiments show that the procedure does a good job of estimating the sizes of sediment measured in those studies. Being able to do a good job of predicting sediment particle sizes with an erosion computer program allows someone to see which land management practices would be most beneficial at reducing the movement of soil particles that are carrying polluting chemicals. Technical Abstract: The USDA-ARS Water Erosion Prediction Project is a major effort to improve estimates of soil detachment, transport, and deposition on an agricultural hillslope, as well as 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 paper describes the mathematical equations that predict the sediment particle sorting in WEPP for both interrill and rill areas on a hillslope. Discussion on the advantages and disadvantages of the techniques is presented. The amount of predicted particle sorting as affected by various model input parameters was examined in a sensitivity analysis. Soil type, random roughness, rainfall intensity, slope length, slope gradient, and slope shape can all affect the predicted size distribution of sediment leaving a hillslope. Comparison of model results to measured data show that the technique described in this paper can represent the trends in sediment particle sorting observed in field experiments.