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I RISKPRO'S SESOIL for Windows User's Guide 4JJ 3.4 Sediment,Washload Cycle I In pollutant transport models, estimates of erosion and sediment yield on watersheds may be needed in order to compute the removal of sorbed chemicals on eroded sediments. A mayor I factor in this process is the surface runoff (i e , rainwater which does not infiltrate the soil and may carry dissolved pollutant). Surface runoff is computed as part of the hydrologic cycle. Erosion is a function of the rate of surface runoff and several other factors. These factors ' include the impact of raindrops, which detaches soil particles and keeps them in motion as overland flow, surface-features such as =vegetation_and- roughness; and infiltration,capacity. Because of the difficulty in directly' measuring washload using water quality monitoring f techniques, estimation techniques and models are widely employed. The sediment cycle of SESOIL is optional, it can be turned on or off by the user Thus, if ' pollutant surface runoff is considered negligible, the washload cycle can be neglected. ,If the option is used, SESOIL employs EROS, a,theoretical sediment yield model (Foster et al., 1980), which is part of the-CREAMS model (Knisel, 1980, Foster et al., 1980). The erosion ' component considers the basic processes of soil detachment, transport, and deposition. The EROS model uses separate theoretically derived equations for soil detachment and sediment ' transport. Separate equations are needed for these two processes because the relationship of the detachment process to erosion is different from the relationship between erosion and transport. For the detachment process, the model employs the Universal Soil Loss Equation (LISLE) (Wischmeier and Smith, 1978), modified by Foster et al (1980) for single storm events The USLE is applicable for predictions of annual sediment erosiony originating mairily from small watersheds which are subject to sheet and nll erosion Detachment of soil particles occurs when the sediment load already in the overland flow�s-less than the sediment capacity of this-flow T -- - The equation-takes-into account soil-erodibility:(the rate of-soil loss per-storm),-which vanes for different soil types and texture classes The USLE considers topography, since both the length and"the steepness of the land slope affect the rate of rain--induced soil erosion Also, the land cover (e.g , vegetation) and the roughness of the soil surface affect the rate of erosion and the ' rate of overland transport. The USLE includes a parameter called "Manning's n", or roughness coefficient, to model these influences capacity for overland flow, EROS ico orates the Yalin I To model the sediment transport cap y vn _ _ _ _ , nrP__-. _ _ _ Transport Equation (Palin, 1963), modified for nonuniform sediment with a mixture of particle sizes and densities The model estimates the distribution of sediment particles transported as sand, silt, and clay, and the fraction of organic matter in the eroded sediment SESOIL computations of sediment transport are performed for each particle size type, beginning at the I upper end of a slope and routing sediment downslope Page 14