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RISKPRO'S SESOIL for Windows User's Guide The EROS model in SESOIL accounts for several surface features which may divert and slow the overland flow, allowing settling and deposition of the washload These include vegetation, which slows the flow and filters out particles, and topography, which includes surface characteristics such as roughness and the existence of small depressions. Change in slope and loss of water through infiltration into the soil will reduce the flow rate and encourage settling of soil particles. Organic matter is distributed among the particle types based on the proportion I of primary clay in each type (Foster et al , 1980) Soil receiving the deposited sediment is referred to as enriched EROS computes sediment enrichment based on the ratio of the surface area of the sediment and organic matter to that of the surface area of the residual soil (Knisel ' et al , 1983) 3.4.1 Implementation in SESOIL The EROS model uses characteristic rainfall and runoff factors for a storm to compute erosion and sediment transport for that storm (Foster et al , 1980) Hydrologic input to the erosion Icomponent consists of rainfall volume, rainfall erosivity, runoff volume, and the peak rate of runoff for each storm event These terms drive soil detachment and subsequent transport by overland flow. Note that input data for the hydrologic cycle of SESOIL include total monthly ' precipitation, the number of storms per month, and the mean time of each rainfall event. Since SESOIL provides only monthly estimates of hydrologic parameters and in order to couple the SESOIL and EROS models, a statistical method is used to generate the amount of rainfall and duration of each storm for every rainfall event during the month This algorithm employs a model featuring probability distributions to order to estimate the individual storm parameters (Eagleson, 1978, Grayman and Eagleson, 1969) The washload cycle has been implemented with two subroutines in addition to the EROS model, PARAM and STORM, which take the input data for and results generated by the hydrologic cycle and adapt them for use. The PARAM subroutine supports EROS by first retrieving the hydrologic input data (e g , the number of storm events per month and the depth of rainfall) read by SESOIL and then setting specific parameters applicable to the STORM and EROS subroutines The STORM subroutine then uses the PARAM results and statistically generates information about each storm using the algorithm mentioned above Thus, the coupled SESOIL/EROS model does not require any additional hydrologic input parameters for individual storms However, it should be recognized that estimates of rainfall for each storm may be quite different than the actual values IAdditional data needed for the sediment cycle include the washload area, the fraction of sand, silt and clay in the soil, the average slope and slope length of the representative overland flow profile, the soil erodibility factor, the soil loss ratio, the contouring factor, and Manning's n coefficient for soil cover and surface roughness Example values for these parameters can be j I Page 15 j I