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1 <br /> 6 RISKPRO'S SESOIL for Windows User's Guide <br /> 3. SESOIL MODEL DESCRIPTION <br /> I SESOIL is a one-dimensional vertical transport model for the unsaturated soil zone SESOIL <br /> can consider only one compound at a time and is based on mass balance and equilibrium <br /> partitioning of the chemical between different phases (dissolved, sorbed, vapor, and pure). The <br /> SESOIL model was designed to perform long-term simulations of chemical transport and <br /> transformations in the soil. The model uses theoretically derived equations to represent water <br /> transport, sediment transport on the land surface, pollutant transformation, and migration of the <br /> pollutant to the atmosphere and groundwater Climatic data, compartment geometry, and soil <br /> and chemical property data are the major components used in these equations. <br /> The expression "long term" applies to both annual and monthly simulations in SESOIL, and is <br /> used in contrast to "short-term" models which employ a storm-by-storm resolution Some soil <br /> models are designed to estimate pollutant distribution in the soil after each mayor storm event, <br /> and simulate chemical concentrations in the soil on a daily basis (e g , see Patterson et al , <br /> 1984). These models are data intensive, requiring, for example, hourly rainfall input and daily <br /> maximum and minimum temperatures SESOIL, on the other hand, estimates pollutant <br />' distribution in the soil column and on the watershed after a "season", which can be defined by <br /> the user as a year or a month This is accomplished using a statistical water balance analysis <br /> and a washload routine statistically driven within the season This approach saves time for the <br /> model user by reducing the amount of data that must be provided, and also reduces computer <br /> time and resource requirements since fewer computations are required <br /> Two operation options are available for running SESOIL annual estimates (Option A) requiring <br /> annual climatic data, and monthly estimates (Option M) requiring monthly data. It is <br /> recommended that the monthly option always be selected since it will provide a better estimate <br /> of chemical movement through the soil RISKPRO simplifies the task of compiling monthly <br /> input data by extracting pertinent data from on-line databases (see the next section on building <br /> input data files using RISKPRO) Thus, the monthly option is no more difficult to use than the <br /> annual option. Option A is not available in the RISKPRO system, and this option will not be <br /> discussed further in this report with the exception of the hydrologic cycle, which implements the <br /> annual algorithm as described below The annual option has not been changed from the original <br /> model, and those users interested in the annual option are referred to the report by Bonazountas <br /> and Wagner (1984) <br /> TheYrocesses modeled b SESOIL are categorized into three cycles hydrology, sediment, and <br /> P <br /> pollutant transport. Each cycle is a separate sub-model within the SESOIL code Most <br /> Imathematical environmental simulation models may be categorized as stochastic or deterministic <br /> models Both the stochastic and deterministic models are theoretically derived Stochastic <br /> models incorporate the concept of probability or some other measure of uncertainty, while <br /> lie deterministic models describe the system in terms of cause/effect relationships SESOIL <br /> Page S <br /> I <br />