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4 RISKPRO'S SESOIL for Windows User's Guide found in the CREAMS documentation (Knisel' 1980, Foster et al , 1980). Note that the washload area should be less than or equal to the pollutant application area EROS takes the information generated by both the PARAM and STORM subroutines and ' computes estimates of the sediment yield for each month. Information from the sediment cycle, along with information from the hydrologic cycle, is then provided to the pollutant fate cycle, which will be discussed in the next subsection ' The coupled_ SESOIL/EROS model was evaluated by comparing,predictions to-published measured data (Hetrick and Travis, 1988) Two cornfield watersheds and ane grassland watershed were included in the study. The saes differed in their management practices, soil -_- type?ground-cover,—and-meteorology The-model-predictions-were-in-fair-to-good-agreement— with observed data from the three watersheds, except for months where surface runoff came ' from one or two high intensity storms (Hetrick and Travis, 1988) 3.5 Pollutant Fate Cycle ' The pollutant fate cycle focuses on the various chemical transport and transformation processes which may occur in the-soil These processes are summarized in Table 3 1, and are discussed ' in more detail in the subsections that follow The pollutant fate cycle uses calculated results from the hydrologic cycle and the sediment washload cycle. Information from these cycles automatically provided to the pollutant fate cycle 41 In SESOIL, the ultimate fate and distribution of the pollutant is controlled by the processes _interrelated by the mass balance equation (6) below The processes are selectively employed and !- - - - combined�bythe polliWih-Cfate cycle-based--oii-the chemical properties and the simulation- -scenario specified imulation-_scenanospecified by the user The actual quantity or mass of pollutant taking part in any one ' process depends on the competition among all the processes for available pollutant mass Pollutant availability for participation in these processes, and the pollutant rate of migration to the groundwater, depends on its partitioning in the soil between the gas (soil air), dissolved (soil I moisture), and solid (adsorbed to soil) phases 3.5.1 Foundation In SESOIL, any layer (sublayer) can receive a pollutant, store it, and export if to other subcompartments Downward movement of the pollutant occurs only with the soil moisture, while upward movement can occur only by vapor phase diffusion. Like the hydrologic cycle, the pollutant fate cycle is based on a mass balance equation (Eq. [6]) that tracks the pollutant as it moves in the soil moisture between subcompartments Upon reaching and entering a Iayer or sublayer, the model assumes instantaneous uniform distribution of the pollutant throughout that layer or sublayer I Page 16 I