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r e <br /> s <br /> a o <br /> RISKPRO'S SESOIL for Windows User's Guide <br /> (1948), and Cowan (1965) is employed in calculating evapotranspiration '(Eagleson, 1978) I <br /> Surface runoff is derived from the distribution of rainfall intensity and duration, and by use of <br /> the Philip infiltration'equation. The effects of moisture storage are included in the monthly <br /> option in SESOIL, based on the work of Metzger and Eagleson (1980). I <br /> Eagleson's theory assumes a one-dimensional vertical analysis in which all processes are <br /> stationary in the long-term average The expression-"long term" applies to both annual and ' <br /> monthly simulations in SESOIL, and is used in contrast to "short-term" models which employ <br /> a storm-by-storm resolution. Also, Eagleson's approach assumes that the soils are homogeneous <br /> and that the soil column is semi-infinite in relation to the surface processes. Thus, in the <br /> hydrologic cycle of SESOIL, the entire unsaturated soil zone is conceptualized as a single layer <br /> (or compartment)-and-the prediction-for-soil-water-content-is-an-average-value-for-the-entire <br /> unsaturated zone t <br /> While the user can provide different permeability values as input for-each of the four mayor soil <br /> layers for the pollutant cycle in SESOIL, the hydrologic cycle will compute and use the depth- ' <br /> weighted average permeability according to the following formula <br /> K = d <br /> z n d (3) <br /> E ,K=i <br /> where. - <br /> -- _ -- KZ_ _ =-vertically averaged-permeability (cm) <br /> K, = permeability for layer i (cm2) <br /> d depth from-surface to groundwater (cm) <br /> d, = thickness of layer i (cm) ' <br /> Thus, the user should exercise care when applying SESOIL to sites with large vertical variations <br /> in soil properties. The average permeability calculated by Eq (3) in=the hydrologic cycle may ' <br /> not be what the user intended and the resulting computed average soil moisture content may not <br /> be valid. u _ <br /> There is no explicit consideration of snow and ice, which are entered as precipitation. The <br /> model assumes that the water table elevation is constant with no change in`groundwater storage ' <br /> from year to year. Bonazountas et al. (1984) adopted this theory for both annual and monthly <br /> simulations. <br /> Page 10 ' <br /> 1 <br />