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I <br /> RISKPRO'S SESOIL for Windows User's Guide <br /> 3.3 Hydrologic Cycle <br /> The hydrologic cycle is one-dimensional, considers vertical movement only, and focuses on the <br /> role of soil moisture (or interstitial pore water) in the soil compartment. The hydrologic cycle <br />' submodel calculates results for the hydrology of a site and passes these results to both the <br /> sediment washload cycle and the pollutant fate cycle. The hydrologic cycle used in SESOIL is <br />' an adaptation of the water balance dynamics theory of Eagleson (1978). The theory can be <br /> described as a dimensionless analytical representation of an annual water balance. It is itself a <br /> model based on simplified models of interacting hydraulic processes, including terms for the <br />' climate, soil, and vegetation. These processes are coupled through statistically based modeling. <br /> It is beyond the scope of this manual to present the detailed physics and mathematical <br /> expressions of the model The hydrologic cycle is thoroughly described by Eagleson (1978) and <br /> summarized by Bonazountas and Wagner (1984), and is based on the water balance equations <br /> shown below. All of these parameters are expected or mean annual values, and in SESOIL they <br />' are expressed in centimeters. <br /> I P - E - MR = S + G = Y <br /> I = P - S (2) <br />' where- <br /> P = precipitation <br />' E = evapotranspiration <br /> MR = moisture retention <br /> S = surface runoff <br /> I = infiltration <br /> Y = yield <br /> G = groundwater runoff or recharge (includes term for capillary rise) <br /> IBriefly, precipitation is represented by Poisson arrivals of rectangular gamma-distributed <br /> intensity pulses that have random depth and duration Infiltration is described by the Philip <br />' equation (Philip, 1969), which assumes the medium to be effectively semi-infinite, and the <br /> internal soil moisture content at the beginning of each storm and inter-storm period to be <br /> uniform at its long-term average. Percolation to the groundwater is assumed to be steady <br />' throughout each time step of simulation, at a rate determined by the long-term average soil <br /> moisture content. Capillary rise from the water table is assumed to be steady throughout the <br /> time period and to take place at a dry surface. The work of Penman (1963), Van den Honert <br /> 1 <br /> Page 9 <br /> I <br />