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• ABSTRACT: The Hydrologic Evaluation of Landfill Performance (HELP) computer program <br />is aquasi-two-dimensional hydrologic model of water movement across, into, through <br />and out of landfills. The model accepts weather, soil and design data and uses solution <br />techniques that account for surface storage, snowmelt, runoff, infiltration, vegetative <br />growth, evapotranspiration, soil moisture storage, lateral subsurface drainage, leachate <br />recirculation, unsaturated vertical drainage, and leakage through soil, geomembrane or <br />composite liners. Landfill systems including combinations of vegetation, cover soils, <br />waste cells, lateral drain layers, barrier soils, and synthetic geomembrane liners may be <br />modeled. The program was developed to conduct water balance analyses of landfills, <br />cover systems, and solid waste disposal facilities. As sucfi, the model facilitates rapid <br />estimation of the amounts of runoff, evapotranspiration, drainage, leachate collection, and <br />liner leakage that may be expected to result from the operation of a wide variety of <br />landfill designs. The primary purpose of the model is to assist in the comparison of <br />design alternatives as judged by their water balances. The model, applicable to open, <br />partially closed, and fully closed sites, is a tool for both designers and permit writers. <br />The HELP model uses many process descriptions that were previously developed, <br />reported in the literature, and used in other hydrologic models. The optional synthetic <br />weather generator is the WGEN model of the U.S. Department of Agriculture (USDA) <br />Agricultural Research Service (ARS) (Richardson and Wright, 1984). Runoff modeling <br />is based on the USDA Soil Conservation Service (SCS) verve number method presented <br />in Section 4 of. the National Engineering Handbook (USDA, SCS, 1985). Potential <br />• evapotranspiration is modeled by a modified Penman method (Penman, 1963). <br />Evaporation from soil is modeled in the manner developed by Ritchie (1972) and used <br />in various ARS models including the Simulator for Water Resources in Rural Basins <br />(SWRRB) (Arnold et al., 1989) and the Chemicals, Runoff, and Erosion from <br />Agricultural Management System (CREAMS) (I{nisel, 1980). Plant transpiration is ' <br />computed by the Ritchie's (1972) method used in SWRRB and CREAMS. The <br />vegetative growth model was extracted from the SWRRB model. Evaporation of <br />interception, snow and surface water is based on an energy balance. Interception is <br />modeled by the method proposed by Horton (1919). snowmelt modeling is based on the <br />SNOW-17 routine of the National Weather Service River Forecast System (NWSRFS) <br />Snow Accumulation and Ablation Model (Anderson, 1973). The frozen soil submodel <br />is based on a routine used in the CREAMS model (Knisel et al., 1985). Vertical <br />drainage is modeled by Dazcy's (1856) law using the Campbell (1974) equarion for <br />unsaturated hydraulic conductivity based on the Brooks-Corey (1964) relationship. <br />Saturated lateral drainage is modeled by an analytical approximation to the steady-state - <br />solution of the Boussinesq equation employing the Dupuit-Forchheimer (Forchheimer, <br />1930) assumptions. Leakage through geomembtanes is modeled by a series of equations <br />based on the compilations by Giroud et al. (1989, 1992). The processes aze linked <br />together in a sequential order starting at the surface with a surface water balance; then <br />evapotranspiration from the soil profile; and finally drainage and water routing, starting <br />at the surface with infiltration and then proceeding downward through the landfill profile <br />• to the bottom. The solution procedure is applied repetitively for each day as it simulates <br />the water routing throughout the simulation period. <br />. 2 <br />