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profile at the time of water application. In other words, infiltration is limited by the ability of the soil to take water. 0 Since positive drainage will always be maintained on the landfill surface, no ponding of rain or irrigation waters was included in the analyses. The enclosed analyses of a non -prescriptive cover at the FSL were performed utilizing precipitation data recorded at the Western Regional Climate Center's Stockton Station for the years 1993-2002. The magnitude and duration of the rainfall events were estimated in a fashion believed to conservatively represent typical anticipated application. This included defining the time of day for water application to be 0.4 days (9:36 am) and a water application rate of 60 mm/day. Evapotranspiration - LEACHM requires mean weekly pan evaporation data from which daily potential evapotranspiration (DPET) is calculated as one- seventh of the weekly total pan evaporation. Daily potential transpiration is calculated by multiplying the DPET by a crop cover fraction. Daily potential evaporation is then calculated as the difference between DPET and potential transpiration. For modeling purposes; LEACHM assumes that evapotranspiration starts at 0.3 days (7:12 a.m.) and ends 12 hours later at 0.8 day (7:12 p.m.). During this period, potential evapotranspiration is varied sinusoidally with actual evapotranspiration calculated as a function of the potential evapotranspiration, the profile's soil water status, and specified plant properties. Given a stipulated crop cover fraction of 1.0, LEACHM will calculate that all of the evapotranspiration that occurs is associated with transpiration. However, if a value of 0.0 is entered, all soil moisture losses will be calculated to result solely from evaporation. Pan evaporation data was duplicated from the California Department of Water Resources (DWR) Manteca Station for the same 10 -year period as was incorporated in the rainfall record. 4.2.5 VEGETATION CONDITIONS Rather than applying a coefficient to approximate the transpiration effects of plants, LEACHM uses the equation of Nimah and Hanks (1973) to simulate the uptake of water by plant roots. Variables included in the equation and requiring user specification include: crop cover fraction, root water potential (the root potential below which plants are unable to extract water from the soil); root resistance (the depth dependent resistance to upward flow of water within the roots); and the root distribution (expressed as a nodal percentage of all roots). The vegetation evaluated herein included the native shallow rooting grasses and sparse native shrub species, which were estimated to provide a conservative 60% -6- C:\2003-033\EDOM HILL ALTERNATIVE COVER.D006/16/2005 Geologic Associates