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actual evapotranspiration calculated as a function of the potential evapotranspiration, the <br /> profile's soil water status, and specified plant properties. <br /> Given a stipulated crop cover fraction of 1.0, LEACHM will calculate that all of the <br /> evapotranspiration that occurs is associated with transpiration. However, if a value of 0.0 <br /> is entered, all soil moisture losses will be calculated to result solely from evaporation. <br /> Pan evaporation data was duplicated from the California Department of Water Resources <br /> (DWR)Manteca Station for the same 10-year period as was incorporated in the rainfall <br /> record. <br /> 4.2.5 VEGETATION CONDITIONS <br /> Rather than applying a coefficient to approximate the transpiration effects of plants, <br /> LEACHM uses the equation of Nimah and Hanks (1973)to simulate the uptake of water <br /> by plant roots. Variables included in the equation and requiring user specification <br /> include: crop cover fraction, root water potential (the root potential below which plants <br /> are unable to extract water from the soil); root resistance(the depth dependent resistance <br /> to upward flow of water within the roots); and the root distribution(expressed as a nodal <br /> percentage of all roots). <br /> The vegetation evaluated herein included the native shallow rooting grasses and sparse <br /> native shrub species, which were estimated to provide a conservative 60% coverage over <br /> the land. This coverage was selected based on review of the development of vegetation <br /> on the natural landscape adjacent to the site, and is considered typical of the native <br /> vegetation in the area. <br /> The additional vegetation values listed below were also specified in the final cover <br /> analyses. <br /> VARIABLE SPECIFIED VALUE <br /> Wilting point -1500 Kpa <br /> Root Flow Resistance 1.00 <br /> Root Condition Mature <br /> 4.3 VARIABLE SENSITIVITY ANALYSIS <br /> The following discussion addresses the sensitivity of model results to changes in the <br /> critical variables discussed above, and also includes a brief analysis of several less <br /> sensitive input variables. A summary of this sensitivity analysis is included in Table 3. <br /> 4.3.1 COVER THICKNESS <br /> As shown in Table 3, negative flux potential increases as the cover section thins. <br /> Although a 3-foot thick cover is expected to perform even better than a 4-foot section, <br /> -6- <br /> C:\20050082\FOR WARD/FomardCOVER.DOC\7/13/2005 <br /> Geologic Associates <br />