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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 />11 <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 />C.XZ0OMOSZWORW ARD/ForwardCO V ERD 007/13/1005 <br />in <br />Geologic Associates <br />