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Hydrological system and available data DRAFT 9 <br />Evapotranspiration (ET) estimates for each of the major stream drainages were made. <br />ET was assumed to occur in areas where the groundwater levels in the valley fill <br />deposits are near ground surface. In most stream drainages, this is limited to a <br />relatively narrow zone along the intermittent reaches of the streams where groundwater <br />occurs close to the ground surface. The reaches within each drainage basin where ET <br />occurs were identified based on stream flow measurements and field observations (Pre- <br />feasibility Hydrology Report, WMC, August 2005), and measured in GIS. The total <br />length of intermittent stream in each basin as then multiplied by the average width of the <br />vegetated valley, based on aerial photos, which ranged from 50 to 400 ft. Annual ET <br />rates for the area range from 10 to 21.5 inches (Pre-feasibility Hydrology Report, WMC, <br />August 2005). For purposes of the initial estimate of ET, a value of 18 inches per year <br />was used. <br />The estimated valley fill groundwater outflow, stream baseflow, and ET were added <br />together to estimate a total groundwater discharge from each stream valley (Table 2.2). <br />An average recharge rate was estimated for each drainage area in the model by dividing <br />the total groundwater discharge at each stream valley by the drainage area of each <br />stream basin. <br />The results of these calculations are provided in Table 2.2. The resulting estimated <br />recharge rates are in the range of about 0.2 to 0.7 inches per year averaged over the <br />entire drainage areas. These recharge rates are consistent with estimates made by <br />Leonard Rice Consulting Engineers (1979), which ranged from 0.2 to 0.35 inches per <br />year. <br />• <br />2572-R3 <br />Colowyo Coal Company, L.P. <br />Water Management Consultants