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Model development DRAFT 16 <br />0 3.7.1 Evapotranspiration (ET) <br />ET occurs in the model area where the groundwater table is at or near ground surface <br />(e.g., along some stream reaches and at springs and seeps). Estimates of ET for the <br />major drainage basins are presented in the Collom area water balance (Section 2.4). <br />ET is not simulated explicitly into the regional model as a separate discharge <br />mechanism, but is included as a portion of the total model outflow. <br />3.8 Calibration targets <br />Calibration of the regional model was conducted using several different data sources <br />including: <br />• The water balance described in Section 2.4 was used to calibrate against <br />overall model recharge and discharge amounts. <br />• Field observation indicates that certain stream reaches flow intermittently, <br />indicating that the valley fill groundwater level is near ground surface <br />along these reaches. The model was calibrated such that these same <br />reaches had the water table at or near ground surface. <br />• Numerous piezometric head data were available from wells and <br />piezometers completed in various bedrock units (FS, F/G, H, and I <br />sequences) in the Collom area. The modeled heads were calibrated <br />against the measured heads in areas where groundwater levels were <br />considered to represent the area-wide flow system (i.e., outside of <br />perched zones). <br />• Extensive monitoring of water levels was conducted during the pilot <br />dewatering test. The drawdown response in the F/G sequence and other <br />bedrock units during the dewatering test were used for transient model <br />calibration. <br />• <br />2572-R3 Colowyo Coal Company, L.P. <br />Water Management Consultants