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2.7. Model Calibration and Steady State Simulation <br />The model was calibrated to simulate existing conditions. Calibration was <br />performed by varying the values for constant head boundary cells. This process <br />was repeated until the water levels from the model output matched the measured <br />heads at five observation wells (measured on April 11, 2003) to within about 10 <br />percent of observed heads in the observation wells. <br />The mean absolute difference (MAD) between measured and simulated water <br />levels is .73 feet which is less than 10% of the total range of water levels in our <br />five observation wells (8.2 feet). <br />Table 2.7.1 -Model Calibration Table <br />Well Measured <br />Water Elev. Modeled <br />Water Elev. Difference Difference <br />(Absolute) <br />OW 1 4884.35 4885.66 -1.31 1.31 <br />OW2 4888.0 4887.65 0.35 0.35 <br />OW3 4888.0 488.97 0.03 0.03 <br />OW4 4892.5 4890.9 1.60 1.60 <br />OW5 4888.75 4889.1 -0.35 0.35 <br /> Range Mean <br /> 8.15 0.728 <br />The models were run under steady state conditions which estimate heads and <br />flows after infinite time. Steady state models simulate equilibrium conditions for <br />both the baseline existing condition model and the post-development reclamation <br />condition model. Similarly the steady state model runs simulate worst case <br />drawdown and flow conditions under dewatering stresses. The models are <br />created to assess the relative and worst case effects of dewatering the gravel <br />pits on the alluvial aquifer, the river, and ponds nearby. The model is not <br />designed to simulate seasonal variations in river flow, ditch flow, evaporation and <br />recharge. Evaluation of these parameters in seasonally varying transient <br />Gardels Pit <br />Revised Groundwater Modeling Report <br />5of9 <br />