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higher ET calculations during recharge. The increase in ET during recharge is not <br />consistent with actual operations. The slight rise in water level elevations probably <br />does not measurably increase actual ET. <br />To modify the MODFLOW program so as to maintain a constant ET, the ET package <br />was disabled, and ET was modeled as negative recharge. Several different model <br />simulations were run to estimate the impact of changing ET to recharge. Based on <br />the Zone Budget and calibration results the simulation where ET changed to recharge <br />and reduced by 30% was selected as the Optimum Tamarack Recharge Model. <br />5. Analysis of the model was conducted to estimate the sensitivity of head calibration <br />and river recharge to changes in model parameters such as hydraulic conductivity, <br />specific yield, river constant head boundary and recharge <br />Preliminary Model Results and Conclusions <br />Several general conclusions affecting recharge operations were derived from this <br />preliminary modeling effort <br />Two distinctive groundwater flow processes are acting at this site. During <br />pumping and recharge of this very transmissive aquifer, a local hydrologic system <br />is created where the recharge mound and the pumping cone of depression merge <br />and dominate the local flow regime. This local flow system results in significant <br />re- pumping of recharge. Immediately after pumping ceases, the eastward, <br />regional flow gradient resumes and dominates the return of recharge water to the <br />South Platte River. <br />• ET parameters do not significantly affect the water level calibration but are <br />critical in determining the groundwater inflow into the South Platte River. Prior to <br />the start of recharge operations, the majority of natural groundwater returns to the <br />South Platte River occur in the winter and fall when ET rates are the lowest. <br />A calibrated SDF method using the modeling results provides a reasonable <br />estimate of river recharge. However, care must be taken to verify the calibrated <br />SDF values as time progress and additional structures are added. For the modeled <br />operations, the recharge was best simulated using a 140 -day SDF for the recharge <br />ponds, 4 -day SDF for the wells, and a 25 -day SDF for the artificial stream system. <br />The modeling team is continually working on expanding and improving the model to <br />better simulate project operations. <br />Page 5 of 6 <br />