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6.0 ET IMPACT ON RIVER AUGEMENTATION CREDITS <br /> • The Original Tamarack Recharge Model was used to estimate the impact on Project augmentation credits <br /> when ET is varied with depth. The goal of this task was to determine how much of the accretions are <br /> consumed by increased native ET before the water reaches the South Platte River. A memorandum <br /> discussing the details of the analysis was prepared in 2004 and based on this analysis CDOW proposed <br /> taking a 5% ET reduction off of augmentation credits as they hit the South Platte River. Summarized <br /> below is the ET analysis (CDOW, April 21, 2004). <br /> The Tamarack Recharge Model was run under four different operational scenarios to evaluate the impact <br /> on Project augmentation credits by varying ET with depth to groundwater: <br /> 1 Current Project configuration with 10 recharge wells and 3 recharge ponds. <br /> 2 Decreed Project configuration with 17 recharge wells and 12 recharge ponds. All recharge goes to <br /> ponds north of the interstate. <br /> 3 Decreed Project configuration with 17 recharge wells and 12 recharge ponds. Recharge is distributed <br /> to all ponds (3 wells going to Pond 4 located south of the interstate). <br /> 4 Decreed Project configuration with 17 recharge wells and 12 recharge ponds. Recharge is distributed <br /> to all ponds (5 wells going to Pond 4 located south of the interstate). <br /> For each scenario all wells were pumped at 2,000 gallons per minute for two time periods: <br /> Period 1: Wells were activated on October 15 and turned off on January 31s <br /> Period 2: Wells were activated on April 15 and turned off on May 15` <br /> Pumping and recharge were operated for one season with Project augmentation credits and increases in <br /> • <br /> ET being tracked for 10,000 days. <br /> Project augmentation credits are defined as the amount of water that is pumped by the Project and <br /> returned to the river at times when recharge accretions to the river exceed pumping depletions from the <br /> river. Increase in ET is defined as the increase in ET resulting from Project recharge operations. Only <br /> increases in ET were included in the calculations. Any decreases in ET caused by lowered water levels <br /> resulting from well pumping were not included in the total Increase in ET calculation. <br /> Project augmentation credits and total increase in ET were calculated as follows: <br /> 1. Zone Budget, a program written by the USGS (Harbaugh, 1990) was used to identify the flows into <br /> and out of the river constant head boundary and ET flow rates out of all the cells in the model. <br /> 2. The model was run under two conditions: 1) run with all Tamarack recharge and pumping included; <br /> and 2) run without any recharge and pumping from the Project. The changes in flows to the river <br /> boundary and in flows out of cells through ET between the two runs were calculated. This change <br /> represents river accretions and the increase in ET due to Project recharge. <br /> 3. An EXCEL spreadsheet was created to download Zone Budget data, and track Project augmentation <br /> credits and increases in ET. <br /> Table 8 summarizes the modeling results. The percent increase in ET varied from 1% to 2% of total <br /> Project water recharged. However, the percent increase in ET stayed relatively constant at 5% of total <br /> Project augmentation credits to the river. The percent increase in ET was also 5% when augmentation <br /> credits and ET increases were set to zero during the following year's pumping periods. <br /> • <br /> 15 'Page <br />