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The monthly average baseflow values for the 1999 through 2005 period are displayed <br />graphically in Figures 30 through 39. The baseflow values depict similar trends as discussed <br />above. <br />4.0 Summary and Conclusions <br />This task developed monthly estimates of stream gains and losses due to groundwater, or <br />baseflow, for seven reaches >11 the main stem of the South Platte River and two tributaries, the <br />Cache la Poudre River and Cherry Creek, within the SPDSS study area for the period 1950 to <br />2005. A spreadsheet-based water balance method termed the Pilot Point method was employed <br />in this analysis. This method uses daily data that are constrained and smoothed to estimate <br />baseflow. It was compared to a mass balance method involving average monthly flows without <br />smootll>11g and found to produce more realistic results due to the additional data processing <br />uzvolved. <br />These baseflow results will be used to 11e1p qualitatively calibrate the alluvial groundwater <br />model being developed wider Task 48, by comparing the trends >11 computed to simulated <br />baseflow over time and also by comparing individual baseflow values in a given reach and time <br />period. <br />This task also provides information on the general trends of t11e stream-growldwater system of <br />the reaches analyzed. In general, baseflow is larger in the South Platte River downstream of the <br />Fort Lupton gage but there are no consistent trends amongst the reaches. Seasonal trends, <br />however, were generally depicted >11 all reaches. <br />The monthly baseflow values ranged from reach to reach based on the daily inflows and <br />outflows included in fl1e analysis for each reach. The largest monthly baseflow gains of the <br />reaches typically occurred in July, August and September and indicate a gaiiliilg stream <br />condition. During this time a groundwater gradient is towards the river which would be <br />expected due to an increase >11 groundwater levels typically observed during an irrigation <br />season in the alluvial aquifer of the South Platte River. T11e filming of t11e largest baseflow <br />during the latter portion of the growing season suggests that muc11 of the irrigation-based <br />recharge infiltrates into the alluvial aquifer and returns to file stream quickly, withal a few <br />months. This pattern was not observed in the South Platte 2 -Denver to Henderson reach. <br />Possible reasons for this difference are due to differences in inflows and outflows within each <br />reach, to the amount and location of irrigation occurring within file reach, to the timing of <br />return flows and to well depletions as a result of pumpiizg. <br />5.0 Recommendations <br />Below are recommendations from file activities performed under Task 46: <br />^ Alluvial aquifer parameters used uz the constraints of maximum gauz or loss are <br />representative values for an entire reach of river and should not be applied as localized <br />values within the river reaches. <br />SPDSS Phase 4 Task 46 Technical Memorandum -Final 16 <br />0~4i 10i0~ <br />