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3.0 Results <br />The estimated monthly gain/loss values for each reach are summarized below and are <br />displayed graphically in a series of hydrograplls. Average annual baseflow and associated <br />mass balance values for t11e study period (1950 to 2005) are shown for each reach in Fib res 3 <br />through 11. Average monthly baseflow and mass balance values for 1991 through 1994 for each <br />reach >11 Fibres 12 through 20. This is the steady-state calibration period t11at will be used in <br />t11e alluvial groundwater flow model being developed under Task 48. The average monthly <br />values are also presented as gain/loss per river Hole in Fibres 21 through 30. The time series of <br />monthly baseflow results corresponding to the transient model calibration period (1999 through <br />2005) are presented for each reach in Fibres 31 through 39. Each set of graphs (average annual, <br />average monthly 1991-1994, average monthly 1991-1994 per river nine, and average mont111y <br />1999-2005) include the same range on t11e axes to facilitate comparison between reaches. <br />Positive flows shown on all b apps represent gaining stream conditions and negative flows <br />represent losing stream conditions. Flow values may not transition smoot111y between adjacent <br />reaches since the values represent the average baseflow over the entire reach. The baseflow <br />values show much smoother trends than the mass balance values due to t11e data processing <br />steps associated with the Pilot Point method. The difference between the baseflow and mass <br />balance curves represent other flows that are not quantified. <br />Data files developed to estimate stream gain/loss have been provided to the State wit11 the F>11a1 <br />TM, under separate cover. The monthly average baseflow data presented in Fibres 12 through <br />20 and Figures 31-39 will be used to help qualitatively calibrate the steady state and transient <br />alluvial groundwater models, respectively, by comparing the computed to simulated <br />grotmdwater gain/loss in a given reach and tinge period. In addition, the results of this task <br />will provide a useful addition to the tulderstanding of the South Platte River surface water - <br />grotuldwater system. <br />The average annual baseflow results are generally positive for the seven Sout11 Platte reaches <br />and the two tributary reaches, indicating that an arulual basis these rivers are gaining flow from <br />the alluvial aquifer. Table 8 summarizes the average mass balance and estimated baseflow from <br />each reach for the full period of record. Tl1e column labeled Other Gaul-Loss represents the <br />difference between the mass balance and estimated baseflow values. The hig11 values for the <br />South Platte 7 - Balzac to Julesburg reach (Table 8) is likely due to the relatively long length of <br />this reach (98 nines) compared to the other reaches. The South Platte 4 -Fort Lupton to Kersey <br />reach also shows high values but is not long at 39 miles. The higher baseflow values >11 this <br />reach may be related to a relatively large amount of surface water-based irrigation u1 this area <br />that would provide return flow water as baseflow. The negative mass balance value for t11e <br />South Platte 5 -Kersey to Weldona reach (Table 8) demonstrates the effect of a few very low <br />annual values that are dampened out in t11e associated baseflow value for this reach. <br />SPDSS Phase 4 Task 46 Technical Memorandum -Final 14 <br />0~4i 10i0~ <br />