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partly a result of tlne travel time (lag) of streamflow from flee upstream end of the reach to the <br />downstream end, partly a result of irrigation runoff, and partly a result of inflows from <br />ungaged locations. <br />To account for streamflow travel time within each reach, and to help smooth flee results, aone- <br />to two-day averaging was applied to the mass balance data for each reach. Tlne lag applied to <br />each reach is listed Table 5. <br />Table 5 -Short-Term Lagging Applied to Gain/Loss Study Reaches <br />Stud Reach Short-Term La in (da s <br />South Platte 1 -Waterton to Denver 1 <br />South Platte 2 -Denver to Henderson 1 <br />Soufli Platte 3 -Henderson to Fort Lu Eton 1 <br />South Platte ~ -Fort Lu Eton to Kerse 1 <br />South Platte 5 -Kerse to Weldona 1 <br />South Platte 6 -Weldona to Balzac 1 <br />South Platte 7 -Balzac to Julesbur r 2 <br />Cache la Poudre - Ft Collins to Greele 1 <br />Chei7 Creek - Franktown to Denver 1 <br />The number of days used for the short-term averaging was deternnined by analyzing the <br />number of days between the peak flows at tlne upstream and downstream gages for each reach <br />during high flow events. The number of days differed for each reach and was generally <br />dependent on the length of flee reach. Figure 2 shows an example of the daily mass balance for a <br />reach (South Platte 4 -Fort Lupton to Kersey) for a single year (1977) and the effect of applyiing <br />the short-term average. As shown u1 this figure, the short-term averaging reduces the more <br />extreme peak daily mass balance flow values. <br />2.3 Constraint Determination <br />A small portion of very large positive (stream gain) or negative (stream loss) daily mass balance <br />values remained in the data set after the short-term averaging was applied. Due to the relatively <br />slow movement of groundwater, these large values are not expected to be caused by stream <br />gains or losses. Instead, the extreme values are likely a result of inflows or outflows that are not <br />included in tlne daily mass balance calculations, such as inflow from farm rwnoff, ungaged <br />tributaries, storm precipitation runoff, surface water return flows or ungaged diversions. Tlne <br />computed monthly ga>ln/loss values would be biased by including these outlier flows in the <br />calculations. To avoid biasing the monthly gain/loss results, the short-term averaged daily <br />flow data was constrained using estimates of the maximum possible gain or loss ground water <br />flow for each reach. <br />Flow constraints were estimated using Darcy's law and the Glover equation for the maximum <br />gain and the maximum loss, respectively. Two different methods were required to deternl>lne <br />the maximum gain and maximum loss constra>nts because different hydrologic processes occur <br />during gaining and losing conditions and these are better represented by tlne different <br />approaches. <br />SPDSS Phase 4 Task 46 Technical Memorandum -Final 10 <br />0~4i 10i0~ <br />