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The flow constraints were calculated to represent the highest possible flow value for a given <br />day for each reach, not to represent an average flow value typically observed. Average flow <br />results could overly constrain the daily flow data. The flow constraints were calculated to <br />represent flow into and out of fl1e alluvial aquifer from both sides of the river. Representative <br />values for aquifer properties and 1ydraulic gradients were obtained from published available <br />sources. Values of hydraulic conductivity, average saturated thickness values and alluvial <br />length were obtained from data presented in the SPDSS Phase 3 Task 43.3 TM (CDM, 2006). <br />Hydraulic gradient values along the study reaches were estimated from maps presented in <br />Hurr and Schneider (1972a-f) and the CDM Task 44.3 TM (CDM, 2006). <br />Maximum Gain Constraint <br />Darcy's law was used to estimate the maximum groundwater gain expected for eac11 river <br />reach. Darcy's law assumes constant flow conditions and uniform aquifer properties. It was <br />utilized to estimate the maximum flow constraint because, u1 general, t11e largest gaols to the <br />river are observed during t11e irrigation season w11en the aquifer hydraulic gradient 11as been <br />elevated byirrigation-based recharge and shallow groundwater flow is towards the river. <br />Darcy's law is defined as: <br />Q=KA (dh/ dl) <br />Where: <br />Q =Flow through a cross sectional area (cubic feet per second [cfs] ) <br />K =Hydraulic conductivity (ft/day) [multiplied by a constant to convert to ft/sec] <br />A =Cross-sectional area (ft~) (River Reach Lengtll~Average Saturated Thiclaless) <br />dh/ dl =Hydraulic gradient <br />For purposes of fllis analysis, dh/ dl was calculated as the gradient from a point 0.5 mile from <br />the stream to the stream edge parallel to the average direction of groundwater flow using water <br />table maps from Hurr and Schneider (1972a-f) and CDM (2006). The values used in the Darcy's <br />Law calculations for each reach are presented >IZ Appendix D Table D1. <br />Constraints for the maximum gain are presented >11 Table 6. Results for the reaches along the <br />South Platte River varied between 182 cfs and 574 cfs. An exception is the maximum gain of <br />1041 cfs for the South Platte 7 -Balzac to Julesburg reach. This reach is almost twice as long as <br />the other maulstem reaches. When converted to gaol per river Hole the maximum constraints <br />range from 11 to 13 cfs/nine for the ma>1lstem reaches (Table 6). T11e lower gain per river Hole <br />values are in the three upstream reaches, from Waterton to Fort Lupton, and also downstream <br />of the Balzac gage. The higher values, in the noddle reaches of fl1e mainstem, correlate to areas <br />where the floodplauz of the South Platte River is wider and more irrigation activity occurs. It is <br />also possible that the increased gain could be in part due to greater contribution from the <br />Denver Bas>11 bedrock aquifers. The maximum gain constraints for the Cache la Poudre River <br />and Cherry Creek reaches were 522 cfs and 303 cfs, respectively, equating to gains of 9 and 8 <br />cfs/nine, respectively (Table 6). <br />SPDSS Phase 4 Task 46 Technical Memorandum -Final 11 <br />0~4i 10i0~ <br />