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Measured -- February 1972 <br />Measured -- February 1982 <br />8c <br />aquifer system, the gain -loss relation between the river and aquifer <br />varies seasonally and is a function of streamflow, canal diver- <br />sions, and hydraulic gradient. Although the river typically experi- <br />ences a net gain in flow in the study area, streamflow losses to the <br />aquifer can be substantial during the seasonally high streamflow in <br />May and June. The losses during seasonal high flows can vary sub- <br />stantially depending on whether the year has been wet or dry. <br />During dry years, less streamflow is available for diversion into the <br />canal, and the hydraulic gradient between the aquifer and stream <br />decreases because aquifer recharge from surface water applications <br />and canal leakage decreases. Therefore, losses from the river to the <br />MO" 9_r ft y / y a T WH ModN BowN�ry 0 7 MM <br />Measured -- February 1995 . Simulated. =- February 1995. <br />EXPLANATION <br />- - - - -- 2,000— Salinity, in milligrams per mer <br />monitoring wer <br />Figure 8. Measured and simulated salinity in the alluvial aquifer, February 1972, 1982, and 1995. <br />The model was not calibrated to streamflow at the down- <br />stream end of the study area because streamflow was not measured <br />at this location except during the 1971 -72 period. Model simulations <br />of streamflow gains and losses (Figure 10) are consistent with the <br />current understanding of interactions between the river and the <br />alluvial aquifer. A substantial percentage of the river is diverted at <br />the upstream end of the study site by the Fort Lyon Canal. At the <br />downstream end of the study area, streamflow recovery is supplied <br />primarily by irrigation return flow from the aquifer. According to <br />the current understanding and conceptual model of the stream- <br />10 <br />°c 6 <br />a� 4 <br />Range of Error in Salinity (mg/L) <br />Figure 9. Frequency distribution of errors in model - simulated salin- <br />ity (simulated minus measured) for individual wells sampled in <br />February 1972,1982, and 1995. <br />0.8 <br />0.6 <br />E 0.4 <br />0.2 <br />� 0 <br />t7 <br />-0.2 <br />-0.4 <br />Simulated -- February 1972 <br />Simulated -- February 1982 <br />Losing River <br />r ti n n o0 0"'0 0' a o a a rn <br />Year <br />Figure 10. Simulated average monthly streamflow gains or losses in <br />the Arkansas River at the downstream end of the study area. <br />81 <br />gginggg SS %nt --g =M 8Sg <br />V <br />A <br />