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and precipitation that deep percolates. No ground water is estimated to enter the basin from <br />the Sangre de Cristo Mountains. For the Rio Grande and Conejos Rivers Schroeder used the <br />river package of the MODFLOW to simulate seepage to and from the aquifer. <br />Rim inflows from the San Juan Mountains (344 kaf/yr) and Sangre de Cristo mountains (312 <br />kaf/yr) were computed for the water years 1970-1987 using the method developed by Emery <br />(1973). A portion of the estimated rim inflows are diverted for irrigation purposes. <br />Recorded diversions of rim inflows for irrigation were estimated to be 7 percent consumed <br />by ditch losses, and then 60 percent of the on farm deliveries are consumed. The rim inflows <br />presented by Schroeder also included the recharge estimated from irrigation ditches off the <br />Rio Grande and Conejos River (214 kaf/yr). Irrigation ditch recharge was estimated in the <br />ground water model preprocessor program by using various efficiencies to estimate ground <br />water recharge. For instance, ditch seepage losses of 30 percent, on farm deep percolation of <br />25 percent and any diversions in excess of net irrigation requirements were estimated to <br />recharge the unconfined aquifer. The total rim inflows including irrigation ditch recharge is <br />estimated at 870 kaf/yr. <br />Ground water inflows from the San Juan Mountains were estimated at 124.7 kaf/yr using the <br />General Head Boundary Package of the McDonald-Harbaugh model, which uses Darcy's <br />Law. Schroeder estimated a hydraulic conductivity of about 10 gallons per square foot per <br />day, and a gradient of 50 feet per two miles. Aquifer thickness was not specifically <br />mentioned, however Schroeder indicated that all layers modeled were allowed to receive the <br />San Juan mountain ground water inflows. <br />A portion of the ground water from flowing wells and from wells pumped for irrigation is <br />estimated to return back to the ground water aquifer. Schroeder estimated that 60 percent of <br />the flow of flowing wells returned (81.7 kaf/yr), and 30 percent of the pumped irrigation <br />wells (110.5 kaf/yr) returned to the ground water. <br />Precipitation inflow (90 kaf/yr) to the San Luis Valley aquifer is based on an average of <br />0.525 inch/year over the valley. Precipitation was weighted to be slightly higher over <br />irrigated areas and areas where the water table was within two feet of the surface. <br />5.2.2 Consumptive Uses -Consumptive use of ground water in the San Luis Valley was <br />determined for native vegetation, municipal uses, and crop lands. Schroeder reports ground <br />water discharges for native vegetation of 605.6 kaf/yr. He also modeled the net depletion of <br />municipal diversions to be 3.9 kaf/yr. <br />Et of ground water for crop lands was estimated as ground water discharge for irrigation <br />wells (pumped 368.3 kaf/yr * 70% efficiency) and flowing wells (136.2 kaf/yr * 40% <br />efficiency). The net result is 312.3 kaf/yr of ground water consumptive use for irrigation <br />purposes. Ground water discharge from irrigation wells is computed in the preprocessor <br />program as the amount of water needed to meet the full irrigation demand of crops after <br />surface water supplies have been applied. Flowing well discharge in the model is based on a <br />review of surveys conducted by other investigators. <br />5.2.3 Change in Storage - Schroeder's water budget is for a steady state condition, which by <br />definition means inflows equal outflows so there is no change in storage. <br />5.2.4 Outflows -Outflows from the San Luis Valley ground water basin consist of surface water, <br />and ground water. Surface water outflows consist of springs (19.2 kaf/yr) and net gain in <br />river flow (41 kaf/yr) for a total outflow to surface water of 60.2 kaf/yr. The amount of <br />rg_task8-1_2001-08.doc 12 of 19 Apri16, 2001 <br />