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<br />MWSI Project <br />Phase II Conjunctive Use Summary Report. DRAFT <br /> <br />August 22, 1995 <br /> <br />years long (1953-1956), the payback period was assumed at 2 years, This suggests that during <br />the first two years of a drought, borrowing from the system could still be allowed. The payback <br />must then occur in the remaining 2 years of the drought. Any actual threshold levels used in a <br />conjunctive use plan for borrowing from surface storage and payback would be specific to the <br />partici pating surface water system and would be determined by the surface system owner. <br /> <br />The conjunctive use reservoir was sized based upon the volume required to meet peaks if <br />groundwater pumping is at average annual rates, The percent of use assumed for each month is <br />plotted and the average annual pumping is indicated in Figure 6, The volume shown above the <br />average pumping line is the volume of storage required, which was determined to be 20 percent <br />of the annual volwne, This 20 percent is multiplied by the conjunctive use demand to determine <br />reservoir size, Hence, a 30,000 acre-foot demand would require a 6,000 acre-foot reservoir as a <br />minimum, and a 90,000 acre-foot demand requires an 18,000 acre-foot reservoir as a minimum. <br />there may be some advantage to reservoir storage beyond that required for peaking. However, <br />model runs indicate the benefit is moderate, and single sites are not generally available on the <br />plains to achieve storage much beyond that required for peaking. <br /> <br />Sensitivity to Recharge Rate <br /> <br />Under these assumptions, the model was then run to review sensitivity to recharge rates, <br />For a borrow limit of 80,000 acre-feet, the model was run for recharge rates of 33%, 50%, 67%, <br />and 84% for conjunctive use demands varying from 30,000 acre.feet to 120,000 acre. feet. The <br />results are shown in Table 5, <br /> <br />I <br />"r <br />I <br /> <br />53 <br />