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<br />,tl ~ ~j <br /> <br />conditions, the critical period hydrograph was selected for determining storage in excess of <br />that required to annually provide 7.5 maf. <br /> <br />Criticll oeriods of record. An investigation of the critical periods of record was undertaken <br />to support the surplus guidelines studies. The critical period is defined as that period of <br />hydrologic record in which Lake Mead would experience the greatest reduction in storage <br />over the entire hydrologic record under specific release conditions. The studies demonstrate <br />that the beginning of the critical period of record corresponds to 1953 hydrology and that the <br />duration is dependent on system wide depletion levels. For levels of system-wide depletion <br />less than about 11.8 maf, the duration of the critical period is shown to be 12 years. For <br />levels of depletion equal to and greater than about II. 8 maf, the duration is shown to be <br />20 years. <br /> <br />Using the above information, a surplus strategy was developed which determined surplus as <br />storage in excess of that required to annually provide 7.5 maf consumptive use in the Lower <br />Division through the critical period of record. <br /> <br />Surolus threshold storal!e reouirement. The surplus threshold storage requirement is <br />defined as that amount of water required to be in storage at the beginning of the year which <br />is sufficient to satisfy 7.5 maf of deliveries to the lower basin each year during the critical <br />period without the potential for the Lake Mead storage to drop below an assumed target <br />protection level. Shortage was assumed to be determined at a threshold elevation of <br />1120 feet in Lake Mead, which is designed to protect against storage levels falling below <br />elevation 1050 feet (level of SNWS intakes) or about 7.255 maf. <br /> <br />The surplus threshold storage requirement necessarily increases as depletion levels increase. <br />An iteration of model runs was conducted to approximate the appropriate reservoir storage <br />required at the start of the critical period for seven constant depletion levels. Figures 2 to 8 <br />in the attached Draft Appendix Surplus Determination Studies show the storage required in <br />Lake Mead entering the critical period to protect against Lake Mead storage dropping below <br />about elevation 1050 feet. <br /> <br />The surplus threshold storage requirement values were plotted against constant system-wide <br />depletion levels and a straight line interpolation was developed as shown in Figure I. The <br />surplus threshold storage requirement appears to increases as the critical period of record <br />shifts from a 12 year critical period to a 20 year critical period and depletions increase from <br />the current 10.5 maf to 12.2 maf in year 2030. <br /> <br />To apply this method the average use during the critical period of record would be used to <br />determine the surplus threshold storage requirement. As shown in Table I, in 1995 total use <br />in the Upper and Lower Division equals 10,689 thousand acre-feet (Kaf). However to <br />determine the storage required to provide for a 12 year critical period, the average use over <br />the next 12 years would be used or 11,151 Kaf. As shown on Figure 2 the surplus threshold <br />storage requirement in Lake Mead starts out at about 50% capacity and increases sharply <br /> <br />2 <br />