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<br />'. <br /> <br />'. <br /> <br />critical period of record. By setting the surplus threshold storage requirement equal to the <br />storage required to provide scheduled deliveries through the critical period plus an assumed <br />target protection level, a surplus would be determined only when there was sufficient <br />mainstream water available for release to satisfy annual consumptive use in the Lower <br />Division through the critical period of record. Such water would be in excess of 7.5 maf <br />annually required and could be determined as surplus. <br /> <br />CRITICAL PERIODS OF RECORD. An investigation of the critical periods of record for <br />. Lake Mead was undertaken. The studies found that the beginning of the critical period of <br />record corresponds to 1953 hydrology and that the duration is dependent on system wide <br />depletion levels. For levels of system-wide depletion less than about 11.8 maf, the duration <br />of the critical period is shown to be 12 years. For levels of depletion equal to and greater <br />than about I I. 8 maf, the duration is shown to be 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 />SURPLUS THRESHOLD STORAGE REQUIREMENT. A Surplus Threshold Storage <br />Requirement was then defined as that amount of water required to be in storage at the <br />beginning of the year which would be sufficient to satisfy 7.5 maf of deliveries to the lower <br />basin each year during the critical period without the potential for the Lake Mead storage to <br />drop below an assumed target protection level. Shortage was assumed to be determined at a <br />threshold elevation of 1120 feet in Lake Mead, which is designed to protect against storage <br />levels falling below 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 />show the storage required in Lake Mead entering the critical period to protect against Lake <br />Mead storage dropping below 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 9. The <br />surplus threshold storage requirement appears to increases as the critical period of record <br />shifts from a I~ 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 2, in 1995 total use <br />in the Upper and Lower Division equals 10,689 thousand acre-feet (Kat). 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 9 the surplus threshold <br />storage requirement in Lake Mead starts out at about 50% capacity and increases sharply <br /> <br />20 <br />