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<br />O()183~ <br /> <br />which assures that Mexican Treaty obligations of 1.5 million acre- <br />feet/year are met, that California's entitlement through the All- <br />American Canal is satisfied, and that sufficient water is available <br />for diversion through the Gila Gravity Main Canal. MWD diversions <br />through the Colorado River Aqueduct were also reduced to their <br />ultimate allotment of 550,000 acre-feet/year from current levels <br />of about 1.2 million acre-feet. <br /> <br />Table V presents the mean monthly and annual quantities of water <br />involved in the water budget for the base run. These serve as a <br />basis for modifying depletions for future runs. The upper bound- <br />ary and the ungaged tributary flows are constant for all runs. <br />Since the base run was intended to duplicate results of the Biennial <br />Report, reservoir storage was permitted to change. The change in <br />volume, including bank storage, is shown and must be considered in <br />adjusting depletions for future runs. The mass balance error of <br />the base run using the figures shown, is 10,000 acre-feet on an <br />annual basis. <br /> <br />Evaporation amounts for the base run are also compared to those <br />assuming the initial contents represent the average content dur- <br />ing the run. The differences are seen to be relatively slight, <br />and are typical of those experienced with the various runs. <br /> <br />The maximum change in storage was desired to be less than 2 million <br />acre-feet, including bank storage, during the 30 years. This is <br />equivalent to 67,000 acre-feet/year or slightly more than 5,000 acre- <br />feet/month, which is well within the accuracy of the evaporation <br />rates used. Because it is impossible to determine the evaporation <br />amounts exactly prior to a run, the allowable change provided a <br />"cushion" in estimating. It was found that estimates were suffi- <br />ciently accurate to stay within the maximum for most conditions. <br />A smaller tolerance would have required several runs to meet it. <br /> <br />Finally, although the maximum change was not usually exceeded, <br />there was a shift in storage among the reservoirs. This shift <br />is primarily a function of the sequence of inflows and the pat- <br />tern of depletions. Storage changes are tabulated for all <br />reported runs. <br /> <br />(5) Interpretation of results - As previously noted, all sta- <br />tistics are time weighted and the sequence of boundary and ungaged <br />tributary inputs represents but one possible sequence. Although <br />numbers are computed month-by-month, conclusions based on the <br />sequence of hydrologic events may be misleading. Their cautious <br />use in advised. Thus, the statement that under given developments, <br />the salinity will be at a certain level in 1980 cannot be modified <br />with a measure of confidence. This would require generating data <br /> <br />21 <br />