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<br />III. Methodology <br /> <br />The <br />for <br /> <br />concept paper offered the <br />identification of surplus <br /> <br />following justification and methodology outline <br /> <br />water: <br /> <br />.. <br /> <br />"There is little or no risk to reservoir storage under an <br />operating plan decision to meet fully the needs of the Basin <br />States for the operating plan year if reservoir system storage <br />recovers during the 60-month period under a probable minimum water <br />supply scenario. especially if required flood control releases in <br />excess of downstream needs. during that period. exceed the <br />increased water use provided for by the decision or if surplus <br />deliveries are made to Mexico." (Emphasis added) <br /> <br />This explanation was adopted as the methodology for this proposal. <br /> <br />IV. Hydrology <br /> <br />This study tests the effects of applying various probability levels of <br />hydrology to the Colorado River system. These hydrologys were derived from <br />the computed virgin flows for the Colorado River at Lee Perry as reported in <br />the Upper Colorado River Commission Forty Second Annual Report. Virgin flows <br />are reported for 'water years from 1896 through 1990. The 1991 virgin flow has <br />been initially estimated to be 11.2 million acre-feet by the Commission's <br />Chief Engineer. <br /> <br />The concept paper refers to determination of surplus governed by a "60-month <br />period under a probable minimum water supply scenario." This proposal <br />utilized the minimum probable five year hydrology to be the Sf. probability <br />nonexceedence .. <br /> <br />A fivB year moving average was applied to the virgin flo~s as displayed in <br />Table 1. The resulting five year moving averages were ranked, ~nd \,'eible <br />plotting positions applied to the values. The ranked data were treated with <br />five distribution methods including normal, log normal, log pearson, gumbel, <br />and leblein. These distributions are displayed in Pigure 1. The results <br />indicate that a log pearson distribution provides the best overall fit, but a <br />log normal distribution provides the best fit below the 25% probability of <br />nonexceedence as can be observed in Figure 2. Since the major emphasis of <br />this proposal would be in the low probability range, the log normal <br />distribution was selected. Figure 3 displays only the ranked virgin flows of <br /> <br />2 <br />