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I. I . . 4 cricical ?eriod of record. The formula, considering total ~ater su~~ly, available reservoir storage, and total deMands, i. shown below: Allowable .ARn,l Upper S&lin Depletion . (Useable Reservoir Storage ~ number of years in critical period) . average annual streamflow at Lees Ferry 1/ for critical period - annual delivery co Lower Sasin . annual shortage Calculation of the 5.8 MAl Up~er S..in de~leti~n limit is ex~lained a. follows: auervoirs Lake Powll Fllll1na Gorce Navajo Blue He.. Monow Point Cr,.tal Storage 27,000 3,789 1,709 941 117 2S ~ (l.OOO's of acre-feet) Minimum Pover Pool 6,124 - 273 673 192 75 12 Available 20,876 3,516 1,036 749 42 13 26,232 The bottom line figure, 26.232 MAl, is added to che total virlin flow duriac the mo.t critical 34-year period (1931-1964) at 439.417 MAP. Thi. equals 465.649 MAl. In es.ence, this mesns chat che storage project adds water durina the .oet cr1t~ ical period flow. Therefore, the most cricical period flow witb the storage project is 465.649 MAl. divided by 34 years. which equals 13.695 KAY; this tlow, les. Colorado River Com~act and Mexiean Treaty deliveries. equals 5.446 MAl. This final numbet re~resents U~~er Basin ae~letions assuming no shortages. If an average shortage of 6.11 percent is assumed (2.1 MAP of M&I uses with no shortage and 3.7 MAP of irrigation with an aver~ge shortage of 10 percent). we divide 5.446 MAP by 93.89 percent which gives the 5.8 MAl a~~er Basin pos.ible depletion. The effect of sediment accumulation in Lake Powell ~as also evaluated. Preliminary planning .tudie. indicate that the total IOO-year volume of accumu- lated sediment in Lake Povell would be 8.54 MAl. 3.9 MAl of which would be expected to deposit above the minimum power pool. Consideration at chis volume of sediment in the above equation changes the com~uced depletion by only 2 percent. .'~ere are two different hydrology data sets available that were used with the formula to ..algate and verify the present 5.8 MAP decerminacion. The hydrology data seU qe used with the fomula to determine average streamflow and the critieal period. These are discussed below: I. Virgin Flows - Virgin flows are com~uted. undepleted flows at Lee Ferry. They are available on an annual basis for the period 1896 to 1981. 1/ For hydrologic data interpretation purposes, Lees Ferry is a point on the Colorado River just above the Paria River, where a river flow gauge 1s located. Lee Ferry represents a eompact point between the Upper and Lover Colorado River Basins. No flow gauge is located at Lee Ferry; Upper Colorado River flows are computed from the Lees Ferry gauge and a flow gauge at the mouth of the Paria River.