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~j)nnsn WATER SUPPLY FOR THE SAN JUAN-CmMA PROJECT 5 j>,..-"' The balance of 1,697,000 acre-feet anticipated to be made by year 2062 would be for potential projects not presently identified. Pre- liminary data for the section 2 projects shown in Public Law 485, exclusive of those identified in the foregoing tabulation, indicate a total depletion of about 800,000 acre-feet. There will probably.be some additional use by non-Federal developments for municipal and industrial purposes in various parts of the Upper Colorado River Basin. The studies show that with an average annual depletion of 6.2 million acre-feet including reservoir losses (2020 conditions), the aver- age annual release from Glen Canyon Reservoir would be about 9.1 million acre-feet. It is the Bureau's position that, to realize upper. basin uses which would deplete the flows at Lee Ferry beyond an average of about 6,200,000 acre-feet annually would require addi- tional holdover storage reservoirs in the upper basin, and that, as need therefor can be demonstrated, it is presumed that the .additional reservoirs contemplated in the ultimate plan will be authorized, construe Led, and operated. In summary, the Bureau estimates that authorized storage units provide sufficient storage for expected de-, pletions in the upper basin during the next 100 years. Co'mments and conclusions Water operation studies show how a project is expected to be operated under future water conditions. They assume a repetition of past flow conditions for a particular period. This is the most accurate and reliable method available for predicting the future water supply situation and planning the project operation. While it can be said that the longest period of record, assuming reliable data, will produce the most accurate estimate of the future average annual flow conditions, it is the extended drought conditions which occur during the overall period which become the critical factor in project design, in determining the storage needed, etc. '1'he first conclusion that can be drawn from my examination of the operation studies summarized above is that there are no appreciable dIscrepancies in these studies so far as physical data are concerned. The different end results are produced by using different periods of study, different assumptions, etc. Of course, the studies were made for different purposes and had different objectives. For example, the State of Colorado had the Hill study made be- cause of its concern as to whether water was available in the quantities proposed for transmountain diversion and at the same time meet the future needs in that part of the State west of the Continental Divide. The Hill study was intended to produce a safe or conservative estimate of water available. Yet the results of the Hill study are more favor- able to the upper baSin than the results of the Bureau of ReclamatiOn studies which are used for planning purposes. Mr. Hill concluded on the basis of a 22-year period of study during which the historicai runoff at Lee Ferry "averaged only 11.7 million acre-feet per year and assuming 21 million acre-feet of reservoir capacity, that upper basin depletions could amount to 6.2 million acre-feet per year. In com- parison, the Bureau came up with the same depletion figure for plan- ning purposes on the basis of a 54-year period of study (during which the historical flow at Lee Ferry averaged about 13,620,000 acre-feet) with the storage capacity provided by the authorized units (26,853,000 acre-feet initially, with 22,443,000 acre-feet remaining in the year 2062). 10389-l61~ c.". . "~ ~:~_: '\,~:' -',:" ~, J',~- "<"c '_.i.'