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0U2Q31 34 ASSEMBLY INTERIM COMMITTEE OJ:'! WATER It can be secn the amounts vary greatly from the dry periods to the wet periods and the 1909-62 average is substantially less than the average from 1909-29, which was the flow used for consideration of the Boulder Canyon Project Act. Of course. the selection of water periods can effectively determine whether or not a surplus is indicated for a given area. Care must be exercised in dealiug with Colorado River flow statistics so as to not mislead. The total main stream supply on the basis of the entire period 1896- 1962 as estimated by the B1<reau of Reclamation is summarized as follows: Million A.F. Virgin flow at Lee Ferry __________________________________________ 14.9 Historic net gain Glen Canyon to Hoover Dam _______________________ .9 Undepleted flow Bill Williams River _~___________~__________________ .1 Present depletions Glen Canyon to Hoover Dam ______________________ .2 Total long-time average virgin water supply __________________________ 16.1 Item Upper Basin depletion __________________________ Channel, control, and evaporation losses __________ Useable water supply ___________________________ The differences in the above supply figures and those of the Colorado River Board are shown on the chart on the following page prepared by the Colorado River Board of California.H The bureau figures are based upon the 1896-1962 runoff period while the Colorado River Board figures are based on the period of 1922-1962. According to the Department of the Interior in addition to the differences as to runoff there are three other reasons for the differences between the estimates of the two agencies. Each, to a large extent results from judgment factors: 1. The CRB chart estimates a faster rate of depletion by the Upper Basin states than does the bureau. 2. The CRB chart reflects a higher estimate of future channel and control losses below Parker Dam than does the bureau. 3. The CRB chart reflects a lower estimate of useable water supply due primarily to different assumption as to reservoir operations. The breakdown of these three separate reasons, each of which reflects the availability of additional water for use at and below Hoover Dam of the bureau estimates for the period 1975-2000, is as follows: Differernce (1000 A.F.) 1990 500 365 390 1975 400 335 500 13000 270 365 445 Total ______________________________________ 1,235 1,25.5. 1,080 11 A detailed explanation of the criteria utilized in the board's estimates can be found in the appendix to the hearing transcript, August 13, 1964.