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<br />Lake Mead is 'banker' <br />for project shortages, <br />but equity is limited <br /> <br />co <br /> <br />~. <br /> <br />C".; <br />r-: <br /> <br />~ <br /> <br />Lake Mead will be the Cen. <br />tral Arizona Project's water <br />bank. When runoff into the <br />Colorado River is insufficient to <br />supply Arizona with its full right <br />to 2.8 million acre-feet of water, <br />the difference will come from <br />water stored in Lake Mead, <br />behind Hoover Dam. <br />The water bank, however, has <br />a limit. This is because at some <br />future time, the CAP, with other <br />users, will withdraw water faster <br />than it is replaced. <br />Regardless, the CAP should be <br />able to make 80me withdrawals <br />each year, with the Quantity of <br />water it gets dep~ndent upon <br />annual runoff into the Colorado <br />River. <br />Since 1906, runoff has ranged <br />from 6 million to 24 million acre. <br />feet per year and has averaged <br />about 15 million. <br />Other factors in how much <br />water the CAP gets are how the <br />Bureau of Reclamation manages <br />the river and how rapidly Upper <br />Basin states put their Colorado <br />River. entitlements to use. <br />. . . <br />The CAP, when it goes into <br />operation, will have a large water <br />reserve in Lake Mead because <br />runoff since 1978 has filled it and <br />other Colorado River reservoirs. <br />The West appears to be in a <br />cycle of wet years, which means <br />good runoff. The cycle appar- <br />ently began in 1978. No one <br />knows when this will change, but <br />the wet and dry cycles have <br />lasted for about 25 to 35 years. <br />When the river's runoff goes <br />into a dry cycle, the CAP will <br />depend upon its Lake Mead <br />bank account for about 1 million <br />acre-feet of water per year, which <br />will enable Arizona to use its full <br />entitlement of 2.8 million acre- <br />feet. <br />. . . <br />The total annual consumption <br />of water from Lake Mead will be <br />about 9.5 million acre-feet. Be- <br />sides Arizona's 2.8 million acre- <br />feet, this includes California's <br />4.4 million, Nevada's 300,000, <br />Mexico's 1.5 million and water <br />10S868, mainly through evapora- <br />tion, 500,000. <br />Some of the losses are made <br />up by runoff between Lake Mead <br />and Lake Powell, 360 miles <br />upstream, so water releases from ! <br />Lake Mead will total about <br />9.2 million acre-feet per year. <br />Most of the water from Lake <br />Mead first will have entered <br />Lake Powell behind Glen Can- <br />yon Dam, built to help the Upper' <br />Basin meet a legal requirement <br />that it supply no less than <br />75 million acre-feet of water to <br />the Lower Basin in any consecu- <br />tive IO-year period. <br />In addition, the Upper Basin <br /> <br />'must Q!.2vide 750,000 acre-feet <br />per-year--iQ.: :mee:rtli€t-~U.S:- <br />obligiifiOli'or 1.5 million acre-feet <br />~-iinnu'ally. <br />Un{ler the Bureau of Reclama- <br />tion's current operating criteria <br />for the Colorado River, the <br />Upper Basin is, required to <br />release a minimum-'or 8.23.:mil- <br />lion acre-feet per year from Lake <br />Powen - intO Lake Mead. The <br />Upper Basin also is credited with <br />20,000 acre-feet annually from <br />the Paria River, which flows into <br />the Colorado River downstream <br />from Lake Powell. <br />Hence, the differenc.f!..btt\y:eep <br />demands onWe"Mead and <br />wlfat 'flows in from upstream is <br />about 1 million acre-feet. Lake <br />Mead will supply this difference <br />from water in storage and, in the <br />view of the Arizona Department <br />of Water Resources, could come <br />close to doing it every year if <br />runoff into the Colorado River is <br />average during the 5O~year CAP <br />repayment period. <br />. . . <br />The Bureau of Reclamation <br />wants to manage Lake Mead so <br />it always will be able to provide <br />'water for the generation of <br />electricity in the Hoover Dam <br />powerhouse. <br />Arizona agrees with that goal, <br />hut not with the hureau's plan <br />for doing it. The bureau wants to <br />declare a water shortage when <br />the level of Lake Mead drops to <br />1,124 feet above sea level, or 41 <br />feet above the minimum eleva- <br />tion of 1,083 feet at which power <br />can be produced. <br />Arizona contends that more <br />water will be available in Lake <br />Mead than planned for by the <br />bureau because the Upper Basin <br />state8 will not increase their use <br />of water as fast as the bureau <br />predicts. <br />Regardlesa, should runoff into <br />the river enter a dry cycle to the <br />point where the choice is be. <br />. tween water deliveries and power <br />production, the former has prior- <br />ity over the latter. The states <br />could demand their water re- <br />gardless of power production. <br />I Storage capacities in Lake <br />I Mead (see chart above) will drop <br />f in future years because of silt <br />; carried into the lake. By Jan. 1 of <br />each year, 5.35 million acre-feet' <br />of vacant space must be available <br />in Lake Mead and Upper Basin <br />reservoirs to provide space for <br />runoff. <br />This year, because of the high <br />runoff in the river, the Bureau of <br />Reclamation expects to have <br />about 5.75 million acre-feet of <br />vacant space by Jan. 1. <br />This is a flood-control require- <br />ment, which has priority over <br />water deliveries and power gen- <br />eration. <br />