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<br />livery to Mexico. The interim deficit of water to
<br />the systel;n will be replaced by water savings
<br />resulting1from the construction of a
<br />concrete~lined canal generally parallel to the
<br />first 49-,flile reach of the existing unlined
<br />Coachell" Canal. The water saved. estimated at
<br />about 132,000 acre.feet annually, will repre-
<br />sent a part of California's entitlement. How-
<br />ever, until the water saved is required by
<br />these usets, it can supplement or replace
<br />water from storage that has been released to
<br />Mexico anD not counted as part of the sched-
<br />uled treat)/ deliveries. Plans also call for the
<br />permanent replacement of reject brine water
<br />from the desalting plant.
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<br />
<br />Tributaries;
<br />Tables lC-4 through lC-8 show water
<br />uses by selected tributary areas, by States,
<br />, and by type of use. Onsite consumptive use
<br />in 1971 was estimated to be about 3.8 million
<br />acre-feet. ey 1975, consumptive use was
<br />about 4.5 million acre-feet as a result of a
<br />substantial iincrease in both irrigated acreage
<br />and population. Over half of the consumptive
<br />use is satisfied from ground water overdraft.
<br />Irrigated larjd was estimated to be about
<br />954,000 aqres in 1971, and 1,090,000 acres
<br />in 1975. G4in in population has been on the
<br />magnitude of about 100,000 new residents for
<br />each year dQrinii: the period. Most of the in-
<br />crease in water use. irrigated land, and
<br />population h~S occurred in the Gila River Basin.
<br />
<br />Gila River
<br />Consumptive use for the irrigation of crops
<br />represents a~out 85 percent of the total water
<br />use in the Gi'la River Basin. Estimated an-
<br />nual consumbtive use per area for the entire
<br />basin during ;the 5-year period averaged
<br />about 3.5 acte-feet, varying from less than 1
<br />acre-foot per ;acre in parts of New Mexico to
<br />over 4 acre-f~et in the western portion of the
<br />basin. Crop cOnsumptive use varied consid-
<br />erably from y~ar to year on the basis of
<br />climatic conditions. Favorable economic
<br />conditions forifarming led to an increase in
<br />.
<br />irrigated land iof about 127.,000 acres.
<br />The consumptive use of water for municipal
<br />and industrial! purposes is estimated to have
<br />increased abo~t 42,000 acre.feet during the
<br />
<br />5-year period.
<br />Water supply conditions were charac-
<br />terized by exceptionally poor runoff in 1971
<br />and 1974. near normal runoff in 1972 and
<br />1975, and the occurrence of a major flood in
<br />1973. In addition to replenishing storage re-
<br />servoirs in the basin. the 1973 runoff produced
<br />an outflow below Painted Rock Dam of
<br />412.700 acre-feet during the water year.
<br />About 100,000 acre-feet of the outflow
<br />reached the Colorado River mainstream. Es.
<br />timated diversions during the 5-year period av-
<br />eraged about 5.6 million acre-feet, of which
<br />4.1 million acre-feet were from ground-water
<br />pumpage. The recent "Inventory of Re-
<br />sources and Uses, Arizona State Water Plan,
<br />Phase I-July 1975," prepared by the
<br />Arizona Water Commission and based on
<br />1970 development conditions, estimated an.
<br />nual ground-water overdraft to exceed 1.8 mil-
<br />lion acre-feet. In general; increased water uses
<br />within the basin since 1970 have added to
<br />the overdraft. The Central Arizona Project,
<br />scheduled for completion in 1985, would di-
<br />vert the remaining portion of the Arizona en-
<br />titlement of Colorado River water to central
<br />Arizona. reducing ground-water pumpage and
<br />consequently the overdraft.
<br />
<br />Other Tributary Areas
<br />Outside the Gila River Basin, and within
<br />the remaining tributary areas to the Colorado
<br />River mainstream, water resources are gener-
<br />ally limited and their development is less in-
<br />tensive. As shown in tables LC-4 through
<br />LC-8, total estimated consumptive use within
<br />the area increased from about 437,000
<br />.. acre-feet in 1971 to 475,000 acre-feet in
<br />1975. A lack of adequate surface-water stor-
<br />age facilities tends to make irrigated acreage
<br />subject to fluctuation from year to'year based
<br />on the variable and somewhat undependable
<br />runoff. Localized ground water overdrafts occur
<br />in parts of the area. With the exception of .
<br />las Vegas Valley, population is predomi-
<br />nantly rural. In las Vegas Valley, municipal
<br />and industrial demands are increaSing
<br />rapidly; however, these demands are being met
<br />by increased diversion from Lake Mead, as
<br />shown in table LC-3, and reliance on ground-
<br />water pumpage is being reduced.
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