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U.S. GEOLOGICAL SURVEY
Data Series 340

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Discussion of Database Update

The major differences within the DVRFS model boundary between the previous and updated databases are that (1) estimated AR for high-water-use crops were based on new estimates of crop consumptive use and (2) the updated database extends ground-water withdrawal estimates through 2003. The updated database still shows the same overall spatial and temporal trends. In the three hydrographic areas with significant irrigation, the “likely” AR estimates for high-water-use crops are 1.5 ft/yr lower in Pahrump Valley, identical in Sand Spring Valley (known locally as Penoyer Valley), and 1 ft/yr lower in the Amargosa Desert (table 1). An overall decrease in the irrigation water use estimate results from adjusting the application rate. Estimates between the previous and updated databases decreased by 0 to 7 percent before 1978, and by 8 to 13 percent from 1979 to 1998. For example, the irrigation water use in 1998 decreased by about 13 percent from the previous estimate (60,500 acre-ft) to the updated estimate (52,800 acre-ft). Prior to 1979 estimates more heavily relied on literature values rather than on the crop coefficient method and thus were not influenced as significantly by the current evaluation (Moreo and others, 2003).

In the extended time frame of the updated database, the total ground-water withdrawal decreased from a maximum of about 69,800 acre-ft in 1998 to 62,900 acre-ft in 1999 to 55,500 acre-ft in 2000; from 2001 to 2003, the total ground-water withdrawal remained relatively constant. In 2003, about 55,700 acre-ft of ground water was pumped in the DVRFS, of which 69 percent was used for irrigation, 13 percent for domestic, and 18 percent for public supply, commercial, and mining activities (fig. 2).

Between 1998 and 2003 total ground-water withdrawal from Pahrump Valley decreased by 10,000 acre-ft, irrigation use decreased by 12,000, and public supply and domestic water use increased by about 2,000 acre-ft (fig. 3). This decrease in total water use was attributed primarily to agricultural fields removed from production in Pahrump Valley where irrigated acreage decreased from about 3,100 acres in 1998 to 2,400 acres in 1999 to 1,600 acres in 2000. By 2003, 1,700 acres had been converted from agriculture to residential land use. The maximum ground-water withdrawal in Pahrump Valley occurred during 1968 when 8,100 acres of cotton and alfalfa were irrigated with 46,000 acre-ft.

Total ground-water withdrawal from Amargosa Desert decreased from 1998 (21,100 acre-ft) to 2001 (14,100 acre-ft), and then increased through 2003 (17,600 acre-ft). Total ground-water withdrawal averaged 16,800 acre-ft from 1994 through 2003 and ranged from 14,100 to 21,100 acre-ft. The annual variation in total ground-water withdrawal is attributed primarily to crop and irrigation cycles (fig. 3). Alfalfa fields typically are fallowed 2 years during a 7-year period. Other fields are irrigated once every 5 years to demonstrate beneficial use and maintain water rights.

In 2003, 12 percent of reported irrigated acreage for Amargosa Desert was metered, including two of the larger fields, 125 and 112 acres. Application rates for these two fields were about 5 and 6 ft, respectively. These rates were on the low end of 5 to 9 ft/yr range estimated for the Amargosa Desert (table 1). It is unclear whether these application rates are representative of application rates in other fields that are not metered.

Ground-water withdrawals for nonirrigation use in Amargosa Desert ranged from 11 to 16 percent of total ground-water withdrawals between 1998 and 2003. Ground-water use for mining decreased from 2,400 acre-ft (11 percent of total water use) in 1998 to 1,200 acre-ft (7 percent of total water use) in 2003 because of the closure of the Bullfrog Mine owned by Barrick Gold Corporation. Domestic and public supply water use from 1998 to 2003 was less than 5 percent of the total water use.

Other significant areas of ground-water withdrawal are Penoyer Valley and NTS (fig. 3). Irrigation is the only major water use in Penoyer Valley, and did not change from 1998 to 2003 (about 12,600 acre-ft). Ground-water withdrawals supporting NTS activities ranged from about 670 acre-ft to 1,040 acre-ft between 1998 and 2003.

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