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E-M:/ From Science News Online

Drink up!
  Science News Online
  Week of Feb. 23, 2008; Vol. 173, No. 8
  Going Down: Climate change, water use threaten Lake Mead
  Sid Perkins
  If climate changes as expected, and future water use goes unchecked, 
  there's a 50 percent chance that Lake Mead—one of the 
  southwestern United States' key reservoirs—will become 
  functionally dry in the next couple of decades, a new study suggests.
  [IMAGE]  BATHTUB RING. Water levels in Lake Mead have dropped 
  significantly in recent years, as chronicled by the white mineral 
  deposits on rocks that were formerly submerged.
  Besides providing water for millions, flow from Lake Mead—the 
  reservoir formed as the Colorado River collects behind Hoover 
  Dam—generates prodigious amounts of hydroelectric power. Over 
  the past century, on average, about 18.5 cubic kilometers of water 
  flowed into Lake Mead each year, says Tim P. Barnett, a climatologist 
  at the Scripps Institution of Oceanography in La Jolla, Calif. Of 
  that amount, about 2.1 km3 evaporate into the dry desert air or soak 
  into the ground beneath the lake each year. What's left in the lake 
  is more than spoken for: The amount of water drawn from Lake Mead 
  this year to meet demand in cities as far-flung as Los Angeles and 
  San Diego will exceed 16.6 km3.
  And the situation will likely get worse, Barnett and colleague David 
  W. Pierce speculate in an upcoming Water Resources Research. By 2030, 
  the researchers note, annual demand for Lake Mead's water is 
  projected to rise to 17.4 km3. Also, some climate studies suggest 
  that the Colorado's flow will drop between 10 and 30 percent in the 
  next 30 to 50 years. Using these data, as well as weather simulations 
  that impose random but reasonable annual variations in river flow 
  volume, Barnett and Pierce used a computer model to estimate the 
  remaining useful life of the Lake Mead reservoir.
  Thanks in part to the worst drought in the Southwest in the past 500 
  years (SN: 6/26/04, p. 406), Lake Mead is now at about 50 percent 
  capacity. If current allocations of water persist, there's a 50 
  percent chance that by 2023 Lake Mead won't provide water without 
  pumping, and a 10 percent chance that it won't by 2013. Moreover, 
  there's a 50 percent chance that Hoover Dam won't be able to generate 
  power by 2017, the researchers estimate.
  "We were stunned at the magnitude of the problem and how fast it was 
  coming at us," says Barnett.
  Results of the new study are "fairly provocative, an eye-opener," 
  says Connie Woodhouse, a climatologist at the University of Arizona 
  in Tucson. Using estimates of river flow based on an average of the 
  past century may be optimistic, she adds, because tree 
  ring–based reconstructions of the region's climate suggest that 
  the 20th century was one of the wettest in the past 500 years. "The 
  more we learn about the Colorado River and its hydrology, the more 
  worried we need to be," says Peter H. Gleick, a hydrologist at the 
  Pacific Institute in Oakland, Calif.
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  Barnett., T.P., and D.W. Pierce. In press. When will Lake Mead go 
  dry? Water Resources Research.
  Further Readings:
  Perkins, S. 2007.  Tree rings tell tale of megadroughts. Science News 
  171(June 23):397. Available to subscribers at 
  ______. 2004. Long dry spell. Science News 165(June 26):406. 
  Available to subscribers at 
  Tim P. Barnett
  Mail Code 0224
  Scripps Institution of Oceanography
  University of California, San Diego
  9500 Gilman Drive
  La Jolla CA, 92093
  Peter H. Gleick
  Pacific Institute
  654 13th Street, Preservation Park
  Oakland, CA 94612
  Connie Woodhouse
  University of Arizona
  Department of Geography and Regional Development
  412 Harvill Building, Box #2
  Tucson, AZ 85721-0076
  From Science News, Vol. 173, No. 8, Feb. 23, 2008, p. 115.
  Copyright (c) 2008 Science Service.  All rights reserved.
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