The Colorado River Basin is more prone to drought than had been thought, a panel of experts reported yesterday, and as the climate warms and the population in the region grows, pressure on water supplies will become greater. The severe droughts the region suffered in the 1990s and early 2000s would not stand out in the record of the last few centuries, the panel said, and the future presents “a sobering prospect for elected officials and water managers.” The panel said residents of the region should prepare for more frequent and more severe dry spells, and “costly, controversial and unavoidable trade-offs” in water use.

The data discussed in the report have been published before in scholarly journals and elsewhere. But Ernest T. Smerdon, a former dean of the College of Engineering and Mines at the University of Arizona, who led the panel, said its members hoped with this publication to pull the findings into a single document that ordinary people could understand. Severe droughts will recur, Dr. Smerdon said, “and we better be prepared. That is the message.” He spoke at a news conference yesterday in Las Vegas, where the report was made public.

The panel recommended an “action-oriented” study of water use patterns and demands, including drought planning, population projections and possible effects of transferring water to urban areas from agriculture, still the dominant consumer. Dialogue between policy makers and scientists who study water issues should be “a permanent fixture within the basin,” it said. The panel, organized by the National Research Council, the research arm of the National Academy of Science, noted that the water allocation agreement for the basin, the Colorado River Compact, was negotiated in 1922 based on river flow records dating to the 1890s, when gauging stations were established. The agreement assumed that the annual river flow was 16.4 million acre feet — enough to cover 16.4 million acres to a depth of one foot.

But for some time, the panel said, researchers have known that the early 20th century was unusually wet and that 15 million acre feet was a more accurate estimate of the flow. Recent studies based on tree rings put the figure lower still — as low as 13 million acre feet — and suggest that “drought episodes are a recurrent and integral feature of the region’s climate.” Because trees grow more when it is wet, scientists use tree ring size as an indicator of water abundance. The report says the federal Bureau of Reclamation and other agencies requested the panel’s review in the wake of the new findings.

Global warming is already making things worse, the experts said. For one thing, warmer weather means less precipitation in the form of snow, which is stored in the region’s mountain snowpack. And the snowpack itself forms later and melts sooner each winter. As a result, the steady reliability of snowpack water storage is compromised. Also, warmer weather itself increases consumer, environmental and agricultural demands for water. Rainfall patterns are difficult to predict, another panel member, Connie A. Woodhouse, a geographer at the University of Arizona, said at the news conference. But the report said it was probable that the region would experience less precipitation over all in a warmer world.

Cloud-seeding, water desalinization and improved underground water storage have yet to emerge as solutions, the report said, and even conservation, while helpful, “is no panacea,” Dr. Smerdon said. The report, which is available at www.nationalacademies.org, notes that the basin, 240,000 square miles in Wyoming, Utah, Colorado, New Mexico, Arizona, Nevada, California and Mexico, has seen rapid population growth in recent decades. Until about 30 years ago, the panel wrote, growing demands for water were met through building dams and reservoirs. But today, the report says, “prospects for constructing additional large dams in the Colorado River basin have diminished.”

Instead, “there is going to have to be some kind of reallocation of who gets the water,” said Richard Seager, a climate expert at the Lamont Doherty Earth Observatory, who was not involved in the panel’s report. Dr. Seager, who studies the drought history of North America, said that it was “silly” to put golf courses in the region’s desert areas and that hotels and other businesses were already installing water-conserving toilets and other fixtures. But he added, referring to cattle and cotton raising, “Let’s think whether it makes sense to have all this subsidized agriculture in the region, people who aren’t even paying the full cost of the water they do use.”
(Por Cornelia Dean, The N.Y.Times, 22/02/2007)