Global warming to hurt Colorado River, report says

Global warming will worsen drought and reduce flows on the Colorado River, a key water source for several Western states, according to a report released Wednesday.

The study, prepared by a National Research Council committee, paints a sobering picture as the water needs of a rapidly expanding population test the limits of a river system strained by the effects of climate change.

“The basin is going to face increasingly costly, controversial and unavoidable trade-off choices,” said Ernest Smerdon, chairman of the panel of academicians and scientists who wrote the report. “Increasing demands are impeding the region’s ability to cope with droughts and water shortages.”

Such measures as conservation, desalination and water recycling would be helpful, the authors said, but wouldn’t offer a panacea.

The report, which examined climate modeling and tree ring data, reaffirms a more pessimistic assessment of river hydrology that has emerged in recent years.

Scientists have concluded that the Colorado river system, which supplies water to 25 million people and several million acres of crop and ranch land, has been drier and more prone to severe drought than was the case in the early 20th century, when the river’s flows were divvied among the seven basin states.

It turns out that period was unusually wet, prompting an overly generous estimate of how much water would be available to farms and cities. Ancient tree rings, which provide graphic evidence of past precipitation patterns, indicate it had been three centuries since the basin was last awash with that much water.

The most recent drought began in 2000 and has left the river’s biggest reservoirs, Lake Mead and Lake Powell, roughly half empty.

Global warming will only make matters worse, said Connie Woodhouse, a University of Arizona associate professor of geography who helped write the report. “It’s going to enhance the droughts” that are part of the natural climate cycle.

Although the river’s flows returned to normal in 2005 after five years of dramatic lows, they fell again last year. Federal scientists say that if they remain below average or even average, Lake Mead might never completely refill.

The study, which was sponsored by the National Academies, the U.S. Bureau of Reclamation and several water agencies, notes that temperatures have risen in the Western United States over the past century and are expected to keep climbing.

“All of the models are predicting very significant warming in the future,” said Smerdon, a former engineering college dean at the University of Arizona in Tucson.

The report acknowledges some uncertainty over whether climate change would alter the amount of precipitation in the basin. But as the mercury rises, water demands would increase. There would be more evaporation from croplands and reservoirs, and wild land vegetation would suck more moisture from the soil, reducing runoff.

The U.S. Department of Interior, which oversees the Colorado’s management, is reviewing a drought plan the states put together last year. Because California holds some of the most senior rights to the river, Arizona, Nevada and Mexico would experience cuts in deliveries before California, said Roger Patterson, assistant general manager of the Metropolitan Water District of Southern California.