PHOENIX — Climate change and a growing demand for water could drain two of the nation's largest man-made reservoirs within 13 years, depriving several Southwestern states of key water sources, scientists warn.

Researchers at San Diego's Scripps Institution of Oceanography said Wednesday that there's a 50 percent chance that lakes Mead and Powell will dry up by 2021 and a 10 percent chance the lakes will run out of usable water by 2013.

"We were surprised that it was so soon," said climate scientist David Pierce, co-author of the institution's study that detailed the findings.

The study, which was released Tuesday, found that if current conditions persist, there's a 50 percent chance the reservoirs will no longer be able to generate hydropower by 2017.

Lake Mead, on the Arizona-Nevada border and the West's largest storage reservoir, and Lake Powell, on the Arizona-Utah border, have been hit hard by a regional drought and are half full. Both lakes were created by dams built on the Colorado River, which provides water for about 27 million people in seven states.

Researchers said that if Lake Mead water levels drop below 1,000 feet, Nevada would lose access to all its river allocation, Arizona would lose much of the water that flows through the Central Arizona Project Canal, and power production would cease before the lake level reached bottom.

Larry Dozier, deputy general manager at the Central Arizona Project, which supplies Colorado River water to the Phoenix and Tucson areas, called the Scripps study "absurd."

"I think they must have made some pretty outrageous assumptions to come up with some outrageous conclusions," he said. He said his agency's own study of the water levels in the two lakes showed they were in no danger of drying up.

"You can't get there from here," he said. "You can't make it go dry in that situation using any rational set of assumptions."

Pierce said the conclusions in the Scripps study are based partially on an estimated reduction in runoff of 20 percent over the next 50 years. He said that figure was used because it split the difference between the 10 percent to 30 percent decrease in runoff the Intergovernmental Panel on Climate Change predicts will occur over the next 50 years.

Scott Huntley, a spokesman for the regional Southern Nevada Water Authority in Las Vegas, said 90 percent of the region's water comes from the Colorado River and that government officials are committed to not letting the Lake Mead reservoir dry up.

He pointed to an agreement signed in December by the seven states and Interior Secretary Dirk Kempthorne to conserve and share scarce water if the Colorado River drought continues.