SAN FRANCISCO - Chances are about even that Lake Mead, the prime source of water for the desert city of Las Vegas, will run dry in 13 years if usage is not cut back, according to study released on Tuesday.

The finding is the latest warning about water woes threatening the future of the fast-growing US casino capital and comes amid a sustained drought in the American West.

The study by two researchers at the Scripps Institution of Oceanography at the University of California San Diego calculates a 10 percent chance that Lake Mead will run dry in six years and a 50 percent probability it will be gone by 2021 absent other changes.

"Our reaction was frankly one of being stunned," study co-author Tim Barnett, a marine research physicist, said in an interview. "We had not expected the problem to be so severe and so up close to us in time."

Climate change - both man-made and natural variation - strong human demand and evaporation are all factors affecting water in the lake. "The biggest change right now is taking more water from the bucket than we are putting into it," Barnett said.

The uncertainty about when and if the lake will run dry stems from the natural fluctuations of the Colorado River, which feeds the lake, the researcher said. In recent months the flow has been above average, he said, after years below average.

The West has suffered years of drought with the Colorado supplying less water to Lake Mead, which serves Nevada, California, Arizona and northern Mexico.

The lake created by Hoover Dam provides 90 percent of Las Vegas' water and is less than half full, giving the edge of the lake a bath tub ring visible even far away by air.

Scott Huntley, a spokesman for the Southern Nevada Water Authority, said his agency overseeing the Las Vegas area's water was also concerned about reliance on Lake Mead as the major source for Las Vegas and officials were seeking alternate sources.

"While we wholeheartedly support the authors' call for greater urban water conservation, it is important to also remember that agriculture uses four-fifths of the Colorado River's flows, so meaningful solutions cannot be borne solely by urban users," he said.