April 20 (Bloomberg) -- Farms, businesses and residents who rely on the Colorado River may face water supply cuts as early as next year as drought and global warming reduce snowmelt while demand rises, a study said.

The water supply for 27 million people in the southwestern U.S. and Mexico can’t be sustained, said Tim Barnett, a marine physicist with the Scripps Institution of Oceanography, a research center in San Diego. Supplies will fall short even with new conservation rules, said Barnett, who co-wrote the study appearing today in the Proceedings of the National Academy of Sciences.

Storage reservoirs along the 1,450-mile (2,330 kilometer) river that flows through the Grand Canyon are at 54 percent of capacity, down from 89 percent in 2000, according to the U.S. Bureau of Reclamation. Drought has reduced snowfall in the region the last nine years to the lowest of any period that length in the past century.

“It’s clear with natural climate change or human-induced climate change that we’re not going to be able to deliver all that water,” Barnett said today in an interview. “The only way out of these climate-change impacts is simply not to take as much water out of the river.”

Global warming from fossil fuel emissions such as coal- fired power plants will reduce rain and snow runoff 10 percent to 30 percent by 2050, Barnett said in the study. A 20 percent reduction in runoff may lead to water shortfalls in one of every three years beginning in 2010, Barnett said.

Missed Deliveries

By 2050, river managers will have to cut supply about 88 percent of the time, with an average shortfall of about 1.26 million acre-feet of water annually, the report said. The river provides about 12.7 million acre-feet of water a year to homes, businesses and to irrigate 3 million acres (1.2 million hectares) of farmland, including California’s Imperial Valley.

An acre-foot of water is 326,000 gallons, enough to supply two households for one year.

“We’ve published these kinds of results before,” Terry Fulp, the U.S. Bureau of Reclamation’s deputy regional director for the lower Colorado region, said in an interview. “The bottom line is, as we know, the system is over-allocated.”

The Colorado River starts on the western slope of the Rocky Mountains and empties in the Gulf of California, passing through five U.S. states. Heavy use for farm irrigation has desiccated the lower river in Mexico and it no longer consistently reaches the sea.

Seven states are allocated a portion of the river’s water, including California. In February, Governor Arnold Schwarzenegger declared a state of emergency, saying three years of drier-than-normal weather have intensified the state’s drought.

River Is ‘Fully Subscribed’

The four states in the upper basin of the river, Utah, Colorado, Wyoming and New Mexico, will by 2060 be seeking an additional 1.5 million acre-feet of water a year to meet urban demand, Fulp said.

“From our numbers, the river’s being fully subscribed right now,” Barnett said. “I don’t see where they’re going to get that water.”

In late 2007, California, Arizona and Nevada took measures to prevent large reservoirs, including Lake Mead and Lake Powell, from dropping too low. The states agreed to supply cuts if the water level in Lake Mead falls to 1,075 feet. The reservoir is now 29 feet above that.

The soonest river managers are likely to cut supplies under the agreement is between 2011 and 2012, Fulp said.

Rising Temperatures, Earlier Snowmelt

The Colorado River Basin, which stretches from Wyoming to Mexico, is at the center of climate change in the western U.S., according to the Natural Resources Defense Council, a New York- based environmental group. Since the 1970s, rising temperatures have contributed to earlier snowmelts and the basin has warmed more than any other area in the contiguous U.S., the group said.

As a result, the volume of water in the basin is decreasing. Lake Powell is only 52 percent full and Lake Mead is 46 percent full, according to the Bureau of Reclamation.

A 2008 Scripps Institution study said Lake Mead had a 50 percent chance of going dry within 20 years without conservation. In today’s study, Barnett and co-author David Pierce, a Scripps climate researcher, focused on how global warming will affect water supplies.

“We may be sustainable now, but we won’t be for very much longer,” Barnett said. “I’d like to see people sit down and start taking this seriously now before there’s a crisis at hand.”

To contact the reporter on this story: Jim Efstathiou Jr. in New York at jefstathiou@bloomberg.net

Last Updated: April 20, 2009 17:00 EDT