April 21 (Bloomberg) -- Global warming is drying up rivers that provide water to residents of China, India, West Africa and the Southwest U.S., threatening food supplies, scientists said.

Stream flows in 45 of the largest rivers, such as the Yellow River in China and India’s Ganges, fell in the last half- century, according to the study by the National Center for Atmospheric Research, or NCAR, in Boulder, Colorado. Higher flows, conversely, were found in sparsely populated areas near the Arctic Ocean where snow and ice are rapidly melting.

While dams and the diversion of water for agriculture and industry can affect river discharge, the study found that changing precipitation patterns and faster evaporation related to warming weather worldwide led to reduced flows. In a separate study yesterday, scientists warned of water supply cuts from the Colorado River as early as next year.

“Reduced runoff is increasing the pressure on freshwater resources in much of the world, especially with more demand for water as population increases,” said Aiguo Dai, an NCAR scientist and the report’s lead author. “Freshwater being a vital resource, the downward trends are a great concern.”

The results will be published May 15 in the American Meteorological Society’s Journal of Climate.

Dai examined stream flows from 1948 to 2004 in 925 of the largest, ocean-reaching rivers. Of the top 200, such as the Ganges and the Congo in West Africa, stream flow in 45 fell as much as 25 percent below the long-term average, the report said. Nineteen rivers had upward trends.

Pacific Discharge

Overall, annual freshwater discharge into the Pacific Ocean fell by about 6 percent, or about the same amount that flows out of the Mississippi River each year. Annual flow into the Indian Ocean dropped by about 3 percent.

The flow of freshwater affects ocean circulation patterns, which help regulate Earth’s climate, the report said. While the recent changes are relatively small, the freshwater balance in the ocean needs to be monitored for long-term changes, Dai said.

Annual river discharge into the Arctic Ocean rose about 10 percent. Those higher flows are associated with less snow and ice cover, not higher precipitation, the report said.

“As climate change inevitably continues in coming decades, we are likely to see greater impacts on many rivers and water resources that society has come to rely on,” Kevin Trenberth, an NCAR scientist and co-author of the study, said in a statement.

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, said Tim Barnett, a marine physicist with the Scripps Institution of Oceanography in San Diego.

Colorado River

In yesterday’s report on the Colorado River, Barnett said a 20 percent reduction in runoff may lead to water shortfalls in the Southwest U.S. in one of every three years beginning in 2010.

Storage reservoirs along the 1,450-mile (2,330 kilometers) 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.

In the U.S., lower rainfall and higher demand led to a 14 percent declined in the flow of the Columbia River during the study period, according to today’s report.

“In the West and almost worldwide, the zero-sum game of water management is now becoming readily apparent,” Brad Udall, head of the University of Colorado’s Western Water Assessment, said in an interview. “All decisions are now becoming not what’s the best decision but what’s the least worst.”

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

Last Updated: April 21, 2009 12:58 EDT