Climate change drying up big rivers, study finds

* More rivers lose water than gain, even with ice melt

* Flow decreases as population needs increase

WASHINGTON, April 21 (Reuters) - Rivers in some of the world's most populated regions are losing water, many because of climate change, researchers reported on Tuesday.

Affected rivers include the Yellow River in northern China, the Ganges in India, the Niger in West Africa, and the Colorado in the southwestern United States.

When added to the effects from damming, irrigation and other water use, these changes could add up to a threat to future supplies of food and water, the researchers reported in the American Meteorological Society's Journal of Climate.

"Reduced runoff is increasing the pressure on freshwater resources in much of the world, especially with more demand for water as population increases," Aiguo Dai of the National Center for Atmospheric Research in Boulder, Colorado, who led the study, said in a statement.

"Freshwater being a vital resource, the downward trends are a great concern."

Dai's team looked at records of river flow in 925 big rivers from 1948 to 2004, finding significant changes in about a third of the world's largest rivers.

Rivers with decreased flow outnumbered those with increased flow by 2.5 to 1, they said.

For instance, annual freshwater discharge into the Pacific Ocean fell by about 6 percent, or 526 cubic kilometers -- about the equivalent volume of water that flows out of the Mississippi River each year.

Annual river flow into the Indian Ocean dropped by about 3 percent during the 56-year period, or 140 cubic kilometers.

The Columbia River in the U.S. Northwest lost about 14 percent of its volume from 1948 to 2004, largely because of reduced precipitation and higher water usage in the West, Dai's team said.

But the Mississippi River drains 22 percent more water because of increased precipitation across the U.S. Midwest since 1948, they said.

Annual discharge from melting ice into the Arctic Ocean also rose about 10 percent, or 460 cubic kilometers.

"Also, there is evidence that the rapid warming since the 1970s has caused an earlier onset of spring that induces earlier snowmelt and associated peak streamflow in the western United States and New England and earlier breakup of river-ice in Russian Arctic rivers and many Canadian rivers," the researchers wrote.

"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," said NCAR scientist Kevin Trenberth, who worked on the study.

(Reporting by Maggie Fox, editing by Vicki Allen)