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Rivers in some of the world's most populated regions are losing water - many due to climate change - according to a new study of global stream flow.

The research, to be published next month in the American Meteorological Society's Journal of Climate, was led by scientists at the National Center for Atmospheric Research (NCAR) in the US, and warns that the reduced flows could potentially threaten future supplies of food and water.

The scientists, who examined stream flow from 1948 to 2004, found significant changes in about one-third of the world's largest rivers. Of those, rivers with decreased flow outnumbered those with increased flow by about two-and-a-half times.

Several of the rivers channeling less water serve large populations,including the Yellow River in northern China, the Ganges in India, the Niger in West Africa and the Colorado in the southwestern United States.

In contrast, the scientists reported greater stream flow over sparsely populated areas near the Arctic Ocean, where snow and ice are rapidly melting.

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

Many factors can affect river discharge, including dams and the diversion of water for agriculture and industry - but the reduced flows in many cases appeared to be related to global climate change, which is altering precipitation patterns and increasing the rate of evaporation.

The results are consistent with previous research by Dai and others showing widespread drying and increased drought over many land areas.

The study raises a wide variety of concerns. For example, freshwater flow from the world's rivers affects global ocean circulation patterns, which are driven by changes in salinity and temperature, and which play a vital role in regulating the world's climate.

Although the recent changes in the freshwater discharge are relatively small and may only have impacts around major river mouths, Dai said the freshwater balance in the global oceans needs to be monitored for any long-term changes.

Conflicting studies

Scientists have long been uncertain about the impacts of global warming on the world's major rivers.

While recent studies with computer models indicate that many rivers outside the Arctic could lose water due to decreased rainfall and increased temperatures, earlier, less comprehensive analyses of major rivers have suggested that global stream flow was increasing.

Dai and his co-authors analyzed the flows of 925 of the planet's largest rivers, combining actual measurements with computer-based stream flow models to fill in data gaps. The rivers in the study drain water from
every major landmass except Antarctica and Greenland, and account for 73 percent of the world's total stream flow.

Overall, the study found that, from 1948 to 2004, annual freshwater discharge into the Pacific Ocean fell by about 6 percent, or 526 cubic kilometers.

The annual flow into the Indian Ocean dropped by about 3 percent, or 140 cubic kilometers.

In contrast, annual river discharge into the Arctic Ocean rose about 10 percent, or 460 cubic kilometers.

The impacts of melting

Some rivers, such as the Brahmaputra in South Asia and the Yangtze in China, have shown stable or increasing flows - but they could lose volume in future decades with the gradual disappearance of the Himalayan glaciers feeding them, the authors warned.

"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, a co-author
of the study.