Some of the developing world's largest rivers are drying up because of climate change, threatening water supplies in some of the most populous places on Earth, say scientists.
Researchers from the US-based National Center for Atmospheric Research (NCAR) analysed data combined with computer models to assess flow in 925 rivers — nearly three quarters of the world's running water supply — between 1948 and 2004.
A third of these had registered a change in flow and most of them — including the Niger in West Africa, the Ganges in South Asia and the Yellow River in China — were dryer.
"Reduced run-off is increasing the pressure on freshwater resources in much of the world, especially with more demand for water as population increases. Freshwater being a vital resource, the downward trends are a great concern," said Aiguo Dai, a scientist at NCAR and lead author of the research.
Rivers are losing water for a variety of possible reasons, say the researchers, including the installation of dams and the use of water for agriculture. But in many cases the decrease in flow is because of climate change, which is altering rainfall patterns and increasing evaporation because of higher temperatures.
"The prospects generally are for rainfalls, when they occur, to be heavier and with greater risk of flooding and with longer dryer spells in between, so water management becomes much more difficult," Kevin Trenberth, head of the Climate Analysis Section at NCAR, told the UK's The Guardian newspaper.
Flow has increased in some developing world rivers. The Brahmaputra in India and China's Yangtze River are stable or have higher flows than in the past but this might not last long, say the scientists, as the Himalayan glaciers that feed them are disappearing.
As well as endangering water supplies, the decreased river flow could affect the world's climate. If less freshwater is discharged into the oceans they become saltier, which could affect salinity- and temperature-driven ocean circulation patterns that in turn play a fundamental role in climate regulation.
The research will be published in the American Meteorological Society's Journal of Climate next month (15 May).