By Jonathan Amos
Science reporter, BBC News, San Francisco
Africa has experienced a significant drying in the past three years, new satellite data reveals.
The Grace twins weigh the changes in the storage of water on land
The volume of water lost from the land amounts to 334 cubic km, which is almost as much as all Africans have consumed over the period.
The data comes from Nasa spacecraft that can detect changes in gravity caused by water as it cycles between the sea, the atmosphere and the land.
Experts stress no firm conclusions should be drawn from the short study.
Professor Jay Famiglietti from the University of California-Irvine said much longer time series were needed to detect real trends and any signal that might indicate a significant shift in climate.
"There are natural climate variations, the natural ups and downs," he explained.
"Another big factor is human control of the water cycle - reservoir management, the storage of water on continents.
"Groundwater mining leads to heavy depletions of water. Wetland drainage, river diversion projects - all of those factors contribute to these storage variations that we see and we'll be working on trying to sort those out over the next few years," he told the BBC.
'Big and small'
The information was obtained by the Grace (Gravity Recovery and Climate Experiment) mission, a pair of satellites that observe changes in mass by monitoring tiny fluctuations in the pull of gravity as they fly over the Earth.
The huge volumes of water that are shifted around the globe in the hydrological cycle result in anomalies in the gravity field that Grace can map and monitor over time.
The variations will reflect changes in water stored in rivers, lakes, reservoirs; in floodplains as snow and ice; and underground in soils and aquifers.
Grace scientists have been doing this for more than 50 major drainage basins that cover most of Earth's land area.
Several African basins show significant drying over the study period - locations such as the Congo, Zambezi and the Nile. The Congo's decrease was equivalent to 260 cu km (about 60 trillion gallons) of water, and was almost certainly the result of reservoirs being let down; it was a consequence of human management of water resources.
"This is the first time we've been able to track these variations with this frequency at these different scales," said Professor Famiglietti.
"We're just starting to understand what's big and what's small, and what the causes are. It's an amazing data set."
The Grace mission is a joint venture between Nasa and the German Aerospace Centre (DLR). It has already been used to weigh the ice sitting on the Greenland and Antarctic landmasses and demonstrate the billions of tonnes lost to melting each year.
The gravity data returned from the spacecraft is also being used to study ocean circulation to improve climate models.
The satellites obtain their data by executing a carefully calibrated pursuit in orbit.
As one spacecraft lurches and drags through the Earth's uneven gravity field, the second follows 210 km behind, measuring changes in their separation to the nearest micron (a thousandth of a millimetre).
It is the size of those changes detected by the twins that describes the nature and scale of the gravity anomalies over which they pass.
"You can think of them as two automobile-sized objects, one of them in Los Angeles and the other in San Diego, and they're measuring the distance between each other to about the size of a red blood cell," said Dr Michael Watkins, the Grace project scientist at Nasa's Jet Propulsion Laboratory.
The researchers were describing their work at the American Geophysical Union Fall Meeting here in San Francisco.
When the team returns next year, Grace should have been joined in orbit by a European gravity mission called Goce. The Gravity field and steady-state Ocean Circulation Explorer is set to launch from Russia towards the end of 2007.
In contrast to Grace, Goce will monitor changes in the acceleration due to gravity by studying the behaviour of mass pairs inside itself.