Page last updated at 18:17 GMT, Tuesday, 15 December 2009

Satellites weigh California water

By Jonathan Amos
Science correspondent, BBC News, San Francisco

California's Central Valley
The California heartland has shed 30 cubic km of water since 2003

Nasa satellites have weighed the water lost by the US State of California's heartland since 2003.

The Sacramento and San Joaquin River Basins which support the highly productive Central Valley have shed over 30 cubic km of water in that time.

The data comes from the Grace mission which detects changes in gravity caused by water as it cycles between the sea, the atmosphere and the land.

It illustrates the impact of a drought but also excessive irrigation use.

"The numbers we're getting out of this analysis point to groundwater use at unsustainable rates," said Professor Jay Famiglietti of the University of California, Irvine.

"It's leading to declining water tables, decreased crop sizes, and continued land subsidence - something that has been going on in the Central Valley for decades."

Professor Famiglietti has been describing the California situation here at the American Geophysical Union's (AGU) Fall Meeting, the world's largest annual gathering of Earth scientists.

It is big issue because of California's importance to food production in the US.

Its Central Valley is one of the major agricultural regions in the world.

It grows more than 250 different crops, accounting for a little under a tenth of all the food produced in the US by value. But the Central Valley also accounts for about a sixth of all the irrigated land in the US, making the region the second most pumped aquifer in America.

"Hopefully the drought will end and we'll have more groundwater recharge and the southern part of the valley will get more surface water allocation," Professor Famiglietti told BBC News.

"But if that doesn't happen, the rates that we're pumping just cannot continue. We'll have more and more farmers whose wells go dry, [and] who won't be able to irrigate their crops. All of that has implications for the amount of food we're able to produce."

Artist's impression of Grace satellite in orbit
The Grace satellites provide a twin eye on Earth's gravity field

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, to demonstrate the billions of tonnes lost to melting each year.

Grace has also observed the continent-wide drying and wetting of Europe and Africa, and only this year detailed the rapidly declining water table in northwest India.

The gravity data returned from the spacecraft is also being used to study ocean circulation to improve climate models.

The satellites obtain their information 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 - and consequently the changes in mass - over which they pass.

Scientists were not quite sure how valuable Grace's data would be when the pair were launched in 2002, but there have now been a number of seminal scientific papers published on the back of what the spacecraft have observed.

"The beauty and the rewarding thing is when the data proves itself to be so valuable that it actually goes into an operational and a management system, and I think you are seeing Grace in year eight of its mission go into that phase," commented Dr Michael Watkins, the Grace project scientist.

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