In March 2002, NASA launched twin satellites most people have never heard of. They are called GRACE, for gravity recovery and climate experiment. By bouncing signals back and forth between them and the ground, they measure the gravitational field at various points on Earth, which can be uneven.

They are operated in cooperation with the German Space Operations Center, the Jet Propulsion Lab in California, and the University of Texas at Austin's Center for Space Research.

Besides studying Earth's gravitational field for pure scientific information, the twin GRACE satellites have proven to be invaluable in measuring many of Earth’s resources, especially its fresh water.

GRACE can, for example, help predict major flooding, such as the Missouri River flooding of 2011, as much as three years early because the system can determine saturation of the ground by water and thus its predisposition to flooding.

In recent weeks, GRACE has completed a study of the world's 37 largest aquifers and found that more than a third of them are being used at unsustainable rates — they are being drained far faster than natural processes can restore them.

The satellites can measure slight changes in the gravitational field at any point on Earth. When water in an aquifer decreases, the amount of gravitational pull in that area also decreases.

Flying about 137 miles from each other, the satellites found that 13 of the 37 aquifers, from California to the Middle East to China, were running out of water.

Eight of the 13 were determined to be overstressed, with no replenishment at all. Five were declared extremely stressed or highly stressed, with some replenishment but not enough to sustain their use.

An overused aquifer will collapse and no longer retain inflows of water.

Alexandra Richey, lead author on the study, which was published in the journal Water Resources Research, said she worries about what will happen when water supplies run out in areas with social and political tensions, like the Middle East.

The Arabian Aquifer System supplies water to 60 million people in the Middle East and was declared in the study to be the most overstressed water supply in the world. The Indus Basin aquifer in India and Pakistan is the second most overstressed system. Pakistan, according to a NASA source, is withdrawing water from the system, which also flows under India.

The GRACE satellites were recently used to calculate how much water California needs to recover from its drought. The satellites showed that the state needs 11 trillion gallons of water to fall as rain or snow to recover, which is 50% more than the largest aquifer in the United States can hold.

Furthermore, the satellites have shown that the deficit in California is increasing dramatically.