India's Groundwater Disappearing at Alarming Rate

Farming is a thirsty business on the Indian subcontinent. But how thirsty, exactly? For the first time, satellite remote sensing of a 2000-kilometer swath running from eastern Pakistan across northern India and into Bangladesh has put a solid number on how quickly the region is depleting its groundwater. The number "is big," says hydrologist James Famiglietti of the University of California, Irvine--big as in 54 cubic kilometers of groundwater lost per year from the world's most intensively irrigated region hosting 600 million people. "I don't think anybody knew how quickly it was being depleted over that large an area."

The big picture of Indian groundwater comes from the Gravity Recovery and Climate Experiment (GRACE) satellite mission, launched in March 2002 as a joint effort by the U.S. National Aeronautics and Space Administration and the German Aerospace Center. Actually two satellites orbiting in tandem 220 kilometers apart, GRACE measures subtle variations in the pull of Earth's gravity by using microwaves to precisely gauge the changing distance between the two spacecraft.

As the lead spacecraft passes over a patch of anomalously strong gravity, it accelerates ahead of the trailing spacecraft. Once past the anomaly, the lead satellite slows back down. Then the trailing spacecraft accelerates and again closes on the leader. By making repeated passes over the same spot, GRACE measures changes in Earth's gravity, which are mainly due to water moving on and under the surface. Most famously, GRACE has recorded the shrinking of ice sheets; it has also detected shifting ocean currents, the desiccation of droughts, and the draining of large lakes.

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