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.
Outside of wasting ice sheets, the world's largest broad-scale decline in gravity during GRACE's first 6 years came across a 2.7-million-square-kilometer, east-west swath centered on New Delhi. That's according to a study in press in Geophysical Research Letters by geophysicists Virendra Tiwari of the National Geophysical Research Institute in Hyderabad, India; John Wahr of the University of Colorado, Boulder; and Sean Swenson of the National Center for Atmospheric Research in Boulder. Adjusted for natural variations due to changing precipitation and evaporation, the decline in gravity that GRACE determined equates to a net loss of 54 plus or minus 9 cubic kilometers of groundwater per year, the group reports. That would produce a fall in the water table of about 10 centimeters per year averaged over the entire region.
A falling water table across the northern Indian subcontinent comes as no great surprise. The GRACE region of sharp groundwater depletion coincides with the world's most intensely irrigated land: Fifty percent to more than 75% of the land is equipped for irrigation with pumped groundwater or reservoir water. And then there are those 600 million people drawing heavily on groundwater. But, the group calculates, the GRACE-determined depletion rate implies that groundwater was being pumped out 70% faster in this decade than the Central Ground Water Board of India estimated it was in the mid-1990s. The apparent surge in withdrawal would have been large enough to turn a once-stable water table into a falling one that demands ever-deeper wells and bigger pumps and may draw in salty or polluted water.
GRACE "has shown us we can do a pretty reasonable job from space" gauging groundwater depletion, says Famiglietti. "We can help regional water managers by giving them a holistic view of a whole system." Still, across the subcontinent, no one knows how far down the water goes. They just know, as Famiglietti notes, that "it's not bottomless."