Using NASA satellite data, scientists have found thatgroundwater levels in northern India have been declining by as muchas one foot per year over the past decade. Researchers concluded theloss is almost entirely due to human activity.
More than 26 cubic miles of groundwater disappeared from aquifers inareas of Haryana, Punjab, Rajasthan and the nation's capitolterritory of Delhi, between 2002 and 2008. This is enough water tofill Lake Mead, the largest manmade reservoir in the United States,three times.
A team of hydrologists led by Matt Rodell of NASA's Goddard SpaceFlight Center in Greenbelt, Md., found that northern India'sunderground water supply is being pumped and consumed by humanactivities, such as irrigating cropland, and is draining aquifersfaster than natural processes can replenish them. The results of thisresearch were published today in Nature.
The finding is based on data from NASA's Gravity Recovery and ClimateExperiment (GRACE), a pair of satellites that sense changes inEarth's gravity field and associated mass distribution, includingwater masses stored above or below Earth's surface. As the twinsatellites orbit 300 miles above Earth's surface, their positionschange relative to each other in response to variations in the pullof gravity.
Changes in underground water masses affect gravity enough to provide asignal that can be measured by the GRACE spacecraft. After accountingfor other mass variations, such changes in gravity are translatedinto an equivalent change in water.
"Using GRACE satellite observations, we can observe and monitor waterstorage changes in critical areas of the world, from one month to thenext, without leaving our desks," said study co-author IsabellaVelicogna of NASA's Jet Propulsion Laboratory in Pasadena, Calif.,and the University of California, Irvine.
Groundwater comes from the natural percolation of precipitation andother surface waters down through Earth's soil and rock, accumulatingin cavities and layers of porous rock, gravel, sand or clay.Groundwater levels respond slowly to changes in weather and can takemonths or years to replenish once pumped for irrigation or otheruses.
Data provided by India's Ministry of Water Resources to theNASA-funded researchers suggested groundwater use across India wasexceeding natural replenishment, but the regional rate of depletionwas unknown. Rodell and colleagues analyzed six years of monthlyGRACE data for northern India to produce a time series of waterstorage changes beneath the land surface.
"We don't know the absolute volume of water in the northern Indianaquifers, but GRACE provides strong evidence that current rates ofwater extraction are not sustainable," said Rodell. "The region hasbecome dependent on irrigation to maximize agricultural productivity.If measures are not taken to ensure sustainable groundwater usage,the consequences for the 114 million residents of the region mayinclude a collapse of agricultural output and severe shortages ofpotable water."
Researchers examined data and models of soil moisture, lake andreservoir storage, vegetation and glaciers in the nearby Himalayas inorder to confirm that the apparent groundwater trend was real. Theloss is particularly alarming because it occurred when there were nounusual trends in rainfall. In fact, rainfall was slightly abovenormal for the period. The only influence they couldn't rule out washuman.
"For the first time, we can observe water use on land with noadditional ground-based data collection," said co-author JamesFamiglietti of the University of California, Irvine. "This iscritical because in many developing countries, where hydrologicaldata are both sparse and hard to access, space-based methods provideperhaps the only opportunity to assess changes in fresh wateravailability across large regions."
Source: NASA/Goddard Space Flight Center