Satellites Unlock Secret to Northern India's Vanishing Water
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WASHINGTON -- Using NASA satellite data, scientists have found that
groundwater levels in northern India have been declining by as much
as one foot per year over the past decade. Researchers concluded the
loss is almost entirely due to human activity.
More than 26 cubic miles of groundwater disappeared from aquifers in
areas of Haryana, Punjab, Rajasthan and the nation's capitol
territory of Delhi, between 2002 and 2008. This is enough water to
fill Lake Mead, the largest manmade reservoir in the United States,
three times.
A team of hydrologists led by Matt Rodell of NASA's Goddard Space
Flight Center in Greenbelt, Md., found that northern India's
underground water supply is being pumped and consumed by human
activities, such as irrigating cropland, and is draining aquifers
faster than natural processes can replenish them. The results of this
research were published today in Nature.
The finding is based on data from NASA's Gravity Recovery and Climate
Experiment (GRACE), a pair of satellites that sense changes in
Earth's gravity field and associated mass distribution, including
water masses stored above or below Earth's surface. As the twin
satellites orbit 300 miles above Earth's surface, their positions
change relative to each other in response to variations in the pull
of gravity.
Changes in underground water masses affect gravity enough to provide a
signal that can be measured by the GRACE spacecraft. After accounting
for other mass variations, such changes in gravity are translated
into an equivalent change in water.
"Using GRACE satellite observations, we can observe and monitor water
storage changes in critical areas of the world, from one month to the
next, without leaving our desks," said study co-author Isabella
Velicogna of NASA's Jet Propulsion Laboratory in Pasadena, Calif.,
and the University of California, Irvine.
Groundwater comes from the natural percolation of precipitation and
other surface waters down through Earth's soil and rock, accumulating
in cavities and layers of porous rock, gravel, sand or clay.
Groundwater levels respond slowly to changes in weather and can take
months or years to replenish once pumped for irrigation or other
uses.
Data provided by India's Ministry of Water Resources to the
NASA-funded researchers suggested groundwater use across India was
exceeding natural replenishment, but the regional rate of depletion
was unknown. Rodell and colleagues analyzed six years of monthly
GRACE data for northern India to produce a time series of water
storage changes beneath the land surface.
"We don't know the absolute volume of water in the northern Indian
aquifers, but GRACE provides strong evidence that current rates of
water extraction are not sustainable," said Rodell. "The region has
become 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 may
include a collapse of agricultural output and severe shortages of
potable water."
Researchers examined data and models of soil moisture, lake and
reservoir storage, vegetation and glaciers in the nearby Himalayas in
order to confirm that the apparent groundwater trend was real. The
loss is particularly alarming because it occurred when there were no
unusual trends in rainfall. In fact, rainfall was slightly above
normal for the period. The only influence they couldn't rule out was
human.
"For the first time, we can observe water use on land with no
additional ground-based data collection," said co-author James
Famiglietti of the University of California, Irvine. "This is
critical because in many developing countries, where hydrological
data are both sparse and hard to access, space-based methods provide
perhaps the only opportunity to assess changes in fresh water
availability across large regions."
GRACE is a partnership between NASA and the German Aerospace Center,
DLR. The University of Texas Center for Space Research in Austin has
overall GRACE mission responsibility. GRACE was launched in 2002.
For more information, please visit:
http://www.nasa.gov/topics/
For more information about NASA and agency programs, visit:
http://www.nasa.gov
