Using NASA satellite data, scientists have found that ground water 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 ground water 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 in the journal 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," says study co-author Isabella
Velicogna of NASA's Jet Propulsion Laboratory in Pasadena,
Calif., and the University
of California, Irvine.
Ground water 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.
Ground water 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 ground
water use across India
was exceeding natural replenishment, but the regional rate of depletion was
unknown. Rodell and colleagues analyzed 6 years of monthly GRACE data for
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," says 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 ground water 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," says 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."
Satellites Unlock Secret to Northern India's Vanishing Ground Water
August 25, 2009