Beneath northern India's irrigated fields of wheat, rice, and
barley ... beneath its densely populated cities of Jaiphur and New
Delhi, the groundwater has been disappearing. Halfway around the
world, hydrologists, including Matt Rodell of NASA, have been
hunting for it.
Where is northern India's underground water supply going?
According to Rodell and colleagues, it is being pumped and consumed
by human activities -- principally to irrigate cropland -- faster
than the aquifers can be replenished by natural processes. They
based their conclusions -- published in the August 20 issue of
Nature -- on observations from NASA's Gravity Recovery and Climate
Experiment (GRACE).
"If measures are not taken to ensure sustainable groundwater
usage, consequences for the 114 million residents of the region may
include a collapse of agricultural output and severe shortages of
potable water," said Rodell, who is based at NASA's Goddard Space
Flight Center in Greenbelt, Md.
Groundwater comes from the natural percolation of precipitation
and other surface waters down through Earth's soil and rock,
accumulating in aquifers -- cavities and layers of porous rock,
gravel, sand, or clay. In some of these subterranean reservoirs, the
water may be thousands to millions of years old; in others, water
levels decline and rise again naturally each year.
Groundwater levels do not respond to changes in weather as
rapidly as lakes, streams, and rivers do. So when groundwater is
pumped for irrigation or other uses, recharge to the original levels
can take months or years.
Changes in underground water masses affect gravity enough to
provide a signal, such that changes in gravity can be translated
into a measurement of an equivalent change in water.
"Water below the surface can hide from the naked eye, but not
from GRACE," said Rodell. The twin satellites of GRACE can sense
tiny changes in Earth's gravity field and associated mass
distribution, including water masses stored above or below Earth's
surface. As the satellites orbit 300 miles above Earth's surface,
their positions change -- relative to each other -- in response to
variations in the pull of gravity. The satellites fly roughly 137
miles apart, and microwave ranging systems measure every microscopic
change in the distance between the two.
With previous research in the United States having proven the
accuracy of GRACE in detecting groundwater, Rodell and colleagues
Isabella Velicogna, of NASA's Jet Propulsion Laboratory and the
University of California-Irvine, and James Famiglietti, of
UC-Irvine, were looking for a region where they could apply the new
technique.
"Using GRACE satellite observations, we can observe and monitor
water changes in critical areas of the world, from one month to the
next, without leaving our desks," said Velicogna. "These satellites
provide a window to underground water storage changes."
The northern Indian states of Rajasthan, Punjab and Haryana have
all of the ingredients for groundwater depletion: staggering
population growth, rapid economic development and water-hungry
farms, which account for about 95 percent of groundwater use in the
region.
Data provided by India's Ministry of Water Resources suggested
groundwater use was exceeding natural replenishment, but the
regional rate of depletion was unknown. Rodell and colleagues had
their case study. The team analyzed six years of monthly GRACE
gravity data for northern India to produce a time series of water
storage changes beneath the region's land surface.
They found that groundwater levels have been declining by an
average of one meter every three years (one foot per year). More
than 109 cubic km (26 cubic miles) of groundwater disappeared
between 2002 and 2008 -- double the capacity of India's largest
surface water reservoir, the Upper Wainganga, and triple that of
Lake Mead, the largest man-made reservoir in the United States.
"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, so we could be looking at more than a water crisis."
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 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. Nothing unusual showed up in the natural environment.
The only influence they couldn't rule out was human.
"At its core, this dilemma is an age-old cycle of human need and
activity -- particularly the need for irrigation to produce food,"
said Bridget Scanlon, a hydrologist at the Jackson School of
Geosciences at the University of Texas in Austin. "That cycle is now
overwhelming fresh water reserves all over the world. Even one
region's water problem has implications beyond its borders."
"For the first time, we can observe water use on land with no
additional ground-based data collection," Famiglietti said. "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."
SOURCE: NASA