A simulation based on data from the GRACE satellites and historical
weather records reveals the effects of this year’s drought in Texas
(driest conditions shown in dark red). A new analysis of data from the
GRACE satellites reveals worldwide changes in groundwater.
Credit: NASA/National Drought Mitigation Center at the University of Nebraska-Lincoln
SAN FRANCISCO —
Groundwater levels have dropped in many places across the globe over the
past nine years, a pair of gravity-monitoring satellites finds. This
trend raises concerns that farmers are pumping too much water out of the
ground in dry regions.
Water has been disappearing beneath
southern Argentina, western Australia and stretches of the United
States. The decline is especially pronounced in parts of California,
India, the Middle East and China, where expanding agriculture has
increased water demand.
“Groundwater is being depleted at a rapid
clip in virtually of all of the major aquifers in the world's arid and
semiarid regions,” says Jay Famiglietti, a hydrologist at the University
of California Center for Hydrologic Modeling in Irvine, whose team
presented the new trends December 6 at a meeting of the American
Geophysical Union.
Famiglietti and his colleagues detect water
hidden below the surface using the modern equivalent of a dowsing rod: a
pair of car-sized satellites, nicknamed Tom and Jerry, that are
especially sensitive to the tug of gravity from below.
As the
spacecraft chase each other around the planet like their cat and mouse
namesakes, they are pulled apart and pushed together by areas of higher
or lower gravity. Mountains and other large concentrations of mass have a
big, obvious effect that’s consistent from month to month. But water
moves around over time, creating small gravity fluctuations that the
satellites’ orbital motions respond to.
It takes a lot of flow to
noticeably change the distance between the satellites. After
subtracting the contributions of snowpack, rivers,
lakes and soil moisture, scientists can detect changes in groundwater
greater than a centimeter over an
area about the size of Illinois.
This joint mission between NASA
and the German Aerospace Center — called the Gravity Recovery and
Climate Experiment, or GRACE — has been creating monthly snapshots of
global groundwater since 2002. The trends now identified in this data
help fill in monitoring gaps and confirm problems in places where
official groundwater information is unreliable or nonexistent.
“GRACE
is very good for areas of the world where we don’t have good ground
observations,” says Marc Bierkens, a hydrologist who studies groundwater
at Utrecht University in the Netherlands.
China, for example, has
been shown to underestimate groundwater use. The country lacks the
nationwide network of monitoring wells found in the United States.
GRACE’s measurements suggest that water levels have been dropping six or
seven centimeters per year beneath plains in the northeast.
In
some areas, short-term climate variability may be to blame. For example,
the plains of Patagonia in Argentina and areas across the southeastern
United States — areas that have been hit hard by droughts — store less
groundwater today than they did in 2002.
But there’s little doubt
as to what’s behind the biggest drops: farming. An agricultural boom in
northern India has helped to squeeze nearly 18 cubic kilometers of
water from the ground every year
That’s
enough water to fill more than seven million Olympic swimming pools. And
in California’s Central Valley, which supports about one-sixth of the
nation’s irrigated land, the ground has been sinking for decades as
landowners drill more wells and pull out almost 4 cubic kilometers of
water per year.
“People are using
groundwater faster than it can be naturally recharged,” says Matthew
Rodell, a hydrologist and GRACE team member at NASA’s Goddard Space
Flight Center in Greenbelt, Md.
Agricultural pressures are
particularly worrisome in places like the Middle East, another hotspot
on the new GRACE map. Water pumped out of the Arabian aquifer beneath
Saudi Arabia and surrounding countries today fell as rain thousands of
years ago. Once this fossil water disappears, there’s little new
rainfall to replenish it.
Climate change will only worsen the
problem, says Famiglietti. Precipitation patterns are becoming more
extreme, increasing the severity of droughts. Wet areas are also
becoming wetter and dry areas drier, which may accelerate declines in
groundwater in some places over the coming years.
But even as the
researchers sound the alarm, they don’t know how loud to crank up the
volume. GRACE reveals only changes in groundwater. It doesn’t divulge
how much water is left.
“We don’t really know how stressed the
world’s largest aquifers are,” says Sasha Richey of the University of
California Center for Hydrologic Modeling.
Some reservoirs, like
the giant Nubian Aquifer that underlies North Africa, may be large
enough to meet demand for centuries. But few reliable estimates exist of
the amount of groundwater stored in the world’s aquifers.
Despite
the uncertainties, Leonard Konikow, a hydrogeologist at the U.S.
Geological Survey in Reston, Va., says that water use has become
unsustainable in many places. Better irrigation systems that use less
water could help to curb the problem, he says. So could channeling water
during especially wet periods into aquifers instead of letting it run
off into the ocean.
“There are too many areas in the world where
groundwater development far exceeds a sustainable level,” says Konikow.
“Something will have to change.”
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