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GRACE’s twin satellites (illustrated here) dance toward and away from each other
depending on the strength of the gravitational pull they feel from below. Onboard
instruments can detect a micrometer change in distance between the spacecraft.
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 about what’s
behind the biggest drops: farming. An
agricultural boom in northern India has
helped to squeeze nearly 18 cubic kilo-
meters of water from the ground every
year (SN: 9/12/09, p. 5). That’s enough
water to fill more than 7 million Olym-
pic 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 (SN: 1/16/10, p. 14).
Back Story | DRY AS TEXAS TOAST
Useful for tracking long-term groundwater trends
in areas with poor monitoring, GRACE data can
also yield insights about places with well-tracked
conditions. For the continental United States, a
computer combining GRACE data with meteorological records can simulate the movement of
rainwater into and out of the ground. One simulation’s output, shown here, reveals the situation
on November 28, which saw supplies in many
locations reduced to levels seen only 2 percent
of the time since 1948 (wetness percentile).
increasing the severity of droughts. Wet
areas are becoming wetter and dry areas
drier, which may accelerate declines in
groundwater in some places.
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
FROM TOP: NASA; NASA/NATIONAL DROUGHT MITIGATION CENTER AT THE UNIVERSIT Y OF NEBRASKA-LINCOLN