In the News
“ If you asked me to tell you what those fins do just by looking at them, I’d give you a thousand functions, and walking wouldn’t
be one of them. ” — NEIL SHUBIN, PAGE 12
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GRACE mission finds supply
falling mostly due to irrigation
By Devin Powell
SAN FRANCISCO — Ground- water levels have dropped in many places across the globe over the last 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 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 half-ton satellites, nicknamed
Tom and Jerry, that are especially sensitive to the tug of gravity from below.
As the spacecraft chase each other
The Middle East
Water woes Groundwater levels have fallen substantially over the last decade in certain
arid and semiarid regions (labeled above). Some declines can be blamed on short-term climate
events, but the biggest seem to be caused by unsustainable pumping of water for irrigation.
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, the scientists can detect a
centimeter rise or fall in groundwater
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
identified in this data help fill in moni-
toring gaps and confirm problems in
places where official groundwater infor-
mation is unreliable or nonexistent.