GRACE-FO Will Help Monitor Droughts

You may not
notice water in the ground under your feet, but it
plays an important role in
keeping you alive. Plants draw
water from soil into their roots and use it to
grow. If
there’s not enough, the resulting drought may have impacts that
across local water supplies, regional agriculture and
even international food
prices. NASA’s Gravity Recovery and
Climate Experiment (GRACE) mission was the first
system to directly measure global changes in the water stored
underground in the world’s largest aquifers. GRACE Follow-On,
scheduled to launch
this month, will continue this important

really are no remote sensing products that are
equivalent to GRACE data in
giving a snapshot of conditions in
deep aquifers,” said Brian Wardlow, director
of the Center for
Advanced Land Management Information Technologies at the
University of Nebraska, Lincoln. Wardlow is familiar with the
value of this
information in understanding drought. He worked
at the National Drought
Mitigation Center (NDMC) in Lincoln
when the original GRACE data were first
incorporated into
experimental products of the weekly U.S.
Drought Monitor

The maps show soil moisture at three underground levels:
the top few inches of
soil, the top few feet/one meter of soil
(the root zone), and aquifers.

The U.S.
Drought Monitor maps are one of the nation’s most
important tools for tracking
drought across the United States.
They are widely used by
decision makers at federal and state
levels. For example, the current U.S. Farm
Bill designates the
Drought Monitor’s county-by-county drought rankings as the
for judging eligibility for some federal disaster
assistance programs.

Climatologist Brian Fuchs of NDMC, one of the team of
scientists who author the drought maps, explained that soil
moisture is sparsely
measured on the ground. “Some states have
their own [ground] networks, but
the data is spotty, and that
makes it very difficult to use the products in a
assessment,” he said. “GRACE covers the entire country, and
helps us to get an idea of where areas of wetness and dryness

The NDMC also considered GRACE data in producing other NDMC
resources such as the Quick
Drought Response Index
(QuickDRI), a NASA-supported
project with multiple institutional partners. QuickDRI is an
early warning
system for flash droughts — rapidly developing
losses of soil moisture due to
heat waves and lack of rain.
The onset of a flash drought is hard to detect at
level, but its impacts on agriculture may be as harsh as those
of a
seasonal drought.

the Data into the Maps

launched in 2002, the science team knew the data
would be useful
for measuring the depletion of large aquifers.
However, most hydrologists
were unfamiliar with the
measurement. One of the few exceptions was
Matt Rodell, now
chief of the Hydrological Sciences Laboratory at NASA’s
Goddard Space Flight Center in Greenbelt, Maryland. Rodell had
recently done his doctoral research on how the new GRACE data
could be used for
monitoring underground water.

GRACE-FO, like GRACE, is designed to measure changes in
gravitational pull that result from changes in mass on Earth.
More than 99 percent of Earth’s mean gravitational pull does
not change from one month to the next. That’s because it comes
from the mass of the solid Earth itself — its surface and
interior — and that rarely moves, or moves very slowly. Water,
on the other hand, moves continually nearly everywhere: Snow
falls, ocean currents flow, ice melts and so on. As the twin
GRACE-FO satellites orbit Earth, one closely following the
other, the changes in mass below change the distance between
the two satellites very slightly. The record of these changes
is analyzed to create monthly global maps of changes and
redistribution of Earth’s mass near the surface.

Since the techniques used by GRACE record the
total change in
mass from month to month, not whether that change occurred
ground, near the surface in the soil, or well below
ground, Rodell and Jay
Famiglietti, now at NASA’s Jet
Propulsion Laboratory in Pasadena, California,
needed to
combine the GRACE data with meteorological and other data to
the underground water changes from above-ground and
near-surface changes. After
several experiments with the
combined observations, Rodell and Famiglietti were
able to
isolate the signal of changing water in aquifers for the first
ever using remote sensing data.

Another obstacle
between GRACE and operational drought
monitoring was a mismatch in time and
spatial scales. GRACE
data products were produced once a month with a
resolution of
about 115 square miles (300 square kilometers). Water managers
need new data every few days to keep up with changing soil
moisture, and they would
prefer to have it on the scale of
their county, district or similar smaller area.
Rodell and his
team at Goddard assimilated the
GRACE data and a long-term
record of weather observations such as
temperature and solar radiation into a NASA hydrology model,
the Catchment Land Surface Model. It is a numerical
model of water and energy
processes on land, developed for
research on the global water cycle. With
numerical modeling techniques and data analysis, the Goddard
was able to downscale the GRACE data, that is, to adapt
its larger time and
space scales to “fit” the model’s finer

The team generated experimental drought indicators from
downscaled data each week from 2011 until the GRACE mission
ended in October


Because the original GRACE mission ended after
the end of the
growing season last fall, NDMC’s Fuchs said that its loss has
not yet been strongly felt by the Drought Monitor mappers. “We
would be
looking for GRACE in the growing season, where soil
moisture is pertinent to
determining crops and vegetation,” he
said. “In a couple more months,
people will be asking, ‘Where
are these data?'”

Those people will most likely have their answer
before the
2019 growing season, as GRACE Follow-On begins delivering
maps. Fuchs said, “If it’s going to behave as GRACE
did in the past, it
will be a valuable asset.”

For more information on GRACE-FO, see:

News Media Contact

Alan Buis
Jet Propulsion Laboratory, Pasadena, California

Written by Carol Rasmussen
NASA’s Earth Science News Team


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