Recent space observations of freshwater storage by the Gravity
Recovery and Climate Experiment (GRACE) are providing a new
picture of how Earth's most precious natural resource is
distributed globally and how it is changing.
Researchers are using the mission's almost five-year data
record to estimate seasonal water storage variations in more
than 50 river basins that cover most of Earth's land area. The
variations reflect changes in water stored in rivers, lakes and
reservoirs; in floodplains as snow and ice; and underground in
soils and aquifers.
"GRACE is providing a first-ever look at the distribution of
freshwater storage on the continents," said Dr. Jay
Famiglietti, professor of Earth System Science, University of
California, Irvine. "With longer time series, we can
distinguish long-term trends from natural seasonal variations
and track how water availability responds to natural climate
variations and climate change."
Several African basins, such as the Congo, Zambezi and Nile,
show significant drying over the past five years. In the United
States, the Mississippi and Colorado River basins show water
storage increases during that time. Such information is vital
for managing water resources in vulnerable parts of Africa and
Southeast Asia, since increasing populations and standards of
living place demands on water resources that are often
unsustainable. The data can be used to make more informed
regional water-management decisions.
The twin GRACE satellites monitor tiny month-to-month changes
in Earth's gravity field that are primarily caused by the
movement of water in Earth's land, ocean, ice and atmosphere
reservoirs. Hydrologists are analyzing GRACE data to identify
possible trends in precipitation changes, groundwater depletion
and snow and glacier melt rates, and to understand their
underlying causes.
Dr. Matt Rodell, a hydrologist at NASA's Goddard Space Flight
Center, Greenbelt, Md., said the data correspond well with
ground observations. As a result, hydrologists can now apply
GRACE data in ways that will impact regional water
management. "GRACE data improve our understanding of the water
cycle and simulations of soil moisture, snow and groundwater in
computer models," he said. "This is a key step toward better
weather, stream flow, flood, drought and water resource
forecasts worldwide."
Dr. Michael Watkins, GRACE project scientist at NASA's Jet
Propulsion Laboratory, Pasadena, Calif., said GRACE is the only
element in NASA's broad water cycle research program that
measures changes in all types of water storage. "GRACE detects
water storage changes from Earth's surface to its deepest
aquifers. Water can't hide from it," he said.
The mission's abilities to detect water are particularly vital
for the emerging field of groundwater remote sensing. "Remote
sensing of groundwater has been a Holy Grail for hydrologists
because it is stored beneath the surface and is not detected by
most sensors," said Famiglietti. "Outside of the United States
and a few other developed nations, it is not well monitored.
It's been speculated that many of Earth's key aquifers are
being depleted due to over-exploitation, but a lack of data has
hampered efforts to quantify how aquifer levels are changing
and take the steps necessary to avoid depleting them. With
additional data, such as measurements of surface water and soil
moisture, we can use GRACE to solve this problem."
GRACE is also allowing scientists to estimate another key
component of the water cycle for the first time: water
discharged by freshwater streams from Earth's continents.
Stream flow measurements are often not shared for economic,
political or national defense reasons. GRACE measurements of
the total water discharged by continental streams are important
for monitoring the availability of freshwater and understanding
how surface water runoff from continents contributes to rises
in global sea level.
Scientists from NASA and the University of California, Irvine,
are presenting their research today during the American
Geophysical Union meeting in San Francisco.
GRACE is a partnership between NASA and the German Aerospace
Center (DLR). The University of Texas Center for Space
Research, Austin, has overall mission responsibility. JPL
developed the two Grace satellites. DLR provided the launch,
and the GeoForschungsZentrum Potsdam, Germany, operates the
GRACE mission.
For more information about GRACE, see
http://www.csr.utexas.edu/grace/. For more on NASA water and
energy cycle research, visit http://watercycle.gsfc.nasa.gov/index.php. Additional
information on NASA news from the American Geophysical Union
conference is at http://www.nasa.gov/agu.
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