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Two hydrologists, Matt Rodell at NASA Goddard
Space Flight Center, and Jay Famiglietti at the University of California,
Irvine are studying the GRACE technique, and building on some of Wahr’s
earlier work. “In 1997, Jay (my supervisor at the University of
Texas at the time) told me about a new NASA satellite mission being planned
that would measure the gravity with high enough precision to detect mass
variations caused by water storage changes,” Rodell recalls. “I
was skeptical at first, but after learning more, I decided to focus my
doctoral research on that topic.” His Ph.D. now completed, Rodell
is working in the Hydrological Sciences Branch at NASA’s Goddard
Space Flight Center, continuing his research on this application for
GRACE data.
Rodell and Famiglietti conducted two initial studies of the GRACE technique.
In their first experiment, the idea was to test the limits of the proposed
GRACE technique and see over how small an area the satellite could be
expected to detect changes in water storage. To do this the scientists
needed to know, first, how sensitive to changes in Earth’s gravity
did GRACE’s designers think the satellites would be, and second,
what kinds of water storage changes was GRACE likely to encounter in
the real world. Because not much real-world data exists on changes in
water storage, the scientists turned to computer model simulations of
seasonal and climatic changes in water storage from twenty different
river basins of varying size all around the world.
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Rodell and Famiglietti compared these modeled river environments
to the GRACE team’s projections of the satellites’ capabilities
and found that data from GRACE should be able to determine monthly water
storage changes for areas of approximately 200,000 square kilometers
or larger. “Water storage changes in the whole Mississippi River
Basin, for example, should be no problem for GRACE,” explained
Rodell. “It’s a large area, and there are large changes in
water storage. The Salt Lake Basin, on the other hand, is too small and
dry for GRACE to be able to detect changes in that region.”
The scientists relied on modeled water storage changes because for many
parts of the world, ground-based observations don’t exist. But
in their second study, the scientists wanted to go beyond models and
to verify the GRACE technique against actual observed data, so they focused
on the state of Illinois where extensive long-term records of water storage
data exist. Illinois has an area of 145,800 square kilometers (below
the minimum threshold for GRACE to detect monthly water storage changes)
so the scientists had to scale up the results obtained in Illinois to
larger regions by assuming conditions in the surrounding area are similar
to those in Illinois. Again, the results were encouraging and indicated
that GRACE would successfully detect water storage changes in areas larger
than 200,000 square kilometers.
Generally speaking, the GRACE technique will be more accurate for larger
areas over longer time intervals. For example, GRACE will be able to
detect seasonal (three-month) and annual changes in water storage in
an area the size of Illinois, but not month-to-month changes. In contrast,
in an area the size of the Mississippi River basin (3,165,500 square
kilometers), GRACE will likely be able to detect water storage changes
at monthly, seasonal, and annual time intervals. In many locations, changes
between seasons have the highest magnitude, and thus seasonal water storage
change may be easiest to detect using the GRACE technique.
Challenges and Limitations to Using the GRACE Technique
Earth's Weighty Wellsprings
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To find out over how small an area GRACE could accurately monitor water storage changes, Matt Rodell and Jay Famiglietti modeled
twenty river basins around the world (areas shown in green above) and compared the modeled hydrological data to scientists’
predictions of GRACE’s capabilities. They confirmed that the GRACE technique is increasingly effective as the size of the
area studied increases. (Map by Alex McClung based on data by Matt Rodell.) |