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The first steps in the analysis of GRACE
data provide an estimate of total water storage change that includes
all groundwater, soil moisture, snow, ice, and surface waters. When combined
with additional information, the GRACE technique may be taken a step
further and used to focus solely on groundwater changes. This raises
the very exciting possibility of tracking changes in aquifer water storage
from space. Rodell and Famiglietti demonstrated this potential by examining
data from the High Plains Aquifer located in the midwestern United States.
“GRACE will provide estimates of changes in total terrestrial
water storage over large areas [greater than 200,000 square kilometers]
on a monthly basis,” Rodell states. “We hydrologists need
to find a way to [separate] those total changes into changes in soil
moisture, snow, groundwater, etc.”
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The hydrologists want to take GRACE’s
measure of continental water storage and break it down into its component
parts so they can determine how much groundwater is stored in aquifers.
In their study, Rodell and Famiglietti found that for the High Plains
region, the variability of snow and surface water made insignificant
contributions to total water storage variability when compared to the
contributions made by groundwater and soil moisture. This means that
soil moisture is the sole component that has to be removed from the gravity
data to get at elusive groundwater changes. It turns out that the U.S.
High Plains Aquifer has been well sampled and studied, and so scientists
have long-term, detailed records on soil moisture change in the region.
So, by subtracting the soil moisture contribution, the remaining change
in GRACE’s measure of total water storage is likely due to changes
in groundwater.
Rodell and Famiglietti’s study shows that the GRACE technique
will allow for estimates of annual groundwater change over the High Plains
that are within about 8.7 millimeters of their actual value. This level
of accuracy may only represent a modest improvement for a well-sampled
aquifer like the High Plains Aquifer, but there are many places in the
world, such as northern Africa, India, and the Arabian peninsula, where
an estimate of the water levels within a centimeter or less of the actual
value would be extremely valuable. GRACE may help to reveal groundwater
depletion in areas of the world where such measurements are not systematically
recorded or where they are not disclosed for political reasons. Even
over more extensively sampled areas, the prospect of a technique that
is less labor intensive and does not require an extensive network of
wells makes GRACE an attractive option.
Overall, the GRACE technique offers an objective, unbiased method for
monitoring water storage changes on a global scale. According to GRACE
Principal Investigator Byron Tapley, of the University of Texas, “GRACE
is our first real chance to demonstrate the measurement of gravity as
a new remote-sensing tool that will play a major role in our understanding
of the Earth and its climate.”
Planning
for the Future
Challenges
and Limitations to Using the GRACE Technique
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Earth’s total water storage
is an indicator and predictor of climate variability, but water storage
is not well understood at this time. The first steps in the analysis of
GRACE data produce an estimate of total terrestrial water storage change
for all components pictured above: snow, surface water (such as lakes and
rivers), soil moisture, and groundwater. Additional analysis isolates the
changes in groundwater from the total estimate. (Image by Alex McClung.) |