NASA's Grace Helps Monitor U.S. Drought

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Written by:
Kelly Helm Smith, National Drought Mitigation Center; and Adam Voiland, NASA's Earth Science
News Team

Contact:
Alan Buis 818-354-0474
Jet Propulsion Laboratory, Pasadena, Calif.
Alan.Buis@jpl.nasa.gov

News release: 2011-365 Nov. 30, 2011

NASA's Grace Helps Monitor U.S. Drought

The full version of this story with accompanying images is at:
http://www.jpl.nasa.gov/news/news.cfm?release=2011-365&cid=release_2011-365

The record-breaking drought in Texas that has fueled wildfires, decimated crops and forced cattle
sales has also reduced groundwater levels in much of the state to the lowest levels in more than 60
years, according to new national maps produced by NASA using data from the NASA/German
Aerospace Center Gravity Recovery and Climate Experiment (Grace) mission. The map are
distributed by the National Drought Mitigation Center at the University of Nebraska-Lincoln.

The latest groundwater map, released on Nov. 29, shows large patches of maroon over eastern Texas,
indicating severely depressed groundwater levels. The maps, publicly available on the Drought
Center's website at http://go.unl.edu/mqk , are generated weekly by NASA's Goddard Space Flight
Center in Greenbelt, Md., using Grace gravity field data calculated at NASA's Jet Propulsion
Laboratory, Pasadena, Calif., and the University of Texas Center for Space Research, Austin.

"Texas groundwater will take months or longer to recharge," said Matt Rodell, a hydrologist based at
Goddard. "Even if we have a major rainfall event, most of the water runs off. It takes a longer period
of sustained greater-than-average precipitation to recharge aquifers significantly."

The twin Grace satellites, which JPL developed and manages for NASA, detect small changes in
Earth's gravity field caused primarily by the redistribution of water on and beneath the land surface.
The paired satellites travel about 137 miles (220 kilometers) apart and record small changes in the
distance separating them as they encounter variations in Earth's gravitational field.

To make the maps, scientists use a sophisticated computer model that combines measurements of
water storage from Grace with a long-term meteorological dataset to generate a continuous record of
soil moisture and groundwater that stretches back to 1948. Grace data go back to 2002. The
meteorological data include precipitation, temperature, solar radiation and other ground- and space-
based measurements.

The color-coded maps show how much water is stored now as a probability of occurrence in the 63-
year record. The maroon shading over eastern Texas, for example, shows that the level of dryness
over the last week occurred less than two percent of the time between 1948 and the present.

The groundwater maps aren't the only maps based on Grace data that the Drought Center publishes
each week. The Drought Center also distributes soil moisture maps that show moisture changes in the
root zone down to about 3 feet (1 meter) below the surface, as well as surface soil moisture maps that
show changes within the top inch (2 centimeters) of the land.

"All of these maps offer policymakers new information into subsurface water fluctuations at regional
to national scales that has not been available in the past," said the Drought Center's Brian Wardlow.
The maps provide finer resolution or are more consistently available than other similar sources of
information, and having the maps for the three different levels should help decision makers
distinguish between short-term and long-term droughts.

"These maps would be impossible to generate using only ground-based observations," said Rodell.
"There are groundwater wells all around the United States, and the U.S. Geological Survey does keep
records from some of those wells, but it's not spatially continuous and there are some big gaps."

The maps also offer farmers, ranchers, water resource managers and even individual homeowners a
new tool to monitor the health of critical groundwater resources. "People rely on groundwater for
irrigation, for domestic water supply, and for industrial uses, but there's little information available on
regional to national scales on groundwater storage variability and how that has responded to a
drought," Rodell said. "Over a long-term dry period, there will be an effect on groundwater storage
and groundwater levels. It's going to drop quite a bit, people's wells could dry out, and it takes time to
recover."

The maps are the result of a NASA-funded project at the Drought Center and NASA Goddard to
make it easier for the weekly U.S. Drought Monitor to incorporate data from the Grace satellites. The
groundwater and soil moisture maps are updated each Tuesday.

To learn more about the Grace mission, visit: http://www.csr.utexas.edu/grace/ and
http://grace.jpl.nasa.gov/ .

JPL is a division of the California Institute of Technology in Pasadena.

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