"Water is heavy, and as a result snow, groundwater, soil moisture and surface water affect the orbits of the GRACE satellites enough to be detected using the inter-satellite range [separation] measurements," Rodell said.
That information, along with measurements from the GPS instruments and accelerometers on the satellite that account for movements not related to the Earth's gravitational field, are all combined within a supercomputer to create a new map of the Earth's gravity field each month.
Other things that affect the gravitational pull, like the oceans and the atmosphere, are subtracted out, and the difference between the new map and the average map allows scientists to see where on Earth water levels are above or below normal, from the Ogallala Aquifer to the Greenland ice sheet.
The Drought Monitor, which provides a nationwide picture of drought updated each Thursday, uses the GRACE data, said Mark Svoboda, a climatologist at the University of Nebraska, Lincoln's National Drought Mitigation Center.
GRACE helps the Drought Monitor staff members in three ways, Svoboda said. It gives them national coverage even in areas where they lack on-the-ground data collection; it sees soil moisture, which is relatively poorly monitored yet a key component of drought; and it sees groundwater.
Groundwater "is monitored even less" than soil moisture, Svoboda said.
Seeing what others can't
"GRACE allowed us to go much deeper. It goes through the soil column all the way to groundwater tables," he said.
While the program is very useful, it does have limitations.
Even though the measurement between the satellites is quite fine, because of all the other factors that have to be subtracted out and accounted for, its resolution is only accurate to about 150,000 square kilometers, an area about the size of Illinois.
That is still useful on a river basin level, but Famiglietti would like to see that number cut down to about 50,000 square kilometers, perhaps with the next version of the GRACE satellites, which are scheduled to launch in 2017.
The bigger issue, though, the scientists said, is that the data do not become available until two to six months after they are collected, due to the amount of time it takes to process them.
This means the information is less usable for managers who want to make decisions based on it.
"It is important to get the data out quickly," Famiglietti said. "We could use the data, say, all over the southern part of the United States and use what's been happening over the past three or four months to predict what the drought situation will be like over the end of the summer.
"But we can't do that right now because we don't have the data, it doesn't come out fast enough."
That's in the works, said NASA's Rodell, noting that a researcher at the University of Texas, Srinivas Bettadpur, is working to improve that delay to one to two weeks after the data get collected.
"It's essential for any real-time operational product that it's producing something daily or weekly," Svoboda said. "That should be the goal."
Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC. www.eenews.net, 202-628-6500