Pioneering a New Technology


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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  20 River Basins

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.

  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.)

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