It’s World Water Week! Did you know, scientists are studying water underground on Earth from space. They’re doing it with the help of NASA’s GRACE satellites. Why is it important to study our water supply?
Earth’s Weighty Wellsprings
Water storage refers to groundwater, soil moisture, snow, ice, and surface waters. Groundwater is the portion of the water residing in underground aquifers. Scientists know that changes in climate and weather influence water storage, and vice versa, but they don’t fully understand how the relationship works. As a result, predicting water storage changes is difficult, even with sophisticated computer models. Scientists need more observations, but these are not easy to make over large areas. Ground-based measurements require lots of work and only describe water storage for a single location. Because of these difficulties, we don’t regularly and methodically survey the world’s aquifer systems, which means it’s tedious at best (and impossible in many cases) to assess regional changes in groundwater levels.
Two NASA missions offer a new perspective on this problem, and allow regional- and global-scale observations of the Earth, contributing a wealth of new information on water movement on and beneath the surface. The Advanced Microwave Scanning Radiometer for EOS (AMSR-E), onboard the Aqua spacecraft, has the ability to determine how much moisture is in surface soil. This observation gives scientists a more complete picture of the hydrologic cycle than they’ve ever had before, but it is far from a complete picture. AMSR-E is unable to penetrate beyond the top few centimeters of soil, so scientists still lack critical information on what’s going on in deeper soil moisture or aquifers.
The image above shows the many processes of the Earth’s hydrologic cycle that contribute to total changes in water storage. Because a large portion of the Earth’s usable fresh water is located in underground aquifers, scientists are interested in determining how groundwater supplies are changing with time. GRACE offers an effective new means of studying the entire water column from space, and will be especially useful for looking at groundwater storage changes. (Image Courtesy NASA GSFC)
“GRACE is really the only instrumentation in space that can tell you much about deep water storage,” says Michael Watkins, Project Scientist for GRACE at NASA’s Jet Propulsion Laboratory (JPL). “These data are a key missing element that we can combine with these soil-moisture measuring missions [such as AMSR-E] to get a much better handle on the hydrologic cycle.”
Unlike most satellite remote sensors, GRACE doesn’t measure the electromagnetic energy reflected back to it from the Earth’s surface. Instead, as GRACE’s two satellites fly in tandem around the Earth, the distance between the two spacecraft to changes in response to variations in the Earth’s mass—and therefore gravity—on the surface below them. A device on the spacecraft can detect changes in the distance between the satellites as small as one millionth of a meter (smaller than a human red blood cell) and records this information along with the satellites’ exact position over the planet. The GRACE Science Team collects the data and translates these changes in distance into monthly maps of the Earth’s average gravity field.
GRACE takes advantage of the fundamental physical relationship between the mass of an object and the gravitational force exerted by that object—the greater the object’s mass, the stronger its gravitational field. If the mass (like underground water) in an object (such as the Earth) is free to move around, then the gravitational field of that object will change as the location of its center of mass changes. It turns out that over a time period of one month, water movement under the continents is one of the major causes of changes in the Earth’s mass distribution, and therefore its gravity field. The GRACE team aims to take advantage of this relationship between mass and gravity to track changes in Earth’s water storage.
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.
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.
Read rest of the research: http://earthobservatory.nasa.gov/Features/WeighingWater/weighing2.php