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.


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  Components of Terrestrial Water Storage

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.


The graphics above help to illustrate how the positions of the two GRACE satellites change in response to variations in Earth’s gravity field. In the first drawing, the two spacecraft pass over the ocean and neither is affected. In the second drawing, the lead spacecraft encounters a change in gravity over the more dense land mass and pulls away from the trailing spacecraft, which is still over water. In the third drawing, the lead spacecraft moves back over water but now the trailing spacecraft changes position in response to the greater pull of gravity over the land mass.

Please note that these drawings are not to scale. In reality, the GRACE satellites are spaced about 220 km apart and changes in distance between them would be undetectable by human eyes. GRACE has an onboard microwave ranging system that makes extremely precise, continuous measurements of the distance between the two spacecraft as they orbit the Earth. (Graphics courtesy Chris Meaney, NASA GSFC)