Weighing Earth's Water from Space
Posted on: Thursday, 8 January 2004, 06:00 CST
NASA -- With vast oceans covering about 70 percent of Earth's surface, it would seem that our planet is home to a nearly boundless water supply. But in reality, only 2.5 percent of Earth's water is fresh, and only one-third of that small percentage is accessible.
Since the amount of consumable water on Earth is limited, scientists and natural resource managers are looking for ways to monitor our water resources.
NASA's twin Gravity Recovery And Climate Experiments (GRACE) satellites are using a new technique to address this problem. Launched in 2002, GRACE is serving as a "divining rod" in space, using changes in Earth's gravitational field to track the amount and movement of water both on and under our planet's surface.
The most abundant and available source of fresh water is groundwater, stored in underground reservoirs called aquifers. Scientists know that there is a relationship between water storage, climate and weather, but the dynamics of the relationship are still mysterious.
Even using computer models, predicting water storage changes is difficult, and there was previously no feasible method of assessing regional changes in groundwater levels.
NASA's Advanced Microwave Scanning Radiometer for EOS (AMSR-E), on board the Aqua spacecraft, can measure the amount of moisture in soil. Deeper soil moisture and aquifers are out of its reach, however, because it is unable to see past the top few centimeters of soil.
"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. "These data are a key missing element that we can combine with soil-moisture measuring missions [such as AMSR-E] to get a much better handle on the hydrologic cycle."
The GRACE technique relies on the fundamental physical principle that the greater an object's mass, the stronger its gravitational field. Water movement under the continents is one of the major causes of changes in the Earth's mass distribution.
As the twin GRACE satellites fly in tandem above the Earth's surface, the distance between the two spacecraft changes in response to changes in the gravitational field.
The spacecraft can sense the changes in distance, and the GRACE science team translates these data into monthly maps of the Earth's average gravity field, shedding light on shifts in the locations and amounts of groundwater.
Information collected from GRACE will enhance forecast accuracy, enabling the population to plan for extreme events such as floods and drought. It will also improve our ability to regulate our water resources, helping to ensure that enough water is available to meet society's needs.
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On the Net:
http://earthobservatory.nasa.gov/Study/WeighingWater/
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