The GRACE Tellus program has been running for nine years now and is a collaboration of the US and German space agencies (NASA and DLR) whose key partners are the University of Texas Center for Space Research, Geoforschungszentrum Potsdam and the Jet Propulsion Laboratory.
Its twin satellites, launched 17 March 2002, are making detailed measurements of Earth's gravity field over land and ice and over the oceans to investigate Earth's water reservoirs .
WHAT IS 'EQUIVALENT WATER THICKNESS'?
The observed monthly changes in gravity are caused by monthly changes in mass. The mass changes can be thought of as concentrated in a very thin layer of water at the surface, whose thickness changes. In reality, much of the monthly change in gravity is indeed caused by changes in water storage in hydrologic reservoirs, by moving ocean, atmospheric and cryospheric masses, and by exchanges among these reservoirs. Their vertical extent is measured in centimeters, much smaller than the radius of the Earth or the horizontal scales of the changes, which are measured in kilometers. Some changes in gravity are caused by mass redistribution in the 'solid' Earth, such as that following a large earthquake, or that due to glacial isostatic adjustment; in those cases the concept of 'equivalent water thickness' does not apply, even though it is possible to compute the quantity...
As GRACE travels over areas of snow and ice sheets such as Greenland and Antarctica, changes in mass will be recorded. This information, along with measurements from the ground and other satellites will enable scientists to determine if these areas are growing or shrinking. Knowledge of this mass variation is key to understanding the effects of climate change and sea level rise.
WATER
LAND
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