As another tool in measuring and understanding changes in the climate, a pair of satellites is keeping a close eye on minute changes in the Earth’s gravity. GRACE, the Gravity Recovery and Climate Experiment, is a joint US (NASA)/German project taking place 500 km above the Earth.
In an April 1 Columbia Water Center Seminar, two researchers from the Center for Sustainable Water Resources, Bureau of Economic Geology, University of Texas Austin, Bridget R. Scanlon and Laurent Longuevergne, spoke on their work with GRACE data and subsurface tracer studies, and what they bring to the climate science field.
The basis of GRACE is that the two satellites, in polar orbit 220 km apart, measure not changes on the Earth itself, but capture how changes in the planet’s gravitational field affect the satellites themselves. In this way, GRACE has provided real-time information on the distribution of water around the planet since 2002.
Basically, the presence of a water mass changes the Earth’s gravity field in itself, and also exerts pressure on the planet’s crust, which taken together can indicate increases and decreases in water on the continental land masses. This method measures water at all depths: canopy, surface, soil moisture, unconfined subsurface aquifers, and to the deepest levels, confined aquifers.
Scanlon and Longuevergne explained that there are variables present in the raw data, such as solar pressure and atmosphere, that must be corrected for in order to measure changes on the planet’s land surface. The results provide a picture of global water balance shifts, but can also be refined to a regional level to provide an accurate picture even where direct sampling data is not available.
As complementary research, Scanlon and Longuevergne use subsurface tracers to gage changes in groundwater recharge over time, through the measurement of chloride residues. If moisture is leaving the groundwater system though evapotranspiration rather than mechanisms such as irrigation or runoff, then chloride deposits are left behind in the soil.
These techniques, along with other methods of scientific research, contribute to the understanding of long-term trends in water storage – seeing where on the planet there are increases or decreases in ground water – which in turn can help inform policies to adapt to and mitigate these changes.
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