Saying GRACE Over Groundwater

GRACE in this case is Gravity Recovery and Climate Experiment.  Hydrologists have used a pair of gravity sensing satellites to measure changes in the amount of groundwater in the Sacramento and San Joaquin  basins of California.  GRACE relies on the interplay of two nine year-old twin satellites that monitor each other while orbiting the Earth.  The two satellites, each the size of a small car, travel in polar orbits about 135 miles apart.  Each bombards the other with microwaves calibrating the distance between them down to the intervals of less than the width of a human hair.

If the mass below the path of the leading satellite increases - - because, say, the lower Mississippi basin is water logged - - that satellite speeds up, and the distance between the two grows.  Then the mass tugs on both, and the distance shortens.  It increases again as the forward satellite moves out of range while the trailing satellite is held back.  The measurements of the distance between the craft translate to a measurement of surface mass in any given region.  GRACE sees all of the change in ice, all of the change in snow and water storage, all of the surface water, all of the soil moisture, all of the groundwater.

Recent findings indicate that from October 2003 to March 2010 aquifers under the Sacramento-San Joaquin Valley in California were drawn down by 25 million acre-feet - - almost enough to fill Lake Mead, the nation's largest reservoir (the estimate is not without controversy among water managers in California).

GRACE is under the management of Dr. Jay S. Famiglietti of the University of California Center for Hydrologic Modeling.