Science: Groundwater Depletion Is Detected from Space

For decades, groundwater measurements in the United States had been made from points on the Earth's surface — by taking real-time soundings at 1,383 of the United States Geological Survey's observation wells and daily readings at 5,908 others. Those readings are supplemented by measuring water levels in hundreds of thousands of other wells, trenches and excavations.

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 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 waterlogged — 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.

For more, see Groundwater Depletion Is Detected from Space by Felicity Barringer, May 30, 2011 at NYTimes.com.