Some 285 miles above the Earth's surface, a pair of NASA satellites can measure changes in groundwater levels at river-basin scale. And what they're telling us about Texas and much of the South is disturbing. The satellites are identifying hot spots that blanket East Texas and much of the Panhandle, where groundwater supplies may be depleted within decades.
Development and agriculture are draining aquifers that aren't being recharged because of persistent drought. Take a gander at the red spots on the map above and you'll notice entire regions of the state, measuring thousands of square miles, are at risk of running out. Given the increasing demands placed on groundwater and decreasing rainfall, perhaps this should not be surprising. What's stunning, though, is the fact that there is no real clearinghouse of large, river-basin level groundwater data. Much of what we can draw from is more granular.
That's what makes NASA's Gravity Recovery and Climate Experiment (GRACE) so fascinating. It gives nationwide -- even worldwide -- snapshots of groundwater levels over the course of a decade. It would appear that the moisture is moving north, where record-breaking rainfall and floods are becoming more common, while the South only gets drier, according to a study in the journal Science.
GRACE doesn't really see these changes so much as it experiences them. Twin satellites orbiting within roughly 100 miles of each other are constantly measuring subtle changes in the distance between them. Those changes are influenced by gravity. Because water has more mass, it exerts more gravity and will alter the satellites' speed and their distance from one another. What they're detecting, essentially, are changes in the distribution of water and its mass beneath the surface in a way well-by-well measurements never could.
"[There is a] very dire situation that we face right now in the United States that, frankly, I don't think many people recognize," the paper's author, Jay Famiglietti, director of the UC Center for Hydrologic Modeling at the University of California, Irvine, told Scientific American. "I'm talking about the very rapid rates of groundwater depletion."