The GRACE gravity-hunting satellites have nailed another significant observation: Groundwater levels in California’s agriculturally rich Central Valley have dropped dramatically since 2003.
GRACE is a pair of satellites that zoom constantly around Earth, the distance between them varying a tiny bit as underlying gravity – say, a big mountain range – tugs the leading satellite ahead ever so slightly. The mission, a joint effort of NASA and the German Aerospace Center, has made fundamental discoveries about how quickly the Greenland and Antarctic ice sheets are losing mass, and about groundwater depletion in regions such as India.
At the AGU meeting today, Jay Famiglietti of the University of California, Irvine, presented new numbers on California, which has been suffering through a drought since 2006. GRACE data show that over a period of 66 months, the water stored in the basins of the Sacramento and San Joaquin rivers, which together cover 154,000 square kilometers, decreased by more than 31 cubic kilometers – nearly the volume of the massive Lake Mead reservoir. Of that, nearly two-thirds, or 20 cubic kilometers, could be attributed to groundwater loss from the Central Valley.
“That’s a ton of water,” says Famiglietti, “and it has huge implications.” The Valley produces more than 250 different agricultural crops and accounts for one-sixth of the irrigated land in the United States.
Further monitoring by GRACE may help improve drought predictions for the state and help regional managers better handle their ever-scarcer water resources, he says.