Groundwater Monitoring with GRACE Satellites
From "NASA Satellites Track Vanishing Groundwater," in National Geographic:
In 2002 NASA and the German Research Institute for Aviation and Space Flight launched two satellites—together known as the Gravity Recovery and Climate Experiment (GRACE)—which measure changes in Earth's gravity.
Large pockets of water and ice on Earth exert a stronger gravitational pull on the satellites than areas without water. By studying maps of Earth's gravitational field made with data from GRACE, scientists are able to monitor fluctuations in groundwater over time and highlight where aquifers are being depleted faster than replenished.
From "NASA Data Reveal Major Groundwater Loss in California," at the NASA Jet Propulsion Laboratory website (December 2009):
New space observations reveal that since October 2003, the aquifers for California's primary agricultural region -- the Central Valley -- and its major mountain water source -- the Sierra Nevadas -- have lost nearly enough water combined to fill Lake Mead, America's largest reservoir. The findings, based on data from the NASA/German Aerospace Center Gravity Recovery and Climate Experiment (Grace), reflect California's extended drought and increased rates of groundwater being pumped for human uses, such as irrigation.
[Update] From "California's massive groundwater overdraft newly revealed," at Peter Gleick's City Brights blog:
Between October 2003 and March 2009, more than 24 million acre-feet (30 cubic kilometers) of groundwater were pumped out of California's Central Valley. This is overdraft of groundwater -- the pumping of groundwater faster than nature recharges it.
Most of the overdraft is occurring in the San Joaquin Valley and it is occurring at a rate far faster than previously reported by the California Department of Water Resources. This rate of overpumping is more than 4.4 million acre-feet per year -- more than three times above DWR's previous estimates.
Already the satellites have passed the five-year lifetime of the planned mission, and a follow-up mission is not planned for at least ten years.
"GRACE could remain up for three to four more years, but it could fail at any time. Unless a gap-filling mission is launched, the scientific community will lose the long-term data, which is needed to separate these long-term trends from year-to-year variability," NCAR's [National Center for Atmospheric Research's Sean] Swenson said.
More on GRACE at the NASA Earth Observatory website.