New York Times reported that scientists have been using small variations in the Earth’s gravity to identify trouble spots around the globe where people are making unsustainable demands on groundwater, one of the planet’s main sources of fresh water.

They found problems in places as disparate as North Africa, northern India, northeastern China and the Sacramento-San Joaquin Valley in California, heartland of that state’s $30 billion agricultural industry.

Jay S. Famiglietti, director of the University of California’s Center for Hydrologic Modeling, said the center’s Gravity Recovery and Climate Experiment, known as Grace, relies on the interplay of two nine-year-old twin satellites that monitor each other while orbiting the Earth, thereby producing some of the most precise data ever on the planet’s gravitational variations. The results are redefining the field of hydrology, which itself has grown more critical as climate change and population growth draw down the world’s fresh water supplies.

According to the findings from October 2003 to March 2010, aquifers under the California’s Central Valley were drawn down by 25 million acre-feet — almost enough to fill Lake Mead, the nation’s largest reservoir.

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. But now the satellite technology allows the real time monitoring from space. This may be the best data about groundwater that is available. Harvesting and disseminating all of the information about aquifers as they dry up and shortages loom is the best use of space technology.

Separating groundwater from other kinds of moisture affecting gravity requires a little calculation and the inclusion of information on precipitation and surface runoff obtained from surface studies or computer models.

Because the climate change is first going to be felt on water shortages the groundwater needs to be managed carefully. We have population growth, we have widespread groundwater contamination, and we satellites showing us we are depleting most of groundwater.