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, north-eastern China and the Sacramento-San Joaquin Valley, the heartland of California’s $30 billion agricultural industry.
Jay Famiglietti, the director of the University of California’s centre for hydrologic modelling, said the centre’s gravity recovery and climate experiment, known as ”Grace”, relies on the interplay of 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 has grown more critical as climate change and population growth draw down the world’s fresh water supplies.
Grace sees ”all of the change in ice, all of the change in snow and water storage, all of the surface water, all of the soil moisture, all of the groundwater”, Mr Famiglietti said.
Yet even as the data signal looming shortages, policy makers have been relatively wary of embracing the findings.
California water managers have been sceptical of a recent finding by Mr Famiglietti that from October 2003 to March last year, aquifers under the state’s Central Valley were drawn down by 25 million acre-feet - almost enough to fill Lake Mead, the nation’s largest reservoir.
Greg Zlotnick, a board member of the Association of California Water Agencies, said the managers feared that the data could be marshalled to someone else’s advantage in California’s tug of war over scarce water supplies.
There are other sensitivities in arid regions around the world where groundwater basins are often shared by unfriendly neighbours - India and Pakistan, Tunisia and Libya, or Israel, Jordan, Lebanon, Syria and the Palestinian territories - that are prone to suspecting one another of excessive use of this shared resource.
The two satellites, each the size of a small car, travel in polar orbits about 217 kilometres apart. Each bombards the other with microwaves calibrating the distance between them down to intervals of less than the width of a human hair.
The measurements of the distance between the craft translate to a measurement of surface mass in any given region.
The data was beautifully simple, Mr Famiglietti said. From one moment to the next, ”it gives you just one number”, he said. ”It’s like getting on a scale.”
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.
