India's vanishing groundwater
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The pace of groundwater depletion in northern India is greater than anyone expected and mirrors trends seen in many other regions, including China and the western United States, says Sandra Postel, director of the Global Water Policy Project, based in Los Lunas, N.M. When groundwater disappears or becomes too difficult to pump, people who now support themselves on the land will become economic refugees, she contends. In many parts of the world, Postel adds, “water problems are becoming very serious, very fast.”
The AFP reports that 80 percent of India is threatened with drought this year. The region is experiencing a weak monsoon season. Monsoons bring rainfall to the area on a fairly predictable schedule. (A rough rule of thumb: When we have El Niño years off the US West Coast, as we're having this year, the other side of the Pacific tends to have weaker-than-usual monsoons.)
But India's prime minister, Manmohan Singh, says that, after two years of good harvest, grain stores were adequate to stave off famine brought on by crop failures.
Bloomberg News gives some context to the issue of water groundwater withdrawals worldwide. Forecasts aren't cheery:
About a fifth of water used globally comes from under the ground, the Stockholm International Water Institute has said. Withdrawals are predicted to increase 50 percent by 2025 in developing countries, and 18 percent in developed countries, according to the policy group based in the Swedish capital.
But how do we know what's happening to India's underground water supply anyway? By definition, you can't see it, and we have little to no on-the-ground data.
The answer: Gravity. Scientists infer groundwater levels from variations in Earth's gravitational field measured by satellites.
Richard Kerr of Science Now explains how it works:
As the lead spacecraft passes over a patch of anomalously strong gravity, it accelerates ahead of the trailing spacecraft. Once past the anomaly, the lead satellite slows back down. Then the trailing spacecraft accelerates and again closes on the leader. By making repeated passes over the same spot, GRACE [the Gravity Recovery and Climate Experiment satellite mission] measures changes in Earth's gravity, which are mainly due to water moving on and under the surface. Most famously, GRACE has recorded the shrinking of ice sheets; it has also detected shifting ocean currents, the desiccation of droughts, and the draining of large lakes. Outside of wasting ice sheets, the world's largest broad-scale decline in gravity during GRACE's first 6 years came across a 2.7-million-square-kilometer, east-west swath centered on New Delhi.
“This is the first time that we have been able to go into the region with essentially no data on the ground and be able to come up with a pretty reasonable number for the rate of groundwater depletion.”