A NASA study has found that data have found that groundwater levels in northern India have been declining at an alarming rate, by as much as one foot per year over the last decade.
Beneath northern India's irrigated fields of wheat, rice, and barley ... beneath its densely populated cities of Jaiphur and New Delhi, the groundwater has been disappearing, said the US space agency in a statement.
Where is northern India's underground water supply going? According to Matt Rodell and colleagues, it is being pumped and consumed by human activities -- principally to irrigate cropland -- faster than the aquifers can be replenished by natural processes. They based their conclusions -- published in the August 20 issue of Nature -- on observations from NASA's Gravity Recovery and Climate Experiment (GRACE).
The GRACE is a satellite system launched in 2002 by NASA and the German Aerospace Center and allows scientists to estimate changes in groundwater storage by measuring tiny variations in the Earth's gravitational pull.
The loss is particularly alarming because it occurred when there were no unusual trends in rainfall. In fact, rainfall was slightly above normal for the period.
The researchers examined data and models of soil moisture, lake and reservoir storage, vegetation and glaciers in the nearby Himalayas, in order to confirm that the apparent groundwater trend was real. Nothing unusual showed up in the natural environment.
The only influence they couldn't rule out was human.
"If measures are not taken to ensure sustainable groundwater usage, consequences for the 114 million residents of the region may include a collapse of agricultural output and severe shortages of potable water," said Rodell, who is based at NASA's Goddard Space Flight Center in Greenbelt, Maryland.
Groundwater comes from the natural percolation of precipitation and other surface waters down through Earth's soil and rock, accumulating in aquifers -- cavities and layers of porous rock, gravel, sand, or clay. In some of these subterranean reservoirs, the water may be thousands to millions of years old; in others, water levels decline and rise again naturally each year.
Groundwater levels do not respond to changes in weather as rapidly as lakes, streams, and rivers do. So when groundwater is pumped for irrigation or other uses, recharge to the original levels can take months or years.
"Using GRACE satellite observations, we can observe and monitor water changes in critical areas of the world, from one month to the next, without leaving our desks," said Velicogna. "These satellites provide a window to underground water storage changes."
The northern Indian states of Rajasthan, Punjab and Haryana have all of the ingredients for groundwater depletion: staggering population growth, rapid economic development and water-hungry farms, which account for about 95 percent of groundwater use in the region.
Data provided by India's Ministry of Water Resources suggested groundwater use was exceeding natural replenishment, but the regional rate of depletion was unknown. Rodell and colleagues had their case study. The team analyzed six years of monthly GRACE gravity data for northern India to produce a time series of water storage changes beneath the region's land surface.
They found that groundwater levels have been declining by an average of one meter every three years (one foot per year). More than 109 cubic km (26 cubic miles) of groundwater disappeared between 2002 and 2008 -- double the capacity of India's largest surface water reservoir, the Upper Wainganga, and triple that of Lake Mead, the largest man-made reservoir in the United States.
"At its core, this dilemma is an age-old cycle of human need and activity -- particularly the need for irrigation to produce food," said Bridget Scanlon, a hydrologist at the Jackson School of Geosciences at the University of Texas in Austin. "That cycle is now overwhelming fresh water reserves all over the world. Even one region's water problem has implications beyond its borders."
"For the first time, we can observe water use on land with no additional ground-based data collection," Famiglietti said. "This is critical because in many developing countries, where hydrological data are both sparse and hard to access, space-based methods provide perhaps the only opportunity to assess changes in fresh water availability across large regions."
Another recent study based on GRACE data, using results from a 1,200-mile swath across eastern Pakistan, northern India and into Bangladesh, showed about 1.9 million cubic feet of groundwater lost per year.
"This is probably the largest rate of groundwater loss in any comparable-sized region on Earth," that study said.