New data show quickening loss of groundwater beneath India: increased crop irrigation is depleting region's aquifers.Irrigation in northern India in recent decades has pulled water from the ground faster than the region's soaking monsoon rains can replenish it. And satellite data reveal that the pace of extraction has accelerated during that time, scientists report in two new studies.
In an area that is home to about 10 percent of the world's people, that could be a recipe for disaster, policy experts say. A growing population with an increasing standard of living will boost the demand for groundwater, a trend that could eventually lead to reduced agricultural yields, shortages of potable water and an increase in societal unrest.
Northern India and the surrounding areas--a 2,000-kilometer-long swath that rims the Himalayas from Pakistan to Bangladesh--are home to more than 600 million people. The region is also one of the most heavily irrigated areas in the world, says Virendra M. Tiwari, a geophysicist at the National Geophysical Research Institute in Hyderabad, India, and coauthor of a new report to appear in an upcoming Geophysical Research Letters. Government policies put in place in the 1960s to boost agricultural productivity nearly tripled the amount of irrigated acreage in India between 1970 and 1999, previous research has found.
In the mid-1990s, India's Central Ground Water Board estimated that farmers pulled more than 172 cubic kilometers of water each year from aquifers in portions of the study region in northeastern India, southern Nepal and western Bangladesh, Tiwari says. That's more than three times the capacity of India's largest surface reservoir. New data gleaned from gravity-measuring satellites suggest that the annual rate of extraction in that region has jumped more than 60 percent since then, Tiwari and colleagues report.
Researchers estimate that monsoon rains supply, at most, 246 cubic kilometers of precipitation to the region each year, Tiwari says. So, during the mid-1990s, groundwater supply--which largely comes from rainfall that soaks into the ground--was sufficient to meet agricultural demands. But data gathered from April 2002 to June 2008 by the Gravity Recovery and Climate Experiment's two satellites show that irrigation now extracts substantially more water than is replenished each year.
GRACE, a joint mission of NASA and DLR, the German aerospace center, is designed to map changes in Earth's gravitational field (SN: 1/4/03, p. 6). The craft can discern movements of groundwater--which, Tiwari says, often flows away from a region or evaporates after being pumped from aquifers.
Across northern India and nearby regions, including parts of Afghanistan, the net loss of groundwater averaged 54 cubic kilometers per year between 2002 and 2008, he and colleagues estimate. As a result, the water table, the upper water surface in the aquifers, fell 10 centimeters or so per year. This loss of groundwater has about the same volume as the water that melted from Alaska's glaciers during the period, he notes.
A separate analysis of GRACE data, focused on northwestern India, also reveals groundwater depletion. From August 2002 to October 2008, farmers pumped an average of 17.7 cubic kilometers of water a year from aquifers under three Indian states, says hydrologist Matthew Rodell of NASA's Goddard Space Flight Center in Greenbelt, Md. In that arid region, home to more than 114 million people, the water table fell an average of 33 centimeters per year, the team reports online August 12 in Nature.
Because rainfall in the region was normal during the study period, all of the water movement detected by the GRACE satellites is presumed to have come from groundwater depletion, Rodell says. The net loss of groundwater from northwestern India's aquifers is almost three times the volume of Lake Mead, which supplies water for many parts of the southwestern United States.
The pace of groundwater depletion in northern India is greater than 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."
Many governments often aren't forthcoming about groundwater or other resources within their borders, so using remote sensing data is the only way to track usage trends for those resources, says Jay Famiglietti, a hydrologist at the University of California, Irvine and coauthor of the Nature report. "Big movements of water can't hide from GRACE" he notes.
GRACE detects shifts in water storage indirectly. The two craft orbit the planet along the same path, with one traveling about 200 kilometers ahead of the other. As the first craft in the pair approaches a gravitational anomaly on Earth's surface--say, a mountain range made of dense rock or a water body--it is pulled forward in its orbit. After the first craft passes over the anomaly, it is pulled backward. The second craft is simultaneously pulled forward as it approaches the anomaly. The magnitude of subtle changes in distance between the two craft reveals the gravitational anomaly's size.
Scientists have used GRACE to measure ice loss from Greenland and Antarctica (SN: 12/17/05, p. 387), changes in Amazon Basin water levels (SN: 8/7/04, p. 94) and even the movement of tectonic plates that occurred during the massive quake off the western coast of Indonesia in December 2004 (SN: 1/7/06, p. 6).
Back Story | MINING AQUIFERS FOR FARMING
Underground water supplies in many of the globe's most heavily irrigated regions, highlighted here in shades of purple, are draining faster than they can be replenished. Overall only about 10 percent of the world's agricultural food production depends on groundwater for crop irrigation, but some regions are much more reliant on aquifers for farming.
A United States: About 43 percent of irrigated lands depend on groundwater. The percentage is especially high in arid regions such as the Great Plains and California.
B India: Water for more than 50 percent of irrigated farmland, an area responsible for between 70 and 80 percent of the nation's crop yields, comes from aquifers.
C China: A booming economy and rising standards of living are straining water supplies here, where aquifers supply water to about 27 percent of irrigated farmlands.