India's water vanishing
Using satellite data from the US space agency NASA, scientists have found that groundwater levels in northern India have been declining by as much as 30 centimetres per year over the past decade.india Updated: Aug 14, 2009 01:45 IST
Using satellite data from the US space agency NASA, American scientists have found that groundwater levels in northern India have been declining by as much as 30 centimetres per year over the past decade. Researchers concluded the loss is almost entirely due to human activity.
A National Aeronautics and Space Agency (NASA) press release said more than 65 cubic kilometres of groundwater disappeared from aquifers in Haryana, Punjab, Rajasthan and the national capital region of Delhi between 2002 and 2008. This is enough water to fill Lake Mead, the largest manmade reservoir in the US, three times.
A team of hydrologists led by Matt Rodell of NASA's Goddard Space Flight Center in Greenbelt, Maryland, found that northern India's underground water supply is being pumped and consumed by human activities, such as irrigating cropland, and is draining aquifers faster than natural processes can replenish them.
The results of this research were published Wednesday in the journal Nature.
The finding is based on data from NASA's Gravity Recovery and Climate Experiment (GRACE), a pair of satellites that sense changes in earth's gravity field and associated mass distribution, including water masses stored above or below earth's surface.
As the twin satellites orbit 300 miles above earth's surface, their positions change relative to each other in response to variations in the pull of gravity.
Changes in underground water masses affect gravity enough to provide a signal that can be measured by the GRACE spacecraft. After accounting for other mass variations, such changes in gravity are translated into an equivalent change in water.
NASA scientists said that using GRACE satellite observations, "we can observe and monitor water storage changes in critical areas of the world, from one month to the next, without leaving our desks".
Groundwater comes from the natural percolation of precipitation and other surface waters down through earth's soil and rock, accumulating in cavities and layers of porous rock, gravel, sand or clay. Groundwater levels respond slowly to changes in weather and can take months or years to replenish once pumped for irrigation or other uses.
Data provided by India's ministry of water resources to the NASA-funded researchers suggested groundwater use across India was exceeding natural replenishment, but the regional rate of depletion was unknown. Rodell and colleagues analysed six years of monthly GRACE data for northern India to produce a time series of water storage changes beneath the land surface.
"We don't know the absolute volume of water in the northern Indian aquifers, but GRACE provides strong evidence that current rates of water extraction are not sustainable," the NASA release quoted Rodell as saying.
"The region has become dependent on irrigation to maximise agricultural productivity. If measures are not taken to ensure sustainable groundwater usage, the consequences for the 114 million residents of the region may include a collapse of agricultural output and severe shortages of potable water," the study warned.
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. 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 only influence they couldn't rule out was human.
The researchers said that for many developing countries, where hydrological data is both sparse and hard to access, "space-based methods provide perhaps the only opportunity to assess changes in fresh water availability across large regions".