Bangalore, Aug 13 - 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'.