'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,' said the study leader and NASA scientist Matt Rodell.

'The region has become dependent on irrigation to maximize farm output,' said Rodell.

'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.'

Although less than a third of farmland there is irrigated, crop irrigation accounts for up to 95 percent of groundwater use. 'If farmers could shift away from water-intensive crops, such as rice, and implement more efficient irrigation methods, that would help.'

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.

The loss was particularly alarming because it occurred when there was no unusual trend in rainfall. In fact, rainfall was slightly above normal for the period.

The only influence they couldn't rule out was human.

Changes in underground water mass affect gravity enough to provide a signal that can be measured by the Grace spacecraft. After accounting for other variations, such changes in gravity are translated into an equivalent change in water.