A global survey of groundwater levels appears to show underground supplies are dwindling.

The research, examining groundwater reserves by measuring rainfall and other water sources against evaporation and removal for agriculture and other uses, finds that the rate of depletion more than doubled between 1960 and 2000, to 283 cubic kilometres annually from 126 cubic kilometres per year.

Depletion hotspots include northwest India, northeastern China, northeast Pakistan, California's Central Valley and the midwestern U.S., according to the report.

Groundwater represents roughly 30 per cent of all fresh water in the world. Its depletion is leading to an increase in sea levels, as people remove groundwater, leaving it then to evaporate and fall as rain.

The researchers predict current practices will lead to a crisis, as so much of the world's agriculture relies on groundwater.

"If you let the population grow by extending the irrigated areas using groundwater that is not being recharged, then you will run into a wall at a certain point in time, and you will have hunger and social unrest to go with it," said Marc Bierkens of Utrecht University in the Netherlands, the lead author of the study. "That is something that you can see coming for miles."

The research is to be published in Geophysical Research Letters, a journal of the American Geophysical Union.