Almost one-quarter of the world’s population lives in regions where groundwater is being used up faster than it can be replenished, concludes a comprehensive global analysis of groundwater depletion, published this week in Nature.

The world's oldest and largest acquifers, according to the study, have supplied civilization with water for agricultural and industrial use for thousands of years, but are now under threat from over-extraction and the underground reservoirs can no longer replenish themselves at a sustainable rate.

“This overuse can lead to decreased groundwater availability for both drinking water and growing food,” says Tom Gleeson, a hydrogeologist at McGill University in Montreal, Quebec, and lead author of the study. Eventually, he adds, it “can lead to dried up streams and ecological impacts”.

Gleeson said irrigation for agriculture drives much of the demand and is the main driver for the fragility of the acquifers. According to the study, India, Pakistan, Saudi Arabia, Iran, Mexico, and the United States lead the global pack of water-thirsty nations.

The researchers, from McGill University in Montreal and Utrecht University in the Netherlands, combined groundwater usage data from around the globe with computer models of underground water resources to come up with a measure of water usage relative to supply.

In addition, the scientists calculated how much stress each source of groundwater is under and looked in detail at the water flows needed to sustain the health of ecosystems such as grasses, trees and streams.

“To my knowledge, this is the first water-stress index that actually accounts for preserving the health of the environment,” says Jay Famiglietti, a hydrologist at the University of California, Irvine, who was not involved in the study. “That’s a critical step.”

According to Reuters, "Gleeson said limits on water extraction, more efficient irrigation and the promotion of different diets, with less or no meat, could make these water resources more sustainable."