Many of the world’s most important food-producing regions depend on freshwater from massive underground aquifers that have built up over thousands of years. The Ogallala Aquifer in the midwestern United States. The Upper Ganges, sustaining India and Pakistan.

Going, going… gone? (Kevin Clark-AP)

Yet many of those aquifers are now being sucked dry by irrigation and other uses faster than they can be replenished by rainwater, according to a new study in Nature. It’s unclear when many of these aquifers will be completely emptied — scientists are still trying to measure how much “fossil water” these aquifers actually hold. But it’s a worrisome trend: About 1.7 billion people rely on aquifers that are rapidly being depleted. And once they’re gone, it would take thousands of years to refill them.

The Nature study, published by researchers at McGill and Utrecht University in the Netherlands, offers a map showing the regions where the use of water from these aquifers vastly exceeds the rate at which they’re being refilled by rain.

The map is a bit complicated, but it essentially compares the usage footprint with the actual rainfall a particular aquifer gets. Blue areas receive more rain than is being used up by humans. The Floridian Aquifer in the southeastern United States, for instance, can get quickly refilled by a big storm (though it still faces problems with saltwater contamination and overuse). Russia has plenty of freshwater. But orange or red areas indicate places where irrigation and drinking water use is drawing out more water from the aquifers than the rain can refill.

In some areas, the imbalance is staggering. Take, for instance, the Upper Ganges in northern India, which sustains farm irrigation in both India and Pakistan. The underground reservoir there would essentially need 54 times as much rain as it currently gets to replenish the water that’s being used by farmers and the local population. (That’s what the gray “footprint” at the bottom of the map shows.)

In the United States, aquifers are taking on increasing importance as food production expands and drought becomes a nagging issue. In regions like western Kansas, where farmers don’t get enough rain for their crops, they depend on irrigation, using freshwater from the Ogallala Aquifer. That’s especially true this year, amid the massive U.S. drought.

About 27 percent of U.S. irrigated farmland depends on the Ogallala aquifer, and it’s a key region for livestock, corn, wheat, and soy. But it’s slowly getting depleted. In some counties, the water table is dropping by as much as two feet per year. And, as David Biello notes, once the Ogallala gets drained, it would take about 6,000 years to recharge with rainfall.

Is it possible to stop this from happening? Possibly. Across the High Plains, farmers have been experimenting for years with various water conservation practices, such as crop rotation, as well as more-efficient watering techniques like center pivot or drip irrigation. States like Kansas are enacting conservation measures. Those practices have helped slow the rate of depletion in the Ogallala, but they haven’t stopped it, as shown in the map above.

Others are putting their hopes in technological advances — new crop breeds that can use water more efficiently. Recently, I talked to Clay Scott, a corn grower in western Kansas who volunteered to plant two plots of Monsanto’s genetically-engineered DroughtGard hybrid corn among his 3,000 acres of regular corn this year. Like many farmers in the region, Scott relies on the Ogallala aquifer — especially when drought hits — and he’s trying everything he can to reduce his water usage. He’s hoping that this new engineered corn, which carries a gene that enables it to draw water more gradually from the soil, can allow him to rely less on irrigation.

“We’re trying to see if we can maintain yields but reduce our water usage,” Scott said. “In this region, that would be a game changer.”  He’ll know how the genetically modified corn did after weighing the harvest in the fall.

And with the global population soaring past 7 billion, this is one of the biggest questions the world is now facing. Can better conservation practices and new technology enable farmers to keep feeding the planet without depleting its most important water resources?