Out of sight, out of mind means deep trouble when it comes to the reserves of freshwater stored underground. New numbers are out on the rate of groundwater depletion around the globe, and if they hold up to further scrutiny, the world is almost certainly facing a future of food shortages.
In an upcoming issue of Geophysical Research Letters, a journal of the American Geophysical Union, Professor Marc Bierkens of Utrecht University in the Netherlands and his colleagues estimate that the rate at which humanity is pumping dry the underground reservoirs that hundreds of millions of people depend upon for food and drinking water more than doubled between 1960 and 2000.
Using global groundwater data and hydrologic modeling to calculate the volumes of water added to and withdrawn from underground aquifers, the researchers estimate that groundwater depletion rose from 33 trillion gallons (126 billion cubic meters) per year in 1960 to 75 trillion gallons (283 billion cubic meters) per year in 2000. The rate increased more or less linearly from 1960 up to the early 1990’s, but then climbed sharply–likely due to escalating groundwater use in China and India.
Although the research team doesn’t delve into the implications of their findings, a lot is at stake–especially for the world’s food supply. Irrigation, which accounts for 70 percent of world water use, is the principal cause of the groundwater depletion. About 40 percent of the world’s food supply comes from the 18 percent of farmland that’s irrigated, making irrigated farming a cornerstone of global food security. But in recent decades as more farmers have turned from rivers to groundwater for their water supply, groundwater pumping in many areas has become unsustainable.
Just as a bank account shrinks when withdrawals exceed deposits, so does a groundwater account. Water budgets are badly out of balance, throwing many regions into water debt. In effect, farmers are using some of tomorrow’s water to meet today’s food demands.
It takes approximately 396,300 gallons (1,500 cubic meters) of water to produce one ton of grain. (That’s an approximate average based on the water needed to produce rice, wheat and corn.) So the estimated 75 trillion gallons (283 billion cubic meters) of groundwater depleted in 2000 is sufficient to produce 188.6 million tons of grain. That’s enough to feed 943 million people a subsistence diet for a year.
The highest rates of water loss are occurring in critical regions of irrigated farming–including the north plain of China, northwestern India, and the central valley of California. In most areas groundwater is not monitored or regulated, so as increasing numbers of wells extract ever more water, the “tragedy of the commons” is playing out on a large scale.
A little over a decade ago, I took a stab at estimating groundwater depletion using national-level data for China, India, the United States, North Africa and Saudi Arabia. For these regions, I came up with a net loss of 43 trillion gallons (164 billion cubic meters) a year. Adding in a ballpark estimate that inclusion of the rest of the world would bump this figure up by 25 percent, I arrived at a global depletion rate of 53 trillion gallons (200 billion cubic meters) per year–not too far off from the 283 billion figure of this current study, which may lend a little confidence to both.
This global assessment comes on the heels of regional studies that also corroborate the severity of the groundwater depletion problem. Using new satellite technology called GRACE – the Gravity Recovery & Climate Experiment – scientists with the University of California at Irvine and the National Atmospheric and Space Administration have estimated that California’s Central Valley – the fruit and vegetable bowl for the nation – has lost a volume of groundwater equivalent to about two-thirds the volume of Lake Mead. Scientists with the U.S. Geological Survey have estimated that as of 2005, a volume of water equivalent to two-thirds of Lake Erie had been depleted from the Ogallala Aquifer, which supplies 30 percent of the groundwater used for U.S. irrigation. And in a 2009 study in Nature, again using the GRACE technology, scientists estimated that northern India, which encompasses that nation’s breadbasket, is losing groundwater at a rate of 54 billion cubic meters per year, placing the livelihoods of 114 million people in that region at risk.
With modern satellite capabilities and new modeling and monitoring techniques, it is now possible to know what is happening to our water supplies underground. The picture is not good. The challenge now is to encourage the adoption of more efficient irrigation systems, more appropriate cropping patterns, and other measures to bring the world’s groundwater accounts into balance. The future food security of hundreds of millions of people depends on this.
Sandra Postel is director of the Global Water Policy Project and lead water expert for National Geographic’s Freshwater Initiative. She is the author of several acclaimed books, including the award-winning Last Oasis, a Pew Scholar in Conservation and the Environment, and one of the “Scientific American 50.”
[This post has been reformatted for Water Currents.]