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Aquifer Depletion Essayscorer

Scientists do not know how much groundwater is left.

Hydro Resources drills a well near Sublette, Kansas. The Ogallala Aquifer provides nearly all of the irrigation water in the High Plains, one of America’s top grain regions. The Ogallala is being depleted in Kansas, and some areas have only a few decades of water remaining. Click image to enlarge. Photo © Brian Lehmann / Circle of Blue

By Brett Walton
Circle of Blue

Population growth and agriculture are putting unsustainable demands on the world’s largest aquifer systems, particularly those in the planet’s dry midsection, according to the broadest assessment to date of global groundwater-storage trends.

Water reserves in 21 of the 37 largest aquifers have declined since 2003, according to a study led by researchers at the University of California, Irvine, who analyzed data from NASA’s GRACE satellite mission. Moreover, 13 of the aquifers are depleted to the point that regional water availability is threatened.

“We don’t really know how much groundwater we have.”

–Jay Famiglietti, professor
University of California, Irvine

More than 2 billion people rely on aquifers as their primary water source. The water held underground in layers of rock and soil is an essential emergency supply during droughts, when rivers and streams shrivel, as is the case today in California. Too much groundwater pumping can cause rivers to dry up, the land surface to sink, and wetlands to evaporate.

Key groundwater basins on every inhabited continent are being drained, according to the study.

From northern China to the Middle East, from North Africa to the Central Valley of California, a common and unsettling story is unfolding: the effort to produce massive grain and food surpluses that will feed billions and to supply drinking water to the largest knots of humanity on the planet is taxing aquifers beyond their capacity.

Yet despite groundwater’s prominence, surprisingly little is known about the size of reserves, according to a separate study accompanying the analysis of GRACE data that points out the rudimentary state of scientific knowledge about aquifer storage. The two studies were published online today in the journal Water Resources Research.

“We don’t really know how much groundwater we have,” Jay Famiglietti, a co-author on both papers, told Circle of Blue. “It’s something scientists have been talking about for a long time, but it needs to be said clearly.”

The lack of knowledge — due to the cost and complexity of groundwater-monitoring systems — is unacceptable, Famiglietti said. Acquiring the information necessary for a fully formed understanding of the world’s water resources requires a concerted effort that places the value of water on par with the earth’s wealth of minerals and fossil fuels.

“We need to embark on a major hydrogeological exploration of the world’s aquifers,” said Famiglietti, a professor of Earth System Science at the University of California, Irvine, and senior water scientist at NASA Jet Propulsion Laboratory. “I don’t see how we can get around it. We need to explore the world’s aquifers as if they were oil reservoirs.”

Stocks and Flows

The studies bring together two aspects of groundwater that are rarely considered together: the flow of water into and out of an aquifer and the stock of water sitting underground.

A bank account is a common analogy. An account holder must know both deposits and withdrawals (the flows) and the account balance (the stock).

An irrigation system in Ningxia, China. The groundwater resources on the North China Plain are slowly being drained. Click image to enlarge. Photo © J. Carl Ganter / Circle of Blue

The GRACE satellite mission, which launched in 2002, is helpful for assessing flows. The pair of satellites translates changes in the Earth’s gravitational field into changes in water storage — water being so heavy that it alters gravitational pull.

GRACE will detect changes in the total amount of water that is stored underground, a measure that incorporates both human withdrawals — for irrigation, household use, or mining — and natural variations due to drought or flood. Water withdrawal statistics, compiled by local or national agencies and a typical source of information for researchers, do not provide such a full picture.

“Traditional use statistics show withdrawals only,” said Alexandra Richey, lead author of both studies. “But all factors have to be at play when considering groundwater sustainability.”

According to the study, which used data from 2003 to 2013, the three basins with the highest depletion rates cover a variety of ecosystems and manmade pressures:

  • Depletion in the Ganges Basin, in northern India, is fueled by dense cities and expansive irrigated fields.
  • In the Arabian Aquifer, which underlies Saudi Arabia and several other Gulf nations, irrigated agriculture is the primary stressor.
  • In the Canning Basin of northwest Australia, the mining industry is the biggest groundwater user.

Balancing the Check Book

In 1904, the Texas Supreme Court concluded that groundwater was too “secret, occult, and concealed” to regulate. More than a century later, knowledge about the account balance in most of the world’s aquifers is little better than in the early days of the automobile.

According to the study on groundwater stocks, commonly accepted estimates of groundwater storage have little relationship to actual conditions. Estimates developed in 1969 and 1974, which are still cited today, assumed that each aquifer had the same characteristics, that the depth to water and the water-holding capacity of the soil were the same. They are not.

By altering the assumptions of those original studies, based on findings from regional groundwater studies — which are smaller in scope but have more detailed information on local conditions — the research team found a much emptier balance sheet.

“We have less groundwater than we thought,” Famiglietti said. “A lot less.”

The regional estimates were between 10 and 1,000 times lower than the rough estimates from 40 years ago, an astonishing difference.

Knowing how much water an aquifer holds is important for understanding its resilience to periods of depletion.

A large storage capacity allows for more pumping before the aquifer is drained. The study draws a comparison between two aquifers in North Africa — the Nubian and the Sahara. Both have similar rates of depletion, according to GRACE data. But the Sahara has roughly 10 years until 90 percent of the water is gone, according to the lowest storage estimate. The Nubian has 13,000 years of water, according to its lowest estimate.

There are several limits to the analysis.

First, GRACE data express average conditions across a large expanse of land. Significant variations, both in water levels and water use, within an aquifer are blurred. The Ogallala Aquifer, in the U.S. High Plains, is one example. GRACE data show a net increase in water storage in the Ogallala. That is entirely due to gains in the northern half of the Aquifer in Nebraska. Parts of the Aquifer’s southern end — in Kansas, Oklahoma, and Texas — have only a few decades of usable water left. On the aquifer’s margin, the water has already run out.

Second, the GRACE satellites can measure only one dimension of water: the quantity. Other physical attributes such as pollution, or economic factors such as the cost of pumping from increasingly deeper depths, are beyond the mission’s purview. Arsenic pollution or extremely deep reserves that are expensive to pump would, in effect, decrease the amount of available water in an aquifer.

Next Steps

The old, rudimentary estimates of groundwater storage have persisted not because of a lack of interest, Richey said. Other factors are at play.

“Aquifers are hard to study,” explained Richey, whose doctoral dissertation is based on the research in the two studies. “You need to drill wells and get ground-based measurements. That’s expensive and requires a lot of wells. The oil industry does it because there’s a lot of money in oil.”

Of the dozens of regional studies that the research team looked at, Richey could recall only one, in the Ogallala Aquifer, that used observational data rather than estimates to assess aquifer storage.

Richey thinks that groundwater could be the perfect opening for citizen science, a public data-gathering program. After all, millions of wells are already operating, pumping water for homes, businesses, and crops.

“In my dream, there is a common platform for groundwater users to upload their groundwater information,” she said.

NASA’s GRACE satellite mission, launched in 2002, measures changes in the amount of water held underground, in aquifers. Most of the world’s largest aquifers are being depleted. Click infographic to enlarge. Graphic © Kaye LaFond / Circle of Blue

To learn more about global water wars, watch Parched.

Aquifers provide us freshwater that makes up for surface water lost from drought-depleted lakes, rivers, and reservoirs. We are drawing down these hidden, mostly nonrenewable groundwater supplies at unsustainable rates in the western United States and in several dry regions globally, threatening our future.

We are at our best when we can see a threat or challenge ahead. If flood waters are rising, an enemy is rushing at us, or a highway exit appears just ahead of a traffic jam, we see the looming crisis and respond.

We are not as adept when threats—or threatened resources—are invisible. Some of us have trouble realizing why invisible carbon emissions are changing the chemistry of the atmosphere and warming the planet. Because the surface of the sea is all we see, it's difficult to understand that we already have taken most of the large fish from the ocean, diminishing a major source of food. Neither of these crises are visible—they are largely out of sight, out of mind—so it's difficult to get excited and respond. Disappearing groundwater is another out-of-sight crisis.

Groundwater comes from aquifers—spongelike gravel and sand-filled underground reservoirs—and we see this water only when it flows from springs and wells. In the United States we rely on this hidden—and shrinking—water supply to meet half our needs, and as drought shrinks surface water in lakes, rivers, and reservoirs, we rely on groundwater from aquifers even more. Some shallow aquifers recharge from surface water, but deeper aquifers contain ancient water locked in the earth by changes in geology thousands or millions of years ago. These aquifers typically cannot recharge, and once this "fossil" water is gone, it is gone forever—potentially changing how and where we can live and grow food, among other things.

A severe drought in California—now approaching four years long—has depleted snowpacks, rivers, and lakes, and groundwater use has soared to make up the shortfall. A new report from Stanford University says that nearly 60 percent of the state's water needs are now met by groundwater, up from 40 percent in years when normal amounts of rain and snow fall.

Relying on groundwater to make up for shrinking surface water supplies comes at a rising price, and this hidden water found in California's Central Valley aquifers is the focus of what amounts to a new gold rush. Well-drillers are working overtime, and as Brian Clark Howard reported here last week, farmers and homeowners short of water now must wait in line more than a year for their new wells.

In most years, aquifers recharge as rainfall and streamflow seep into unpaved ground. But during drought the water table—the depth at which water is found below the surface—drops as water is pumped from the ground faster than it can recharge. As Howard reported, Central Valley wells that used to strike water at 500 feet deep must now be drilled down 1,000 feet or more, at a cost of more than $300,000 for a single well. And as aquifers are depleted, the land also begins to subside, or sink.

Unlike those in other western states, Californians know little about their groundwater supply because well-drilling records are kept secret from public view, and there is no statewide policy limiting groundwater use. State legislators are contemplating a measure that would regulate and limit groundwater use, but even if it passes, compliance plans wouldn't be required until 2020, and full restrictions wouldn't kick in until 2040. California property owners now can pump as much water as they want from under the ground they own.

California's Central Valley isn't the only place in the U.S. where groundwater supplies are declining. Aquifers in the Colorado River Basin and the southern Great Plains also suffer severe depletion. Studies show that about half the groundwater depletion nationwide is from irrigation. Agriculture is the leading use of water in the U.S. and around the world, and globally irrigated farming takes more than 60 percent of the available freshwater.

The Colorado River Basin, which supplies water to 40 million people in seven states, is losing water at dramatic rates, and most of the losses are groundwater. A new satellite study from the University of California, Irvine and NASA indicates that the Colorado River Basin lost 65 cubic kilometers (15.6 cubic miles) of water from 2004 to 2013. That is twice the amount stored in Lake Mead, the largest reservoir in the U.S., which can hold two years' worth of Colorado River runoff. As Jay Famiglietti, a NASA scientist and study co-author wrote here, groundwater made up 75 percent of the water lost in the basin.

Farther east, the Ogallala Aquifer under the High Plains is also shrinking because of too much demand. When the Dust Bowl overtook the Great Plains in the 1930s, the Ogallala had been discovered only recently, and for the most part it wasn't tapped then to help ease the drought. But large-scale center-pivot irrigation transformed crop production on the plains after World War II, allowing water-thirsty crops like corn and alfalfa for feeding livestock.

But severe drought threatens the southern plains again, and water is being unsustainably drawn from the southern Ogallala Aquifer. The northern Ogallala, found near the surface in Nebraska, is replenished by surface runoff from rivers originating in the Rockies. But farther south in Texas and New Mexico, water lies hundreds of feet below the surface, and does not recharge. Sandra Postel wrote here last month that the Ogallala Aquifer water level in the Texas Panhandle has dropped by up to 15 feet in the past decade, with more than three-quarters of that loss having come during the drought of the past five years. A recent Kansas State University study said that if farmers in Kansas keep irrigating at present rates, 69 percent of the Ogallala Aquifer will be gone in 50 years.

This coincides with a nationwide trend of groundwater declines. A 2013 study of 40 aquifers across the United States by the U.S. Geological Survey reports that the rate of groundwater depletion has increased dramatically since 2000, with almost 25 cubic kilometers (six cubic miles) of water per year being pumped from the ground. This compares to about 9.2 cubic kilometers (1.48 cubic miles) average withdrawal per year from 1900 to 2008.

Scarce groundwater supplies also are being used for energy. A recent study from CERES, an organization that advocates sustainable business practices, indicated that competition for water by hydraulic fracturing—a water-intensive drilling process for oil and gas known as "fracking"—already occurs in dry regions of the United States. The February report said that more than half of all fracking wells in the U.S. are being drilled in regions experiencing drought, and that more than one-third of the wells are in regions suffering groundwater depletion.

Satellites have allowed us to more accurately understand groundwater supplies and depletion rates. Until these satellites, called GRACE (Gravity Recovery and Climate Experiment), were launched by NASA, we couldn't see or measure this developing invisible crisis. GRACE has given us an improved picture of groundwater worldwide, revealing how supplies are shrinking in several regions vulnerable to drought: northern India, the North China Plain, and the Middle East among them.

As drought worsens groundwater depletion, water supplies for people and farming shrink, and this scarcity can set the table for social unrest. Saudi Arabia, which a few decades ago began pumping deep underground aquifers to grow wheat in the desert, has since abandoned the plan, in order to conserve what groundwater supplies remain, relying instead on imported wheat to feed the people of this arid land.

Managing and conserving groundwater supplies becomes an urgent challenge as drought depletes our surface supplies. Because groundwater is a common resource—available to anyone with well—drilling equipment-cooperation and collaboration will be crucial as we try to protect this shrinking line of defense against a future of water scarcity.

Dennis Dimick grew up on a hilly Oregon farm named Spring Hill, where groundwater from a spring provided his family's—and the farm's—water supply. He is National Geographic's Executive Editor for the Environment. You can follow him onTwitter,Instagram, and flickr.

The National Geographic Society supports a project to restore freshwater ecosystems. You can find out more about Change the Course here, and how by pledging to reduce your own water footprint you can restore 1,000 gallons of water to the Colorado River.


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