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Agriculture is depleting world aquifers, new satellite measurements show

Hard to say if the glass is half full or half empty, as the saying goes but the level of this nonrenewable resource is dropping fast.

Among crops leading to the most depletion globally in 2010, wheat makes up 22 percent of global groundwater depletion for irrigation.

Unsustainable pumping of groundwater for irrigated agriculture is acclerating rapidly around the world, according to new research that matches crop production statistics against high-tech measurements of aquifer drawdowns.

Agriculture’s heavy demand on the world’s freshwater resources is well understood from the output end — of all water consumption for all uses, the United Nations estimates, 70 percent goes to produce food.

But the problem has been more difficult at the sourcing end, which requires distinguishing between perpetually replenished surface water from lakes and streams on the one hand, and essentially nonrenewable underground reserves on the other.

Quantifying the impact of withdrawals from aquifers has become a little easier since the introduction about 15 years ago of the satellite program known as GRACE, for Gravity Recovery and Climate Experiment, developed in a collaboration of the U.S. and German space programs.

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Using a pair of satellites equipped with sensors that measure changes in the earth’s gravitational field, scientists can now visualize what’s going on with changing water volumes far below the surface (also, for that matter, with water locked in polar ice sheets).

For a paper published last week in the prestigious journal Nature, an international team of researchers — led by scientists at the UK’s Institute for Sustainable Resources and NASA’s Goddard Institute for Space Studies — looked at the gap between the rapid rate at which water is being withdrawn from aquifers and the very slow pace at which it is returned, essentially via rainfall and surface water filtering down through soil. Because most of the returns occur on time scales of many decades, at least, the gap amounts to a long-term depletion of groundwater resource.

But where is the water going?

To answer that question, the team made what appears to be the first effort to overlay depletion data with country-by-country statistics on agricultural output, to see how much of the loss could be attributed to food production.

They called the resulting measurement GWD — groundwater depletion for irrigation — and the numbers were rather grim in terms of the acceleration rate.

Depletion up 24 percent in just 10 years

In the year 2000, GWD was estimated at 19.47 cubic kilometers. By 2010, the endpoint of the analysis, it had risen to 24.14 km3 — an increase of 24 percent in just one decade. (If that volume measure seems unimpressively small, note that one cubic kilometer is 26.42 billion gallons.)

Of course, agricultural depletion is not uniform across the globe. About two-thirds of the GWD calculated for 2010 was in just four countries: India (7.35 km3), Iran (3.33 km3), Pakistan (2.75 km3) and China (2.40 km3). Almost 85 percent occurred in 10 nations — the top four plus the United States (1.62 km3), Mexico (1.11 km3), Libya (.25 km3), Turkey (.20 km3), and Italy (.20 km3).

During the decade that ended in 2010, the acceleration of GWD was most rapid in India (23 percent), China (102 percent) and the United States (31 percent).

Nor does all agriculture contribute equally, the researchers found:

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The crops leading to the most depletion globally in 2010, both because of their large production and high GWD intensity, are wheat (22% of global GWD, or 65 km3/year) rice (17%), sugar crops (7%), cotton (7%) and maize (5%).

As a result of this, GWD itself is further concentrated within parts of the producing countries where output is highest:

Most GWD is concentrated in a few regions that rely significantly on overexploited aquifers to grow crops, mainly the USA, Mexico, the Middle East and North Africa, India, Pakistan and China, including almost all the major breadbaskets and population centres of the planet [my emphasis].

Both distribution factors raise obvious issues of food security — and so does a third, which the paper addresses at some length:  

Food and water security

Because many of the crops driving GWD trends are globally traded commodities, the threats to agriculture posed by overconsumption of groundwater for irrigation are also threats to the economies of exporting countries, and to essential food supplies in importing countries.

Indeed, many countries where GWD is accelerating are both exporters and importers — the U.S., Mexico, Iran, Saudi Arabia and China are in the top tier on both sides of the trade ledger — and  therefore face the dual risk of losing production capacity and access to food produced elsewhere.  

This embedding of groundwater in globalized commodities also results in a “virtual water trade,” in which this most fundamental and local of resources is bought and sold across borders within what you might consider the “packaging” of grain, fiber and sugar. And this adds yet another dimension of insecurity:

A vast majority of the world’s population lives in countries sourcing nearly all their staple crop imports from partners who deplete groundwater to produce these crops, highlighting risks for global food and water security. Some countries, such as the USA, Mexico, Iran and China, are particularly exposed to these risks because they both produce and import food irrigated from rapidly depleting aquifers.

The big and unanswered question in this area, of course, is how much water remains in the world’s aquifers. GRACE can measure changes in volumes but not the volumes themselves.

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Even where the situation has been studied closely, as with the Ogallala aquifer underlying the central U.S., and those beneath California’s Central Valley, the complexity and sheer scale of the geology and hydrology involved defy efforts to measure and forecast the available resource with confidence.

So we simply don’t know whether the glass is half full or half empty. We just know that it’s emptying out rapidly, that the rate is accelerating, and that replenishment will take may generations.

And now we are coming to know more about how heavily the depletion is being driven by agriculture, driven in turn by population growth and rising living standards around the world.

The paper suggests that its findings could be used to

help target efforts to improve the sustainability of water use and food production. Solutions to minimize GWD could include, in the producing countries, water-saving strategies such as improving irrigation efficiency and growing more drought-resistant crops, together with targeted measures, such as metering and regulation of groundwater pumping. These policy efforts need to be further supported by local analysis that takes into account socio-economic, cultural and environmental aspects.

All worthy aspirations, and I imagine the world’s governments will be taking them up as soon as they’ve finished fixing that little problem of global warming.

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The full paper, “Groundwater depletion embedded in international food trade,” can be found here but access is not free.