23 September 2010

Dangerous dependence on virtual water deepens

Posted by koneil

Global map of groundwater depletion, where 1000 on the legend is equal to one cubic kilometer of depletion per year. Courtesy: Marc Bierkens (*Correction)


The rate of global groundwater depletion has been on the rise, warning of a potential disaster for an increasingly globalized agricultural system says Marc Bierkens of Utrecht University in Utrecht, the Netherlands.

In an upcoming study, Bierkens and his colleagues find that not only is global groundwater extraction outstripping its natural recharge rate, this disparity has been increasing.

Groundwater represents about 30 percent of the available fresh water on the planet, with surface water accounting for only one percent. The rest of the potable, agriculture friendly supply is locked up in glaciers or the polar ice caps. This means that any reduction in the availability of groundwater supplies could have profound effects for a growing human population.

Using a database of global groundwater availability and estimates of groundwater usage and recharge rates, Bierkens and his team have been able to model both where, and how quickly, groundwater stores are being depleted. They find that the rate at which global groundwater stocks are being used has more than doubled between 1960 and 2000, increasing the amount lost from 126 to 283 cubic kilometers (30 to 68 cubic miles) of water per year. Because the total amount of groundwater in the world is unknown, it’s hard to say how fast the global supply would vanish at this rate. But, if water was siphoned as rapidly from the Great Lakes, they would go bone-dry in around 80 years.

“The rate of depletion increased almost linearly from the 1960s to the early 1990s,” says Bierkens, of Utrecht’s Physical Geography Department. “But then you see a sharp increase which is related to the increase of upcoming economies and population numbers; mainly in India and China.”

It is not just that there are more thirsty and hungry people catalyzing groundwater depletion, using the water for drinking water, irrigation and industry. It’s also, says Bierkens, that as a population grows, “people start to live in locations where there is less precipitation. If you want to feed those people, you need to find water somewhere else.”

In another study, Paolo D’Odorico of the University of Virginia in Charlottesville and his colleagues model a world which is increasingly dependent on a globalized water supply.

D’Odorico says that we are moving towards a system of trading what is called ‘virtual water’. He is referring to a network where food is produced in a region that has water for irrigation, and then sold to feed inhabitants of other regions.  The study by him and colleagues at the Politecnico di Torino, in Turin, Italy, finds that as we become ever more efficient at producing food in areas with a plentiful water supply and shipping to regions lacking sufficient water to feed a hungry population, we reduce our ability to cope with shocks to the network, like droughts or crop failure.

“The exchange of virtual water allows the system to sustain a larger population and to make an apparently more efficient use of the existing resources, with less water resources in water-rich regions remaining unutilized,” the researchers report in their paper.

The 2006 soy bean crop failed due to the drought in the Midwest. Courtesy: Lars Plougmann

When taken together, Bierkens’ and D’Odorico’s works erect warning signs that we may be living on another kind of virtual water.  “You’re living on loaned money, in this case loaned water,” says Bierkens. “It’s a water debt you build up because these aquifers are not being recharged, but it allows you to raise your standard of living. I don’t want to insult you, but it sounds a bit like how some people in the U.S. and Europe live when it comes to money.”

Indeed, Bierkens’ model shows the highest rates of depletion in some of the world’s major agricultural centers, including California’s central valley, northwest India, northeastern China, northeast Pakistan, and the midwest United States. These are thirsty regions with high levels of production. But, above merely supplying nearby populations, they are all exporters of agricultural products.

According to the United States Department of Agriculture, the U.S. agriculture trade turned a net profit of nearly $27 billion in 2009 [xls]. The European Union’s commission on Agriculture and Rural Development says India had a surplus of $4 billion, mostly from exports of rice and soybeans. China, while remaining an overall importer of agricultural products, is still a major exporter of corn – corn which is predominantly grown in the slowly dehydrating northeast. And so, the regions currently using up their essentially non-renewable stores of groundwater the fastest are also helping to feed people in other parts of the world.

The end result is a burgeoning population whose existence depends on virtual water.

“If you let the population grow by extending the irrigated areas using groundwater that is not being recharged, then you will run into a wall at a certain point in time, and you will have hunger and social unrest to go with it,” says Bierkens. “That is something that you can see coming for miles.”

While such grim consequences of groundwater depletion would hit hardest locally, says D’Odorico, “if groundwater depletion affects one of the major producers of food in the global market, the impacts of that depletion would be felt worldwide.”

Colin Schultz – AGU science writer

Paolo D’Odorico, Francesco Laio, and Luca Ridolfi (2010). Does globalization of water reduce societal resilience to drought? Geophysical Research Letters, 37 : 10.1029/2010GL043167

Marc.F.P. Bierkens et al (2010). A worldwide view of groundwater depletion Geophysical Research Letters : 10.1029/2010GL044571

*Correction: The photo caption originally read “A global map of groundwater depletion, measured in cubic meters of water per year.” It has been updated to fix an error made when converting the units from cubic kilometers to cubic meters.