The Bartlett


Gone to ground

Today’s food system is depleting groundwater reserves faster than they can be replenished. What happens next could determine the future of global food security. Words: Sophie Arie


In the recent years when California was hit by drought, hosepipes were banned and gardens turned to a crisp, but many of the crops in the fields – crops that require far more water than people – kept on growing. How did the farmers do it?

By pumping vast quantities of water up from aquifers, huge natural reserves of water stored up to a kilometre beneath the fertile fields of California’s Central Valley.  With climate change increasing the risk of drought in many parts of the world, this “groundwater”provides the world’s farmers with a brilliant fall-back option when surface water runs dry.

The problem is that global demand for most crops – from wheat, rice and maize to soya – has soared in recent decades, just as severe droughts have become more common. The result is that farmers in many parts of the world are now routinely using groundwater. Some 40% of all irrigation now comes from underground aquifers which, unlike rivers and reservoirs, form slowly over hundreds of years and may not fill up again in our lifetime.

Take India, for example: in just a few decades groundwater levels in some aquifers may become too deep to be exploitable.  “We have been using water as if it is renewable, when it is not,” says Dr Carole Dalin, Senior Research Fellow at The Bartlett’s Institute for Sustainable Resources. “Aquifers should be kept in reserve so that food supplies do not suddenly stop when there is a drought.”

A groundbreaking study by Dalin and her colleagues across different disciplines in Europe and the US published in March 2017 in Nature, shows just how dependent the world has become on food that can only be produced using a source of water that is fast running out. Food supply chains are long and complex, so even if one country doesn’t deplete its own groundwater, it may import its food from one that does.  

By analysing land use, food trade data and the best estimates of groundwater depletion worldwide, Dalin’s team were able to calculate how much groundwater is used to grow food crops (grouped into 26 categories) in each country and then to track those crops (and the commodities they are made into) from their country of origin to where they are consumed.  

The team found that “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 those crops”. Pakistan, the USA and India together account for two-thirds of all the crops other countries import that depend on this unsustainable water source. And that dependence has been growing fast. Between 2000 and 2010, exports of groundwater-depleting crops from the three largest exporters increased by 70%, 57% and 100% respectively.

Down is the new up

Global groundwater depletion (GWD) has increased by 22% in 10 years, from 240 km³ in 2000 to 292 km³ in 2010. This is mostly owing to increases in its use in China (102%), the USA (31%) and India (23%). Most GWD is concentrated in a few regions that rely significantly on overexploited aquifers to grow crops, including almost all the worldís major breadbaskets and population centres.

Source: “Groundwater Depletion Embedded in International Food Trade” (Nature 2017)

The study’s findings were so eye-opening that the paper was among the top 4% of Nature’s most-read articles. “Countries are importing water indirectly via food,” says Dalin, who calls this a ‘virtual water trade’. “We now know the groundwater depletion associated with each crop.”

Dalin hopes the research will help countries become aware of how insecure their sources of food are and inspire policymakers, consumers and farmers to seek more sustainable options. “There is no easy solution though,” she admits. “You could say you’re going to tax groundwater use, but then food prices would go up.”

“We knew there was significant groundwater depletion,” says Yoshihide Wada, Deputy Director of the Water Programme at the International Institute for Applied Systems Analysis in Austria, who created the hydrological model the team used to estimate water use for individual crops in every grid cell (50km by 50km) of the global land area.

“But we didn’t know that countries like Japan, which has no depletion itself, were contributing to global groundwater depletion.”Wada says that, while people thought countries should work on this individually, now it’s clear they need to try to resolve this globally. “It will take something like the carbon credits system for trading greenhouse gas emissions. We need that type of solution,” he says. Exactly how much water is stored in the world’s underground aquifers is not known and is too expensive to investigate.

On top of this, most countries do not control, keep or share records of how much their farmers use, so researchers have to go to extraordinary lengths to estimate groundwater depletion and recharge. Nevertheless, around the world, hydrologists have been measuring groundwater use for years and warning of the alarming rate at which it is being drained.

At the same time, food security and climate experts have been charting how land is used in agriculture worldwide and which crops are produced and traded between which countries. “People in the [academic] ‘water community’ and the [academic] ‘land-use community’ have been working on this in parallel,” says Thomas Kastner, Senior Scientist at the Senckenberg Biodiversity and Climate Research Centre in Germany, who helped refine the calculations using an algorithm he has developed to trace imported food back to its origin. “What’s been really interesting is that now the two [academic communities] are working together and realising the potential.”

Import / export

The food exports of Pakistan, the USA and India alone account for more than two-thirds of all GWD embedded in food trade.

Top 5 exporters of GWD crops (2010)  
1.  Pakistan 
2.  USA
3. India
4. Mexico
5. Iran

Top 5 importers of GWD crops (2010)
1. China
2. USA
3. Iran
4. Mexico
5. Japan 

Source: “Groundwater Depletion Embedded in International Food Trade” (Nature 2017)

Dalin worked with Michael J. Puma, a food security expert at Columbia University who works on predictability for water and food systems at NASA’s Goddard Institute for Space Studies in New York, to analyse United Nations Food and Agriculture Organization data on crop production and trade in agricultural commodities.

They then combined this with groundwater depletion calculations from Wada’s hydrological model, which is itself based on painstaking collection of the best available data from academic institutes, the United Nations and governments on everything from precipitation and temperature to groundwater depletion and recharge.

The team validated their findings against those of NASA’s Gravity Recovery and Climate Experience project, which calculates groundwater depletion by measuring gravity levels on the sub-surface of the earth, which vary depending on how much water is under the surface.  If groundwater resources were exhausted in major food-producing regions, agricultural production would fall and food prices would rise sharply, all else being equal, Puma explains.

Yet, governments and international institutions do not have any plans in place to address this situation.  “The biggest challenge for this type of [academic] work now is crossing between physical and economic disciplines,” says Puma. “We are beginning this conversation with economists. They are now receptive to understanding the sudden weaknesses in the food system.”