Image copyright Getty Images Image caption Many of the crops imported and exported from the US are grown with non-sustainable water supplies

The global market for foodstuffs is depleting water sources in many parts of the world quicker than they can naturally be refilled.

The complex trade is increasing pressure on non-renewable groundwater, mainly used for irrigating crops such as rice, wheat and cotton.

Pakistan, the US and India are the countries exporting the most food grown with unsustainable water.

Researchers say that without action, food supplies will be threatened.

Around 43% of the water used to irrigate crops around the world comes from underground aquifers, as opposed to rivers and lakes. Many of these sources are being used up quicker than they can be refilled from rainfall.

Back in 2000, experts believed that non-renewable resources sustained 20% of global irrigation. In the 10 years to 2010, this increased by more than a fifth.

While scientists have long known about the depletion of groundwater, this new study sets out to understand how supplies are impacted by the booming international trade in food and crops.

The vast majority of the world's populations live in countries that source nearly all their staple crop imports from nations who deplete significant amounts of groundwater to irrigate these foodstuffs.

The researchers found that some 11% of the non-renewable groundwater used for irrigation is embedded in the the global food trade. Two-thirds of this are accounted for by Pakistan, the US and India.

Image copyright Getty Images Image caption Rice-growing in Pakistan is often irrigated with unsustainable groundwater

Over the decade from the year 2000, the use of non-renewable groundwater has doubled in China and increased significantly in India and the US. The crops using the biggest amounts of this water are wheat, rice, sugar crops, cotton and maize.

However, the web of responsibility is a complex one.

The US, Mexico, Iran, Saudi Arabia and China are among the top 10 users of unsustainable water in agriculture. However, they are also among the top importers of crops grown with these dwindling resources.

So, Iran, for example, mainly imports rice from Pakistan irrigated by the Upper Ganges and Lower Indus aquifers. These water sources have extraction rates up to 50 times higher than required for sustainable use. Iran in turn exports perennial crops irrigated by the Persian aquifer that has been extracted at a mere 20 times the rate that is sustainable.

"The depletion rate is alarming - we have these clusters of countries that are at risk both from domestic production and imports," said lead author Dr Carole Dalin from University College London.

"If the reserve of water runs out the price of food will be affected and it will affect almost all the world's population."

Many developed countries are aware of issues in the depletion of groundwater and have put measures in place, such as urban water restrictions in California during the recent years of drought. However, in developing nations, the mechanisms to restrict water may not exist.

"Pakistan for instance is quite complex," said Dr Dalin. "They can make good money out of exporting rice, but the framework is not really there to account for the impact on the environment. It is true that eventually it will affect the production there."

The researchers argue that while governments need to have greater awareness about the impacts of production on water resources, consumers in richer countries should also think about water when considering the foods that they buy.

"The products that consumers buy at a supermarket may have very different environmental impacts depending on where they are produced and how they are irrigated," said co-author Yoshihide Wada, from the International Institute for Applied Systems Analysis.

"In order to help consumers make more sustainable choices about their food, producers should consider adding water labels that make these impacts clear."

The paper has been published in the journal Nature.

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