For most of us, our household water consumption is a mere drop in the ocean compared to the amount of water needed to make food. Approximately 80% of available freshwater is used for food production, compared to 5% for household use. As globalization continues apace, much of our food comes from far-flung locations, making us dependent on foreign water resources.

In some countries, for example China, this importing of food, and hence virtual water, has allowed populations to grow far beyond the level that could be supported by the local water supply. Other countries, like Brazil, sustain a net export of food and virtual water.

To better understand this trade in virtual water, Paolo D'Odorico from the University of Virginia, US, and his colleagues reconstructed 23 years of global trade data for 309 crops and animal products. For each product they estimated the virtual water consumed and then totted up the transfers of virtual water between different countries for all of the years.

Their results, which are published in Environmental Research Letters (ERL), show a huge increase in the trade of virtual water and a rise in the number of trading links from 1986–2008. For example, by 2008, mainland China imported 158 cubic kilometres more water than it did in 1986 – an increase of 870%. The UK now guzzles 33 cubic kilometres more virtual water than before – an increase of 77%. Brazil, meanwhile, exports 143 cubic kilometres more than in 1986, an increase of nearly 270%.

The majority of virtual-water trading occurs between a handful of key players. "We have shown that most of the virtual-water flow occurs through a small percentage of links and that the virtual-water network is dominated by a few countries, while the rest of the world – most notably, the African continent – remains only marginally involved in the global virtual water trade," said D'Odorico.

The countries with the most virtual-water links are the US, Canada, Australia, Brazil, China, India and most of western Europe. Nations also fall into clusters or communities that tend to trade with each other frequently. Sometimes the reason for this grouping is obvious – for example central and North America are neighbours. In other cases there are historical or political reasons for connections, for example strong trade between the Russian Federation and Cuba, and weak links between the US and Cuba due to trade embargos. But many communities contain countries that are on opposite sides of the world and have no obvious political or historical reasons to trade. So what causes these countries to cluster into a community?

While one possibility is that these nations have similar population levels or wealth, the data did not support this hypothesis. "While there is a link between GDP and the number of trading partners that a country has, the strength of the virtual-water trade is not a clear function of GDP," explained D'Odorico.

Whatever the reason, it is important that we keep monitoring the virtual-water trade. "Understanding the drivers of virtual-water trade is fundamental to predicting changes in patterns and their implications on regional food security," said D'Odorico.