Humanity uses around 9,089 billion cubic metres of water a year, mostly for agriculture, according to a new study.
The study also maps the international flow of water embodied in agricultural and industrial commodities traded around the world.
Professor Arjen Hoekstra and Dr Mesfin Mekonnen from the Proceedings of the National Academy of Sciences.
The researchers define the water footprint (WF) as the amount of water used by humans.
Their analysis improves on previous analyses by, among other things, dividing water resources into three categories - blue (surface and ground water), green (rainwater) and gray (polluter water)
Hoekstra and Mekonnen found the global annual average WF in the period 1996-2005 was 9,087 giga cubic metres per year and agricultural production contributed to 92 per cent of this.
They also found that the average consumer in the United States had a WF of 2,842 cubic metres per year, compared to those in China and India, which had WFs of 1,071 and 1,089 cubic metres per year, respectively.
“A general trend is that industrialised countries have a larger WF related to consumption of industrial products than developing countries,” the researchers write.
Hoekstra and Mekonnen found that cereal products was responsible for 27 per cent of this per capita WF, while meat and milk products were responsible for 22 and 7 per cent, respectively.
While the Chinese WF is still relatively small, the researchers say that the country’s rapid growth means it is increasingly likely to rely on water resources elsewhere - as evidenced by its policy to buy or lease lands in Africa to secure their food supply.
Hoekstra and Mekonnen also found that 2,320 giga cubic metres of the total WF flowed between countries.
“The study shows that about one-fifth of the global WF in the period 1996-2005 was not meant for domestic consumption but for export,” they write.
The biggest net exporters of this “virtual water” included big agricultural exporters such as the US and Australia. The biggest net virtual water importers included Japan and Europe.
Professor John Williams of the Wentworth Group of Scientists welcomes the research.
“Water isn’t just the litres we consume to shower in,” says Williams. “When we have a lettuce to eat we are probably consuming 600 or 700 litres of water.”
“Really they’re saying if you export agriculture, you export water - and also nutrient, which I’d be every bit concerned about.”
But, he says, just measuring the amount of water used to produce goods and services doesn’t go far enough in informing us about the sustainability of certain practices, for example, in agriculture.
Williams gives the example of a lettuce grown in an irrigation system that depletes a river compared to 1 kilogram of beef that came from well managed savannah pasture land.
While the water embodied in the lettuce is much lower than in the beef, the environmental impact of growing the lettuce is much greater due to the type of irrigation system being used, he says.
