Food grown on irrigated land will, naturally, have a larger water footprint than what is cultivated in fields that just rely on the rain. Nations with warm climates tend to use more water, as do countries which consume a lot of meat: some 22,000 litres of water are used to produce just 1 kilogram of beef, compared with just 1,000 litres for each kilogram of grain.

Every time a country imports something, it also imports the virtual water embedded in its production. Importing that kilogram of grain, for instance, means importing 1,000 litres of virtual water.

There is little trade in real water because it is heavy and expensive to carry long distances. But trade in virtual water happens all the time, and it is estimated that it accounts for about 15 per cent of the water people use. There are wide differences between countries. The United States, Canada, Australia, Argentina and Thailand are all big virtual water exporters, while Japan, Sri Lanka, Italy, the Republic of Korea and the Netherlands are large importers. Exporters place large demands on their own water resources; importers effectively shift part of their demand elsewhere.