Irrigation

Agricultural irrigation
Mike Boyatt, Agstock, Science Photo Library

The biggest demand on the world's water is irrigated farming. It takes two-thirds of all the water abstracted from our rivers and underground reserves. This is largely thanks to the development of super-yielding crop varieties - what scientists a generation ago called the 'green revolution'.

Back in the 1960s and 1970s, the great global fear was that the world would not be able to feed itself. The planet's population was set to double in 30 years. The Californian biologist Paul Ehrlich wrote a best-selling book called the Population Bomb in which he said 'The battle to feed the world is over … billions will die in the 1980s.'

But it didn't happen. Since then, the world's population has doubled. And so has food output. A new generation of crops like rice, wheat and corn have kept the world fed, but those crops are extremely thirsty. So while the world grows twice as much food as it did a generation ago, it takes three times as much water to do it. We thought we were going to run out of land to grow food; instead, we are running out of water.

The amount of water needed to grow our everyday food is staggering. To grow a kilogramme of rice takes between 2000 and 5000 litres, or enough to fill between 10 and 20 bathtubs - that's more water than many households use in a week. It takes 20,000 litres to fill a kilogramme jar of coffee; up to 4000 litres to grow the fodder that will deliver a litre of cow's milk; 5000 litres for a kilogramme of cheese; and up to 11,000 litres to make a quarter-pound hamburger. In such ways, I reckon I indirectly consume a hundred times my own weight in water every day. (My daughter, who is a vegetarian, consumes only half as much.)