Few people, when they slice up a tomato for a sandwich, or tuck into an apple, give a thought to the amount of water that went into the production of those foods. But the water that we consume indirectly, through the food we eat and goods that we use, dwarfs our water consumption for drinking and washing.

About 70 per cent of the water consumed by people in the UK, for example, comes from overseas, according to Waterwise, a UK government-funded body that aims to reduce water usage by businesses. It is imported in the form of food, clothing, computers and cars. These imports give people in developed countries a far higher “water footprint” than people in poor countries.