In Barcelona, water costs 90 per cent more than it does in Valencia, farther down the Spanish coast, where the commodity is scarce. As more global water shortages occur, such anomalies will have to disappear.

Annual world water use has risen sixfold during the past century, more than double the rate of population growth. But water supply, most of it from the sea, is the same now as it was 10,000 years ago. Based on United Nations population projections, Credit Suisse estimates that, by 2025, almost two-thirds of the global population will live in countries where water will be a scarce commodity.