It used to be known as “drizzle city”, but now Sao Paolo – South America's biggest conurbation – is now being compared with a desert. In what should be the middle of a rainy season, it is so dry that its 20 million inhabitants face having their water cut off for five days a week.

The giant Cantareira reservoir system, which supplies nine million people, is now only 5 per cent full, and predicted to run dry in April. The smaller Alto Trietê system, which serves three million people in the city, is a little better off, but still only 15 per cent full. With the dry season due, it is calculated that only severe rationing can stop the city's water from running out altogether before the rains start again in November.

Some economies have so far been made.The daily amount extracted from reservoirs has been cut by 22 per cent, and some residents are already having their water cut off for 16 hours a day. But far more is needed, and there have been warnings that, before long, the taps will only run on two days each week.

The vast forest generates its own rain, which then falls across the country.

Already, better-off citizens are buying large tanks in the hope of being able to hoard water. Many apartment blocks are trucking the stuff in at great cost; others are trying to drill wells. But most of the people can resort to none of these measures. Water riots could be on the cards.

Years of maladministration and neglect play some role in this. The gathering crisis was ignored in the run-up to last year's World Cup, and in the elections that followed it. So does lack of maintenance: it is estimated that 40 per cent of the Brazil's water supplies are lost through leaking pipes and outdated infrastructure.