Water is rationed as reservoirs fall to record lows. While the water company blames drought, critics say the crisis was avoidable

Less than a month before São Paulo hosts the opening game of the World Cup, the biggest city in South America is struggling to overcome a water shortage following one of the driest summers on record.

Residents in some districts say their water is shut off at night or at weekends. Levels in the reservoir system that supplies 9 million people have plummeted to record lows.

For months, falling water levels in the Cantareira system – five interconnected reservoirs that supply 47% of São Paulo's 20 million people – have made front-page news. According to the website of Sabesp, a water company, the Cantareira system was at just 8.6% of its capacity on 13 May.

With the opening game of the World Cup, between Brazil and Croatia, being played in São Paulo on 12 June, the crisis has exposed the difficulties in guaranteeing supply to a city that grew as quickly and chaotically as this one, and to what critics say is an overdependence on one reservoir system.

Experts are divided as to whether more investments by Sabesp could have helped avert shortages. But São Paulo state environmental prosecutors in the nearby city of Piracicaba, supplied by the same reservoir system, have launched an inquiry and claim that the crisis could have been avoided.

"There are a series of investments that if they had been done would have reduced the dependence on the Cantareira system," said prosecutor Ivan Castanheiro. The water shortage has already killed 20 tonnes of fish in the Piracicaba River. "This has already been an ecological disaster," he said.

Sabesp blamed the driest summer since 1930 for the water shortage. "The rain rate for the months of December, January and February was well below the lowest recorded in 84 years, when measurement began," the company said in an email to the Guardian. "It has never rained so little." In February, Sabesp said, 73mm of rain fell, compared to an average of 202.6mm.

The company, which is 50.3% owned by the state government of São Paulo, has introduced measures to guarantee supply – including a bonus for customers who economise on water, the transfer of water from other reservoir systems for consumers who depended on the Cantareira system, and an advertising campaign to encourage people to reduce consumption.

Sabesp is also considering financial penalties for consumers who use too much water. "We do not call it a fine, we call it a tariff increase," said a spokesman.

Residents in poorer suburbs situated on the edge of the city, or who live in higher areas, said their water supply is regularly shut down. "Every night, from around 9pm until 5am, they switch off the water. This has been going on for three months," said Rosali Junqueira, 53, in Casa Verde, North São Paulo.

"There is no more water in the taps after 10pm, and it only comes back at 7am," said Ana Paula Bispo, in Jaguaré in the west of the city. "In my house there is a lack of water every weekend," said Alexandra Machado in Jardim Ângela, in the far south of the city. "They are rationing. We go without water for three, four days."

Monica Porto, a professor of water quality and environmental engineering at the University of São Paulo, said the shortages could be because Sabesp is reducing water pressure at night. "This is a way to reduce consumption that has less impact," she said.

Reducing the water pressure also reduces leaks and consequently overall consumption, said Antonio Zuffo, a professor at the civil engineering, architecture and urbanism department at Campinas State University. "It is a hidden rationing – it is what we called 'white rationing'," he said.

But Sabesp denied that it has reduced pressure in the network. "What we have identified are isolated complaints," said a spokesman. "We have 27 million consumers and we have around 1,000 complaints of lack of water a day."

Benedito Braga, president of the World Water Council and a professor of civil and environmental engineering at the University of São Paulo, said the principal causes of the shortage were the drought and high consumption by a population used to an abundance of water in Brazil's rainy south-east.

"The Cantareira system has operated properly since it started; it's just that this anomaly brought it to our attention," Braga said. There are no easy alternatives for its water supply and the Tietê river that flows through the city is heavily polluted. "São Paulo is located in a very sparse place, water wise."

Work on inter-basin transfers from nearby water basins that could have reduced São Paulo's dependence on the Cantareira system has been held up by Brazil's slow environmental licensing procedures, he said. Edison Lobão, Brazil's mines and energy minister, said the Cantareira system lacked investment.

On 15 May Sabesp started pumping 400bn litres of what is called "dead water" – water that is normally left at the bottom of reservoirs. Prosecutors argue that using this dead water could involve public health risks. Both Sabesp and Braga insist it is safe. Using the dead water guarantees supplies until October, when summer rains should begin again, Brega said.

"There will not be rationing in the Cup. We will get through the cup tranquilly," said Professor Porto. She argued that São Paulo's water problem was part of a wider worldwide trend. "Programmes for reducing consumption are becoming more necessary for all cities," she said. "Unfortunately SP is learning this the hard way."

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