São Paulo Nearly Ran Dry in January. Next Wednesday Decides Its Water Limit for October.
BRAZIL · SÃO PAULO
Key Facts
- —The city São Paulo is the largest city in the Americas, about 21 million people in its metropolitan area — more than New York or Mexico City. One reservoir network, the Cantareira system, supplies roughly nine million of them.
- —Why it matters In 2014 and 2015 Cantareira came within weeks of running dry and the city rationed water for months. Brazil has managed it by fixed rule ever since.
- —How the rule works The volume the city may draw is set once a month, by the reservoir level on the last working day of the month before. Above 40 percent the utility may take 31 cubic metres a second; between 30 and 40 percent, only 27.
- —What happened On 22 September the system stood at 40.6 percent — six tenths of a point above the line. The reading that decides October is taken on Wednesday 30 September.
- —What turns on it Four cubic metres a second, which over a month is about ten billion litres. In January this same system was below 20 percent.
São Paulo is the biggest city in the Western Hemisphere, and a great deal of its drinking water arrives from a single chain of reservoirs in the mountains north of it. The Cantareira system serves around nine million people. Next Wednesday morning someone will read a gauge, and the number they write down will set how much of that water the city is allowed to take for the whole of October.
On 22 September the system held 40.6 percent of its usable volume — 398.89 million cubic metres — according to the daily figures the state water utility Sabesp publishes each morning. It had risen two tenths of a point in a day. The line that matters is 40 percent. The margin is 0.6.
Why a single reading decides a month
Most countries manage a water shortage by announcing restrictions when things look bad. Brazil, after the crisis of a decade ago, chose to take the judgement out of it.
In 2014 and 2015 Cantareira fell so low that engineers pumped the “dead volume” from beneath the intake pipes, and the city went months with pressure cut at night and taps running dry in some districts. The system came within weeks of failing outright. What followed was a joint federal and state resolution, agreed in 2017, that fixed the rules in advance so nobody has to argue about them during a drought.
Under that resolution the reservoir level is read on the last working day of each month, and that one figure sets the withdrawal limit for the month that follows. Between 30 and 40 percent the system sits in what is called the alert band, and the utility may take 27 cubic metres a second. Above 40 percent it moves to the attention band, and the limit rises to 31.
Four cubic metres a second sounds small. Run for a month, it is about ten billion litres — roughly what a city of a million people drinks and washes with in that time.
September has been governed by the alert band, because on the last working day of August the level sat between 30 and 40 percent. The next reading falls on Wednesday 30 September.
The month that changed the arithmetic
That the question is close at all is the work of an extraordinary September.
Sabesp’s rainfall record for the Cantareira basin shows 273 millimetres of rain so far this month, against a long-run September average of 78.8. That is about three and a half times the normal amount. Brazilian outlets citing the national meteorological institute have called it the wettest September in São Paulo since records began in 1943.
The turnaround is steep. In January the same system was operating below 20 percent, and Brazil’s national water agency warned it might drop into the emergency band. It recovered into the attention band by April, slid back to alert as the dry season went on, and in July the agency approved a temporary extra withdrawal to shore up supply. Brazil’s federal disaster-monitoring centre, working from average rainfall, expected the system to finish September at about 36 percent. The rain beat that by nearly five points.
One number, six reservoirs, uneven filling
Cantareira is not a lake. It is six reservoirs linked by tunnels and canals, and the headline percentage is an average across all of them. They have not refilled evenly.
On 22 September, Águas Claras stood at 81.8 percent, Cachoeira at 69.6, and the Jaguari reservoir feeding in from the Paraíba do Sul river at 65.2. But Atibainha was at 30.9 percent, Paiva Castro at 35.6, and Jaguari/Jacareí — much the largest of the six, holding 317.5 million cubic metres by itself — at 39.3.
In other words the composite figure is being carried across the 40 percent line partly by reservoirs that were never the problem, while the largest one, and the one under most stress, remain below it.
What comes next
The reading on Wednesday 30 September sets October, and nothing after that can change it until the end of October sets November.
Crossing the line would not mean the drought is over. It changes one thing: how much water the utility is permitted to draw for four weeks. It has no effect on what falls in October, and southeastern Brazil’s rainy season is only starting.
Falling short would not mean shortages either. Twenty-seven cubic metres a second is what the city has been supplied at all month, and nobody in São Paulo has noticed.
What the week does show is how finely the water supply of the largest city in the Americas is now calibrated — managed to a threshold so narrow that eight days of weather decide which side of it nine million people spend October on. After 2015, that is by design. It is still a thin margin.
Frequently Asked Questions
What is the Cantareira system?
A chain of six reservoirs in the mountains north of São Paulo, linked by tunnels and canals, that supplies drinking water to about nine million people in the metropolitan area. It is the city’s single largest source.
What happens if it drops below 40 percent on 30 September?
The system stays in the alert band for October and the utility may withdraw 27 cubic metres a second rather than 31. It is a limit on how much water may be taken, not a rationing order for households.
Is São Paulo running out of water?
Not at present. The system is at its highest level in months after the wettest September on record, and the city has been supplied normally throughout. The concern is how narrow the margins have become, not an immediate shortage.
Sources: Sabesp, Portal dos Mananciais, reading of 22 September 2026; Joint Resolution 925/2017 as applied by Brazil’s national water agency (ANA) and the São Paulo state water agency; Cemaden hydrological projection for the Cantareira system, September 2026.
This article was produced by The Rio Times’ automated newsroom system. How we use AI · Report an error
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