Sao Paulo, Brazil's drought-hit megacity of 20 million, has about two months of guaranteed water supply remaining as it taps into the second of three emergency reserves, officials say.
The city began using its second so-called "technical reserve" ten days ago to prevent a water crisis after reservoirs reached critically low levels last month.
This is the first time the state has resorted to using the reserves, experts say.
But an expected increase in water usage during the upcoming Christmas and New Year's holidays could easily reduce the time the reserve will last.
After that period, there is no certainty over the water supply available to Brazil's wealthiest city and financial centre, said Marussia Whately, a water resources specialist at environmental NGO Instituto Socioambiental.
If rain does not replenish the Cantareira system - the main group of reservoirs that supply Sao Paulo - the city could run dry, she said.
A third and final technical reserve might be used, but it is difficult to access and mixed with silt that could make pumping it to users difficult, according to Vicente Andreu, the president of the water regulatory agency ANA.
"I believe that, technically, it would be unviable. But if it doesn't rain, we won't have an alternative but to get water from the mud," Mr Andreu said at a hearing about the water crisis in Brasilia's Lower House of Congress earlier this month.
Brazil's southeast region is suffering its worst drought in at least 80 years after an unusually dry year left rivers and reservoirs at critically low levels.
Antonio Nobre, a leading climate scientist at INPE, Brazil's National Space Research Institute, has linked Brazil's worsening drought to global warming and deforestation in the Amazon.
Both are drastically reducing the release of billions of litres of water by rainforest trees, which reduces rainfall further south, he said.