Brazilians are hoarding water in their apartments, collecting rain water and taking other emergency measures to prepare for forced rationing that appears likely and could leave taps dry for up to five days a week because of a drought.

In Sao Paulo, the country's largest city with a metropolitan area of 20 million people, the main reservoir is at just 6% of capacity with the peak of the rainy season now past.

Other cities in Brazil's heavily populated southeast, such as Rio de Janeiro, face less dire shortages but could also see rationing.

Uncertainty over the drought and its consequences on jobs, public health and overall quality of life have further darkened Brazilians' mood at a time when the economy is struggling and President Dilma Rousseff's popularity is at an all-time low.

After January rains disappointed, and incentives to cut consumption fell short, Sao Paulo officials warned their next step could be to shut off customers' water supply for as many as five days a week - a measure that would likely last until the next rainy season starts in October, if not longer.

State officials say they have not yet decided whether or when to implement such rationing, in part because they are still hoping for heavy rains in February and March.

But many residents aren't taking their chances and stores selling large water storage containers are struggling to keep up with demand.

Thunderstorms in recent days have caused lakes to rise a bit.

Still, independent projections suggest that Sao Paulo's main Cantareira reservoir could run out of water as soon as April without drastic cuts to consumption.

As such, the race is on to secure water while it lasts.

Large hospitals in Sao Paulo are installing in-house water treatment and recycling centres, among other measures, to make sure they can still carry out surgeries and other essential tasks if regular supply stops.

A member of Rousseff's Cabinet said earlier this month on condition of anonymity that some degree of water rationing is expected in Brazil's three largest metropolitan areas - Sao Paulo, Rio de Janeiro and Belo Horizonte, with a combined population of 40 million people.

The government has already started to feel the blow back

On Wednesday there were protests in the streets of Sao Paulo against water rationing.