Halfway through the rainy season, the key reservoir for the hemisphere’s largest city holds just 6 percent of its capacity, and experts warn that Sao Paulo authorities must take urgent steps to prevent the worst drought in more than 80 years from drying it out.

The system of reservoirs and rivers that provide water to millions have received less rainfall than hoped during the first weeks of the wet season, raising fears they won’t be replenished.

Rainfall during the first two weeks of January totaled just 2.9 inches, well below the historic average for the month of 10.7 inches.

The biggest problem is in the Cantareira water system, shown above, which is the largest of six reservoirs that provide water to some 6 million of the 20 million people living in the metropolitan area of Sao Paulo city. Cantareira is now down to 6 percent of its capacity.

“The water supply situation is critical,” said Mario Thadeu Leme de Barros, head of the University of Sao Paulo’s hydraulic engineering and environmental department.

He said officials need to consider a range of steps, among them implementing water rationing but also encouraging the use of more efficient appliances, lowering water pressure in the system and doing better at repairing leaks.

“Sao Paulo’s water situation is in the intensive care unit,” he said.