Rainfall deficit in Brazil has steadily increased in recent decades

Amount of rain falling on reservoirs and other runoff collection systems is declining in all regions, according to studies.

By Elton Alisson, in São Carlos

Agência FAPESP – The rainfall deficit throughout Brazil has steadily increased in recent decades and has worsened sharply in the past few years. Brazil’s Southeast region, for example, suffered the worst drought for 70 years in 2014-15, and in mid-August, the driest period of the year, the region will have less water than in 2014.

These are among the main findings of studies performed by the National Space Research Institute (INPE).

Some of the results of INPE’s research were presented in a lecture on the problem of drought in Southeast Brazil, held on July 17, during the 67th Annual Meeting of the Brazilian Society for the Advancement of Science (SBPC). The conference lasted until July 18 and was held at the Federal University of São Carlos (UFSCar) in São Paulo State.

“We have a stark rainfall deficit throughout the country, and this is a very grave situation. The amount of rain entering runoff collection systems is diminishing, so that our water bank account is more and more in the red,” said INPE researcher Paulo Nobre.

A study by researchers at the institute compared data from rainfall records for the period 1960-90 with data for 2015 to estimate Brazil’s current “water bank account balance”. Their calculations show a deficit corresponding to 6 cubic meters of rainfall per square meter of surface area in the North region.

The Northeast has a deficit of approximately 4 cubic meters per square meter, while the South is nearly breaking even.

The Southeast also has a large overdraft, as the deficit in the region has reached 3.5 cubic meters per square meter.

“This represents substantial amounts of water that simply hasn’t entered the hydrological cycle. It’s not water that was used to promote plant growth or for human consumption,” Nobre said.

In another study the researchers analyzed summer rainfall in the Southeast from the 1960s until recently. They found records of 1,000 mm on at least two occasions in a single month between 1960 and 1980.

Between 1980 and 2000, heavy rain became less frequent, rarely exceeding 900 mm. In the first decade of this century and in recent years, summer rainfall in the Southeast rarely surpassed 100 mm.

“Since 2010 it’s been raining below average in the Southeast,” Nobre said. “As a result, water levels have fallen in the region’s reservoirs. To make matters worse, we had a major drought in 2014-15.”

The amount of rain falling on the Cantareira reservoir system, which supplies water to the city of São Paulo and has become an emblem of drought in the state, has decreased decade by decade.

“The 2014-15 drought was an extreme event at the tail end of a long-term decline that has led to steadily lower reservoir levels in the last 20 years,” Nobre said.

Rising temperatures

According to Nobre, rising temperatures in the Southeast and other parts of Brazil are among the factors contributing to this year’s rainfall depression, the worst recorded in the region since 1945.

When Nobre and co-workers analyzed average temperatures in all regions of Brazil between 1960 and 2010, they detected a clear upward trend during the period.

“Brazil is getting warmer year by year. This is largely a reflection of the rise in global temperatures due to increasing concentrations of greenhouse gases in the atmosphere, among other factors,” Nobre said.

Rising temperatures in the atmosphere rapidly lead to extreme hydrological events such as drought and flooding, he explained, because water vapor from the surface dissolves faster as the air heats up and larger clouds are formed, causing more intense rain.

“Intense rain affects planetary circulation, so that precipitation increases sharply in some places and gives way to drought in others,” Nobre said.

The increase in emissions of greenhouse gases such as carbon dioxide (CO2), combined with rising temperatures, tends to lead to even worse water crises, he added.

Using the Brazilian Earth System Model (BESM) developed with support from FAPESP, the researchers at INPE simulated a fourfold increase in the amount of CO2 currently found in the atmosphere over Brazil – 300 parts per million – to estimate what would happen to the dynamics of the climate.

Analysis of the simulation showed that the presence of CO2 at 1,200 ppm in the atmosphere would cause an increase in the number of consecutive dry days in Brazil. Drought would be more frequent in the Southeast, and there would be an increase in the occurrence of long dry periods in the Northeast, the Amazon and South America in general.

In contrast, days of heavy rain would be more frequent, albeit at scattered intervals during longer dry periods.

“The projections suggest that Brazil’s future climate will feature more situations such as the one we’re in now, with flooding in the Itajaí and Tubarão river valleys in Santa Catarina, overflowing of the Madeira in the Amazon, and more frequent droughts in the Northeast and Southeast,” Nobre said.