The disappearance of water from the Urucuia aquifer, the source of the São Francisco, intrigues scientists

Charged clouds cover the sun and flow for the first time in six months in western Bahia. The geraizeiros, traditional communities that inhabit the valleys between the famous plateaus of the region, welcome the rain showers that water their manioc, corn and bean plantations.

Still, many of them feel insecure. Several springs that always irrigated their lands during periods of prolonged drought, common in the region, have disappeared. And farmers who have always counted on the stability of these watercourses now have to be content with harvesting less food.

Since the years 1980, an area the size of the state of Alagoas —or a quarter of the cerrado forest of the plateaus of western Bahia— was deforested and transformed into a sea of ​​corn, soybean and cotton farms. A growing number of these large estates use irrigation water in their crops, which allows them to operate uninterruptedly throughout the year, having more crops.

They draw water from the Urucuia aquifer, a gigantic reservoir that it is below the plateaus and the rivers that cross them. The geraizeiros, who make their living from cultivating the most rugged lands and unsuitable for the mechanized agriculture that is practiced on the plateaus, have long argued that large-scale irrigation is stealing water from their springs in the valleys. But so far, little attention has been paid to the problem.

Recently, however, farmers have gained allies: hydrologists have been able to prove that the Urucuia aquifer is declining and that the rivers of western Bahia are drying up.

Some scientists claim that these changes may be weakening the springs, although no one has yet studied why they are harmed. Experts have yet to reach a consensus on who, or what, would be the biggest culprit in the decrease in this water source, although everyone agrees that agribusiness has at least some of the responsibility.

Last September, by the side of a river near his farm, the geraizeiro Eldo Moreira Barreto contemplated the current that snaked through a small oasis covered with tall green grass, surrounded by a leafy riparian forest.

Less than a hundred meters away, a forest of thorny, leafless trees, probably in hibernation, covered the undulating land as far as the eye could see.

“Look, what a beauty of palm tree!”, he said, pointing to a buriti tree. “These trees only grow in wet areas, like this one. When they are well, the springs are healthy.”

But the spring is no longer as vibrant and healthy as in the past. And that worries him because it is the source of irrigation for his land and for the village of Praia, a few kilometers away.

Barreto went to the spring along with a team of community volunteers to prune the shrubs that grow near the fence erected around the footpath. It serves to keep livestock away, avoiding trampling on delicate vegetation and soft soil.

He claims that cutting the vegetation around the fence prevents fires, common in the biome during the drier months, but also caused by people interested in deforesting, enter the path and damage the springs.

Hung Kiang Chang, professor of geology at Unesp (São Paulo State University), says that protecting riparian forests, as locals are doing, can help prolong the life of a dying spring. But, on the other hand, such efforts are not able to reverse the probable cause of its decline, that is, the depletion of the Urucuia aquifer. If he is not interrupted, he says, the work of Barreto and his community will have been in vain.

At the beginning of 2020, Chang, the researcher Roger Dias Gonçalves and two other colleagues from Germany published a study on the aquifer using data obtained from NASA satellites called Grace (the acronym for the Climate and Gravity Recovery Experiment).

The satellites, which measure gravitational attraction, provided material so that researchers could deduce the mass of water that had been in the water table for 10 years ( since when the satellite was launched, in 2002, until 2002). They found that during this period the aquifer lost about 10 cubic km of water.

O depletion of aquifer reserves may explain the decrease in the flow of springs, such as those used by farmers in Praia, says Chang. “My suspicion is that a small drop in the water table would probably affect springs,” he says. These effects have been described in some scientific articles.

In 2014, Gonçalves and two collaborators published an article that examined official records of 35 years of historic marks on river levels. They concluded that, between 1980 and 2014, the flow of the three main rivers during the dry season—when they depend exclusively on of the aquifer — plummeted 12%.

Some scientists question this number. But still, all experts agree that river flows and the water table are falling.

Determining what causes these changes therefore has important policy implications. For example, to predict the future of water supply, not just in Bahia, but along the entire course of the São Francisco, as the Urucuia is the most significant source of water in the river which spreads over thousands of kilometers in five states and helps generate one-tenth of Brazil’s hydroelectric energy.

Many hydrologists agree that the reduction in rainfall and the extension of monoculture latifundiums have played a role in decreasing water levels. Urucuia, but they differ as to the proportion in which this occurs. “We don’t have definitive answers,” says Chang. “We’re just getting started.”

The rainfall in western Bahia has increased since 1980, but since the beginning of the 1980, has been slowly decreasing, causing the recent drought. Compared to the decade of 80, the period since 1980 has been shown % drier.

Eduardo Marques, geologist and professor at the Federal University of Viçosa, author of an article on the aquifer, says that “a decade of decreased precipitation” is likely the primary cause of recent water stress.

Chang and Gonçalves disagree. “This has nothing to do with the rains”, says Gonçalves. The gigantic water loss detected in their studies and satellite data occurred over a period of 10 years in which precipitation barely changed. “The only thing that can explain this drop is the extraction “, says Chang.

The State requires grants, as the permissions for the pumping of water are known, which stipulate maximum flow rates for companies. Chang explains, however, that supervision is minimal in Bahia and that it is likely that much of the pumping is done in artesian wells or floating pumps in the plateau rivers, thus being underreported or even non-existent in state control.

Marco Heil Costa, scientist “I think the two factors are equally important,” he says. He and his colleague Marques have just completed a study on the Urucuia aquifer, funded by Aiba (Agriculturists and Irrigantes da Bahia).

Scientists also diverge on the advisability of limiting large-scale irrigation. Costa and Marques argue that regulators should place a moratorium on grants granted in more densely populated areas. irrigated, as parts of the Rio Grande basin, one of the main tributaries of the São Francisco.

In these areas, they say, it is not possible to extract water from the underground reservoir in a responsible manner. On the contrary, increasing irrigation in other parts of the plateaus would be acceptable.

Chang is opposed. For him, it is not yet time to stop extraction, even in the most explored and irrigated areas. “First we need to understand the functioning of the aquifer as a whole, as well as the dynamics of interaction between surface and underground water”, he argues.

Many researchers have been working to improve the models and, thus, better understand the water flows in rivers and underground in western Bahia. These studies, they point out, will make it possible to quantify the effects of agriculture in the region.

Gonçalves notes, however, with concern that, when these studies find the definitive answers, the Urucuia may already be irreparably damaged—too late to contain the damage.

“We may have definite answers within ten years,” he says, as the aquifer reserves “work with geological time.” “They can take thousands of years to recover”, he concludes.

The journalists traveled with the support of the Pulitzer Center.

2020

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