- A combination of wet El Niño weather and human-induced climate change were key drivers of the worst flooding event in Brazil’s Rio Grande do Sul state earlier this year.
- The flooding affected 90% of the state and displaced more than half a million people.
- Poor land management is also responsible for the region’s vulnerability to floods, as current agricultural practices in the highlands favor runoff and reduce the soil’s ability to soak up water, with lowlands particularly exposed to high waters.
- While some scientists are still deciphering the causes and behavior of the floodwaters, other experts are working to rehabilitate farmland, tackle soil erosion, and source native seeds for ecological restoration.
Santa Maria, RIO GRANDE DO SUL— On the evening of April 29, Jean Paolo Gomes Minella, a hydrologist at the Federal University of Santa Maria in southern Brazil headed to his river monitoring station an hour’s drive away from his home. It was raining, so water would be trickling into the Guarda-Mor River from all around the watershed, and he wanted to be there to measure the sediments filtering in.
“We live like frogs,” Minella says, noting how his team springs into action whenever rain is forecast — most of the time. Uncharacteristically, his team had skipped the last three days of downpours, so he was anxious not to miss the fourth opportunity to collect data.
Little did he know that the rains he was chasing would leave an unprecedented trail of destruction and indelible trauma. After an hour or so of data collection, at midnight, the floodwaters came. As he stood in chest-high water, Minella realized he would potentially be the only person in the world able to capture the data as the flood happened.
All across Brazil’s Rio Grande do Sul state, rains saturated the land. Creeks, tributaries and rivers spilled over. After three days, on May 2, all that hydrological excess would march eastward into Porto Alegre, the state capital, and cause the worst flood to have ever hit the state.
Six months after the disaster, residents statewide are still grappling with the consequences. New research, including Minella’s, is slowly making sense of the floods’ devastating impact and how to move forward, while other scientists are working to recover ecosystems. At the same time, farmers in the region are having to rethink their practices if they want to prevent similar disasters in the future.
Drivers of extreme weather
Between April 29 and May 2, several towns across the state recorded more than 300 millimeters (12 inches) of rain: two months’ worth of precipitation in just four days. In central parts of the state, waters rose then subsided in a matter of hours. Further east, Porto Alegre’s flood protection system failed, so the city stayed underwater for three weeks. According to sources from the Extraordinary Secretariat for the Reconstruction of Rio Grande do Sul, the government agency created in June to help with recovery, the disaster led to 183 deaths, displaced nearly 600,000 people and affected 90% of the state.
Several reports have indicated that the combination of unusually rainy weather induced by El Niño and climate change was probably the main driver of the catastrophic floods. An analysis by World Weather Attribution estimated that human-induced climate change made the April-May floods, considered a once-a-century event, more than twice as likely to occur and about 6-9% more intense than normally expected. The last historic flood that hit the state was in 1941.
Climate change is also increasing the frequency of floods, says Fernando Mainardi Fan, a hydrologist at the Federal University of Rio Grande do Sul, noting that the 2024 flood came hot on the heels of a smaller flood just last year. “What happened is perfectly matched with the climate change projections. It will happen again.”
Poor land management also worsened the flood’s impacts, experts say. Between 1985-2022, agricultural encroachment has reduced forest area in the state, as well as the native Pampa grasslands by about a third. In the past few years the agribusiness-friendly government has also weakened environmental laws meant to protect native vegetation on farmlands, leaving ecosystems more vulnerable to environmental shocks.
Analyzing the data he collected during the flood, Minella found that excessive runoff also contributed to the event’s severity. Intensive agriculture practiced across the highlands surrounding the Jacuí River Basin in central Rio Grande do Sul has compacted soils, reducing their capacity to absorb water and removed natural vegetation that slows down runoff.
On the first night of the flood, Minella’s measurements showed the water current speeds were orders of magnitude above the baseline, while the river’s haziness indicated it carried a lot of sediment. The fast-moving runoff gouged riverbanks and littered the scene with felled trees and car-sized boulders.
Cultivation practices on the plateau “generate runoff that impacts the lower part of the catchment,” Minella says. “It’s an important and very difficult message,” one that farmers might not want to hear.
Short-term recovery
Days after the flood, the federal government released emergency cash aid and mobilized donations of household supplies and food for affected families. The administration earmarked 51 billion reais ($9 billion) to kick-start recovery. In an emailed response to Mongabay, Fernanda Costa Corezola, director at the Secretariat for Support for Reconstruction of Rio Grande do Sul, assistance for agriculture recovery largely includes debt relief for farmers and new credit in preparation for the next crop cycle. But experts say this isn’t enough.
Many households say they haven’t received any help. Norma Isabel Franke, a 53-year-old farmer in the central part of the state, forked out her life savings to start over after mudslides damaged her home and destroyed her farm. But she says she accepts that the cash-strapped government can’t do much, because so many households were affected.
Across the state, farmers like her are having difficulty bouncing back. According to the State Department of Agriculture, more than 48,000 producers and nearly 3.2 million hectares (7.9 million acres) of land — an area larger than Belgium — were affected by the floods. The National Confederation of Municipalities estimated that Rio Grande do Sul’s agricultural and livestock sectors suffered a combined loss of $600 million.
Walk or drive over the tapestry of farmlands in the valley, and the destruction is evident. On steeper terrain around the headwaters, the topsoil has been washed away. On lower ground, plots are buried under meters-thick dirt.
Not every farm can be recovered. Ricardo Bergamo Schenato, a soil scientist at the Federal University of Santa Maria, is visiting farms to evaluate the flood’s aftermath. Recovering the soil structure and fertility is the first step in restoring denuded plots. Although farmers are turning to quick fixes such as conventional fertilizers, these can’t really improve soil quality, Schenato says. Using cover crops, in particular nitrogen-fixing legumes and millet, which have short life cycles and quickly decay into humus, can help. “Organic matter is central in this process of soil recovery,” Schenato tells Mongabay.
Even with cover crops, there’s no guarantee that the soil will completely regain its previous productivity. “Will we have the same yield potential in these areas? That is a question that we have no answer to,” Schenato says. “We are certain that in some areas we will not achieve the same yield for five to 10 years. And in some plots, it will not be possible to cultivate anymore.”
Healing the earth
Several researchers at the Federal University of Santa Maria are currently working to restore native biodiversity in disaster-prone areas.
Forestry engineer Fabrício Jaques Sutili is leading efforts to rehabilitate riverbanks, starting with sites near the university campus and in the Quarto Colonia region in the central part of the state. His team wields living “construction material,” a mix of native plants whose root network fortifies the soil against erosion. Natural vegetation along the riverbanks promotes groundwater storage and helps slow down water flow. Many of the seeds for these species are commercially unavailable, so researchers have to gather them themselves from the wild and propagate them in greenhouses.
Forestry scientist Ana Paula Rovedder is collecting native seeds for ecological restoration, some of which have commercial value, working with Schenato to convince farmers to start a seed production supply chain. It could be a win-win solution: seed producers would earn some income from sales while regenerating the land.
“Restoration of native vegetation is a must,” says Valério Pillar, a vegetation ecologist at the Federal University of Rio Grande do Sul, who’s not involved in the projects. Though native vegetation can’t avert flooding entirely, he adds, “it could reduce the damage.” Floodwaters just a few inches lower would make the difference for some neighborhoods staying dry, or for flood protection systems, such as Porto Alegre’s, not being overwhelmed.
However, agroforestry solutions can take a long time to pay off. “The impact of this extreme event is so great that it will be very difficult for us to rebuild the capacity of these native forests,” Rovedder says. “It will take time.”
Revamping agricultural practices can help with flood resilience. Broad-based arrays, which are strips of soil mounds, cut horizontally across slopes and act as hydrological speed bumps, slowing down runoff. According to Minella’s team’s research, if farms in the highlands adopted them, peak runoff could decrease by as much as a third.
Minella says the government should incentivize broad-based array construction, as they provide an environmental service, including to urban communities miles away. “This is the point — farmers will receive money to decrease the floods,” he says. He’s currently seeking government sponsors for the project.
However, getting farmer buy-in is another matter, as farmers in the highlands who inadvertently exacerbated the flooding were the least affected by it.
Changing the topography can disrupt farming operations, as most equipment is optimized for flat and smooth surfaces. “The amount of maneuvering that you have to do … requires a lot of planning,” says Alice Prates Bisso Dambroz, Minella’s graduate student who also comes from a family of soy producers. Crops can be fussy to care for: farmers operate on tight schedules to match the pace of the seasons and strive for cutthroat efficiency to work around the whims of the weather. Any extra inconvenience is costly. At the end of the day, farmers are most concerned about making a profit. “That’s kind of the thought,” Dambroz says, “‘Do I need the infiltration terrace right now?’”
According to government statements, entities such as the Ministry of Environment and Climate Change and the Brazilian Agricultural Research Corporation (Embrapa) have long been involved in encouraging farmers to adopt environmentally friendly agricultural practices. The policy impact, though, isn’t clear; many of their strategies are based on technological assistance, require voluntary farmer buy-in, and don’t provide direct incentives for participation.
Whether the farmers know it or not, these uneasy choices weigh on those living at lower elevations, who themselves have tough calls to make. Silvana Viera Venturini, a 50-year-old dairy farmer who lost half of her riverside pastures to the floods, is keeping on through sheer “Brazilian grit,” she says. She hasn’t received financial help from the government, and she doesn’t expect it. She says she hopes the government will reengineer the river altogether — either through straightening the river channel or deepening the bed — so it never floods again.
However, Minella says that would be costly and expose populations downstream to unknown consequences. “It’s impossible to control the river,” he says. The more straightforward solution, he says, is to relocate away from floodplains, away from any risk of high water in the first place.
The flood in Rio Grande do Sul was just the latest wake-up call to the urgency of addressing climate change in everyday policy and action. It speaks to the broader challenge of “recognizing climate change and its impacts on life from now on,” says Corezola from the state reconstruction secretariat.
In the past, the Brazilian government hasn’t always acted in the best interest of the climate, and now, the current administration faces a hostile Congress to pass environment-friendly policy. But it’s becoming increasingly harder and costly to ignore environmental catastrophes like the May flood. “It is necessary to change culture towards behavior adapted to climate change,” Corezola says. “We can no longer repeat what happened in Rio Grande do Sul.”
This story was partially supported by the Pulitzer Center.
Citations:
Londero, A. L., Minella, J. P., Schneider, F. J., Deuschle, D., Menezes, D., Evrard, O., … Merten, G. H. (2021). Quantifying the impact of no‐till on runoff in southern Brazil at hillslope and catchment scales. Hydrological Processes, 35(3). doi:10.1002/hyp.14094
Werle, L. (2024). Modelagem do efeito do terraceamento para o controle do escoamento superficial numa bacia rural no sul do Brasil (Master’s thesis, Federal University of Santa Maria, Santa Maria, Brazil). Retrieved from http://repositorio.ufsm.br/handle/1/32278
Rodrigues, M. (2023). Politics and the environment collide in Brazil: Lula’s first year back in office. Nature. doi:10.1038/d41586-023-04042-x
Banner image: The metropolitan area of Porto Alegre affected by floods earlier this year. Image by Ricardo Stuckert / PR via Wikipedia.
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