The South American Pampas, among the flattest and most fertile grasslands in the world, is overflowing. In the past 5 decades, the region has suffered severe floods that destroyed hundreds of hectares of produce, disrupted infrastructure, and displaced people. In a new study, researchers blame intense agriculture for the problem, showing that as local cropland expanded, so did floods—over the past 40 years, flooded areas doubled in size.
“These cycles of flooding were coming and going, but with each new one, areas [that] never flooded before started to flood.”
“These cycles of flooding were coming and going, but with each new one, areas [that] never flooded before started to flood,” said one of the study’s authors, remote sensing specialist Javier Houspanossian in the Group of Environmental Studies at the National Scientific and Technical Research Council (CONICET) in San Luis, Argentina.
Flanked by the Andes to the west and the Brazilian highlands to the east, the Pampas spreads over parts of Argentina, Brazil, and Uruguay. Home to large cities, it also hosts some of the best farming soils on Earth: Good at absorbing rainwater, they dispense with artificial irrigation. These unique conditions make the area suitable for soy and corn plantations, which have displaced about 16% of native vegetation over the past 2 decades.
In the new study, published in Science, researchers combined satellite images with data on underground water levels as well as observations from several field studies and simulations to understand what is driving increased flooding in the South American flatlands.
Looking at the satellite data, they found that the Pampas has been flooding more intensively than the rest of the continent since 1977, with flooded areas expanding to extremely flat zones with high concentrations of crops. After the turn of the 21st century, when agriculture in the region expanded and consolidated, researchers also identified more occurrences of floods in places that had never historically flooded at all. From this period on, more than three quarters of all newly flooded areas on the continent occurred in the plains.
Data from eight monitoring stations in the Pampas also revealed that the water table—the boundary between water-saturated ground and unsaturated ground—has gone from very deep (6 to 12 meters) in the 1970s to very shallow (0 to 4 meters) after the 2000s, especially in cultivated lands.
Field studies and computer simulations offered an explanation for both the flooding and rising water table, suggesting that the replacement of native vegetation by crops is making the region more prone to flooding. Crops have shallower roots than the ecosystem’s native grasslands and forests, limiting the Pampas’s ability to absorb and retain water.
Plants That Prevent Floods
Plants use their roots to suck up water from the soil, which is then released as vapor into the atmosphere through transpiration. The deeper the plant roots are, the more efficiently the plant takes water out of the ground.
Because the Pampas is so flat, the rainwater that reaches the ground doesn’t spread much and instead penetrates horizontally into the soil, accumulating underground. Although native vegetation can reach down 15 meters and absorb water from those depths, crops have much shallower roots, around 3 meters deep, and can remove water only from shallow zones. In addition, crops are seasonal and are not present to absorb water year-round.
“You can think of it as a bank: The native perennial vegetation is much more effective at taking money out of the bank, and the agricultural shallow-rooted crops with fallow seasons are not,” explained hydrologist Bridget Scanlon from the University of Texas at Austin. “So then you get a buildup of water, and the water table rises, and you can get flooding.”
Scanlon, who was not involved with the new research but studies the effects of land use on groundwater levels, was struck by how drastic the situation is in the Pampas. Similar processes have been well documented in other parts of the world, including the United States and South Africa, she said. But none has shown the same flooding extent as in the Pampas.
Good for Crops, but Not for Too Long
On a positive note, the researchers also observed (in a parallel study) that the rise of the water table in the Pampas might be favorable for agriculture, buffering the negative impacts of the drought season. However, they stressed, this positive effect will last only in the short run.
The authors warn that flooding risk in the Pampas will only keep rising with the expansion of crops and expected effects of climate change, which include more rain. “We no longer need an extraordinary amount of rain to get more floods,” said coauthor and geoscientist Esteban Jobbágy at CONICET. “There will be no space to host the water excess, which is expected to increase particularly during the summers in the coming years.”
Besides the immediate disruptions caused by a flood, groundwater carries a naturally large quantity of salts that, when deposited onto the outer layers of soil over time, make it unsuited for farming. As flooding continues to expand, the negative effects of soil salinity might overcome the temporary benefit currently experienced during the drought season, the researchers said.
Practical, Possible Solutions
“The fact that we can have some control with monitoring the water table is good, but the challenge with this is [determining] who is going to say what the ideal water table level in each position is.”
Simple solutions offer some hope. Introducing crop rotation by planting vegetables with longer roots, like vetch and rye, is one of the options that farmers can adopt. Another solution, the researchers pointed out, is growing trees and even pastures, which naturally absorb more water from the soil and keep the water table down.
It is also important to give farmers the right tools to monitor the underground water levels so they can adjust their crops and prepare for floods. Jobbágy and Houspanossian are currently working on a project to develop low-cost instruments linked to smartphones to measure the water table. “If they [the farmers] can monitor the water table and combine that with more flexibility to plan their land use based on the risk of flooding, they will do better,” said Houspanossian.
But the final say, Jobbágy highlighted, is political and depends on governmental decisions on land use. “The fact that we can have some control with monitoring the water table is good, but the challenge with this is [determining] who is going to say what the ideal water table level in each position is,” he observed. “That is why regulating land use is very important.”
—Sofia Moutinho (@sofiamoutinhoBR), Science Writer