According to an Anglo-Argentine study, the spread of large-scale agriculture over the last four decades has favoured the rise of water tables in the plains of South America. Substitution of native vegetation has been crucial factor
by Matteo Cavallito
The strong expansion of large-scale extensive agriculture is making the plains of South America more vulnerable to flooding. That is a direct effect of the deep transformation these territories have undergone in recent decades. This was revealed by a team of researchers from the University of San Luis, Argentina, and Lancaster University, UK.
Using satellite images and field observations over the past four decades, as well as statistical models and hydrological simulations, the scientists identified trends in groundwater and flood dynamics. Revealing how some seemingly minor but also extremely widespread human-imposed changes to vegetation cover have ended up altering the water cycle in different regions.
Unprecedented agricultural expansion
The surge in global demand for commodities has encouraged the conversion of many grasslands and forests into fields for the cultivation of soya and maize. This agricultural expansion, points out a statement by Lancaster University, has taken place at the impressive rate of 2.1 million hectares per year.
This phenomenon has long raised many concerns about the maintenance of biodiversity and soil degradation. The Anglo-Argentine study, which covered the central plains of South America above the 50th parallel south, focused instead on the hydrological impact.
“The extent and speed of farming expansion across the South American plains over the past four decades provide an unprecedented case of the effects of rainfed farming on hydrology,” says the research published in the journal Science. According to Javier Houspanossian, a researcher at the University of San Luis, “The replacement of native vegetation and pastures with rain-fed croplands in South America’s major grain-producing area has resulted in a significant increase in the number of floods, and the area they affect.”
By evaluating remote-sensing images, the authors observed how flood-prone areas are expanding at a rate of 700 square kilometres per year in the Central Plains. This is, “a phenomenon unseen elsewhere on the continent.” Analyses, in particular, “shows that as annual crops replaced native vegetation and pastures, floods gradually doubled their coverage, increasing their sensitivity to precipitation.”
In detail, the research continues, “Groundwater shifted from deep (12 to 6 meters) to shallow (4 to 0 meters) states, reducing drawdown levels.” Based on available field research, it is possible to state that vegetation change – with its impact on the average length of plant roots – and evapotranspiration in cultivated land are the main causes of the hydrological transformation.
More vulnerable plains
“By replacing deeper rooted trees, plants and grasses with shallow rooted annual crops over such a huge scale this has culminated in seeing the regional water table rise closer to the surface,” said Esteban Jobbágy, a researcher at the Consejo Nacional de Investigaciones Científicas y Técnicas, an Argentine government research organisation, quoted in the Lancaster University statement.
“As the water level rises closer to the surface,” he added, “there is naturally less capacity for the land to absorb heavy rainfall, contributing to making flooding more likely.” Finally, the essential characteristic of the plains also plays a role. The flatness of the soil, in fact, results in very slow water runoff, favouring flooding in “some of the best farming soils on Earth”.
Risks are not limited to South America
The results of the study, in short,“show the escalating flooding risks associated with rainfed agriculture expansion at subcontinental and decadal scales.” The spread of floods, the researchers continue, typically damages the food supply by driving up commodity prices. Not to mention other problems such as soil erosion and salinisation.
The dangerous link between rapid and extensive land use changes and hydrogeological changes does not only affect the South American continent.
The phenomenon, the researchers go on to point out, also represents a potential danger for other areas subject to the expansion of lowland agriculture. That is territories as central Canada, Hungary, Kazakhstan and parts of China and Ukraine. Hence the call for the use of more sustainable practices such as the selection of crops with roots capable of penetrating deeper into the subsoil and the use of crop rotations that are more flexible to changes in groundwater depth.