This study should serve as a “wake-up call,” according to researchers, because it shows how extensive large-scale agriculture has increased the vulnerability of the South American plains to widespread flooding.
Along with other vast flat plain regions of South America, the grasslands of the Argentine Pampas, renowned as the home of the iconic Gaucho, have seen a significant alteration in recent years.
Large tracts of grasslands and woodlands across South American lowlands have been quickly transformed to the production of annual crops like soybean and maize due to growing global demand. A astounding 2.1 million hectares of new land are being converted to agriculture each year.
Environmental concerns around biodiversity and soil degradation from these changes are long-standing. However, a recent study that was just published in Science demonstrates how these transitions to annual crop agriculture, which depends on rainfall rather than irrigation, are also quickly upsetting the water table in the vast, flat Pampas and Chaco plains and raising the likelihood of surface flooding significantly.
In order to identify trends for groundwater and flooding, an international team of researchers from Lancaster University in the UK and San Luis Universities in Argentina used satellite imagery and field observations from the past four decades along with statistical modeling and hydrological simulations. They provided previously unheard-of proof of how human activity can alter the water cycle across vast areas through subtle but pervasive changes to vegetation cover.
“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,” said Dr. Javier Houspanossian of the National University of San Luis, in Argentina. “Fine-resolution remote sensing imagery captured the appearance of new flooded areas, expanding at a rate of approximately 700 square kilometres per year in the central plains, a phenomenon unseen elsewhere on the continent.”
The information showed that floods are gradually expanding in area and becoming more vulnerable to variations in precipitation as shallow-rooted annual crops replace deeper-rooted native plants and pastures. Groundwater, once deep beneath the surface (12-6 metres), is now rising to shallower levels (around 4 metres).
We used a statistical modelling approach that avoided bias and strong assumptions, adding rigour to our findings linking intensive agriculture expansion to increased risk of flooding. These findings should inform new land management policies across these extensive flat rain-fed regions.
Dr. Wlodek Tych
“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 Dr. Esteban Jobbágy of CONICET, in Argentina. “As the water level rises closer to the surface there is naturally less capacity for the land to absorb heavy rainfall, contributing to making flooding more likely.”
The flatness of the terrain, which causes water to move away very slowly, is the key to this sensitivity to shallower water tables. In many cases, the best farming soils on Earth are found in the flattest sedimentary plains.
“In these extremely flat regions we find vegetation changes play a major role in modulating flooding through the capacity of plants to draw down groundwater reserves during dry periods,” said Dr. Jobbágy.
These results, according to the researchers, are a “wake-up call” because they demonstrate how rapidly expanding agriculture across broad plains can disturb the hydrology on a massive scale, raising the risk of flooding.
“These floods are a major concern for the farmers and people living in the region, but also elsewhere as further expansions of these floods could potentially disrupt food supplies and prices,” said Professor Mariana Rufino, formerly of Lancaster University.
“These results should act as a wake-up call that if we are going to make such huge and rapid land-use changes across large flat landscapes then it can transform the hydrology with potential increased risks.”
Researchers claim that in addition to flooding, these human-caused hydrological changes also run the danger of other problems such soil erosion, methane emissions, and salinization of the land.
The researchers argue that the hydrological changes happening in the South American plains also offer lessons for other similar agriculturally intensifying flat regions elsewhere in the world, such as central Canada, Hungary, Kazakhstan, areas of China and the Ukraine.
Professor Peter Atkinson of Lancaster University said: “This research reminds us that the Earth is a delicately balanced system, and that our actions in one domain can have unintended negative consequences in another, in this case fairly uniformly over a vast area.”
Dr. Wlodek Tych of Lancaster University said: “We used a statistical modelling approach that avoided bias and strong assumptions, adding rigour to our findings linking intensive agriculture expansion to increased risk of flooding. These findings should inform new land management policies across these extensive flat rain-fed regions.”
The findings, according to the authors, highlight the urgent need for better land use regulations that support environmentally friendly farming methods and sensible water management plans.
“There is much that can be done at the landscape level if we allocate parts of the land to deep rooted forest patches, and perennial pastures to prevent very shallow areas of ground water,” said Dr. Jobbágy.
Breeding crops with deeper root systems and creating agricultural rotations that are more adaptable to water table depths are additional alternatives. The findings are outlined in the paper ‘Agricultural expansion raises groundwater and increases flooding in the South American plains’ published by Science.
The paper’s authors are Javier Houspanossian and Raul Gimenez of CONICET and the National University of San Luis, in Argentina; Juan Whitworth-Hulse and Esteban Jobbagy of CONICET; Marcelo Nosetto of CONICET and Universidad Nacional de Entre Rios; Wlodek Tych, Peter Atkinson and Mariana Rufino of Lancaster University.
