In the Vitor and Siguas valleys of southwestern Peru, giant landslides have been creeping down hillsides for decades, damaging farmland in the fertile valley basins. For the first time, scientists have documented the cause of these vast movements of earth: large-scale irrigation programs developed to feed modern agriculture on the high plateaus above.
“The findings demonstrate the long-term erosional impacts of irrigation,” said Pascal Lacroix, a geoscientist based in Grenoble, France, at the Research Institute for Development and colead author of the new study. “They also highlight the competition between modern and traditional agriculture.”
Lacroix and his collaborator, Edu Taipe of Peru’s Mining and Metallurgical Geological Institute in Arequipa, visited the valleys in May 2017, having heard of the landslides and their impacts. To investigate further, the researchers analyzed images from the Satellite pour l’Observation de la Terre (SPOT) 6 and SPOT 7 and the Hexagon spy satellite between the period of 1978 and 2016.
Sleuthing the Culprit
Analyzing the satellite data, scientists could see the expansion of cropland on plateaus above the valleys. For at least 3 millennia, farming had been confined to the basins of these valleys, where rivers provided water for crops. “But Peruvians wanted to increase the space for agriculture areas, and they couldn’t do that in the valleys because they are so narrow,” Lacroix said. “So they started irrigating the plateaus above the valleys.”
Vast irrigation programs started in the 1950s, and satellite data detailed the expansion of farmland to cover 105 square kilometers above the Vitor valley and 76 square kilometers above the Siguas valley. During that time, satellite imagery also showed that 12 large, slow-moving landslides had started, ranging in volume from 2 to 80 million cubic meters. The volume of earth moved by the largest landslide was the equivalent of more than 26,000 Olympic swimming pools.
The landslides occurred only on the sides of valleys below irrigated plateaus, and scientists found landslides in the Siguas valley began later than those in Vitor valley. This time difference corresponds to later development of irrigation programs in the region. “The spatial distribution of the landslides, together with the different initiation timing in the two valleys, clearly indicates that the landslides are triggered by irrigation,” Lacroix said. “And when you go there, you see water gushing from the cliffs, and it is clearly from irrigation.”
In both valleys, landslides began about 20 years after irrigation started, and now that earth is moving, it will continue to move, Lacroix said. The cliffs above are feeding the mechanics of the landslides and promoting motion farther down, he added.
Paradoxical Impacts
As the slides move downhill, sometimes as fast as 10 meters a year, they are eating away at agricultural land in the valleys. Satellite imagery shows that in the past 4 decades, 7% of the valley surface has been lost.
The data also show that valuable cropland on valley plateaus is being eroded as valley walls crumble. “The paradox is that modern farming needs extensive irrigation, leading to landslides, and those landslides destroy the modern farmland as well as the older farming areas,” Lacroix said.
Another group of researchers, led by Paul Santi, a professor of geology and geological engineering at the Colorado School of Mines in Golden, is also investigating landslides in the Siguas valley. One of them could soon have impacts on the Pan-American Highway, a major thoroughfare, and another could eventually damage an industrial milk facility that produces approximately half the milk for Peru.
Moving Toward Mitigation
Lacroix and Taipe hope their study, published in December 2019 in Nature Geoscience, will bring the attention of authorities and eventually lead to the deployment of mitigation strategies in the valleys. The findings show that it is important to use water efficiently, for instance, with drip or sprinkler irrigation with adequate dosing, said hydrologist Wouter Buytaert of Imperial College London. “The study highlights that rational use of water is not just a water resources issue but has wider geomorphological implications,” Buytaert said.
Developing sustainable mitigation strategies requires an understanding of how irrigation is triggering earth movement. “The only way to reduce the risk of these disasters is to investigate the processes,” Taipe said. “With this knowledge preventative and mitigation measures can be implemented.”
—Jane Palmer ([email protected]; @JanePalmerComms), Science Writer