Scientists have discovered a new driver of aridification, potentially changing the way droughts are understood around the world.
A new study published Wednesday in Nature by two researchers from Dartmouth College and the Université du Québec de Montréal shows that changes in precipitation concentrations have important effects on water retention in landscapes. When an area receives moisture from a few large wet storms each year, that moisture can overwhelm the soil and form puddles on the surface. These exposed pools are prone to evaporation, causing water that would otherwise reach streams, rivers, and dams to return to the atmosphere.
Combined with a long dry season, the researchers found that these storms dry out the landscape, even though total precipitation does not necessarily change.
“When you ask land to drink from a firehose, whether it’s from high concentrations of precipitation falling from the sky or rapid snowmelt, you’re losing water,” said Justin Mankin, associate professor of geography at Dartmouth College and lead author of the study. “It is a feature of our world that when rainfall is concentrated, less rain falls on land.”
Mankin and his co-author Corey Lesque, a professor of earth and atmospheric sciences at the Université du Québec de Montréal, used several precipitation datasets to determine where the Earth’s annual moisture is concentrated and where annual rain and snow totals are spread across the calendar.
“There are probably two hotspots that are showing the strongest consolidation trends since 1980,” Lesk said. “One of them is the Amazon and its adjacent regions, which are huge hotspots.”
“But another hotspot is almost directly above Wyoming (and Colorado),” he added.
The Amazon and western United States experienced the highest global precipitation concentrations since 1980.
River basins across the American West have been dried up by the “megadrought” that has gripped the region for much of the 21st century, forcing Western states to cut back on water use and make some rather acrimonious renegotiations of dwindling resources. Mankin and Lesk’s new paper adds to a growing body of science showing the dangers of changing water cycles in river basins, where users are accustomed to receiving constant amounts of water at predictable times.
“This method is a powerful combination of direct observation and testing the relationship using computer simulations,” said Brian Schumann, a professor of paleoclimatology at the University of Wyoming who was not involved in the study. “These are not patterns that can be dismissed as unreliable computer predictions. They show that this pattern actually occurs and can be observed.”
Schumann, who has previously studied precipitation concentrations, said the dynamics outlined in Mankin and Lesk’s paper paint a grim picture of Western climate.
“The challenges raised here highlight that the future could be accompanied not only by dangerous flooding, but also by far more severe droughts than in the past,” he said. “Simply put, you can get the same amount of rain and still experience a drought.”
As the Western United States slowly emerges from the worst winter on record, a future El Niño cycle is a possibility, with warm waters in the Pacific Ocean raising temperatures and precipitation in the West, bringing concentrated levels of precipitation and the potential for the drying described in Mankin and Lesk’s study.
Although this is not guaranteed, the relationship between rainfall and drought risk is something Mankin would like to explore in future research.
Since the early 20th century, the American West has blossomed into a vine of federal and state dams and canals intended to store and transport water from where it naturally flows to where it serves cities, farms, and industry.
But this century-old infrastructure and the economy it enables could be “potentially ill-adapted to this rapidly changing climate,” and could yield less water even if several severe storms pack the same amount of moisture, Mankin said.
Although Mankin and Lesk believe that water consolidation is a logical consequence of atmospheric warming, Lesk added, “In reality, it is a new mode of variability, a new way of making precipitation and water cycles in warmer climates harder to predict and harder to manage.”
“This is not just the same thing that Western countries have always dealt with.”
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Jake Bolster reports on Wyoming and the West for Inside Climate News. Previously, she worked as a freelancer covering climate change, energy, and the environment across the United States. He holds a master’s degree in journalism from Columbia University.
