Thirsty plants soak up water that would otherwise end up in the Colorado River, according to a new study. The findings could have important implications for water management in regions that rely on snowmelt for water, including Arizona and California.
More than 1.4 billion people worldwide depend on water from snowmelt-fed mountain rivers. In the United States, more than 10% of the population gets most of their water from the Colorado River alone.
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Most water in mountain ecosystems is lost through a combination of evaporation from the soil and a process called plant transpiration, where plants release water vapor from their leaves. This combined process is known as evapotranspiration. Scientists had thought that plants mostly drew on shallow soil moisture from recent rain or snow, which would mean that hot, dry conditions would reduce evapotranspiration while leaving river flows relatively constant.
But recent studies have uncovered a “drought paradox”: Plants maintain, or even increase, transpiration during dry periods.
To resolve this paradox, Reed Maxwell and Harry Stoneenvironmental engineers at Princeton University’s High Meadows Environmental Institute installed an array of sensors over an 81-hectare area in the East River watershed in Colorado, which flows into the Colorado River. The sensors measured water movement through the snowmelt-to-streamflow pathway over two years: 2023, which had a high snowpack but a warm, dry summer; and 2024, which had a moderate snowpack followed by a cool, wet summer.
They found that even during hot, dry periods – when soil moisture was at record lows – evapotranspiration remained high. This suggests that plants tapped into groundwater reserves when soil moisture was low, and used water that would otherwise end up in the river.
“Dry summer, wet summer; they get their water,” Maxwell told Live Science. “But they find it from other sources. They take it from shallow groundwater.”
Historical temperature and flow data also showed that summer temperatures affected current flow regardless of how much snow had fallen the previous winter. Snowmelt efficiency, the ability for a given amount of snowmelt to produce a certain amount of runoff, has declined over the past century, so the same storm produces less water in the reservoirs as time goes on. It is unclear exactly what is driving this shift, but climate change is part of it.
“We’re seeing it across the Upper Colorado River Basin; a warm summer will actually take a big snowmelt and make it an average snowmelt because of extra water needs from plants,” Maxwell said. “Plants are still meeting their needs; they’re just using other water sources, and those sources are taking away the water that would end up in our reservoirs.”
We could see a 40% decline by mid-century
Brad Udall, senior water and climate scientist at Colorado State University
The work was published as a preprint articlemeaning that it has not yet undergone peer review or been published in a scientific journal.
Brad Udalla senior water and climate scientist at Colorado State University who was not involved in the research said that a major strength of the work was that the researchers directly measured changes in evapotranspiration on an hourly basis, rather than relying solely on computer models.
The findings support a hypothesis that Udall and others are actively investigating: that increased evapotranspiration caused by higher temperatures contributes to reduced river flows, he said.
Over the past century, temperatures in the Colorado River Basin have risen by 2.5 degrees Fahrenheit (1.4 degrees Celsius), according to the study. Over the past seven years, water flow in the basin has dropped by 35%, Udall said. He and others have hypothesized that there will be “increased reductions in flow due to these higher temperatures going forward,” Udall told LiveScience.
That could have a big impact on how much water is available to people who depend on the Colorado River in the future. “That means there will be a lot less water, and we’re already seeing that,” Udall said. “We could see a 40% decline by mid-century.”
New management rules for how the water is divided between the upper and lower basins will enter into force next year. But so far there is no agreement on what these rules should look like, Udall said. Rising temperatures, decreasing river flows and decreasing rainfall will only complicate these negotiations.
Although this study is only one part of how the water cycle in the Upper Colorado River Basin works, it could have important implications for water use decisions in the future, Maxwell said. As summers get drier and warmer, we need to recalibrate our understanding of how much water can be available in the Colorado River, even in years with high snowfall.
“A better water budget that takes into account increases in summer transpiration is a very important factor in figuring out how much water is in the basin before we start dividing it up,” he said.
