Predicting Plant Water Needs in a Warmer, Drier World

  • June 1st, 2020
Professional headshot of Dr. Mukesh Kumar
Dr. Mukesh Kumar

Plants drink up much of the water that falls to Earth. They take what they need before releasing it through tiny holes on the underside of their leaves, just as people release water vapor with every exhale.

How much a plant drinks and the rate at which it releases water, or transpires, depends partly on moisture levels in the air and soil. Global warming will shift this process more than previously predicted, according to new research published in Nature Climate Change and involving The University of Alabama.

In the paper, researchers show current climate models underestimate how severely plants ration their water use in response to dry air, and overestimate the effect of dry soil. The results suggest plants in many regions will lock away less water than expected during hot droughts in the future, leaving more water available to percolate into reservoirs, underground aquifers, rivers, lakes and streams.

“Our results suggest that existing estimates of crop water use, productivity and vulnerability, which are often based on empirical evapotranspiration models, are likely to be incorrect during periods when the atmosphere is dry,” said Dr. Mukesh Kumar, co-author on the paper and UA associate professor of civil, construction and environmental engineering. “The study also points to opportunities for improving predictions of plant vulnerability under hydroclimatic stress by incorporating plant hydraulics in ecosystem models.”

The work was led by Kumar’s former doctoral student at Duke University, Dr. Yanlan Liu, now a postdoctoral scholar at Stanford University. Kumar joined UA in fall 2018 to teach and research hydrology and water resources. Other co-authors include Dr. Alexandra Konings, assistant professor of Earth system science at Stanford University. Other co-authors come from Duke and University of Minnesota, Twin Cities.

The research is good news, yet there is also a dark side to the findings: While water resources may be less diminished, plant growth and carbon uptake will likely suffer more than most models predict.

“Whether plants will fare better in future droughts is a more complex question,” said Liu, who now works in Konings’ lab. “But now we know plants will use less water than expected.”

In some hot spots around the globe, episodes of dangerously humid heat are striking with growing severity and frequency. But as temperatures rise, Konings said, most droughts will be accompanied by relatively dry air. Hotter air can simply hold more water vapor than cooler air, which means the atmosphere becomes less saturated if it heats up without additional water. As a result, while future changes in soil moisture are hard to predict and likely to vary by region, she said, “Atmospheric dryness is going to go through the roof.”

They found the most widely used approaches for estimating evapotranspiration miss about 40 percent of the effect of dry air. This is like a weather forecast that fails to mention wind chill or stifling humidity. The effect is strongest — and current predictions are the most off-base — in places where plants are the least adapted to drought.

“We were surprised that this had such a big effect,” Konings said.

This story was adapted from a version written by Josie Garthwaite of Stanford’s School of Earth, Energy & Environmental Resources (Stanford Earth).

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