Plant physiology and river flooding

Earth system scientists have identified another culprit (other than rain) that causes a river to overflow its banks: leafy plants.

In a study published today in Nature Climate Change, the UCI researchers describe the emerging role of ecophysiology in riparian flooding. As an adaptation to an overabundance of carbon dioxide in the atmosphere, trees, plants and grasses constrict their stomatal pores to regulate the amount of the gas they consume, a mechanism that limits the release of water from leaves through evaporation.

“Plants get more water-efficient and leak less underground soil moisture out through their pores in a carbon-rich atmosphere,” said study co-author Mike Pritchard, UCI assistant professor of Earth system science. “Add this up over billions of leaves in very sunlit, leafy places, especially the tropics, and it means there is a bunch more soil moisture stored up underground, so much so that climate models predict rainfall events will saturate the ground and more rain will run off into rivers.”

Pritchard said this so-called forest effect dominates atmospheric responses to CO₂ on most land masses up to 30 degrees north and south of the equator, which is where most people live. And he noted that this plant-based phenomenon could have a large influence on flooding in the Mississippi River basin.

“I was really interested in the Mississippi because it’s in our own national backyard,” Pritchard said. “It’s a big, complex basin fed by multiple sources, but it kind of looked like the spring rains on the East Coast and the Appalachians were running off more efficiently due to these effects of plant physiology, leading to an earlier peak flow out of the Mississippi than normal, which actually makes sense. The spring rains are able to more easily run off.”

He said the twin effects of plant physiology in the U.S. Southeast and precipitation anomalies caused by atmospheric warming farther north in the Mississippi basin “are both really conspiring to juice up the future flood statistics in equal proportion.”

Collaborating closely with Pritchard and Fowler on the project were James Randerson, UCI Chancellor’s Professor and Ralph J. and Carol M. Cicerone Chair in Earth System Science; and Gabriel Kooperman, assistant professor of geography at the University of Georgia and a former UCI postdoctoral scholar who worked with Randerson and Pritchard. Funding for the study was provided by the U.S. Department of Energy, the National Science Foundation, NASA, and the Gordon and Betty Moore Foundation.

“As the climate warms, an increase atmospheric moisture and an intensification of rainfall may contribute to more frequent flooding in some regions," Kooperman said. "Our study found that plants, in response to higher levels of atmospheric CO2, also contribute to changes in flooding. The stomatal pores on the surfaces of leaves do not open as much when atmospheric CO2 levels are high, which can reduce transpiration and leave more water in the ground. This occurs most in low-latitude regions with dense vegetation and can result in higher soil moisture that can increase flooding frequency, even without rainfall changes from warming.”

Great work by this research team, and an important new publication that adds to what we know about increasing levels of atmospheric CO2.

Image: NASA satellite image of flooding at the confluence of the Illinois, Missouri and Mississippi rivers near St. Louis on Aug. 19, 1993.