New research from the University of Illinois is urging scientists to redefine the term “drought,” specifically when it comes to crop-relevant drought in the U.S. corn belt.

“Plants have to balance water supply and demand. Both are extremely critical, but people overlook the demand side of the equation, especially in the U.S. Corn Belt,” said Kaiyu Guan, who is the principal investigator of two new studies on this subject, Blue Waters Professor in the Department of Natural Resources and Environmental Sciences and the National Center for Supercomputing Applications at the University of Illinois.

This demand is atmospheric dryness, otherwise known as vapor pressure deficit.

The drier the air, the more moisture is sucked out of the pores and stomata in plant leaves. Plants must open stomata to take in carbon dioxide, but if they perceive the atmosphere is too dry, they’ll close pores to avoid drying out. If this closure lasts for too long, it can lead to a decline in photosynthesis, grain yield and plant growth.

The concern is that plants shut down stomata due to atmospheric dryness even when there’s an adequate supply of moisture in the soil.

“If you only consider rainfall and soil moisture, which is how most people think about drought, that’s mostly describing the supply side. Of course, if you have low soil moisture, plants will be stressed by how much water they get. But the supply is often pretty sufficient, especially here in the U.S. Corn Belt,” Guan said. “However, the demand side from the atmosphere can also severely stress plants. We need to pay more attention to that drought signal.”

In both studies, researchers tried to understand the demand side of drought from two angles. One measured landscape water and carbon use and the other used satellite data and model-simulated hydrological changes connected with regional supply.

“In both, we demonstrate VPD is more important than soil moisture to explain the crop drought response in the U.S. Midwest,” Guan said.

This research suggests that modifying the drought concept for crops will be critical for global food security under a changing climate.

“When we look at climate change scenarios, the amount of rainfall is not changing much for the Corn Belt, but we for sure know temperature and VPD will increase here. That means not much will change on the supply side, but demand stress will increase significantly. And that type of stress is so connected to end-of-season crop yield,” Guan said.