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Scientists redefine drought in Corn Belt

URBANA, IL. – As the climate trends warmer and drier, global food security increasingly hinges on the ability of crops to withstand drought. But scientists and producers aren’t focusing on the correct metric when measuring crop-relevant drought, according to new research from University of Illinois scientists, who urge the scientific community to redefine the term.

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

The demand Guan refers to is atmospheric dryness, often expressed as vapor-pressure deficit, according to a university news release. The drier the air, the more moisture is sucked out of pores, or stomata, in plant leaves.

Plants must open stomata to take in carbon dioxide as their food, but if they sense the atmosphere is too dry they’ll close pores to avoid drying out. Keeping stomata closed too long leads to reductions in photosynthesis, plant growth and grain yield.

The kicker is 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,” Guan said. “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. However the demand side from the atmosphere can also severely stress plants. We need to pay more attention to that drought signal.”

Guan’s two recent studies used multiple technological approaches — including field measurements, various sources of satellite data, hydrological model simulations and government crop-yield statistics.

The first study, published in Agricultural and Forest Meteorology, used data from seven sites across the Corn Belt to conclude vapor pressure deficit accounts for almost 90% of the changes in crop-stomatal conductance, a proxy for drought stress. And it accounts for about 85% of changes in gross primary productivity, which is a measure of productivity.

Hyungsuk Kimm, doctoral student in Guan’s group and the study’s lead author, said, “By comparison, soil moisture typically accounts for 6% to 13% of these measures for corn and soybean, and up to 35% when considering time-lag effects.”

In the other study, published in the Journal of Hydrology, Guan’s team focused on grain yield. Yield depends on many factors related to water cycles, but the researchers found that vapor-pressure deficit explains the biggest proportion of variability in crop yield and also provides the earliest warning for yield loss when comparing with other water-cycle metrics and traditional drought indices.

Wang Zhou, postdoctoral researcher in Guan’s group and the study’s lead author, said, “This led us to build a new drought index integrating vapor-pressure deficit, soil moisture and measures of evapotranspiration, which can account for more than 70% of yield variation.”

Adjusting the drought concept for crops will be critical for global food security with a changing climate.

“When we look at climate-change scenarios, the amount of rainfall is not changing much for the Corn Belt,” Guan said. “But we for sure know temperature and vapor-pressure deficit 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.”