Redefining droughts for the U.S. corn belt: The dominant role of atmospheric vapor pressure deficit over soil moisture in regulating stomatal behavior of maize and soybean

Authors: KIMM, HYUNGSUK - University Of Illinois; GUAN, KAIYU - University Of Illinois; GENTINE, PIERRE - Columbia University; WU, JIN - University Of Hong Kong; Bernacchi, Carl; SULMAN, BENJAMIN - University Of California; GRIFFIS, TIMOTHY - University Of Minnesota; LIN, CHANGJIE - Tsinghua University

Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/2/2020
Publication Date: 6/15/2020
Citation: Kimm, H., Guan, K., Gentine, P., Wu, J., Bernacchi, C.J., Sulman, B.N., Griffis, T.J., Lin, C. 2020. Redefining droughts for the U.S. corn belt: The dominant role of atmospheric vapor pressure deficit over soil moisture in regulating stomatal behavior of maize and soybean. Agricultural and Forest Meteorology. 287:107930. https://doi.org/10.1016/j.agrformet.2020.107930.
DOI: https://doi.org/10.1016/j.agrformet.2020.107930

Interpretive Summary: Droughts reduce yields in major agricultural regions of the planet, such as the Midwestern United States where over 90% of the agricultural land depends exclusively on rainfall. A lack of water in the soil is generally associated with the impact of drought on crop yields but a lack of humidity in the atmosphere can also cause plants to experience drought responses. It is unclear what is more important in causing drought responses of crops - low humidity in the air or low soil water. In this experiment a number of approaches were used to better understand the importance of dry air compared with dry soils in crop responses to drought. The data used represents long-term measurements over corn and soybean that includes yields, soil moisture, atmospheric humidity, and water use of crops. The results show that dry air is a much more important factor in determining how plants use water than soil moisture, which means dry air may cause more damage to crop yields than dry soil. This is particularly important because global warming is causing drier atmospheres and this may lead to lower crop yields in the future.

Technical Abstract: The U.S. Corn Belt, the world’s biggest production region for corn and soybean combined, is prone to droughts. Currently 92% of the U.S. Corn Belt croplands are rainfed, and thus are sensitive to interannual climate variability and future climate change. Most prior studies identify the lack of soil moisture as the primary cause of agricultural drought impacts, although water-related stresses are also induced by high atmospheric water demands (i.e., vapor pressure deficit, VPD). Here we empirically attributed the variability of canopy-level stomatal conductance (Gs) and gross primary productivity (GPP) to VPD and soil water supply (i.e. volumetric soil water content, SWC), using eddy-covariance data from seven AmeriFlux eddy covariance sites in maize and soybean fields across the U.S. Corn Belt, which are well represented for the current rainfed part of the Corn Belt croplands. We used three independent approaches, including two statistical models (i.e. a multiple-linear regression model and a semi-empirical, non-linear model) and information theory, to quantify the relationship of Gs (or GPP) with VPD and SWC. The attribution result from the two models shows that VPD explains most of Gs variability (91% and 89%, respectively), and mutual information also attributed 91% of GPP variability to VPD. This finding was robust over the gradients of rainfall and temperature, crop types (maize vs. soybean), and management practices (whether irrigated or not). We reconciled our finding with the previously emphasized importance of precipitation and SWC, by conducting a path analysis, which revealed the causal relationships between precipitation, air temperature (Ta), relative humidity (RH), VPD, SWC, and Gs. We find that precipitation impacts on Gs through reduced RH and Ta to VPD (rather than directly through SWC). With increased VPD robustly projected under climate change, we expect increased crop water stress in the future for the U.S. Corn Belt.