Impact of High Vapor Pressure Deficits on Canopy Temperature and Evapotranspiration in Irrigated and Rainfed Maize.

Recently, several statistical studies have concluded that high air temperatures (Tair) and associated high vapor pressure deficits (vpd) are correlated with significant yield declines in maize.  However, collinearity in climate data can generate model results that are difficult to interpret and make predicting future yields by extrapolating from these results problematic.  Using data from eddy covariance flux towers, we show that in irrigated maize, high vpds (>2 kPA) are associated with maximum daily canopy temperatures that are at or below daily maximum Tair.  In rainfed maize, at higher vpds canopy temperatures can be above or below daily maximum Tair depending on soil water availability.  Our analysis shows that the impact of vpd on evapotranspiration (ET) in maize is diurnally dynamic and asymmetric, and that on a daily basis there is little change in ET rates under well watered conditions at vpds > 2 kPa.  Although higher vpds may be associated with greater ET rates earlier in the day, they are associated with decreasing ET rates later in the day as stomatal closure is occurring, thus limiting the daily impact of high vpd on ET.