Limited impact of vapor pressure deficit on rainfed maize evapotranspiration, CO2 flux, and canopy temperature

Recent studies have suggested that increasing growing season vapor pressure deficit (vpd) will result in greater crop water use and more frequent water stress in rainfed maize (Zea mays L.). In this 3-year study, we used flux tower and biometeorological measurements to examine the impact of diurnal variability of vpd and other climate parameters on latent heat (LE), sensible heat (H) and CO2 fluxes, and canopy temperature in field-grown maize at non-stress (high) and stress (low) volumetric soil water contents (SWC). Increases in vpd impacted LE primarily through midday when both net radiation (Rn) and vpd are increasing but vpd can still be low enough (<1,800 Pa) so that not all Rn is being dissipated as LE. At high SWC, increases in LE at maximum daily vpd (1500-1700 h) were limited because Rn was decreasing at that time. Increases in LE above Rn at maximum vpd depended on H flux toward the canopy, which occurred late in the day and was generally low (<50 W m(-2)). Vpds above 2000 Pa were only recorded at low SWC where CO2 flux was reduced and canopy temperature was greater than air temperature, indicating reduced canopy conductance. However, similar LE occurred at both low and high SWC due to higher potential evapotranspiration at low SWC. Our data suggest that the impact of possible increases in growing season vpd on LE may be limited in rainfed maize.