Disentangling day–night contributions toward altering atmospheric desiccation strength for US croplands

Vapor pressure deficit (VPD) is the difference between saturation and actual water vapor pressures and is driven by interactions between air temperature and humidity. Globally, VPD observations indicate drying of the atmosphere, while air temperature (T) and relative humidity (RH) have changed asymmetrically during daytime versus nighttime. However, whether daytime or nighttime T (T_day and T_night, respectively) and RH (RH_day and RH_night, respectively) have been more important to VPD change is unknown, and so is the seasonal variation of this importance. This research determines the relative contribution of T_day, T_night, RH_day, and RH_night to VPD change observed during 1981–2021 across the conterminous United States. T_day contributed 41% to driving a VPD increase of 2.1 standard deviations (SD), followed by contributions of 25%, 22%, and 12% from RH_day, RH_night, and T_night, respectively. Regions with significant VPD increase have seen increase in T_day (by 1 SD) and T_night (by 0.9 SD) and a decrease in RH_day (by 1 SD) and RH_night (by 3.4 SD). Diurnal asymmetry in warming and drying trends was true for most months, with greater rates of change during the nighttime than daytime. T_day was the dominant driver of the annual mean VPD rise for 70% of the counties, followed by RH_day (18%), RH_night (11%), and T_night (1%). We conclude that warming and drying during the days and nights together explain increased VPD across the US croplands and that climate stressors during days and nights should be viewed independently due to their crop-specific impacts that vary spatially and seasonally.