Observed changes in vapor pressure deficit suggest a systematic drying of the atmosphere in Xinjiang of China

The vapor pressure deficit (VPD) is the difference between the atmospheric water vapor holding capacity at saturation (es) and actual amount of water vapor (ea) in the air at a given temperature. In this study, we investigated the changes in VPD and related variables (air temperature, relative humidity, es and ea), and assessed the relationship between VPD and atmospheric conditions in Xinjiang over the period 1961–2017. The annual VPD records showed increasing trends during all seasons. In terms of spatial distribution, the increasing and decreasing trends of annual VPD were 79% (57% significant) and 21% (9% significant) of the observation stations, respectively. A trend reversal from downward to upward was observed in 1993, and the VPD records indicated rapid drying of the atmosphere in Xinjiang for 1994–2017, especially in the summer months. The differences in es and ea resulted in changes in the VPD, and the ea generally increased less than the es. We suggest that the increasing VPD in Xinjiang is due to increasing es from rising temperatures and decreasing ea from decreasing relative humidity since 1994. The decreased relative humidity has a strong influence on the increased VPD and accounts for ~74% of the variance in Xinjiang. In different environments, the VPD trends followed changes in air temperature and ea, with the greatest VPD increase in the desert, followed by the oasis and mountain environments. The increased VPD observed in Xinjiang between 1994 and 2017 is consistent with the global intensification of drought and adverse ecological effects of the 21st century. Thus, VPD change can be an accurate measure of regional drought and vegetation dynamics in Xinjiang.