Increased atmospheric water stress on gross primary productivity during flash droughts over China from 1961 to 2022

Flash droughts threaten ecosystems substantially because of the fast onset and low predictability. Soil and atmospheric water stress are two main factors reducing ecosystem productivity during flash droughts. However, the long-term trends in the soil and atmospheric water stress on vegetation during flash droughts are unclear. By conducting long-term land surface model simulations, this study investigated the impact of atmospheric and soil water stress on gross primary productivity (GPP) during flash droughts and hot periods of flash droughts, as well as the long-term changes in water stress from 1961 to 2022 over China. The areas dominated by soil and atmospheric stress were 65.2% and 19.9% during flash droughts, respectively. During the hot periods of flash droughts, the areas dominated by atmospheric water stress were raised to 39.4%, and the areas dominated by soil water stress were reduced to 48.7%. During 1961–2022, the frequency, intensity, and duration of flash droughts all showed significant upward trends (p < 0.05) over China. Meanwhile, soil water stress on GPP decreased significantly (p < 0.05), but the atmospheric water stress increased significantly (p < 0.05). Correspondingly, the areas dominated by soil water stress decreased at 0.8%/decade, while the areas dominated by atmospheric water stress rose at 1.6%/decade during hot periods of flash droughts. With sensitivity simulations, we found that the water stress was weakened in the North China plain under irrigated conditions, but the trend was consistent with non-irrigated conditions over China. Our study indicated the importance of atmospheric moisture stress on vegetation productivity during flash droughts under climate warming.