Remote sensing of atmospheric and soil water stress on ecosystem carbon and water use during flash droughts over eastern China.

Flash droughts are often accompanied by large soil and atmospheric moisture deficits, and the concurrence of flash droughts and high temperature may have a great impact on the ecosystem. However, the stress of soil and atmospheric moisture deficits on carbon and water use of the ecosystem during flash droughts, especially during the drought periods with hot conditions, are unclear over a large region. In this study, we decoupled the atmospheric and soil water stress over eastern China by using vegetation productivity data and photosynthetically active radiation data retrieved from the Moderate Resolution Imaging Spectroradiometer (MODIS). The analysis is conducted during flash droughts and their sub-periods that are accompanied by high temperature and intense radiation from 2003 to 2018. The results showed that soil moisture (SM) stress was significantly greater than the vapor pressure deficit (VPD) stress on vegetation productivity in the humid regions of eastern China during flash droughts. However, high VPD controlled the water stress on light use efficiency (LUE) of vegetation over 55 % of the regions. For the hot periods of flash droughts, the area subjected to VPD stress on vegetation productivity significantly increased in semi-arid and semi-humid regions. The concurrent hot and drought conditions also increased water use efficiency (WUE) for most areas, which suggests that the reduction percentage of vegetation productivity is larger than that of evapotranspiration. Our research emphasized the severe impact of compound hot and flash drought conditions on vegetation carbon and water use from a remote sensing perspective.