How land surface characteristics influence the development of flash drought through the drivers of soil moisture and vapor pressure deficit

As global mean temperature rises, extreme drought events are expected to increasingly affect regions of the US that are crucial for agriculture, forestry, and natural ecology. A pressing need is to better understand and anticipate the conditions under which extreme drought causes catastrophic failure to vegetation in these areas. In order to better predict drought impacts on ecosystems, we first must understand how specific drivers, namely, atmospheric aridity and soil water stress, affect land-surface processes during the evolution of flash drought events. In this study, we evaluated when vapor pressure deficit (VPD) and soil moisture thresholds corresponding to photosynthetic shutdown were crossed during flash drought events across different climate zones and land surface characteristics in the US. First, the Dynamic Canopy Biophysical Properties (DCBP) model was used to estimate the thresholds that define reduced photosynthesis by assimilating vegetation phenology data from MODIS to a predictive phenology model. Next, we characterized and quantified flash drought onset, intensity, and duration using the Standardized Evaporative Stress Ratio (SESR) and NLDAS-2 reanalysis. Once periods of flash drought were identified, we investigated how VPD and soil moisture co-evolved across regions and plant functional types. Results demonstrate that croplands and grasslands tend to be more sensitive to soil water limitations than trees across different regions of the US. We found that whether VPD or soil moisture was the primary driver of plant water stress during drought was largely region-specific. The results of this work will help to inform land managers of early warning signals relevant for specific ecosystems under threat of flash drought events.