The features of regional flash droughts in four typical areas over China and the possible mechanisms

Flash droughts have severe impacts on the society and environment due to the sudden onset. Most studies focused on the definitions of flash droughts but rarely investigated the mechanisms. This study investigated the variation of the regional flash droughts in China and the possible mechanisms. The results show that the 6-pentads flash droughts show more regional characteristics, particularly for southwestern China (SWC), the northeastern China (NEC), the northwestern China (NWC), and the middle reaches of the Yellow River (MRYRC) during 2000-2018. The frequency of flash droughts decreases in SWC and increases in NWC, but shows no significant change in NEC and MRYRC. From the perspective of water supply factors and energy supply factors, the possible mechanisms have been further proposed. Since it is abundant of radiation energy in NWC, the potential evaporation is high, but evapotranspiration is low in normal circumstance. Thus, before the onset of a flash drought event, precipitation is needed to increase the soil moisture and then raise the evapotranspiration. Such growth can keep positive anomalies of evapotranspiration during the event. In contrast, the abundant soil moisture in SWC means only the excessive anomaly of potential evaporation triggered by shortwave radiation and temperature is required to support evapotranspiration raising during a flash drought event. Such growth of evapotranspiration can lead to a greater soil water deficit. However, soil moisture content in both NEC and MRYRC is less than that in SWC and radiation energy is less than that in NWC, so the evolution of evapotranspiration is similar to that in NWC, but the need for high potential evaporation is similar to that in SWC. The results of this study can improve our understanding of causal mechanisms of flash drought.