A New Evapotranspiration-Based Drought Index for Flash Drought Identification and Monitoring

Flash droughts, a type of extreme event characterized by the sudden onset and rapid intensification of drought conditions with severe impacts on ecosystems, have become more frequent in recent years due to global warming. The drought index is an effective way to monitor drought and mitigate its negative impact on human production and life. This study presents a new flash drought identification and monitoring method based on the evapotranspiration-based drought index, i.e., the evaporative stress percentile (ESP). This ESP-based method considers both the rate of the rapid intensification and each phase of flash drought development, which allows it to be used quantitative assessment of flash drought characteristics including detailed information on the onset, development, termination, and intensity. The ESP is evaluated using the soil moisture percentile (SMP) derived from the GLDAS-Noah soil moisture data. The results show that there was good agreement between the ESP and SMP across most of China, with correlation coefficient values above 0.8 and MAE values below 10 percentile/week. The ESP was then used to identify flash droughts in China and compared with the Precipitation Anomaly Percentage (PAP) and the SMP for three cases of typical flash drought events in three different regions and years with different land covers. It demonstrates the robustness of the ESP for detecting flash droughts in different geographical regions, for different land cover types, and for different climatic characteristics. This method is applied to characterize historical flash droughts in 1979-2018 in China, and the results show that flash droughts in China occur most frequently in the transitional climate zone between humid and arid regions in Northern China. This study contributes to a better understanding of flash drought development and supports to decision-makers in providing early warnings for flash droughts.