Flash Drought
ESI
The Evaporative Stress Index (ESI) is the standardized anomaly of the actual evapotranspiration (ET) to potential evapotranspiration (PET) ratio derived from the Bureau of Meteorology's land surface water balance model version 7 (AWRA-L v7) daily outputs. ET is an estimate of the total evapotranspiration from vegetation, soil and groundwater, while PET is the theoretical evapotranspiration that could occur if the surface was well supplied with water, therefore representing a measure of the evaporative demand of the atmosphere.
ET and PET are derived from inputs of precipitation, temperature and wind speed from AWRA-L. The ESI is calculated daily on a 4-week running window.
Positive values of the ESI indicate normal to wet conditions while negative values indicate dry conditions. During a flash drought event, the ESI rapidly decreases, initially because of the increased PET, but then also due to the subsequent decrease in actual ET as the surface water supply becomes limited.
Further reading:
Nguyen et al., 2019: Using the evaporative stress index to monitor flash drought in Australia. Environ. Res. Lett. 14 064016
FDI
Flash Drought Index (FDI) indicates the onset and duration of a potential flash drought event. If the FDI=1, this represents the onset of a potential flash drought event. If the FDI>1, this indicates the duration (in days) of a potential flash event. FDI returns to 0 when the drought condition is no longer met. If the FDI<28, the event is identified as short-term dry spells that are less likely to have impacts on vegetation and water resources.
Further reading:
Nguyen et al., 2021: The 2019 flash droughts in subtropical eastern Australia and their association with large-scale climate drivers. Weather Clim. Extrem., 32, 100321