Understanding drought onset: What makes flash droughts different from conventional droughts?

This study examines the timescales of drought onset to understand the differences between rapid onset droughts, called flash droughts, and the more conventional slow-onset droughts. Using a soil moisture-based drought identification approach, we show that soil moisture across most of Australia can transition from near-normal to drought conditions within one month’s time. The median duration for non-rapid drought onset, here called a conventional drought, is 30 days, while the rapid onset drought, here called a flash drought, takes around 15 days, indicating that the difference in onset timescales of the two drought types is relatively small. Further, our findings reveal that changes to precipitation and evaporative conditions during a flash drought onset are not very different from those that cause a conventional drought onset. However, flash drought development is associated with larger magnitude of anomalies of those variables leading to drought conditions. These larger anomalies during flash droughts reduce soil moisture rapidly, with a potential to cause damage to vegetation health without sufficient early warning. Although there is a diversity in the mechanisms causing flash droughts, we show here that the majority of the flash droughts are primarily related to the joint influence of abnormally low precipitation and heightened incoming solar radiation (low cloud cover) and large vapour pressure deficits (low relative humidity). The results emphasise the need to update existing drought monitoring systems to account for more realistic timescales of drought onsets for better early warning and preparedness. • Difference in onset time of flash (FD) and conventional (CD) drought is small. • CDs typically take 4–5 weeks to onset, while FDs take 2–3 weeks. • Process drivers of FDs and CDs are the same. • FD onsets are characterised by significantly higher driver anomalies than CDs. • Rainfall deficits, high solar radiation and high VPD are key features of FD onset.