A new PolyU study published in Nature Communications has found that southeast China – the region spanning from the Yangtze River Delta to Hainan province – is at a higher risk of experiencing more rapid drying, with an increase in the proportion of flash droughts developed within five days by as much as 18.67 per cent during 2000 - 2020.

The finding by Dr Wang Shuo, Assistant Professor of PolyU’s Department of Land Surveying and Geo-Informatics, showed that contrary to the common understanding that the region is vulnerable to typhoons, rainstorms and floods, rapidly developing flash droughts are actually emerging at a faster rate in the region and many parts of the world amid climate change.

Ordinary droughts usually take five to six months or an even longer time to develop to full strength. But for flash droughts, their rapid onset in less than a month and the fast depletion of water availability means there are less early warning indicators for impact preparation, according to Dr Wang.

“Flash drought occurrence is often accompanied by above-average temperatures and a precipitation deficit, which may trigger compound extreme events such as the concurrence of a flash drought and heat wave. Thus, flash droughts can pose even more serious threats to urbanised areas like Hong Kong due to the urban heat island effect,” he said.

Compared with traditional, slowly developing droughts, flash droughts evolve with a relatively fast depletion of soil moisture that may cause an imbalance of ecosystems and agricultural systems.

Based on different data sets that use satellite soil moisture measurements, the study found that although flash droughts are not becoming more frequent in most parts of the world, they are developing at a faster rate (i.e. in a shorter period of time). Out of all flash droughts that occurred in the past two decades, these data sets showed that there was about 33.64 - 46.18 per cent of flash droughts that developed within five days, representing an increase of 3.23 - 19.03 per cent during the period.

“Governments and the public should realise the increasing flash drought risk in addition to commonly known extreme weather events, and adapt to such emerging climate-induced natural disasters. It is crucial to improve traditional drought monitoring systems and indicators for capturing rapidly evolving flash droughts,” Dr Wang said.