How fast does flash drought evolve?
Approximately 33.64−46.18% of flash droughts develop within 5 days for the period 2000−2020, and the rapid onset of flash droughts is further accelerated by the joint influence of soil moisture depletion and atmospheric aridity.
The emerging phenomenon of flash drought has recently attracted widespread attention due to the sudden onset and rapid intensification of drought conditions worldwide, even in humid regions like southern China. Compared with traditional, slowly developing droughts, flash droughts evolve with a relatively fast depletion of soil moisture. In the past two decades, more than one third of flash droughts developed within 5 days. Giving very few early warnings, flash droughts leave human beings unprepared for the severe impacts they bring, e.g., imbalance of ecosystems and agricultural systems.
“The occurrence of flash droughts has been detected in many regions around the world. However, we don’t know how fast flash droughts evolve and what causes the rapid onset speed,” said Dr Wang Shuo, Assistant Professor from the Department of Land Surveying and Geo-Informatics at The Hong Kong Polytechnic University.
In the new study, published in Nature Communications on 3 March 2022, Dr Wang and his research teammates revealed the mechanism causing flash droughts. They found that flash droughts do not appear to be occurring more frequently in most regions of the world, just coming on faster. Approximately 33.64−46.18% of flash droughts develop within 5 days for the period 2000−2020, and there is a significant increasing trend in the proportion of flash droughts with the 5-day onset time globally.
“Flash droughts are most likely to occur in humid and semi-humid regions, including Southeast Asia, East Asia, Amazon Basin, Eastern North America, and Southern South American.” Dr Wang said, “Southeast China is also one of flash drought hot spots. In the past, it is generally believed that northern China is mainly subject to droughts while floods affect southern China. The southeastern part of China is most vulnerable to the impacts of typhoons, storm surges, rainstorms, and floods. Thus, the increasing flash drought risk is overlooked, without any early warning and emergency response measures, posing serious threats to ecosystem protection and sustainable agriculture development.”
Dr Wang explained that “atmospheric aridity creates a perfect condition for the occurrence of flash droughts, and the joint influence of soil moisture depletion and atmospheric aridity further enhances the rapid onset of flash droughts. In other words, low soil moisture combined with high vapor pressure deficit accelerates the decline in soil moisture through land–atmosphere feedbacks. Thus, Southeast China with strong land–atmosphere coupling is most vulnerable to flash droughts.”
The new study contributes to a deeper understanding of the rapid onset development and driving mechanism of flash droughts. Identification of flash drought-prone regions and global hot spots can help policymakers and stakeholders develop flash drought mitigation and risk management strategies. Furthermore, comprehensive assessment of onset development timescales of flash droughts provides insights into the implementation of flash drought forecasts and early warning systems.