Flash Droughts (Centennial)

We provide an overview of recent research into rapid onset “flash droughts” that has led to a deeper understanding of their characteristics and the physical processes that control their evolution over sub-seasonal time scales. The study of flash drought is motivated by recent high-impact events around the world where extreme climate anomalies led to rapid drought intensification and billions of dollars in economic losses due to lower crop yields, damage to natural ecosystems, and other societal impacts.

Flash droughts can be viewed as a class of droughts that are distinguished from their slowly developing counterparts by their rapid rate of intensification. Recent studies have shown that flash droughts are more likely to develop when large rainfall deficits occur alongside other weather anomalies that enhance evaporative demand. Unusually warm temperatures, low humidity, strong winds, and sunny skies can work together to quickly deplete root zone soil moisture due to increased evapotranspiration (ET). If precipitation is insufficient to compensate for the increased evaporative demand, vegetation stress can rapidly develop. Because this scenario is most likely to occur during the growing season, flash droughts strongly impact agriculture and natural ecosystems.

To capture the evolution of a flash drought, it is necessary to use monitoring tools depicting anomalies in soil moisture, ET, and vegetation health. A typical progression during a flash drought given initially adequate soil moisture is for an extended period of enhanced evaporative demand to initially cause an increase in ET as vegetation responds to the anomalous weather conditions, followed by a period of rapidly decreasing soil moisture, a transition to water-limited conditions, reduced ET, and the emergence of visible signs of vegetation moisture stress. This transition from energy-limited to water-limited conditions exemplifies the complex relationship between evaporative demand, soil moisture, ET, and vegetation health.