Flash droughts over the Contiguous United States: Sensitivity of inventories and trends to quantitative definitions

Abstract:

“Flash drought” is no longer a new term when referring to droughts that develop and onset rapidly over a relatively short timescale. Despite the multiple studies that have introduced quantitative and qualitative definitions to flash droughts, there is still no generalizable definition that can incorporate all the types and pathways of flash droughts development. Previous studies have advanced our understanding of flash droughts process but have also created possibility for confusion. Different definitions can lead to different conclusions with respect to frequency of occurrence and trends, especially under climate change. To clarify conceptual differences between definitions, we are comparing five previously published definitions, a new proposed definition based on root zone soil moisture volatility and an operational satellite-based drought monitoring product. Analysis of flash droughts according to different definitions over the contiguous U.S shows conceptual and significant differences in terms of spatial distribution, frequency, and trends. There are few evident trends for the different definitions over the past four decades, unlike air temperature-based definitions that show a significant increase. Results reveal the composite nature of flash droughts that can take different pathways to develop and onset, which requires the careful selection of flash droughts definition in the context of environmental conditions for an accurate monitoring and forecasting of the extreme phenomenon.