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

Authors: OSMAN, M. - Johns Hopkins University; ZAITCHIK, B.F. - Johns Hopkins University; BADR, H. - Johns Hopkins University; CHRISTIAN, J. - University Of Oklahoma; TADESSE, T. - University Of Nebraska; OTKIN, J. - University Of Wisconsin; Anderson, Martha

Submitted to: Hydrology and Earth System Sciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/24/2020
Publication Date: 2/8/2021
Citation: Osman, M., Zaitchik, B., Badr, H., Christian, J., Tadesse, T., Otkin, J., Anderson, M.C. 2021. Flash droughts over the contiguous United States: Sensitivity of inventories and trends to quantitative definitions. Hydrology and Earth System Sciences. 25:565-581. https://doi.org/10.5194/hess-25-565-2021.
DOI: https://doi.org/10.5194/hess-25-565-2021

Interpretive Summary: As a parallel to “flash floods”, the term “flash drought” describes drought conditions that develop rapidly over short timescales, often leaving growers little time to respond and mitigate impacts. Several efforts have been conducted to characterize events that qualify as a “flash drought”, often yielding very different assessments of how frequently these events occur. This manuscript compares spatial and temporal frequency of flash droughts over the United States over the past few decades as depicted by several definitions published in the literature, including a new index based on stock market analysis techniques applied to soil moisture datasets. The results are widely varying, depending on the variables and thresholds used to distinguish rapid onset from more gradual onset events. These results strongly indicate that “flash drought” represents a composite class of events, with several possible pathways all leading to rapidly intensifying drought conditions. To improve our ability to forecast impending onset and develop effective mitigation plans, we will require a convergent definition that encompasses these various pathways and most effectively pinpoints areas of significant agricultural impact.

Technical Abstract: The term “flash drought” is frequently invoked to describe droughts that develop rapidly over a relatively short timescale. Despite extensive and growing research on flash drought processes, predictability, and trends, there is still no standard quantitative definition that encompasses all flash drought characteristics and pathways. Instead, diverse definitions have been proposed, supporting wide-ranging studies of flash drought but creating the potential for confusion as to what the term means and how to characterize it. Use of different definitions might also lead to different conclusions regarding flash drought frequency, predictability, and trends under climate change. In this study, we compared five previously published definitions, a newly proposed definition, and an operational satellite-based drought monitoring product to clarify conceptual differences and to investigate the sensitivity of flash drought inventories and trends to the choice of definition. Our analyses showed that estimates of flash drought frequency, spatial distribution, and seasonality vary across the contiguous U.S. depending upon which definition is used. Definitions also differ in their representation of some of the largest and most widely studied flash droughts of recent years. Trend analysis indicates that definitions that include air temperature show significant increases in flash droughts over the past forty years, but few trends are evident for definitions based on other surface conditions or fluxes. These results indicate that “flash drought” is a composite term that includes several types of event, and that clarity in definition is critical when monitoring, forecasting, or projecting the drought phenomenon.