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Hydrology and Earth System Sciences An interactive open-access journal of the European Geosciences Union
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https://doi.org/10.5194/hess-2020-385
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.
https://doi.org/10.5194/hess-2020-385
© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

  19 Aug 2020

19 Aug 2020

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This preprint is currently under review for the journal HESS.

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

Mahmoud Osman1, Benjamin F. Zaitchik1, Hamada S. Badr1, Jordan I. Christian2, Tsegaye Tadesse3, Jason A. Otkin4, and Martha C. Anderson5 Mahmoud Osman et al.
  • 1Department of Earth and Planetary Sciences, Johns Hopkins University, Baltimore, MD, USA
  • 2School of Meteorology, University of Oklahoma, Norman, OK, USA
  • 3National Drought Mitigation Center, University of Nebraska–Lincoln, NE, USA
  • 4Space Science and Engineering Center, Cooperative Institute for Meteorological Satellite Studies, University of Wisconsin–Madison, WI, USA
  • 5Hydrology and Remote Sensing Laboratory, Agricultural Research Service, USDA, MD, USA

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 indicate that the newly introduced Soil Moisture Volatility Index definition effectively captures flash drought onset in both humid and arid regions. Analyses also showed that estimates of flash drought frequency, spatial distribution, and seasonality vary across the contiguous U.S. depending upon which definition is used. Definitions 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.

Mahmoud Osman et al.

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Mahmoud Osman et al.

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Short summary
Our study of flash droughts' definitions over the U.S. show that published definitions yield markedly different inventories of flash drought geography and frequency. Results suggest that there are several pathways that can lead to events that are characterized as flash droughts. Lack of consensus across definitions helps to explain apparent contradictions in the literature on trends, and indicates that the selection of a definition is important for accurate monitoring of different mechanisms.
Our study of flash droughts' definitions over the U.S. show that published definitions yield...
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