Development of a flash drought intensity index

Authors: OTKIN, J. - University Of Wisconsin; ZHONG, Y. - University Of Wisconsin; HUNT, E. - Atmospheric And Environmental Research; CHRISTIAN, J. - University Of Oklahoma; BASARA, J. - University Of Oklahoma; NGUYEN, H. - Collaborator; WHEELER, M. - Collaborator; FORD, T. - University Of Illinois; HOELL, A. - National Oceanic & Atmospheric Administration (NOAA); SVOBODA, M. - University Of Nebraska; Anderson, Martha

Submitted to: Environmental Research Letters
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/5/2021
Publication Date: 6/9/2021
Citation: Otkin, J., Zhong, Y., Hunt, E.D., Christian, J., Basara, J., Nguyen, H., Wheeler, M., Ford, T.W., Hoell, A., Svoboda, M., Anderson, M.C. 2021. Development of a flash drought intensity index . Environmental Research Letters. 12(6):741. https://doi.org/10.3390/atmos12060741.
DOI: https://doi.org/10.3390/atmos12060741

Interpretive Summary: Flash droughts, such as the event of 2012 which significantly impacted crop yields in the Corn Belt, are a growing concern in agricultural sectors. Crops, pasture, and rangeland conditions can deteriorate rapidly during flash droughts, with little time available for mitigation planning once the drought is in progress. Furthermore, weather forecast models have been unable to reliably forecast flash drought events. The first step in improving monitoring and forecasting capabilities is to develop a robust definition of what we mean by “flash drought”, capturing both the rapid onset and the severity aspects of events that lead to agricultural impacts. This paper describes a new flash drought intensity index (FDII) that specifically considers both intensification rate and severity based on soil moisture percentile estimates from the Noah land surface model. Based on this definition, the study investigates the frequency and spatial distribution of flash drought events over the continental United States over the past 40 years. The behavior of the FDII is reasonable, and well captures recent flash drought events. The definition is flexible and can also be applied to other indicators of flash drought such as evapotranspiration and evaporative demand, enabling a richer description of the full suite of drivers and cascading impacts of rapid onset drought.

Technical Abstract: Flash droughts are characterized by a period of rapid intensification over sub-seasonal time scales that culminates in the rapid emergence of new or worsening drought impacts. This study presents a new flash drought intensity index (FDII) that explicitly accounts for both the unusually rapid rate of intensification and the resultant drought severity when assessing the intensity of flash drought. The FDII framework advances our ability to characterize flash drought because it provides a more complete measure of flash drought intensity than existing methods that do not consider the severity or longevity of the drought conditions that occur after the period of rapid intensification ends. The FDII is computed using two terms measuring the maximum rate of intensification (FD-INT) and the average drought severity (DRO-SEV) relative to a set of thresholds representing the minimum requirements for flash drought. An analysis using soil moisture output from the Noah land surface model from 1979-2017 revealed large regional and interannual variability in the spatial extent and intensity of soil moisture flash drought across the U.S. Overall, FD-INT is larger across the eastern U.S. whereas DRO-SEV is larger over the central and western U.S. Comparison of the FD-INT and DRO-SEV terms showed that they are strongly correlated at regional scales, which indicates that the subsequent drought severity is closely related to the magnitude of the rapid intensification preceding it. This proof-of-concept demonstrates the importance of using metrics that incorporate rapid intensification and drought severity terms when evaluating flash drought characteristics.