Assessing the evolution of soil moisture and vegetation conditions during a flash drought - flash recovery sequence over the south-central United States

Authors: OTKIN, J. - University Of Wisconsin; ZHONG, Y. - University Of Wisconsin; HUNT, E.D. - Atmospheric And Environmental Research; BASARA, J. - University Of Oklahoma; SVOBODA, M. - University Of Nebraska; Anderson, Martha; HAIN, C. - Goddard Space Flight Center

Submitted to: Journal of Hydrometeorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/14/2019
Publication Date: 3/15/2019
Citation: Otkin, J., Zhong, Y., Hunt, E., Basara, J., Svoboda, M., Anderson, M.C., Hain, C. 2019. Assessing the evolution of soil moisture and vegetation conditions during a flash drought - flash recovery sequence over the south-central United States. Journal of Hydrometeorology. 20(3):549-562. https://doi.org/10.1175/JHM-D-18-0171.1.
DOI: https://doi.org/10.1175/JHM-D-18-0171.1

Interpretive Summary: While drought is usually thought to be a slowly progressing phenomenon, recently more attention has been paid to “flash drought” events where the onset of crop stress is relatively rapid leaving little time for mitigative actions. Similarly, drought impacts can be rapidly relieved with well-timed beneficial rains. This paper investigates signals of a flash drought – flash recovery event that occurred over the south-central United States in 2015 as conveyed by a suite of modeled and remotely sensed drought indicators. The satellite-based indicators studied reflected drought impacts on evapotranspiration, soil moisture, and leaf area, and these were compared to soil moisture anomalies generated by three different land-surface models. The study investigated relative timing and magnitude of the drought-recovery signature conveyed by each indicator in relationship to the atmospheric conditions driving the events. Results showed a cascading sequence of responses, with below normal precipitation and above normal evaporative demand leading to decreases in remotely sensed soil moisture and subsequently decreased crop water use and leaf area. This sequence is reasonable, with moisture deficiencies leading to crop stress, which confirm impact that will likely lead to reduced yield. The model output showed similar response, particularly when the three models were averaged together. Recovery rates were more variable across the region. In summary, this study demonstrates how convergence of evidence from multiple drought indicators provides a richer narrative regarding impacts than can be conveyed by a single indicator used in isolation.

Technical Abstract: This study examines the evolution of atmospheric, soil moisture, and vegetation conditions during an unusual flash drought, flash recovery sequence that occurred across the south-central U.S. in 2015. This event was characterized by a period of rapid drought intensification (flash drought) during late summer that was terminated by heavy rainfall at the end of October that eliminated drought conditions during a two-week period (flash recovery). A detailed analysis of the event was performed for select regions. Though the analysis revealed a similar progression of cascading effects in each region, characteristics of the flash drought such as its onset time, rate of intensification, and vegetation impacts differed in each region due to variations in the antecedent conditions and the atmospheric anomalies during its growth. Overall, flash drought signals initially appeared in the near-surface soil moisture and gross primary productivity, followed closely by reductions in evapotranspiration. Total column soil moisture deficits took longer to develop, especially in the western part of the region where heavy rainfall during the spring and early summer led to large moisture surpluses. Large differences were noted in how land surface models in the North American Land Data Assimilation System depicted the evolution of the flash drought, with one model consistently depicting better conditions than the other models. Much smaller differences occurred during the flash recovery period. This study illustrates the need to use multiple datasets to track the evolution of a flash drought.