Changing the Color of Drought

“What value to decision- making is a map that is red everywhere all of the time?”
—Justin Mankin of Dartmouth College, on a recent finding that the U.S. Drought Monitor may not be keeping up with actual drought conditions across the country. The color-coded map, which ranges from white for areas with no drought conditions to red for regions in extreme drought, is used by policymakers and other officials to inform a wide range of actions including water and fire policy and disaster response. Mankin and colleagues studied changes between 2000 (when the Monitor was created as a collaboration between NOAA, the National Drought Mitigation Center, and the U.S. Department of Agriculture) and 2022 for six variables that are used to inform the Monitor: precipitation, runoff, soil moisture, terrestrial water storage, vapor pressure deficit, and near-surface air temperature. They also studied trends in the frequency of the six drought classifications—from “normal or wet conditions” to “exceptional drought”—that the Drought Monitor categorizes based on the frequency they are estimated to occur; the likelihood that an area would experience “exceptional drought” is 2% or less. Lead author Zhiying Li, formerly at Dartmouth and now at Indiana University, points out that “while these percentile thresholds are static, the climate is not. The ongoing aridification and worsening droughts in certain regions may change what was once perceived as an anomaly, making it less of an emergency anymore.” Specifically, the researchers found that what the Drought Monitor considers “exceptional” droughts are occurring much more frequently than the Monitor suggests they should in some areas of the Southwest, the southern Plains, and the Deep South; parts of California experienced “exceptional” drought 18% of the studied time period—nine times more often than the Drought Monitor's probability of such drought (≤2%). “Exceptional” drought regions appear in red on the Monitor, but the droughts they experience now may be more extreme hydrologically than such a drought was in 2000. “What has historically been classified as a severe drought will likely be considered less severe in the future as drought conditions worsen due to human-driven climate change,” Mankin notes. In the study, which was published in AGU Advances, the researchers call for the Drought Monitor to revise its classification system to reflect a changing climate. “Essentially, the amount of time places spend in drought is exceeding federal guidelines that determine how severe a drought is,” Mankin says. “The management system itself needs to adapt or it risks obsolescence.” [Source: Dartmouth College]

Photo Credit: iStock.com/NNehring