Evaluating Flash Drought Detection Utilizing In Situ Soil Moisture Observations

Rapidly intensifying droughts, otherwise known as flash droughts, have been shown to have significant impacts on crop yields, livestock forage production, and natural ecosystems. Therefore, early warning for flash droughts is critical to mitigating those impacts. This study evaluates the ability of in situ soil moisture observations to provide early drought warning at measurement locations across the continental United States. The soil moisture measurements were retrieved from 114 NOAA U.S. Climate Reference Network (USCRN) stations from 2010 to 2018. The hourly measurements were standardized to identify the dryness or wetness of a location relative to climatology. These data were aggregated to weekly values ending on 12Z Tuesday to match the temporal resolution the United States Drought Monitor (USDM), which is a composite drought index constructed from numerous objective drought indicators and reports that are integrated by expert analysis. To match the USDM’s percentile categorization approach and create soil moisture-based drought events, the weekly standardized soil moisture measures are converted to percentiles using an empirical cumulative density function per USCRN station. Standardized soil moisture-based drought events are then compared to flash drought events defined by the USDM with respect to the drought onset identification. It is hypothesized that soil moisture observations will provide early warning for flash drought events because rapidly intensifying drought events are characterized by a quick transition from energy-limited to water-limited conditions, which should correspond to a rapid depletion of soil moisture.