In recent decades, there has been increased attention on drought and the aridification of parts of the western contiguous United States (CONUS) (Bishop et al., 2021). The continually updated U.S. billion-dollar weather and climate disasters data set (NOAA-NCEI, 2021;Smith & Katz, 2013) reports that droughts and heatwaves represent nearly a quarter of all long-term weather and climate disaster costs exceeding 1 billion dollars per event, and that vulnerability to these drought events has increased from 1980 to 2020. Droughts have wide ranging impacts on agriculture, water utilities, power generation, recreation and tourism, aquatic species, forest resources, public health and other sectors (Wlostowski et al., 2022). National scale streamflow studies have found increasing occurrences of streams with no flow in the southern and western CONUS (Zipper et al., 2021), decreases in the magnitude of the lowest flows in the southeastern and parts of western CONUS, and increases in the lowest flows in the northeastern and midwestern CONUS (Dethier et al., 2020;Dudley et al., 2020). Runoff reductions in future years for western regions of the CONUS are likely a result of projected precipitation decreases and temperature increases (McCabe & Wolock, 2021b), especially where the temperature increases lead to reductions in seasonal snowpack and water yield (Barnhart et al., 2016;McCabe et al., 2017;Milly & Dunne, 2020). These studies highlight the need for increased understanding of streamflow drought variability, trends, climatic, landscape and anthropogenic drivers.
Drought is a recurring natural hazard that has substantial human and environmental impacts. Given continued global warming and associated climate change, there is concern that droughts could become more severe and longer lasting. To better monitor and understand drought development and persistence, it is helpful to understand the development and climatic drivers of past droughts. In this study we use monthly runoff percentiles to identify five major drought events in the conterminous United States (CONUS) from 1901 through 2020. For each drought event we examined spatial patterns of departures of mean monthly precipitation, temperature, soil moisture storage, and runoff for 2,107 hydrologic units (HUs) across the CONUS. Results indicated that precipitation deficits have been the primary driver of past major‐drought events and temperature a secondary driver, even of the most recent drought event (September 1999 through September 2015) when positive temperature anomalies occurred over most of the CONUS. Additionally, negative soil moisture storage departures were more negative than runoff departures during the five drought events we examined, which emphasizes the importance of measuring both runoff and soil moisture to monitor drought conditions. We also examined the use of statistical persistence to develop short‐term (i.e., 1 month) forecasts of runoff drought conditions in the CONUS by developing autoregressive integrated moving average (ARIMA) models for each HU. Results indicated that persistence can be used to predict short‐term changes in the spatial pattern of drought and the areal extent of drought, but that predictions of runoff magnitude for any particular site are often poor.
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