Five Decades Of Observed Daily Precipitation Reveal Longer And More Variable Drought Events Across Much Of The Western United States

Multiple lines of evidence suggest climate change will result in increased precipitation variability and consequently more frequent extreme events. These hydroclimatic changes will likely have significant socioecological impacts, especially across water-limited regions. Here we present an analysis of daily meteorological observations from 1976 to 2019 at 337 long-term weather stations distributed across the western United States (US). In addition to widespread warming (0.2 degrees C +/- 0.01 degrees C/decade, daily maximum temperature), we observed trends of reduced annual precipitation (-2.3 +/- 1.5 mm/decade) across most of the region, with increasing interannual variability of precipitation. Critically, daily observations showed that extreme-duration drought became more common, with increases in both the mean and longest dry interval between precipitation events (0.6 +/- 0.2, 2.4 +/- 0.3 days/decade) and greater interannual variability in these dry intervals. These findings indicate that, against a backdrop of warming and drying, large regions of the western US are experiencing intensification of precipitation variability, with likely detrimental consequences for essential ecosystem services.Plain Language SummaryChanges in precipitation have far-reaching consequences for socioecological systems, especially in water-limited regions such as those common in the western US. While the total amount of precipitation is important for these systems, key aspects of precipitation timing, such as the length of dry periods between precipitation events, can strongly influence ecosystem services including ecosystem carbon uptake, productivity for grazing and forage, wildfire frequency, and intensity, and water availability for societal use. In this study, we used daily meteorological data from over three hundred long-term weather stations across the western US to understand changes in precipitation amounts and timing during the period 1976-2019. In addition to widespread warming, we found overall lower precipitation combined with increasing variability in the size of precipitation events, indicating the western US is not only getting hotter and drier, but that systems are experiencing more year-to-year variation in precipitation. We also found that the average time without precipitation has increased during the past 45 years across the southwestern US, and we saw increases in year-to-year fluctuations in these dry periods. Together, these changes will likely have large, but still poorly understood, consequences for social and ecological systems of the western US.