Climate warming is intensifying the hydrologic cycle due to higher water-holding capacity of a warmer atmosphere and changes in atmospheric circulation modes. Changes in annual precipitation sums are important in many ecosystems and are, accordingly, well studied. However, changes in temporal precipitation patterns are leading to increasingly common extreme events, such as longer droughts between rainfall, with significant and poorly understood ecological impacts. Therefore, it is critical to understand precipitation characteristics and their changes at long-term weather stations and at the daily time scale. Most existing studies of precipitation change focus on monthly to seasonal patterns and/or rely on interpolated, gridded datasets.
Here, for a comprehensive synthesis of 463 sites across the western United States (US), we examined trends in the mean and variability of key precipitation attributes including amount, numbers of events, dry intervals between events, and longest dry intervals. Trends in precipitation temporal characteristics were quantified using 5-year moving average windows from 1976 to 2015 and Mann-Kendall nonparametric trend tests. To detect trends across ecological regions, we applied the regional Kendall test and grouped stations by National Ecological Observatory Network domains.
Results/Conclusions
We found (1) the annual longest intervals trended to a northeast-southwest increasing pattern, with longer intervals in most regions except the Northern Plains. The largest increase of longest interval was found in the Desert Southwest, with 7.8 days per decade. (2) The mean of dry intervals during the warm season (May-Oct) became lengthened across the Central Plains (0.6 days per decade), Desert Southwest (2.3 days per decade) and Great Basin (1.2 days per decade), coupled with an increasing interannual variability. (3) The trend of annual number of events and precipitation totals decreased over the spatial domain and were generally opposite with the widespread pattern of variability increased, especially across the Central Plains, Desert Southwest and Southern Rockies/Colorado Plateau.
We conclude that precipitation in the western US changed non-uniformly in space over the past 40 years with a general increase in longest intervals and mean intervals during the warm season. Collectively, our results show that most parts of the western US are becoming drier with lower rainfall totals, decreased numbers of precipitation events and correspondingly longer dry intervals. Importantly, the ecological impacts of these trends are compounded by generally increasing interannual variability across most of the western US.