ABSTRACT
The western United States (the West) undergoes considerable hydrologic variability in response to regional climate fluctuations that are termed anomalous by climate scientists because they depart from long-term average conditions. Regional climate fluctuations persist for seasonal to multidecadal durations, usually in association with larger-scale climate patterns. They play a crucial role in determining regional hydrologic variability by affecting trends of important drivers such as precipitation and temperature, sometimes by promoting particular blends of influential weather events. In California and other regions of the West, much of the annual precipitation is delivered by relatively few very large storms, which are usually atmospheric river events. Besides providing its water supply, these storms also drive year-to-year differences in annual precipitation totals, and cause most of the region’s floods. During years or multiyear periods when these very large storms are absent, the region may fall into drought. Historically, droughts have had a strong presence in the West, but recent droughts have exhibited unusually warm temperatures, likely a harbinger of dry events in future decades when climate change threatens to make overall conditions even warmer. Other early signs of climate change that have been observed include declines of mountain snowpacks, which supply spring and summer runoff for the region. Along with warmer surface temperatures have come higher elevation freezing levels, more rain and less snow, and earlier snowmelt and earlier snowmelt runoff. Anthropogenic climate changes, which are projected to build as greenhouse gas concentrations rise, would result in further warming and amplified hydrologic changes. Global climate models suggest that precipitation may shift toward fewer overall wet days but somewhat increased extreme storm events. Further shifts in snowpack, runoff, and increased moisture loss to the atmosphere would reduce soil moisture and streamflows in summer. Annual discharge in arid western watersheds may decline, which would18 exacerbate dry spells. Heavier winter precipitation events and higher elevation rain/snow transition zones would cause greater flood volumes in some mountain catchments by the latter half of the twenty-first century.
