Streamflow and the Climate Transition Zone in the Western United States

Abstract

Hydroclimatic variability in the western United States (the West) is characterized by a north-south dipole pattern of precipitation and streamflow variance, with centers of opposite association in the Pacific Northwest and the Desert Southwest. These dipole centers tend to react in opposite fashion to tropical Pacific Ocean conditions, and the resulting contrast in precipitation variability is an important component of Western climate. Teleconnection impacts are not as well understood in the transition zone separating the centers of opposite association, located primarily within the semi-arid Intermountain West. This leads to low hydroclimatic predictive capacity in the transition zone region, an area that is extremely important for water supply in the West. In this dissertation, I examine paleohydroclimatic variability in this region using dendrochronology, investigate recent variability through a synoptic climatology approach, and assess future conditions based on climate change projections.Overall, this dissertation's findings confirm that the transition zone region is highly vulnerable to extremes in hydroclimatic variability and underscore the need for improved predictive capacity in the region. In the Snake River headwaters, low- to mid-elevation Pseudotsuga menziesii trees are the strongest recorders of winter precipitation, a vital component of water supply, and the season of precipitation impacting growth is a major component of the overall variability between tree-ring sites in the region. The 415-year reconstruction of Snake River streamflow indicates that extended droughts, more severe than those recorded in the instrumental period, have occurred in the pre-instrumental past. Streamflow in the upper Snake River is strongly linked to Pacific Ocean conditions and sensitive to storm track position. The West's precipitation dipole has a surprisingly narrow transition zone that has shifted in its location over time in some areas but has remained remarkably stationary across Nevada and Utah. Projected climate changes - including warmer temperatures, changing seasonality, reduced snowpack, and changes in the storm track position - highlight the importance of understanding climate-water linkages for future water resource management.