Author: Samuel A. Miller

Publisher:

Published: 2020

Total Pages: 149

ISBN-13:

Warmer temperatures and forest disturbance from bark beetles pose significant threats to disrupt natural flow regimes of mountainous watersheds by affecting snow accumulation, melt, and subsequent runoff generation mechanisms. This dissertation provides three chapters investigating the fate and transport of streamflow from snowmelt-dominated, headwater watersheds in the western U.S. In the first chapter, changes in precipitation and / or streamflow metrics were measured in nine regions of the western U.S. Warmer temperatures have resulted in earlier timing of snowmelt and streamflow in many regions, putting stress on watershed functions in summer months. Significant reduction in summer runoff were detected in eight of the nine regions with larger reductions occurring in watersheds predominantly facing south. The second chapter assessed the impact a recent bark beetle outbreak had on streamflow generation in the southern Rocky Mountains. Watersheds impacted by the bark beetle outbreak buffered some of the effects of warmer temperatures and showed smaller declines in winter precipitation and snowmelt rates, leading to positive changes in the quickflow ratio, or the volume of runoff produced by diurnal streamflow cycles relative to maximum snow water equivalent. The results were contradictory from those expected due to warmer temperatures. The final chapter used natural tracers to learn more about runoff generation from nested watersheds in the Snowy Range, southeast Wyoming. Specific conductance / streamflow relationships provided a way to model the baseflow component of the total hydrograph and had unique hysteretic relationships at annual and daily scales. Watersheds with higher proportions of baseflow may be buffered from reduced snowpack compared to those with higher proportions of direct runoff, which may be more susceptible to greater degrees of stream network contraction in the summer. Slope aspect was found to be highly correlated to the proportion of baseflow in our nested, study watersheds. In future warming scenarios, shaded hillslopes may provide refugia for native species that developed under cooler, wetter scenarios, whereas sunnier areas will be more susceptible to earlier, slower snowmelt and disruption from natural hydrologic flow regimes.