CONCLUSIONS

In this study, we used a singular spectrum analysis and a physically based cold region hydrological model to characterize major hydroclimatic phases over 31 years at a mid-latitude headwater basin in Idaho, USA. Long data records in the basin and a spatially distributed lidar snow depth dataset provided foundational information for understanding both spatial and temporal hydroclimatic variations. The main findings of this research include:
  1. Concomitant negative AO and SST in NiƱo 3.4 regions of the Equatorial Pacific Ocean resulted in extremely wet conditions with observed peak SWE of 315 mm (63%) above normal. The snow cover period was also extended by almost one month, leading to a 314 mm (57%) increase in observed annual runoff and a 13% increase in runoff ratio relative to long-term averages.
  2. Concomitant negative PNA pattern and positive NAO resulted in extremely dry conditions with observed peak SWE being 275 mm (55%) below normal. The snow cover period was shortened by almost one month, leading to a 234 mm (43%) decrease in observed annual runoff and a 13% decrease in runoff ratio relative to long-term averages.
  3. A positive phase of North Atlantic Oscillation (NAO) is more influential than a positive phase of the Pacific North American (PNA) pattern on the observed annual runoff and the modeled rain on snow runoff in the study area. A switch in the phase of the teleconnection patterns of NAO and PNA in 2012 was concomitant with a transition from wet to dry conditions in the basin.
  4. Snowmelt and runoff generated during ROS events can account for up to one-third of the annual runoff. The areas of high snow accumulations (taller vegetation, north facing and steeper slope, and topographic depressions) produced more considerable runoff during ROS events. High ROS runoff is associated with positive AAO with high moisture transport from southern and equatorial latitudes and negative AO with cold air transport from the northern latitudes to the study area.
Linking hydrological variability in a small headwater basin to regional climatic patterns in this study can be used to improve forecast skills and further develop our understanding of land-atmosphere feedbacks. The warm and dry phases with multiple year durations (e.g., phase six) identified in this study can be extended to diagnose the effects of potential future warm and dry conditions on water flow and storage at local scales and develop appropriate water security plans in regions that depend on snowmelt water from high elevations.