Low-frequency oscillations in precipitation, temperature, and runoff on a west facing mountain front: A hydrogeologic interpretation

This paper examines the space-time patterns of annual, interannual, and dec adal components of precipitation, temperature, and runoff (P-T-R) using lon g-record time series across the steep topographic gradient of the Wasatch F ront in northern Utah. This region forms the major drainage area to the Gre at Salt Lake. The approach is to use multichannel singular spectrum analysi s as a means of detecting dominant oscillations and spatial patterns in the data and to discuss the relation to the unique mountain and basin hydrolog ic setting. Results of the analysis show that high-elevation runoff is domi nated by the annual and seasonal harmonics, while low-elevation runoff exhi bits strong interannual and decadal oscillations. For precipitation and tem perature, only the annual/seasonal spectral peaks were found to be signific antly different from the underlying noise floor, and these components incre ase with altitude similar to the mean orographic pattern. Spectral peaks in runoff show a more complex pattern with altitude, with increasing low-freq uency components at intermediate and lower elevation. This pattern is then discussed in terms of basin storage effects and groundwater-stream interact ion. A conceptual hydrogeologic model for the mountain and basin system pro poses how losing streams and deep upwelling groundwater in the alluvial aqu ifer could explain the strong low-frequency component in streams entering t he Great Salt Lake. The phase-plane trajectories of the dominant components for P-T-R are reconstructed as a function of altitude showing the relation of hydrogeologic conditions to the strongest oscillations in mountain runo ff and discharge to the Great Salt Lake. The paper shows that weak interann ual and decadal oscillations in the climate signal are strengthened where g roundwater discharge dominates streamflow.