Evaluating relative effects of groundwater and heat exchange processes on annual stream temperature signals in mountain streams

Numerous studies use heat as a natural tracer of various watershed processes, but few have explored how long-period (i.e., seasonal to annual) stream signals are altered by factors such as groundwater seepage, effective groundwater seepage source depth, riparian shading, streambed substrate (vertical conduction), and stream discharge magnitude. Additionally, little is known about how conservatively altered annual stream signals propagate downstream before various stream corridor heat fluxes diminish their influence. Recently, we developed paired air and water annual temperature signal analysis techniques to infer relative groundwater contribution to stream water and the effective groundwater flow path depth. Here, we further develop these techniques by employing a deterministic stream heat budget model (HFLUX) to investigate the explicit role of various stream heat budget processes on the annual stream temperature signals and how these signals are transformed as the water moves downstream a hypothetical 1 km reach. We analyze the annual temperature regime using three metrics that include the influence of annual air temperature: amplitude ratio (A_R), phase lag (Δφ), and mean ratio (M_R). We then compare multi-year annual signals from a wide range of observed mountain stream locations to likely upstream watershed processes. Groundwater seepage, riparian shade, and stream discharge all exhibited significant influence on the annual stream temperature signals. Streambed substrate showed only minor influence on annual stream temperature signals, indicating conductive heat exchanges are a secondary control on long-period signal dynamics. The individual and combined effects of groundwater seepage and riparian shade varied depending on their location within the hypothetical reach and their magnitude. The effect of stream discharge varied with timing and magnitude. These techniques provide an efficient, cost-effective way for managers of cold-water habitat to identify current habitat conditions, predict future changes, and to prioritize potential climate refugia.