Combining a paired-catchment experiment and process measurements to understand the magnitude and variability of stream temperature changes following forest thinning in a headwater catchment

There are increasing concerns about the effects of climatic warming on stream temperature and the potential role of managing riparian forest cover as a mitigation strategy, especially considering that summers are projected to become warmer and drier in many regions. Developing effective and efficient management strategies requires an understanding of the underlying processes. This study combined a paired-catchment experiment with process-based measurements to understand the relative roles of meteorological and hydrological drivers on headwater stream temperature response to forest thinning. Following removal of 50% of the forest's basal area in a cut block overlapping the stream, daily maximum stream temperatures increased by up to around 5 °C. Above-stream solar radiation generally peaked below 30 Wṡm^-2 prior to harvest, but exceeded 500 Wṡm^-2 following harvest. The sensible and latent heat fluxes increased in magnitude following harvest, with sensible heat representing a minor heat gain and latent heat a somewhat larger loss. Computed bed heat fluxes are subject to significant uncertainty, but appeared to be a major heat sink relative to net radiation during the day both before and after harvest. After stratifying the data into classes based on solar radiation, the post-harvest treatment effect at an upstream site declined with increasing discharge, as expected. At a downstream site, there was an inverted u-shaped relation, which is hypothesized to reflect loss of surface water by bed infiltration between the temperature loggers during extreme low-flow periods, along with subsurface discharge upstream of the downstream logger, which increasingly dominated the temperature signal as streamflow declined. This and other studies highlight the importance of stream-subsurface interactions in controlling stream temperature response to environmental change.