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UBC Theses and Dissertations

Process-based modelling of stream temperature dynamics in a glacially fed river system Browning, Billy


Glacially fed streams face increasing threats due to changes in stream temperature and flow from continued glacier recession and climate change. These systems support unique ecological communities, often highly sensitive to thermal changes. However, understanding the governing processes of stream temperature in large glacially fed stream reaches remains limited, with a lack of literature focused on larger rivers with tributaries. The objective of this study was to quantify the drivers of stream temperature dynamics in a 24 km long reach of the glacially fed Lillooet River in the southern Coast Mountains, BC. To support the application of HydroCouple, a reach-scale process-based stream temperature model, a new stream shading model was developed that used a LiDAR digital surface model to simulate shade from riparian vegetation and a conventional DEM to simulate topographic shade. Based on a comparison with shade interpreted from high-resolution PlanetScope imagery, the model simulated shade accurately both at individual points and for reach-scale averages. The model was used to simulate incident shortwave and longwave radiation at the stream surface. Air temperature, vapour pressure and wind speed were monitored at both landbased and above- stream meteorological stations. While the differences between land-based and above-stream observations were broadly similar to those found in previous studies, there were some distinct patterns that were attributed to the timing of daily maximum water temperature, which occurred near midnight rather than early afternoon, as is more typical for non-glacier-fed streams.HydroCouple was applied without calibration under the assumption that bed heat conduction and hyporheic heat exchange were negligible. The model showed good performance (RMSE = 0.30 oC, ME = 0.03 oC) compared to previous literature. Longitudinal advection of the upstream boundary condition had an overwhelming impact on downstream temperatures, leading to low sensitivity to streamflow or energy exchange variations. Tributaries muted main channel stream temperature variations, typically reducing temperatures by 0.5 oC around midnight (daily maxima) and increasing them by 0.2 oC during the afternoon (daily minima). Net radiation at the stream surface increased stream temperatures by 0.3-0.4 oC during peak solar hours and shifted daytime daily minimum stream temperatures on average 40 minutes later.

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