Groundwater discharge provides critical thermal refuge for cold-water fishes in many river systems in North America, but the impact of future climate change on the timing, magnitude, and temperature of groundwater discharge has not been previously considered. This dissertation investigates the hydraulic and thermal influence of climate change on unconfined aquifers, particularly in the context of the climate and hydrology of the Little Southwest Miramichi River in New Brunswick, Canada. The thermal sensitivity of groundwater to climate change was investigated via (1) analytical, (2) empirical, and (3) numerical methods. Firstly, an analytical solution was developed to the transient, one- dimensional conduction-advection equation subject to nonlinear initial and boundary conditions. Secondly, an empirical ground surface temperature to groundwater temperature transfer function was developed from measured surface and groundwater temperature data and then explicitly coupled to a soil-vegetation-atmosphere-transfer (SVAT) model to investigate the subsurface thermal influence of climate change. Thirdly, a finite element model of coupled groundwater flow and energy transport with pore water phase change (SUTRA) was linked to surficial energy and hydrologic models to investigate the influence of changing precipitation and air temperature on the timing, magnitude, and temperature of groundwater discharge from unconfined aquifers. In general, the simulation results demonstrate that the hydraulic and thermal regimes of shallow aquifers are very sensitive to changing precipitation and air temperature, and that future climate change will likely limit the distribution of suitable cold-water thermal refugia in warming rivers.
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