Abstract. Global climate models project significant changes to air temperature and precipitation regimes in many regions of the Northern Hemisphere. These meteorological changes will have associated impacts to surface and shallow subsurface thermal regimes, which are of interest to practitioners and researchers in many disciplines of the natural sciences. For example, groundwater temperature is critical for providing and sustaining suitable thermal habitat for cold-water salmonids. To investigate the surface and subsurface thermal effects of atmospheric climate change, seven downscaled climate scenarios (2046–2065) for a small forested catchment in New Brunswick, Canada were employed to drive the surface energy and moisture flux model, ForHyM2. Results from these seven simulations indicate that climate change-induced increases in air temperature and changes in snow cover could increase summer surface temperatures (range −0.30 to +3.49 °C, mean +1.49 °C), but decrease winter surface temperatures (range −1.12 to +0.08 °C, mean −0.53 °C) compared to the reference period simulation. Thus, changes to the timing and duration of snow cover will likely decouple changes in mean annual air temperature (mean +2.11 °C) and mean annual ground surface temperature (mean +1.06 °C).

Projected surface temperature data were then used to drive an empirical surface to groundwater temperature transfer function developed from measured surface and groundwater temperature. Results from the empirical transfer function suggest that changes in groundwater temperature will exhibit seasonality at shallow depths (1.5 m), but be seasonally constant and approximately equivalent to the change in the mean annual surface temperature at deeper depths (8.75 m). The simulated increases in future groundwater temperature suggest that the thermal sensitivity of baseflow-dominated streams to decadal climate change may be greater than previous studies have indicated.