Climate Sensitivity of Humid Boreal Forest Stream Water Sourcing Informed by Variation in Air‐Stream Temperature Regression

ABSTRACT Boreal landscapes are undergoing rapid climate change and are predicted to experience large increases in terrestrial carbon mobilization. The fate of this carbon, critical for predicting terrestrial‐to‐aquatic carbon fluxes and landscape carbon balance, is influenced by hydrological pathways regulating water sources and stream temperature in headwater systems. To assess hydrological responses to climate variability, we analysed air–stream temperature (AST) regressions for nine boreal headwater catchments across three regions of a north–south climate transect in eastern Canada, using ice‐free period data spanning 5–7 years. Small, steep catchments in the warmest region exhibited low AST slopes and high intercepts, consistent with greater groundwater contributions and thermal buffering. In contrast, higher AST slopes and lower intercepts in larger, low‐relief catchments in colder regions indicated stronger atmospheric coupling and greater influence of near‐surface flowpaths. Although spatial variation in slope AST and intercept AST was associated with basin area and steepness, interannual variability revealed climate controls on stream water sources and thermal responses that depended on geomorphic setting. In small, steep catchments, greater autumn precipitation prior to peak discharge was associated with higher slope AST and lower intercept AST , consistent with enhanced air–stream coupling and activation of shallow flowpaths. In contrast, snowpack duration influenced AST regressions in larger, low‐relief catchments of colder regions. Longer snowpack periods decreased intercept AST, whereas reduced snowpack enhanced thermal sensitivity to ice‐free air temperature. These results demonstrate that thermal and hydrological responses to climate variability depend on geomorphic structure and baseline stream thermal behaviour. Shallow flowpath contributions in thermally buffered small, steep headwaters appear sensitive to evapotranspiration and precipitation, whereas larger, low‐relief headwaters may be more sensitive to changing snowpacks. Headwater AST regressions indicate coupled thermal and hydrological sensitivity, supporting predictions of hydrological and carbon fluxes in boreal landscapes.