Evaluating Groundwater Contribution to Streams in Moderate‐Relief Catchments Using Paired Air‐Stream Temperature Analysis

ABSTRACT

Groundwater discharge to headwater streams plays a critical role in maintaining flow during dry periods and buffering stream temperatures, yet groundwater‐stream interactions remain poorly understood in low‐ to moderate‐relief headwater catchments. In this study, we used paired analysis of air, stream and groundwater temperature across 53 site‐years collected over 2 years to assess the relative importance and depth of groundwater contributions in four moderate‐relief headwater catchments in a cold, humid region (eastern Canada). Groundwater contributions were characterized using thermal sensitivity (regression slope between air and stream temperatures) as well as amplitude ratios () and phase shifts () between annual air and stream temperature signals. Thermal sensitivity was strongly correlated with amplitude ratios, which captured the relative importance of groundwater contributions, whereas phase shifts captured information on groundwater flow depth and together these metrics were used to define groundwater signatures. Across site‐years, 51% displayed an atmospheric signature, 42% a deep groundwater signature and 7% a shallow groundwater signature. The relative contribution of groundwater to streamflow declined with increasing drainage area, with no groundwater‐driven signatures observed in catchments larger than 10 km ² over the study period. Across catchments, shallow groundwater signatures occurred only in the catchment containing permeable sand sediments near its outlet, suggesting a role of surficial geology in shaping groundwater contributions. Air‐stream temperature relationships varied between the two study years, with several sites shifting from deep groundwater to atmospheric signatures, highlighting interannual variability in groundwater contributions. Overall, results emphasized spatial variability within catchments (driven by drainage area), across catchments (driven by surficial geology) as well as temporal variability (interannual changes) in groundwater‐stream interactions. These findings underscore the importance of considering both spatial and temporal variability when using paired air‐stream temperature analysis to assess groundwater contributions to moderate‐relief headwater streams.