Detection of Groundwater–surface water Contributions in a Subarctic Stream using Distributed Temperature Sensing, Topographic Indices, and Spatial Hydrological Model

Groundwater (GW) seepage into streams plays a crucial role in sustaining streamflow and regulating thermal regimes in boreal and arctic headwaters. However, spatial variation of seepage and interactions with the stream network remain difficult to observe, especially in snow-dominated catchments with seasonal freezing. This study used high-resolution distributed temperature sensing (DTS) to identify groundwater and surface-water contributions along a sub-arctic headwater stream in northern Finland. Stream temperature was continuously monitored at 2 m spatial and 30 min temporal resolution over a 2 km reach. Seasonal slope-based thermal patterns were used to distinguish stream sections dominated by groundwater inflow. In addition, melt-active nights with observed snow cover and elevated stream discharge were used to capture surface and near-surface inputs to the stream. These DTS-derived signals were compared with commonly used terrain-based predictors, including upslope contributing area (UCA) and topographic wetness index (TWI), as well as with lateral inflow simulations from the SpaFHy-2D hydrological model. The results show that topography-based indices captured broad-scale surface convergence but failed to consistently identify local groundwater discharge zones. SpaFHy-2D reproduced the general distribution of major groundwater-influenced reaches but shows local mismatches, particularly in esker-controlled sections. This study highlights the value of in-stream temperature observations and hydrological modeling for detecting groundwater–surface water interactions in cold-region, where strong seasonality and snow-dominated hydrology limit traditional field methods.