Groundwater exfiltration pattern determination in the sub-arctic catchment using thermal imaging, stable water isotopes and fully-integrated groundwater-surface water modelling

Groundwater-surface water (GW-SW) interactions play an important function in northern catchments, but their dynamics and role in water, carbon, and nutrient balances are still poorly understood. In this case study, we (1) explored groundwater-surface water dynamics in a sub-arctic headwater watershed in Pallaslompolo, Finnish Lapland. (2) We compared fully-integrated physically-based simulations of the groundwater and surface water and their interactions with unique spatially distributed field datasets of drone-derived thermal infrared imaging (TIR) and surface peat pore stable water isotopes. Simulations and field observations point to similar GWexfiltration areas, with an agreement between slightly over 50% for stable water isotopes and modeling and more than two thirds for thermal infrared and modeling. The agreement provides evidence that a fully-integrated GW-SW model with a simple parameterization approach can successfully identify the main features of GW-SW interactions in the high-latitude catchment. All methods suggest the complex nature of GW-SW exchange that is focused in topographical lowlands. Our simulation results also highlight how specific locations serve as groundwater exfiltration zones, surface water downwelling zones, or 'mixed' zones, where both interaction types occur at different times according to the hydrological state of the system. Modeling results suggest abrupt GWSW interaction reversals - a phenomenon known to occur in peatland areas. To conclude, the three methods should be treated as complementary when describing and understanding GW-SW interactions within peatlands, but only the fully-integrated physically-based modeling can provide comprehensive estimates for both the spatial and temporal patterns and magnitudes of GW-SW interaction fluxes.