Water resources management is moving towards integration, where groundwater (GW), surface water
(SW) and related aquatic ecosystems are considered one management unit. Because of this paradigm
shift, more information and new tools are needed to understand the ecologically relevant fluxes (water,
heat, solutes) at the GW–SW interface. This study estimated the magnitude, temporal variability and spatial
distribution of water fluxes at the GW–SW interface using a fully integrated hydrological modelling
code (HydroGeoSphere). The model domain comprised a hydrologically complex esker aquifer in Northern
Finland with interconnected lakes, streams and wetlands. The model was calibrated in steady state
for soil hydraulic conductivity and anisotropy and it reproduced the hydraulic head and stream baseflow
distribution throughout the aquifer in both transient and steady state modes.
In a novel analysis, model outputs were compared with the locations and magnitude of GW discharge
to lakes estimated using field techniques. Spatial occurrence of GW–lake interaction was interpreted
from airborne thermal infrared imaging. The observed GW inflow locations coincided well with model
nodes showing positive exchange flux between surface and subsurface domains. Order of magnitude of
simulated GW inflow to lakes showed good agreement with flux values calculated with a stable water
isotope technique. Finally, time series of GW inflow, extracted as model output, showed moderate annual
variability and demonstrated different interannual inflow changes in seepage and drainage lakes of the
aquifer.
Overall, this study demonstrated the ability of a fully integrated numerical model to reproduce
observed GW–SW exchange processes in a complex unconfined aquifer system. The model-based estimates
obtained for GW influx magnitude and spatial distribution, along with information on GW quality
can be used to estimate ecologically relevant fluxes in future water resources management.