Lateral groundwater flow in boreal catchments: Implications on soil moisture and the impacts of landscape characteristics and model resolution

Understanding the lateral movement of water is essential for accurately modeling hydrological and biogeochemical processes and element fluxes within catchments. Traditional land surface and hydrological models often focus on vertical fluxes and river discharge and tend to overlook the dynamic and spatially heterogeneous nature of lateral water flows from land to smaller water bodies. To capture these dynamics, we use the process-based SpaFHy model at very high spatial resolution (e.g., 16m x 16m), leveraging novel remote sensing data to parameterize the model for boreal catchments. We demonstrate how shallow lateral groundwater flow shapes surface soil moisture patterns in a catchment in northwestern Finland. We then explore how catchment-scale hydrological behaviour responds to different representations of stream and ditch networks in a nested subcatchment system in northern Sweden by comparing simulations using the natural stream network with simulations in which human-made ditches are also represented. Finally, to assess the capability to upscale the simulations to larger areas, we investigate how model behaviour changes when simulations are performed at coarser spatial resolutions, highlighting the challenges in conventional conceptualization of groundwater–surface water exchange. This work represents an important step toward improving our understanding and modeling of lateral fluxes and lays the groundwork for future coupling with carbon dynamics, including lateral dissolved organic carbon, across diverse catchments.