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Abstract. Soil moisture plays a key role in soil nutrient and carbon cycling, plant productivity and in energy, water, and greenhouse gas exchanges between the land and the atmosphere. In this study, we used the Spatial Forest Hydrology (SpaFHy) model, in-situ soil moisture measurements and Sentinel-1 SAR-based soil moisture estimates to explore spatiotemporal controls of soil moisture in a subarctic headwater catchment in northwestern Finland. The role of groundwater dynamics and lateral flow on soil moisture was studied through three groundwater model conceptualizations: i) omission of groundwater storage and lateral flow, ii) conceptual TOPMODEL approach based on topographic wetness index, and iii) explicit 2D lateral groundwater flow. The model simulations were compared against continuous point-scale measurements, distributed manual measurements conducted in the study area, and novel SAR-based soil moisture estimates available from the area at high spatial and temporal resolution. Based on model scenarios and model-data comparisons, we assessed when and where the lateral groundwater flow shapes soil moisture, and under which conditions soil moisture variability is driven more by local ecohydrological processes, i.e. the balance of infiltration, drainage and evapotranspiration. The choice of groundwater conceptualization was shown to have a strong impact on the modeled soil moisture dynamics within the catchment. All model conceptualizations captured the observed soil moisture dynamics in the upland forests, but accounting for the lateral groundwater flow was necessary to reproduce the saturated conditions commonly occurring on the peatlands and occasionally on lowland forest grid-cells. We further highlight the potential of integrating multi-scale observations, including spatially explicit remote sensing data, with land surface and hydrological models. The results have broad implications for choosing suitable models for studying ecohydrological and biogeochemical processes as well as earth system feedbacks in subarctic and boreal environments.
Nousu et al. (Thu,) studied this question.