Water flow in the active layer along an arctic slope – An investigation based on a field campaign and model simulations

Abstract. As climate conditions change, the hydrological regime in the active layer is subject to change too. This influences the transport of solutes and the availability of nutrients, e.g. nitrogen particularly, along slopes. There is a lack of understanding the pathways and travel times of water and nutrients along slopes in discontinuous permafrost regions and how to scale changes along transects to the rest of the landscape. This study presents a comprehensive data set of a field site in Disko Island in Greenland aiming at constructing a hydrological model of the area. Data from automated weather stations, geophysical surveys, soil samples and soil sensors and tracer experiments are combined to describe the spatial variability in the field and to serve as input to a two-dimensional model (SUTRA) for simulating water and solute transport in the summer period. The model is calibrated and validated against volumetric water content and breakthrough curves of the applied tracers. Observed and simulated results suggest that the flow velocity in the active layer is directly influenced by annual precipitation patterns leading to water flow during the summer and rapid movement at the end of summer. Yearly travel times for the specific field site are simulated to be approximately 14 m/a and the highest peak velocities are most likely caused by preferential flow paths. The spatial heterogeneities linked to the frost topography seem to control the direction and velocity of flow. The observed discontinuous movement of a conservative tracer suggests that the movement of dissolved nitrogen compounds such as nitrate, being released along the slope in consequence of permafrost thawing, could possibly quickly influence nitrogen cycling at the end of the slope. This may trigger a feedback of climate changes in terms of increasing carbon sequestration due to additional plant growth in these otherwise nitrogen-limited Arctic ecosystems.