Influence of soil moisture content and infiltration on ground temperature and active layer depth in a river terrace in Adventdalen, Svalbard

Abstract: The active layer constitutes an important subsystem of permafrost environments. Thermal andhydrological processes in the active layer determine local phenomena such as erosion, hydrologicaland ecosystem changes, and can have implications for the global carbon-climate feedback.Despite their importance for environmental and climate change, active layer dynamics are stillonly poorly understood. The importance of hydrology for active layer processes is generallywell acknowledged on a conceptual level, but the physical interdependencies between soil moisture,subsurface water flows and active layer depth are largely unresolved. This thesis usedstate-of-the-art numerical modeling to study the influence of ground surface temperature, soilmoisture content and advective heat flow on near-surface permafrost temperatures and activelayer depths. The investigation was performed for a dry, loess-covered river terrace in centralAdventdalen, Svalbard, and fed by high-resolution hydro-climatic field data for the period2000-2014. Nine scenarios were considered in order to independently test the influence ofdifferent initial soil moisture contents (6%, 12%, and 19%) and infiltration patterns (no infiltration,constant infiltration, and early summer peak infiltration). Results indicated that the permafrost-hydrological system at the study site is largely influenced by cryosuction processes due tostrong capillarity of the highly unsaturated soil. Zones of increased ice content developedprimarily near the permafrost table, creating a ‘transition zone’ between the lower part of theactive layer and the upper permafrost. Infiltration based on snow melt and summer precipitationwas found to be negligible for the seasonal active layer development. The active layer depthgenerally decreased with increasing initial soil moisture content due to a higher consumption oflatent heat. However, cryosuction into the permafrost table and water percolation could potentiallycounterbalance latent heat effects, at least in systems characterized by higher soil moisturecontents. Both model simulations and field observations showed a clear tendency of increasingactive layer depth during the study period, whereas inter-annual variations in active layer depthwere comparably small. Given the moisture migration into the ‘transition zone’, the modelresults further suggested that the site might be capable to buffer thaw and thus obscure increasingground surface temperatures to a certain degree. This could have implications for the suitabilityof active layer depth as a proper indicator for climate change.