Snowmelt-mediated isotopic homogenization of shallow till soil

• Main Authors: Muhic, Filip, Ala-Aho, Pertti, Sprenger, Matthias, Klöve, Björn, Marttila, Hannu
• Format: Text
• Language: English
• Published: 2023
• Subjects: Arctic; Lapland
• Online Access: https://doi.org/10.5194/egusphere-2023-884; https://egusphere.copernicus.org/preprints/2023/egusphere-2023-884/

The hydrological cycle of sub-arctic areas is dominated by the snowmelt event. Understanding the mechanisms that control water fluxes during high-volume infiltration events in sub-arctic till soils is needed to assess how future changes in the timing and magnitude of snowmelt can affect soil water storage dynamics. We conducted a tracer experiment with deuterated water to irrigate a plot on a forested hilltop in Lapland, tracked water fluxes of different mobility and monitored how the later snowmelt modifies the labelled soil water storage. We used lysimeters and destructive soil coring for soil water sampling, and monitored and sampled the groundwater. Surface water flow during the tracer experiment was largely controlled by fill-and-spill mechanism. We found that labelled water remained in deeper soil layers over the winter, but the snowmelt event gradually displaced all deuterated water and fully homogenized all water fluxes at the soil-vegetation interface. The conditions required for the full displacement of the old soil water occur only during snowmelt with a persistently high groundwater table. We propose a conceptual model where infiltration into the soil, and eventual soil water replenishment, occurs in three stages. First, unsaturated macropore flow is initiated via surface microtopography and is directed towards the groundwater storage. The second stage is characterized by groundwater level rise through the macropore network, and subsequent pore water saturation and horizontal connectivity of macropores. Shallow subsurface lateral fluxes develop in more permeable shallow soil layers. In the third stage, which materializes during a long period of a high groundwater table and high hydrological connectivity within the soil, the soil water is replenished via enhanced matrix flow and pore-water exchange with the macropore network.