The impact of snow cover changes on source water contributions and associated biogeochemical cycling in high latitude catchments

High latitude regions are experiencing global warming more rapidly and significantly than any other region of the Earth. A warmer climate has already severely altered the cryosphere. Cryospheric changes such as snowpack reduction are known to be strongly coupled with the entire hydrologic cycle. However, relatively little is known about the nexus between snow cover changes, source water contributions to groundwater and surface water bodies and associated biogeochemical cycling in aquatic systems. 

To better understand the rapid changes occurring in cold region environments, we obtained field- and satellite-derived data from two sub-arctic catchments (one glaciated, one unglaciated) in the north-western corner of the Hardangervidda mountain plateau (South Central  Norway). During 2020 and 2021, we analyzed various water sources including streams, lakes, groundwater, snow and ice for environmental tracers (major ions, stable water isotopes, radon-222) and greenhouse gases (GHG; CO₂, CH₄ and N₂O). Combining the environmental tracer data with a Bayesian end-member mixing modelling approach (Popp et al., 2019) allowed us to partition water source contributions to streams and lakes. Moreover, we used the noble gas radon to assess hyporheic exchange flow and short water residence times (Popp et al., 2021). To estimate snow cover anomalies in 2020 and 2021 compared to a five-year mean, we retrieved fractional snow cover durations (fSCDs) from 2016 to 2021 by merging Sentinel-2 and Landsat 8 imagery over Finse and applying a spectral unmixing algorithm (Aalstad et al., 2020). 

According to the satellite-derived data, 2020 was exceptionally snow-rich, while 2021 was a snow-poor year. Initial results suggest that the snow-poor year (2021) resulted in comparatively longer groundwater and stream water residence times. As expected, in 2021, surface waters and groundwaters showed lower fractions of snow and ice meltwater. This signal is, however, less pronounced in the unglaciated catchment. With this approach, we aim to hone our understanding of the response of water source partitioning and associated biogeochemical cycling, particularly greenhouse gas concentrations, to climate change-induced alterations in the snowpack. 

References:

Aalstad, K., Westermann, S., & Bertino, L. (2020). Evaluating satellite retrieved fractional snow-covered area at a high-Arctic site using terrestrial photography. Remote Sensing of Environment, 239, 111618,  http://dx.doi.org/10.1016/j.rse.2019.111618

Popp, A. L., Scheidegger, A., Moeck, C., Brennwald, M. S., & Kipfer, R. (2019). Integrating Bayesian groundwater mixing modeling with on-site helium analysis to identify unknown water sources. Water Resources Research, 55(12), 10602– 10615. https://doi.org/10.1029/2019WR025677

Popp, A. L., Pardo-Alvarez, A., Schilling, O., Musy, S., Peel, M., Purtschert, R., et al. (2021). A framework for untangling transient groundwater mixing and travel times. Water Resources Research, 57. https://doi.org/10.1029/2020WR028362