Groundwater Storage in High Alpine Catchments and Its Contribution to Streamflow

There is limited knowledge about groundwater storage in alpine catchments, although it might strongly influence how these catchments react to earlier snowmelt due to climate change. The objective of the study was to develop and test a method to quantify seasonal groundwater storage in alpine catchments and evaluate how groundwater storage is related to hydrogeological properties. As representative water table observations are challenging to acquire in such environments, we used a water balance approach targeting the main snowmelt period when most groundwater recharge is expected to occur. Based on a detailed quantification of the snow water equivalent at the onset of snowmelt combined with discharge measurements, it is possible to quantify groundwater storage with a low uncertainty even if other terms of the water balance are less well constrained. The application of the method to an 11-km(2) research catchment revealed a large seasonal increase of groundwater storage by 300 mm or 45% of the premelt snow water equivalent. An independent quantification of groundwater storage depletion during the 7-month-long recession period provided a similar value of 330 mm, demonstrating that the stored groundwater is available to sustain streamflow. At the end of the recession, catchment outflow still amounted to 0.9 mm/day with a composite bedrock aquifer providing a disproportionally high share as demonstrated by hydrochemical data. The study demonstrates that high alpine aquifers can seasonally redistribute water and stabilize catchment outflow in an otherwise very dynamic environment and thus might strongly influence the response of such catchments to climate change.