Hydrological changes in high alpine environments detected with relative gravimetry

Here we present the first long-time mass monitoring in periglacial environment with spring gravimetry and correlate it with external weather conditions (rainfall, snow melt) and cleft water discharge to understand water dynamics inside the bedrock.

Water is widely recognized as a preparing and triggering factor in unstable slopes. Pressurized water is documented coincident to alpine rock slope failures, but the quantification of water and of effective destabilizing pressures inside the slope remains unresolved. Gravimetry allows to monitor water mass changes at different resolutions: satellite based gravimetry can detect hydrological changes with kilometer scale, while ground based absolute and relative superconducting gravimeters provide promising results at sub-basins scale. However, only relative spring gravimeters are light and handy enough for extended measurements in high-alpine environments, but example of this use are missing in the literature.

We conducted monthly relative measurements with a spring gravimeter Scintrex CG-5 at 20 stations located at different altitude and slope expositions inside the permafrost affected Kammstollen tunnel (Mount Zugspitze, 2962 m asl, Germany) from 2015 to 2021. Additionally, monitoring with temperature loggers and electrical resistivity detected permafrost degradation, geological mapping provided cleft structure and snowpack simulations quantified water from snowmelt. Due to the low porosity of the local lithology (Wetterstein Limestone with 4-5% effective porosity), we expect perched water to accumulate in single fractures, especially when they are sealed by permafrost.

A clear seasonal trend results from gravimetry, resistivity and temperature measurements, mainly attributable to the hydrological summer-winter cycle. Correlation with the water flow in clefts is also evident, as well with the snowmelt from the models. Uncertainties due to internal drifts of the instrument can be corrected but also show the limitations of this highly sensitive instrument.

Although measuring hydrostatic pressures in single clefts remains an open challenge, this feasibility study is a benchmark showing that relative gravimetry can provide quantitative data on fluid flow and hydrostatic pressure in fractures even in periglacial and mountainous environments.