Till Rehm

In peri- and paraglacial regions, water plays a critical role in the hydrological cycle and slope stability. However, hydrological models often overlook water infiltration into bedrock due to limited knowledge of groundwater dynamics at high elevations. Although the link between water presence and rock slope failures is evident in many cases, proof of hydrostatic pressure buildup at depth is scarce, highlighting another significant research gap. This study aims to decipher the hydrological dynamics and empirically derive hydrostatic pressures in deep bedrock. We present unique decennial meteorological data, snowmelt modeling, and discharge measurements from two rock fractures in a tunnel located at ≈ 55 m depth under the permafrost-affected Zugspitze ridge (2815–2962 m a.s.l.). We developed an empirical hydraulic model and detected flow anomalies by comparing inputs (i.e., snowmelt and rainfall) and outputs (i.e., discharge from fractures, baseflow, and no-flow events). Results show continuous flow during snowmelt and discontinuous events during summer months. Hydraulic conductivities are in the order of 10⁻⁴ m s⁻¹, with variations according to the saturation. Extreme events are likely to fully saturate the fractures and increase their interconnectivity, producing discharges up to 800 L d⁻¹ and 58 L h⁻¹ from one single fracture. Hydrostatic pressures calculated implementing Darcy's falling-head law are 27 ± 6 m during average snowmelt and 40 ± 10 m for extreme events. These pressure levels can weaken or even destabilize rock slopes in rapidly warming alpine environments. With ongoing climate changes, water relevance is expected to increase, with impacts that have yet to be fully assessed. This study advances the understanding of alpine hydrology and geomorphology by providing new insights into deep groundwater processes and their implications for slope stability.

Estimating the amount of snow, its evolution and spatiotemporal distribution in complex high-alpine terrain is currently considered as one of the most important challenges in alpine hydrology and water resources management. This is predominantly caused by the lack of accurate information on the spatiotemporal variations of snow water equivalent (SWE) in vast regions with no sensor to measure SWE beyond local scale. At Mt. Zugspitze, Germany, a superconducting gravimeter senses the gravity effect of the seasonal snow, reflecting the temporal evolution of SWE in a few kilometers scale radius. An introduction into the novel sensor setup will be given including the sensitivity of the integrative gravimetric signal regarding the spatially distributed snowpack and the cryo-hydro-gravimetric signal changes. We used this new observation to evaluate two configurations of the Alpine3D distributed snow model. In the default run, the model was forced with meteorological station data. In the second run, we applied precipitation correction based on an 8 m resolution snow depth image derived from satellite observations (Pléiades). The snow depth image strongly improved the simulation of the snowpack gravity effect during the melt season. This result suggests that satellite observations can enhance SWE analyses in mountains with limited infrastructure.

In 2024, Germany’s national metrology institute, the Physikalisch-Technischne Bundesanstalt (PTB), signed a sponsorship agreement with the Operational Company of the Environmental Research Station Schneefernerhaus (Umweltforschungsstation, UFS) for the operation, maintenance and upgrade of the Bonner sphere-based neutron spectrometer located at the UFS. The UFS Schneefernerhaus was established in 1999 and is Germany’s highest research station at an altitude of 2650 meters, just below the summit of Mt. Zugspitze, where it houses a wide range of scientific instruments for observing weather, climate and climate change.The Bonner Sphere Spectrometer (BSS) system at the UFS Schneefernerhaus has been in operation since 2005, thanks to the cooperation between the UFS Operational Company and the German Research Center for Environmental Health of the Helmholtz Center Munich. The system is used for continuous measurements of the neutron component of secondary cosmic radiation. With an extensive set of polyethylene sphere moderators and spheres with metal shells, the BSS at Schneefernerhaus can detect neutrons with energies ranging from 10-9 MeV to 103 MeV. Thanks to its spectrometric capabilities, the system can provide neutron energy spectra, which is an advantage over the classical neutron monitors used worldwide.The Neutron Radiation Department of PTB is currently working on upgrading the data acquisition hardware and software, data storage, workflow and data analysis of the BSS system towards an automated and robust operation.This presentation introduces methods for error correction and data preparation, incorporating historical data (years 2013 to 2024) from the former team of the Helmholtz Center Munich, and discusses possibilities for disseminating the data to scientific communities.

Monitoring snow water resources is crucial to understand the dynamics of snow-fed mountain rivers. Still, in harsh and remote environments like mountain regions using conventional measurement techniques remains particularly challenging. Cosmic-Ray Neutron Sensing (CRNS) constitutes an emerging method for autonomous and non-invasive monitoring of soil moisture and snow dynamics at intermediate spatial scales of several hectares. The method is therefore promising for monitoring snow water equivalent (SWE) in high alpine locations.The analysis includes two sites at the Zugspitze Massif, differing in elevation and surrounding topographical features. Both sensors have been installed inside existing buildings with steep roofs to avoid snow accumulation, rather than establishing new infrastructure in complex terrain. The CRNS at the Environmental Research Station Schneefernerhaus (UFS, 2656 m a.s.l.) was installed in November 2015 in the Kugelalm located on one of the terraces. A second CRNS was installed in October 2023 in the building of “Zugspitze Geodynamic Observatory Germany” (ZUGOG) operated by the German Research Centre for Geosciences (GFZ) at the summit (2962 m a.s.l.). The CRNS signal is compared against spatially distributed reference SWE based on manual measurements, terrestrial lidar and airborne photogrammetry. Furthermore, Monte Carlo based neutron simulations using the URANOS model and a dedicated modular scenario tool (YULIA) are performed to characterize the local dynamics at the measurement sites.First results prove that CRNS is suitable for monitoring SWE dynamics even at high alpine locations like the Zugspitze Massif. At UFS the neutron counts reveal both extremely dry years, like 2022, but also very snow-rich years, like 2019 and 2024, which were among the wettest since 2015. The high altitude, the shape of the steep topography and the rocky underground with limited soil cover reduce the statistical error and increase the seasonal dynamics in the neutron flux, facilitating CRNS based SWE monitoring. Another noteworthy aspect is, that due to the large measurement footprint of several hectares, CRNS can even be used when installed within existing buildings, thus reducing costs and limiting the environmental impact of the installation.

Monitoring the amount of snow, its spatiotemporal distribution as well as the onset and amount of snow-melt induced runoff generation are key challenges in alpine hydrology. Cryo-hydro-gravimetry is a non-invasive method of observing temporal gravity variations after the reduction of all other geophysical signals as the integral of all cryospheric and hydrological mass variations including snow accumulation and ablation. It has an accuracy of up to 9 decimals on a wide spectrum from high temporal resolution of up to 1 min to several years within footprints up to approx. 50 km². At the Zugspitze Geodynamic Observatory Germany (ZUGOG) with its worldwide unique installation of a superconducting gravimeter at a high-alpine summit (2.962 m a.s.l.), this method is applied for the first time on top of a well-instrumented, snow-dominated catchment. We use this instrumental setup in synthesis with in situ measured data, detailed physically-based snowpack modelling with Alpine3D as well as satellite-based snow depth maps derived by stereo photogrammetry. We will give an introduction into the novel sensor setup and will show first results, including the sensitivity of the integrative gravimetric signal regarding the spatially distributed snowpack and the cryo-hydro-gravimetric signal changes since 2019. The amount of the simulated snow water equivalent within the footprint of the gravimeter correlates well with the gravimetric signal (Pearson correlation coefficient r = 0.98). Based on the applied snowpack modelling approach including the snow depth maps for precipitation scaling, topography information as well as Newton’s Law of Gravitation, the gravimetric signal contribution and footprint can be described spatiotemporally over winter periods.

The lack of accurate information on the spatiotemporal variations of snow water equivalent (SWE) in mountain catchments remains a key problem in snow hydrology and water resources management. This is partly because there is no sensor to measure SWE beyond local scale. At Mt. Zugspitze, Germany, a superconducting gravimeter senses the gravity effect of the seasonal snow, reflecting the temporal evolution of SWE in a few kilometers scale radius. We used this new observation to evaluate two configurations of the Alpine3D distributed snow model. In the default run, the model was forced with meteorological station data. In the second run, we applied precipitation correction based on an 8 m resolution snow depth image derived from satellite observations (Pléiades). The snow depth image strongly improved the simulation of the snowpack gravity effect during the melt season. This result suggests that satellite observations can enhance SWE analyses in mountains with limited infrastructure.

While recent permafrost degradation in Alpine peri- and paraglacial slopes has been documented in several studies, only restricted information is available on the respective hydrology. Water boosts permafrost degradation by advective heat transport and destabilizes periglacial mountain slopes. Even if multiple recent rock slope failures indicate the presence of water, only a few studies provide evidence of water availability and related hydrostatic pressures at bigger depths, showing a significant research gap. This study combines a unique decennial data set of meteorological data, snowmelt modeling, and discharge measurements from two rock fractures in a tunnel located ≈ 55 m under the permafrost-affected N-S facing Zugspitze Ridge (2815–2962 m asl). To decipher the hydrological properties of fractures, we analyze inputs, i.e., snowmelt and rainfall, and outputs, i.e., discharge from fractures, baseflow, and no-flow events, detecting flow anomalies. For summer precipitation events, we developed i) a uniform recession curve, ii) an empirical water storage model, and iii) an approximate hydraulic water pressure model according to Darcy’s falling-head law. Extreme events with up to 800 l/d and 58 l/h are likely to fully saturate the observed fractures with corresponding hydraulic heads of up to 40 ± 10 m and to increase fracture interconnectivity. The average daily discharge during snowmelt, 10 l/h, can lead to hydraulic heads up to 27 ± 6 m. Water dynamics suggest hydraulic conductivities in the range of 10−4 m/s, with variations according to the fracture’s saturation. E.g., no-flow and baseflow events indicate unsaturated and partially saturated conditions. Here, we show an empirical fluid flow approximation model of hydrostatic pressure regimes in high-alpine deep-bedrock fractures. Pressures from water accumulation in bedrock reach levels that can weaken or even destabilize rock slopes. This process can easily outpace thermal conductive warming of active layers in the foreseeable future, provide positive feedback on water infiltration, and is crucial for the stability of the rapidly warming alpine permafrost environments.

While recent permafrost degradation in Alpine peri- and paraglacial slopes has been documented in several studies, only restricted information is available on the respective hydrology. Water boosts permafrost degradation by advective heat transport and destabilizes periglacial mountain slopes. Even if multiple recent rock slope failures indicate the presence of water, only a few studies provide evidence of water availability and related hydrostatic pressures at bigger depths, showing a significant research gap. This study combines a unique decennial data set of meteorological data, snowmelt modeling, and discharge measurements from two rock fractures in a tunnel located ≈ 55 m under the permafrost-affected N-S facing Zugspitze Ridge (2815–2962 m asl). To decipher the hydrological properties of fractures, we analyze inputs, i.e., snowmelt and rainfall, and outputs, i.e., discharge from fractures, baseflow, and no-flow events, detecting flow anomalies. For summer precipitation events, we developed i) a uniform recession curve, ii) an empirical water storage model, and iii) an approximate hydraulic water pressure model according to Darcy’s falling-head law. Extreme events with up to 800 l/d and 58 l/h are likely to fully saturate the observed fractures with corresponding hydraulic heads of up to 40 ± 10 m and to increase fracture interconnectivity. The average daily discharge during snowmelt, 10 l/h, can lead to hydraulic heads up to 27 ± 6 m. Water dynamics suggest hydraulic conductivities in the range of 10−4 m/s, with variations according to the fracture’s saturation. E.g., no-flow and baseflow events indicate unsaturated and partially saturated conditions. Here, we show an empirical fluid flow approximation model of hydrostatic pressure regimes in high-alpine deep-bedrock fractures. Pressures from water accumulation in bedrock reach levels that can weaken or even destabilize rock slopes. This process can easily outpace thermal conductive warming of active layers in the foreseeable future, provide positive feedback on water infiltration, and is crucial for the stability of the rapidly warming alpine permafrost environments.

While recent permafrost degradation in Alpine peri- and paraglacial slopes has been documented in several studies, only restricted information is available on the respective hydrology. Water boosts permafrost degradation by advective heat transport and destabilizes periglacial mountain slopes. Even if multiple recent rock slope failures indicate the presence of water, only a few studies provide evidence of water availability and related hydrostatic pressures at bigger depths, showing a significant research gap. This study combines a unique decennial data set of meteorological data, snowmelt modeling, and discharge measurements from two rock fractures in a tunnel located ≈ 55 m under the permafrost-affected N-S facing Zugspitze Ridge (2815–2962 m asl). To decipher the hydrological properties of fractures, we analyze inputs, i.e., snowmelt and rainfall, and outputs, i.e., discharge from fractures, baseflow, and no-flow events, detecting flow anomalies. For summer precipitation events, we developed i) a uniform recession curve, ii) an empirical water storage model, and iii) an approximate hydraulic water pressure model according to Darcy’s falling-head law. Extreme events with up to 800 l/d and 58 l/h are likely to fully saturate the observed fractures with corresponding hydraulic heads of up to 40 ± 10 m and to increase fracture interconnectivity. The average daily discharge during snowmelt, 10 l/h, can lead to hydraulic heads up to 27 ± 6 m. Water dynamics suggest hydraulic conductivities in the range of 10−4 m/s, with variations according to the fracture’s saturation. E.g., no-flow and baseflow events indicate unsaturated and partially saturated conditions. Here, we show an empirical fluid flow approximation model of hydrostatic pressure regimes in high-alpine deep-bedrock fractures. Pressures from water accumulation in bedrock reach levels that can weaken or even destabilize rock slopes. This process can easily outpace thermal conductive warming of active layers in the foreseeable future, provide positive feedback on water infiltration, and is crucial for the stability of the rapidly warming alpine permafrost environments.