Water flow timing, quantity, and sources in a fractured high mountain permafrost rock wall

Abstract. Water flow in high mountain rock walls is crucial for landscape evolution and slope stability. However, the timing, quantity, and sources of this flow remain poorly understood. In the Mont Blanc massif, tunnels at the Aiguille du Midi peak (3842 m) provide direct access to steep permafrost-affected rock walls. Between May 2022 and October 2023, we monitored water flowing from fractures using a real-time system that measured flow rate, temperature, electrical conductivity, and fluorescence of tracers, alongside meteorological data and ground surface temperatures. The results indicate high surface–subsurface connectivity. The water source is primarily snowmelt, with additional inputs from late-summer rainfall. Electrical conductivity, stable isotopes, and recession curve analysis suggest another source of older subsurface ice. Flow onset was closely tied to air temperatures, with steady diurnal fluctuations appearing once rock surface temperatures exceeded 0 °C. Lag times between daily peaks of flow rate and peaks of air and ground surface temperatures of 3–9 and 0–3 h, respectively, point to rapid unsaturated infiltration conditions. Distinct flow regimes observed in two adjacent fracture systems reflect a complex, heterogeneous network, including sediment-filled fractures with a delayed response. Significant flow rates (often > 10 L h−1) and water temperature often exceeding 5 °C, suggest significant heat transfer by advection, capable of enhancing permafrost degradation. This study provides rare direct observations of fracture flow dynamics in steep permafrost rocks and improves our understanding of water routing and its response to atmospheric forcing. The findings offer valuable constraints for coupled hydrothermal models, permafrost-related hazard assessments, and the potential impact of climate change.