The present and future of alpine groundwater dynamics: Lessons from the Vallon de Réchy (Switzerland)  

Relative to lowland systems, alpine hydrological systems face more rapid and impactful changes due to climate change. In this context of altered meteorological patterns, including decreased snow cover and greater variations in precipitation timing, groundwater’s hydrological buffering role is becoming increasingly significant. A multi-method research approach applied in the Vallon de Réchy reserve (Valais Alps, Switzerland) has provided us with new insights into groundwater as a key component of alpine hydrological systems.The Vallon de Réchy (2182-3149 m.a.s.l., 11 km²) is a non-glaciated, nival-regime alpine headwater catchment with strong elevation gradients and highly heterogeneous geology. Much of our detailed process understanding comes from extensive work in a specific zone, the Tsalet subcatchment (2268-2893 m.a.s.l., 1 km²), where intermittent streams, perennial springs, and extensive unconsolidated sediments have made it a natural laboratory for studying alpine groundwater, developing hydrogeophysical methods, and investigating climate-change sensitivity.Our hydrological, geochemical, and geophysical investigations reveal a highly heterogeneous system in which unconfined aquifers act as hydrological buffers to ensure year-round streamflow. Geochemical and stable isotope analyses provide us with information on the connectivity of different components of the system, as well as variations of end-member contributions to streamflow. By integrating hydrological observations and electrical resistivity tomography (ERT) measurements into numerical models, we have investigated the impacts of climate change on this hydrological system, finding that the currently perennial stream could eventually become intermittent. The site has also played a key role in the development of time-lapse gravimetry (TLG), a non-invasive geophysical method, as a tool for under-monitored hydrogeological systems in mountain regions. TLG has provided unique, quantitative data on seasonal variations in groundwater storage that would be extremely challenging and costly to obtain through traditional methods. While alpine catchments are undoubtedly highly varied, investigations in the Vallon de Réchy integrate novel monitoring approaches that provide evidence for the importance and finite resilience of groundwater as a vital component of alpine headwater catchments.