Hydrological Modeling of Climate Change Impacts on Alpine Catchments: Shifting Snow Dynamics, Runoff Regimes, and the Frequency of Extremes

Abstract. Climate change is fundamentally restructuring the alpine hydrological cycle, yet how shifting snow dynamics interact with aquifer storage to regulate downstream impacts remains poorly constrained. This interaction is particularly critical in mid- to high-altitude headwaters increasingly dominated by rainfall, where sediment accumulations such as rock glaciers act as hydrogeological buffers. To investigate these dynamics, we assess the streamflow response of 15 nested sub-catchments across five study sites in the Austrian Alps, spanning diverse climatic, geomorphic, and geological conditions. High-resolution field observations combined with satellite-derived snow data were used to calibrate a cascading modeling framework that resolves hydrological processes from individual rock glacier springs and small headwaters (<1 km²) to downstream river catchments up to 200 km². Using an ensemble of 40 climate projections across Representative Concentration Pathways (RCP) 2.6, 4.5, and 8.5, we simulate hydrological change through the 21st century. We quantify these impacts in terms of snow and runoff seasonality, magnitude and significance of hydrometeorological index alterations, and shifts in return periods of historical 10-year extremes. Our results indicate a seasonal reorganization of runoff, with the transition from snow-dominated to rain-influenced regimes progressing asynchronously across catchments. This variation in timing is mediated by elevation, morphology, local climate, and bedrock geology, resulting in contrasting responses between headwater and downstream reaches. Headwater catchments are the most vulnerable to these changes, as their localized aquifers are too small to offset declining seasonal snow storage, earlier spring melt, and intensifying summer flow deficit. In contrast, downstream aquifer systems, including karst formations and valley sediments, more effectively dampen these alterations, although they cannot fully compensate for the loss of cryospheric storage. Across all but the highest elevations, the transition toward rain-dominated regimes drives an asymmetrical shift in hydrological extremes, characterized by more pronounced high flows but less severe low flows. As snow-rich winters decline and the snow season shortens, stronger coupling between precipitation and runoff enhances catchment responsiveness, and more frequent rain-on-snow events amplify early-season peak flows. Conversely, increased winter rainfall and early snowmelt sustain aquifer recharge, thereby buffering streamflow and reducing the severity of low-flow events. Together, these responses show that subsurface storage redistributes changes in flow extremes, yet cannot counteract the broader transition toward a flashier, precipitation-driven runoff regime. This heightened responsiveness of catchment runoff to atmospheric forcing underscores the need for adaptive water management strategies that navigate simultaneous changes in seasonal water budgets, short-term variability, and the shifting frequency of extremes.