The Role of Snowmelt on the Recharge Dynamics of a Vadose Alpine Karst

In Alpine karst aquifers, snowmelt plays a crucial role in groundwater recharge, yet the processes governing water flow through the soil and epikarst into the vadose zone and to the phreatic zone remain poorly understood. This study aims at shedding some light on paths and mechanisms for infiltration, flow, and accumulation of snowmelt in comparison to rainwater in karstic systems. In particular, we present results for the upper vadose zone of the Hochschwab karst massif (Eastern Alps, Austria), a crucial water source for the capital of Vienna. We combined geophysical and hydrological methods to distinguish different infiltration processes. Data were collected at a cave (1896 m above sea level) over 3 weeks in March and April 2024, during which 12 infiltration events were observed—nine through diurnal cycles of snowmelt and three mixed with effective rain. Additionally, dry and wet conditions in the following summer were monitored to provide a seasonal comparison of infiltration dynamics. Monitoring included cave drip water at a V-notch weir (discharge, electrical conductivity, and temperature), soil moisture measurements at depths of 5–30 cm, and electrical resistivity tomography (ERT), utilising 96 electrodes between the cave ceiling and the surface. Measurements at the weir in the cave indicate higher flow rates during heavy precipitation than during snowmelt, while the ERT images show the highest saturation during snowmelt, especially in the bedrock. Hence, results show that snowmelt primarily leads to diffuse recharge, with an overall increase in the saturation of the epikarst and rock, while rain events demonstrate a quick recharge pattern. These findings emphasise the importance of snowmelt as a diffuse recharge source contributing to water storage and underline that the integration of multiple sensors is crucial for understanding the variability of recharge processes in Alpine karst systems under different meteorological conditions.