Flow dynamics in a vadose shaft – a case study from the Hochschwab karst massif (Northern Calcareous Alps, Austria)

Karst aquifers are highly vulnerable to contamination due to quick water flow through conduits. Their high heterogeneity and the poorly known infiltration effect of the vadose zone make quantification of recharge processes difficult. This study characterizes the water flow and storage in the upper vadose zone with almost four years monitoring of a permanent stream in a vadose shaft (Furtowischacht). Its small catchment of 4,500 m² is located in a former glaciated high Alpine environment (Hochschwab, Austria). High discharge fluctuations between 0.002 and 19 l/s, relatively high hydrograph recession coefficients, and transit velocities between 0.0015 and 2.4 m/s estimated with salt tracer experiments indicate a highly dynamic discharge behavior. A fast point infiltration through open karren and dolines could be observed for rainfall events and indicates a highly karstified network with a rapid water transmission. Snowmelt periods show only a slower flow component and diffuse infiltration. However, condensation within the conduit system is likely superimposed to this signal. A lumped-parameter rainfall-runoff model is used to simulate the discharge with a dual porosity approach. It indicates a low storage volume, which is in accordance with the estimated storage of 22 m³ (or 5 mm), deduced from the recession analyses. In contrary, the physicochemical parameters argue for some storage capability: 1) After an increase of discharge, electric conductivity reacts with an average delay of 50 min; 2) Partly a piston flow can be recognized. These amounts of water may be stored in the partial soil cover alone and therefore the presence of a hydrologically significant epikarst layer is unclear