Characterization of the water flow regime in the upper vadose zone, Furtowischacht (Hochschwab, Austria)

The Kläfferquellen as the largest springs in the north of the Hochschwab karst massif are an important source of drinking water for the city of Vienna. Due to the vulnerability of karst waters, parts of the Hochschwab Massive (80 km southwest of Vienna) are a spring water protection zone. A better understanding of the karst aquifer recharge is important for the karst water protection management. The upper zone of a karst aquifer, the epikarst plays an important role for the recharge and the regulation of water flow to the vadose zone below.

Hydrological monitoring in an Alpine vadose shaft (Furtowischacht) is the basis of these analyses. In order to quantify the water storage and flow in the upper vadose zone, a Thomson-weir was installed in a small canyon at 100 m below the entrance. Since 2016, electrical conductivity (EC), temperature (T) and water level have been measured at the weir at least every 10 minutes. The discharge shows extreme fluctuations between 0.003 to 19 l/s. Salt tracer experiments indicate tracer travel times between 10 min and 3 hours. The discharge behaviour after a precipitation event can be classified with hydrograph recession analysis in quick, intermediate and slow flow.  Snow melting events show intermediate and slow responses. The water storage within the epikarst can be inferred from the following observations: (1) Reactions of T and EC after the increase of discharge vary between 10 min and 5 h. (2) During summer rain events, EC increases (after a short decrease) and remains at an elevated level for longer period of time. (3) No drying up of the cave brook after long periods of no recharge, where baseflow shows a storage capacity. Single discharge events were successfully modelled with a bucket-type rainfall-runoff model thereby describing the water flow and storage in the upper vadose zone, respectively.