Alpine water cycle unraveled by hydrogeological measurements, isotopes (18O/2H, 3H, 3He, 14C) and tracer gas analyses (CFC-11,-12,-113, SF6)

Alpine regions are becoming more sensitive to climate change and to understand the hydrogeological processes that follow extreme climatic events (flooding, drought, heavy precipitation and fast snow melt), the hydrologic conditions and geologic realities need to be understood. Our research project “Understanding of Extreme Climatological Impacts from Hydrogeological 4D Modelling” (EXTRIG; funded by the Austrian Academy of Sciences) contributes to this challenge by applying an innovative interdisciplinary approach in an Austrian alpine research area (Sibratsgfäll, Vorarlberg) with a catchment of 5 km² situated at an altitude between 800 and 1.400 m based on a cooperation between hydrogeologists, meteorologists, social scientists and the local population.

This poster study emphasizes on preliminary results of applied hydrogeological methods conducted between 2019 and 2021 to understand the local water cycle.

• To determine surface discharge, radar monitoring stations was installed at each of the four main streams, flowing into the main river beneath the village. Calibration of the results was conducted with several salt dilution measurements. Additional salt dilution measurements helped to estimate diffuse surface and groundwater discharge into the main river.
• Precipitation was measured in Sibratsgfäll (906m) and compared with precipitation measurements of two nearby weather stations and with past precipitation measurements of the area since 1990.
• Evapotranspiration was calculated as ET0 with the Hargreaves method in two different approaches, one using the air temperature reconstructed from surrounding weather stations, the other using temperature measurements from the monitoring station in Sibratsgfäll.

The resulting data was used to calculate the amount of water infiltrating into the ground via water balance calculation. The yearly precipitation from December 2019 including November 2020 sums up to 2.600 mm/a and approx. 50% discharges via the main streams. Evapotranspiration can be estimated to be 22 to 32% with a large uncertainty leaving 18 to 28% of precipitating water to diffuse discharge and infiltration into the groundwater. Estimating that surface-near discharge is at least 10%, between 8 and 18% of precipitating water may infiltrate into the ground. Diffuse surface near discharge has shown to be higher after snow melt and in summer, but almost absent during colder periods. Furthermore, a complex network of shallow agricultural drainages may only partially dewater to streams but also contribute to surface-near discharge.

The monitoring of the stable ²H/¹⁸O-isotopes in a meteorological station (906m) and of Flysch-springs close to the mountain ridge of the recharge area allow to differentiate the recharge altitude. The vertical unsaturated infiltration in silt/sand dominated glaciolacustrine sediments were estimated by seasonal variation of ²H/¹⁸O-isotopes in soil-water to be 1m/year approximately. Precipitation in the Flysch dominated area at higher altitudes is transported slope-parallel in the upper part of the glacial sediments. The Mean Residence Time (MRT) of the shallow groundwater (<40m) estimated by a combination of isotopes ²H/¹⁸O, ³H/³He, ¹³C/¹⁴C and tracer gases (CFC, SF₆) indicate ages between some months and 4 years. Deeper (>40m) artesian wells in the western part are dominated by MRT older than 30 years.