Low groundwater level in the Außermontafon aquifer (Austria)

In the period from autumn 2021 to spring 2023, the groundwater level in the Schruns-Tschagguns sub-basin of the Außermontafon aquifer was at an exceptionally low level. Comparable low groundwater levels have occasionally occurred in the past as well. However, the low groundwater level from 2021 to 2023 was temporarily more extreme and especially more prolonged.

The existing Außermontafon groundwater model was used to analyse the causes of this behaviour. Additionally, an extended data analysis was carried out. The work was commissioned by the Department of Water Management of the Vorarlberg Provincial Government. Other participants were the power plant operator illwerke vkw and the local geological office Geognos Bertle ZT GmbH.

Based on an extended data analysis (e.g. double sum analysis, evaluation of balance quantities and of discharge hydrographs on surface waters) and based on the modeling of various scenarios, an increase of colmation in the Ill and/or Litz river bed in summer/autumn 2021 was identified as the main cause of the sharp drop of the groundwater level. Due to the relatively dry hydrological conditions in 2022, there was neither decolmation nor a rise in the groundwater level due to increased recharge rates, which explains the prolonged low groundwater level. With the onset of heavy rainfall in spring 2023, the groundwater level finally recovered and has since returned to its usual level.