Groundwater temperature

To date, there has been no observable overall increase in groundwater temperature across Switzerland. In certain regions of the country, however, a slight rise in groundwater temperatures indicates the impact of climate change. For example, groundwater temperatures are rising by up to 0.06 °C per year in the Alpine region, in unconsolidated rock outside valley floors in the Central Plateau, and in the Jura. Direct human impacts on groundwater are also leading to measurable changes in groundwater temperature in densely populated areas.

The temperature of groundwater near the surface is influenced by air temperature. Groundwater temperature around surface waters is also affected by the temperature of infiltrating river water. In densely populated areas, the rise in groundwater temperature results from warming caused by infrastructure, rock and underground tunnels, underground car parks or direct thermal input into groundwater (e.g. from building cooling). Other direct impacts in urban areas can also have the opposite effect (temperature decrease), as in the case of heat extraction from the subsurface for heating systems.

Current status reflects seasonality

Groundwater temperature is continuously recorded at around 100 National Groundwater Monitoring NAQUA gauging stations across the country. In groundwater close to the surface, which is typical for Switzerland, the groundwater temperature exhibits a characteristic annual cycle which lags about two months behind changes in air temperature. Depending on the nature of the subsoil, the groundwater temperature varies by between one and several degrees Celsius over the course of the year. In 2021, the mean groundwater temperature at the individual measuring stations was between 5 and 15 °C. In accordance with the altitude of the catchment areas, the lowest groundwater temperatures of below 10 °C occurred in the Jura and in the Alps. In the Central Plateau and on the southern side of the Alps, temperatures ranged from 10 to 13 °C. The highest values, i.e. above 13 °C, were recorded in urban areas.

Development over time stable to slightly increasing

It is possible to identify multi-year periods with relatively low and relatively high groundwater temperatures corresponding to the superordinate air temperature patterns. This can been seen in the high annual temperatures recorded at a comparatively larger number of gauging stations. For example, in the heatwave year 2018 and in the subsequent years 2019 and 2020, high groundwater temperatures were recorded in Switzerland at around half of the gauging stations in the QUANT module (groundwater quantity) of the NAQUA National Groundwater Monitoring. There has also been an increase in groundwater temperatures in recent years (2015–2021), particularly in urban areas.