Groundwater temperature

The temperature of groundwater is one of its most important characteristics, and a key factor influencing its hydrochemical and biological state. It affects the dissolved oxygen content of the water, as well as the degree of mineralisation. Changes in groundwater temperature over time can highlight potential impacts of climate change and urbanisation on groundwater quantity and quality.

The water temperature in near-surface groundwater resources is influenced by the air temperature, and along rivers also by the temperature of infiltrating river water. In urban areas, the groundwater temperature is affected by infrastructure such as borehole heat exchangers, groundwater use for cooling and heating, and underground installations (tunnels, basements, pipes).

In a long-term comparison, 2015–19 was a period in which groundwater temperatures were higher than average. This is reflected in a comparatively large number of monitoring sites recording high annual temperatures. For example, in the heatwave year of 2018 and the following year 2019, around half of the monitoring sites in Switzerland serving the QUANT (groundwater quantity) module of the NAQUA National Groundwater Monitoring recorded high groundwater temperatures.

Groundwater temperature from 2000 to 2019. Number of monitoring sites recording low, normal and high annual values. Groundwater temperature normal: annual value is in the middle 80% of long-term annual means; high: annual value is in the top 10% of long-term annual means; low: annual value is in the bottom 10% of long-term annual means.

In near-surface groundwater resources, the groundwater temperature follows a characteristic annual cycle that lags around two months behind the air temperature. This is reflected in a comparatively large number of monitoring sites recording high monthly temperatures in autumn and winter. Depending on the nature of the subsurface, the groundwater temperature varies in a range of one to several degrees Celsius over the course of the year.

Groundwater temperature in 2018 and 2019. Number of monitoring sites recording low, normal and high monthly values. Groundwater temperature normal: monthly value is in the middle 80% of long-term means for the month in question; high: monthly value is in the top 10% of long-term means for the month in question; low: monthly value is in the bottom 10% of long-term means for the month in question.

The mean groundwater temperature at individual monitoring sites in 2018 and 2019 ranged from 5 to 15 °C. Reflecting the altitude of the catchment areas, the lowest groundwater temperatures – under 10 °C – were in the Jura and the Alps, compared with 10 to 13 °C in the Swiss Plateau and to the south of the Alps. The highest values – above 13 °C – were recorded in urban centres.