Temperature of watercourses

Comment

The average temperature in the Rhine near Basel has risen by around 3 °C since the 1950s. A similar change has also been observed in other watercourses in the Swiss Plateau. It is only below glaciers that watercourses have been found to have cooled slightly. The influences contributing to this development include climate change and, depending on the location, discharges of heated water (for example from cooling systems and wastewater treatment plants). The ecological objectives for surface waters in the Water Protection Ordinance specify that the water quality should be such that the temperature conditions are close to natural. The continuous rise in temperatures since the 1950s is therefore evaluated as negative.

Based on the available data, it is not currently possible to determine with certainty the specific impact on biotic communities. However, climate change is leading to longer periods of warm temperatures. For sensitive aquatic organisms, this reduces their tolerance to additional heat, leading to an increased risk of disease. Therefore, the status is assessed as moderate. According to the climate forecast, water temperatures are expected to continue to rise in the coming decades. 

As a measure against rising water temperatures, it is recommended to increase the proportion of shade-providing trees along watercourses, especially small streams, in addition to increasing the diversity of watercourse structures.

Related indicators

• Groundwater temperature

International comparison

In the last century, water temperatures in European rivers rose by 1–3 °C as a result of atmospheric warming and also locally through the discharge of heated water from cooling systems and wastewater treatment plants. There has been a general increase in water temperatures in Europe. However, a direct comparison of temperatures between regions/countries is not meaningful due to the differences in natural conditions (Alps versus lowlands).

Method

The FOEN has been recording water temperatures at selected watercourse monitoring stations operated by the federal hydrometric monitoring network since 1963. Earlier data derives from measurements carried out by the International Commission for the Protection of the Rhine (ICPR). In response to the temperature trend since the late 1980s, the FOEN monitoring network was expanded from 2001 onwards to include monitoring sites on predominantly smaller bodies of water that are virtually unaffected by human intervention. Other federal and cantonal sites will be expanded or integrated in the coming years to provide information on water temperature conditions at NAWA monitoring sites and on smaller watercourses with near-natural or thermal pollution.

All temperature stations are equipped with data loggers. At the main sites, the data is recorded with high temporal resolution and an average is calculated every 10 minutes.

Basis for assessment of the trend