Temperature of watercourses
Water temperature is a key factor for the conditions in watercourses. It influences all metabolic processes, growth and the composition of biocoenoses. If the temperature limits for a given species are exceeded, the species will be unable to survive if it has no means of escaping to cooler waters.
The average temperature in the Rhine at Basel has risen by about 3°C since the 1950s. A similar change has been observed in other watercourses in the Central Plateau. It is only below glaciers that slight cooling of the water has been observed. Among the factors contributing to this development are climate change and, depending on the situation, the introduction of heated water (for example from cooling systems/plants or wastewater treatment plants).
The ecological objectives for surface waters in the Waters Protection Ordinance specify that the water quality should be maintained at temperatures which approximate to the natural conditions. The continuous rise in temperatures since the 1950s is therefore evaluated as negative.
The currently available data and information does not yet allow effects on biocoenoses to be specifically identified. However, due to climate change longer periods of warm temperatures are increasingly being observed. The tolerance of sensitive aquatic organisms to additional temperature rises is being reduced, which leads to a greater risk of disease. For this reason, the state is assessed as medium. Climate forecasts indicate that a further rise in the water temperature can be expected in the coming decades.
To prevent the rise in water temperatures, it is recommended to increase the share of shade-producing tree stands along watercourses, especially small streams – in addition to more diverse waterbody structures.
In the last century the water temperature of European rivers rose by 1-3 °C as a result of atmospheric warming and also locally due to the discharge of heated water from cooling systems/plants and wastewater treatment plants. Europe is thus observing a general rise in the temperature of watercourses. A direct comparison of the temperatures between regions/countries is not appropriate given the differences in natural landscape conditions (Alps vs. lowlands).
The FOEN has collected water temperature data since 1963 at a selected set of watercourse measuring points operated by the hydrometric network of the Confederation. Earlier data derive from measurements conducted by the International Commission for the Protection of the Rhine ICPR. In response to the development of temperatures since the late 1980s, the FOEN network was expanded from 2001 onwards, to include measuring points on smaller bodies of water almost unaffected by human interference. All the measurement sites are equipped with data loggers. The data is recorded with high temporal resolution and an average value is calculated every 10 minutes.
|Targeted trend||Initial value||Final value||Variation in %||Observed trend||Assessment|
|Stabilisation||Average 2000-2002||Average 2019-2021||(1) 5.68%, (2) 4.93%, (3) 6.32%||(1) Growth, (2) Growth, (3) Growth||negative|
|(1) Rhine-Rekingen*, (2) Rhine-Basel*, (3) Rhine-Diepoldsau*|