Federal Office for the Environment FOEN
Federal Office for the Environment FOEN
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Overview maps: explanations
Maps with continuously updated analysis of hydrological data
1. General situation: Flows and water levels
The map shows the current flows of Swiss watercourses and the current water levels of Swiss lakes compared with the monthly percentiles of the long term hourly averages (expressed in percentiles).
The percentiles are calculated every month based on the "long term series" of hourly average measurements. The hourly averages from the start of measurements to date are compiled separately for each month, The percentile defines what proportion of the data is higher or lower than a specific limit. A flow at the 95% percentile means that 95% of the flows measured in the month in question are below that threshold (only 5% are above it).
Example: The flow rate on the Kleine Emme was 7.3 m3/s on 24.09.2010 at 16:50. The following percentiles were previously calculated for the Kleine Emme from the long term series of measurements for September:
- 5%:
- 25%: 4.2 m3/s
- 75%: 13.0 m3/s
- 95%:
The current value of 7.3 m3/s falls into the 25-75% percentile which includes half the flows recorded from the start of measurement in September. The flow rate on the Kleine Emme was therefore average for the time of year on that date.
2. Comparison with flood statistics
The map shows the peak discharge of the past 24 hours for each gauging station, as compared with flood statistics.
HQ stands for the return period of a flood event, e.g. HQ10 is a discharge recurring once in 10 years according to the flood statistics. However, as this is a statistical value, the event may occur more frequently or more rarely.
Example: The flow rate of the Kleine Emme river measured on 22 August 2005 at the gauging station in Littau, Reussbühl, was approx. 650 m3/second. The flood statistics show that a discharge of this magnitude only occurs once in about 65 years (HQ65). Accordingly that day's value was represented on the map by an orange dot (HQ50-HQ100).
3. Current water temperature
The map provides an overview of current water temperatures in Switzerland's rivers (24-hour peak values). Due to the major seasonal fluctuations, two different legends are used for the summer and winter months.
Water temperature is measured at around 70 gauging stations in addition to discharge and water levels.
Example: The water temperature measured in the Kleine Emme river at Littau, Reussbühl, on 23 October 2007 was approx. 7°C. The gauging station was therefore represented on the map by a green dot (6-9° C).
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