Low-flow statistics

The FOEN analyses the data on low water flows statistically and the findings are made available to the public.

Publication of results
Contents of result sheets
Contents of the station reports

In some areas, water resources are now exploited to the full, e.g. as a source of drinking water, for industrial purposes, or for generating hydropower. Yet there is also a widespread desire to ensure that water is used in an ecologically sound manner. In addition, the threat of climate change has also meant that low flows have become a research topic even in Switzerland – a country known as a 'water tower'. The introduction of the Waters Protection Act of 1991, in particular, highlighted the importance of the measurement and analysis of low flows in quantitative water protection.

A wide variety of parameters may be used to describe low-flow events. The FOEN Hydrology Division's low-flow statistics are based on the recommendations of the German Association for Water, Wastewater and Waste (DWA, formerly DVWK, 1982/1992). They include the annual values for the lowest mean discharge over a given number of days x (NMxQ). The parameter NMxQ is of interest in relation to planned withdrawals or releases.

In 2015 the Hydrology Division began working with a new method set which was originally developed for flood statistics. The calculations are similar to those for flood events though the annual maximums are replaced by the low-flow index NMxQ multiplied by (-1).

Hochwasserstatistik am BAFU – Diskussion eines neuen Methodensets (PDF, 791 kB, 07.04.2014)Artikel in "Wasser Energie Luft" 2/2013

Publication of results

Publication of the results is in two stages:

Result sheets which can be used for initial rough estimates of low-flow probabilities are published for all the current discharge measurement stations.
Detailed station reports can be downloaded for selected stations, to be used as a basis for detailed investigations into the topic of low flows. As well as the analyses of low water flows, they also include other low-flow parameters such as duration and deficit volume.

Contents of result sheets

The validated NM7Q are used to generate the result sheets. To ensure that, wherever possible, no low-flow event persists beyond the year end, causing the same event to be included twice in the statistics for two successive years, the system operates with what are known as low-flow years. As a rule, these comprise the periods from 1 June to 31 May for alpine stations and from 1 April to 31 March for non-alpine stations. This classification is generally best suited to Switzerland because low flows rarely occur in the spring.

The return levels (flow for specified return periods or years) are calculated using the generalised extreme value (GEV) distribution.

The front of the result sheet shows the annual NM7Q series as a bar chart and the low-flow probabilities with the estimated distribution function and the measured data as a return level plot. Two tables also list the estimated return levels for the return periods T = 2, 10, 30, 100 and 300 and the five lowest NM7Q with the estimated return period. The back of the sheet contains a range of further information.

Hydrological data and forecasts: The low-flow statistics result sheets can be found for the individual stations.

Hydrological data - FOEN

Contents of the station reports

The detailed station reports are used to recognise non-stationarities in the data and to gain an overview of the calculated low flows, durations and deficit volumes for specified return periods. Two different approaches are also used for the low flows, in order to assess the differences between the approaches.

As well as a purely statistical evaluation of the low-flow data, the station reports also contain rough analyses of the history of the station and influencing factors in the drainage area. This should enable improved classification of the statistical results.

The results of these statistical analyses are only the starting point when answering specific questions. Knowledge of the drainage area's hydrological processes is also important.

Leitfaden Stationsberichte - Niedrigwasserstatistik des BAFU (PDF, 3 MB, 17.05.2024)