Low-flow statistics

The FOEN analyses the data on low water flows statistically; the findings are made available in the form of result sheets.

In many areas, water resources are now exploited to the full, e.g. as a source of drinking water, for industrial purposes, or for hydropower generation. At the same time, there is a widespread desire to ensure that water is used in an ecologically sound manner. In addition, low flows have become a research topic even in Switzerland - a country known as a "water tower" - in view of the threat of climate change. The introduction of the Water 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 the 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 flow monitoring stations.
In future it will be possible to download more detailed station reports which will include an analysis of 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 so-called low-flow years. As a rule these comprise the periods 1^st June - 31^st May for alpine stations and 1^st April - 31^st 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 Generalized Extreme Value Distribution (GEV).

The front of the result sheet shows on the one hand the annual NM7Q series as a bar chart and on the other 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, 20, 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