The term “Q347” was introduced in the Federal Water Protection Act (WPA) in 1992 in order to quantify the concept of “low water flow”. Article 4 of the WPA defines Q347 as “the flow rate which, averaged over ten years, is reached or exceeded on an average of 347 days per year and which is not substantially affected by damming, withdrawal or supply of water”.
This flow rate is mainly used to define the minimum flow that has to be guaranteed in a river at any time of the year if hydraulic projects are to be undertaken. In practical terms, it means that after construction of any hydraulic structure that could have an impact on the river flow (for example a dam or a water catchment), a residual flow should always be guaranteed. The minimum threshold for the residual flow is determined proportionally to the value of Q347, as explained in chapter 2 of the WPA. The maintenance of an adequate residual flow is necessary in order to preserve the health of watercourses, since a stream with a low flow is particularly vulnerable to disturbances. It will be easily affected by high temperatures, which cause a decrease in the dissolved oxygen present in the water. The contamination of a stream with a low flow rate is also more damaging because the presence of the pollutant would be in much higher concentration.
How to calculate Q347 ?
The WPA does not explicitly explain how to calculate the Q347 for a given watercourse, but a publication by the Federal Office for the Environment (FOEN, 2000) gives some indications: The calculation of the Q347 has to be based on average daily flows. The daily flow variability is not taken into account. The simplest way to determine the Q347 is to produce the flow duration curve (FDC) for a period of ten years and extract the flow rate that is reached or exceeded 95% of the time (equivalent to an average of 347 days per year). Practically, you have to sort the average daily flows in descending order, assign a rank to each and calculate the annual frequency for each value as follows:
fr = 365 · r/N
with: fr the annual frequency (in number of days), r the rank and N the number of daily mean discharge data.
Finally, the discharge value corresponding to the Q347 is the mean daily flow rate with an annual frequency fr of 347.
However, some caution must be taken. It is important to have a large period of data to ensure a statistically significant Q347. A trend in the data, for example due to climate change, can affect the calculation. In these cases, the FOEN advises to only use the ten most recent years of data for the calculation.
When no sufficient data is available for the calculation of Q347, there are some methods to approximate it. For example it can be extrapolated with correlated data that covers a larger stretch of time. However, the results of these estimation methods should be considered with precaution. Low water flow rates are extremely variable in both space and time, and the behaviour of one basin cannot usually be applied to another basin.
For more detailed information on this problematic and about the Q347, we highly encourage you to take a look to the two following publications of the FOEN:
– « Débits résiduels convenables – Comment peuvent-ils être déterminés? », instructions, 2000, Office fédéral de l’environnement OFEV
– « Le débit d’étiage Q347, état de la question », 1999, Office fédéral de l’environnement OFEV
– OFEFP (2000). « Débits résiduels convenables – Comment peuvent-ils être déterminés? ». Instructions de l l’Office fédéral de l’environnement, des forêts et du paysage (OFEFP), Berne, VU-2701-F.
– Service hydrologique et géologique national (1999). « Le débit d’étiage Q347, état de la question »,Office fédéral de l’environnement, Berne, LHG-27-D.