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    -René Régnault \ Creseb
    "L’The economy in Brittany is based essentially on two sectors, agriculture and tourism. Our challenge is to reconcile the two, given that water quantity and quality are a major issue..."

An overall approach to setting minimum biological discharges

What minimum discharge downstream of dams is required for the feeding, movement and reproduction of aquatic species? A comprehensive, multicriteria approach has been set up in a river basin in Brittany with the assistance of the Ecohydraulic centre in Toulouse. This type of approach is essential to ensure balanced management of water resources taking the preservation of aquatic environments into account.

The concept of a biological minimum discharge, which first appeared in the 2006 Water law, remains difficult to define. The regulations set minimum thresholds, but do not necessarily correspond to ecological processes based on biological interaction or to variations in levels required at different times in the year. Some methods, e.g. the habitat models proposed in the microhabitat method, may be of help in setting the thresholds. However, if applied individually and not in a comprehensive approach, these tools may not fit the bill, as was the case in the Ellé river basin in Brittany. That is why the Ellé-Isole-Laïta board requested the opinion of the Water scientific and resource centre in Brittany (CRESEB) and the Onema-IMFT-Irstea Ecohydraulic centre.

A comprehensive, multi-criteria approach

The University of Rennes, Agrocampus and the Onema regional office were also brought into the project. A comprehensive approach was adopted, based on successive steps to collect and analyse the information likely to facilitate management decisions capable of reconciling economic activities and the preservation of biological quality in the rivers, notably during low-flow periods. The purpose of this approach was to provide analysis tools simultaneously addressing water resources, their uses, identification of imbalances and the issues involved for aquatic environments and species. Microhabitat methods constitute the last step in this approach. For the Ellé river basin, the procedure was filled out with an analysis of additional data collected by the Onema regional office via the monitoring networks. This data served to link the data series on fish and variations in the discharge. This study and multi-criteria approach produced an initial set of recommendations. The proposed values were roughly 10% of the interannual mean discharge for the biological minimum discharge, 12% of targeted low-flow discharges and 8% of discharge levels during crisis situations. This comprehensive approach with clearly defined steps to produce a biological minimum discharge may subsequently be used on other rivers in Brittany and France, notably in determining the permissible abstraction volumes.

Contact : philippe.baran@onema.fr