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    -Claire Floury \ Rhone-Mediterranean- Corsica water agency
    "An estimate of the annual minimum monthly flow rate with a five-year return period (QMNA5) will be very useful for the status reports on water bodies that each Water agency must produce in 2013..."

Consensus on a new national map of low-flow rates

Increasing attention is paid to low-flow situations in public policies and on the part of research organisations. In the framework of its agreement with Onema, Irstea (formerly Cemagref) recently presented a national map showing reference low-flow rates. The map will provide useful information for efforts to improve the management of water resources in France.

Almost four years of research and work were required to produce the new national map based on models showing the interannual mean flow rate (called QA) and the annual minimum monthly flow rate with a five-year return period (QMNA5). The latter, which corresponds to the minimum monthly flow rate occurring on average once every five years, is the reference value used by the water police. It serves in making decisions on how to process water abstraction or release files (e.g. whether or not an authorisation is required) depending on the sensitivity of the local environment and in setting quality goals for rivers.

By providing these two characteristic rates, the map will assist in ensuring balanced, overall management of water resources, e.g. by indicating the capacity of a river to dilute pollutants. Knowledge on low-flow rates throughout the country is also an essential piece of information in understanding how river basins operate. In addition, the map produced by Irstea and Onema will be an interesting contribution to the 2013 status report for the Water framework directive (WFD).

26 years of data for more accurate results

To produce the map and ensure an accurate calculation of the low-flow statistics, considerable work was put into selecting the hydrometric stations. The total set of reference stations comprises approximately 630 units providing daily data points spanning at least 26 years over the 1970 to 2008 period. The stations are considered to be only slightly influenced by metrological or anthropogenic factors. The map is the combined result of three models developed by Irstea research teams in Lyon, Aix-en-Provence and Antony. The best estimate for each station is used for the map, thus enhancing its accuracy. A data table also provides users with a robustness index (fragile, caution, robust) and a range of uncertainty for the data.

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September 2012