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    -Rémi Oudin \ Environmental agency for the Centre region and the Loire-Brittany basin
    "Considering the two main goals of the observatory, we expect great things from it. Field observations will provide data series spanning several years, which is much more useful than a single data point on the rivers of a department at time T..."

    -Jean-Claude Dumas \ Onema Vienne office
    "Because it is subject to frequent dry periods, the Vienne department was one of the nine test departments for ONDE in 2011. The ROCA and RDOE networks had been extensively developed in the department, with 168 measurement stationsater agency must produce in 2013..."

A new observatory for summer low-flow levels in rivers

Onema has set up a new National low-flow observatory (ONDE) to monitor summer low-flow levels in small rivers. It replaces ROCA (monitoring network for crisis-level river flows). The new observatory will establish a knowledge network addressing this hydrological phenomenon and assist in crisis management.

Following the drought in 2003, ROCA was set up in continental France in 2004 by the former High council on fisheries to monitor extreme low-flow levels in rivers and provide prefects with more information for crisis management. At the same time, certain regions established departmental networks (RDOE) to monitor flow rates and trends in low-flow levels. Unfortunately, after a few years of operation, it became clear that great differences existed in how the networks were organised in the various departments and that the resulting data were very difficult to consolidate on the regional and national levels. To standardise operational techniques and improve hydrological monitoring in the field, ONDE, the National low-flow observatory, was set up in continental France. Its goal is to become a stable knowledge network on summer low-flow levels and to assist in crisis management.

A minimum of 30 monitoring stations in each department

To reach those goals, an important initial step was to correctly size the network. The observatory had to “fit” the hydrological context of the department and fill out the existing monitoring systems. This phase consisted essentially of deciding where to locate the stations, depending on a number of main and secondary criteria that had been modified since the creation of ROCA and the RDOEs. A majority of the long-standing stations were maintained for ONDE, however a certain number were dismantled or moved to other sites. In general, they are located at the head of river basins to gather information on hydrological phenomena not yet covered by existing monitoring systems. Each department must have at least 30 stations, however, many more are required in areas regularly confronted with crisis situations.

To assist in sizing the ONDE observatory, Onema used the results of a series of tests carried out in 2011 by its personnel in nine departments, namely Côte-d’Or, Drôme, Gard, Gers, Marne, Oise, Puy-de-Dôme, Vendée and Vienne. The tests served to collect additional data used to highlight the improvements required in selecting monitoring stations. The full-scale tests also contributed to establishing better guidelines for implementation of the monitoring systems in the field.

A two-level monitoring system

In the field, the water level of rivers and streams is determined visually in three categories (visible, not visible and dry). The system is also organised according to two levels with different observation frequencies and periods. In order to establish a knowledge network based on stable data series, standard monitoring is carried out from May to September. On the 25th day of each month, the network gathers data on the hydrological situation in each department.

During a crisis, however, the second level of monitoring may be activated by the departmental prefect or Onema personnel. Crisis monitoring may take place any time of the year and more frequently than standard monitoring. The increase in the number of observations in the field provides more detailed data used to determine any necessary local action and measures. The data produced by the new observatory will be presented in the Bulletin on the hydrological situation (BSH) according to a standard, national format that is currently being developed.

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