OHMCV (Cévennes-Vivarais Mediterranean Hydrometeorological Observatory, http://www.ohmcv.fr/), labeled as “Environment Research Observatory” in 2002 and “Observation National Service” by INSU since 2006, is led by the Institute for Geosciences and Environmental research (IGE) of Grenoble in collaboration with several laboratories of the CNRS and the Universities of Avignon, Clermont-Ferrand, Grenoble, Montpellier, Nice, and Toulouse as well as with Mines Alès, IFSTTAR Nantes, IRSTEA Lyon and Météo-France.
It brings together the skills of researchers, engineers and technicians from various disciplines (meteorology, hydrology, geophysics, geography, applied mathematics and sociology) to address scientific issues related to extreme hydrometeorological phenomena (in particular intense rainfall and flash floods) affecting the Mediterranean areas in a context of global change.
Due to the scarcity of these events for a given time and place, complementary observation strategies are implemented to monitor the relevant hydrological and social variables:
- a detailed and lasting observation on the Cevennes and the Vivarais (mesoscale site of 32000 km²) of rainfall, soil moisture, water levels and river discharges, sediment monitoring as well as a characterization of the physical properties of the basins by geophysical and geochemical techniques; this physical observation is completed by a social observation of residential trajectories in flood zones, warning processes, daily mobility and the exposure of motorists to flash floods;
- the realization of post-event campaigns on extreme events occurring in all the Mediterranean regions of Western Europe making it possible to collect, by means of physical surveys and interviews with the population shortly after major event, information on the highest water levels, the peak discharges, the flood dynamics, the perception and the knowledge of risk and the behaviours adopted by the population.
The detailed observation strategy in the Cevennes-Vivarais aims to document the hydro-meteorological phenomena from the scale of the plot to those of the watersheds of Vivarais (Cance, Doux, Eyrieux), Ardèche, Cèze, Gardons and of Vidourle. Two of these basins, Ardèche and Gardons, are monitored by the OHMCV on small basins representative of Cevennes and Vivarais in terms of geology and land use. For Ardèche (2388 km²), these are the nested basins of Gazel (3.4 km²), Claduègne (44 km²), and Auzon (116 km²) which have basaltic soils at higher elevations (max 1000 m) and marl-limestone at lower altitudes (min 121 m), land use by scrubland, forests and pastures at higher altitudes, by agricultural and wine-growing activities at lower altitudes. These sites are monitored by IGE Grenoble and IRSTEA Lyon. For Gardons basin (2062 km²), three sub-basins are monitored. Two are located in the Cevennes forest: Tourgueille basin (14.5 km², shale) and Valescure basin (3.9 km², granite). The third, Avène (60 km²), is a partly urbanized basin near Alès with a marked karstic component. These three basins are monitored by UMR ESPACE, Mines Alès and HSM. In addition, since 2015, the OHMCV has integrated the basins of Mont-Lozère, 5 basins of 0.2 to 14.5 km² between 1150 and 1500 m altitude, monitored since the 80s by UMR ESPACE and which have a rather granitic geology and schistous with land use by forests and pastures.
Monitoring of small watersheds by research observation devices is completed at the largest scale by those carried out by operational services (Météo-France, Grand Delta Flood Forecasting Department and EDF).
All of these observations make it possible to conduct studies concerning:
- understanding the hydrological functioning of watersheds subject to intense rainfall in the Mediterranean region;
- improving the hydrometeorological forecasting chain and reducing the societal impact of extreme events;
- the occurrence of these events and their impacts; their evolution in a context of climate change and increasing anthropic pressure.
The “in situ” observations of the OHMCV are also used to calibrate and validate observations from remote sensing. For example, OHMCV produces rainfall reanalysis, fusion of radar and rainfall observations, and is involved in NASA / JAXA’s Global Precipitation Measurement observation project.
OHMCV shares common terrains with the “Cévenols Rivers” site of the “Rhone Basin Long Term Environment Research” which is interested in the basins of the Ardèche, the Cèze and the Gardons on complementary scientific issues concerning the low water levels and the quality of the environment.
OHM-CV is strongly involved in the MISTRALS (Mediterranean Integrated Studies Regional and Local Scales) meta-program and in particular in HyMeX, one of its projects. This international project is dedicated to the study of the water cycle in the Mediterranean, with a particular interest for the evolution of the climatic variability and for the genesis and the predictability of the intense events.