The site of the Hydrogeochemical Observatory of the Environment (OHGE) is the Strengbach watershed (80ha), located in the commune of Aubure (60 km southwest of Strasbourg). It is in a mid-mountain area (altitude between 880 and 1150 m ) in the Vosges massif with steep slopes (10° to 30°). More than 80% of the vegetation cover is made up of exploited forests (80% spruce and 20% beech), on a Hercynian acidic granite substratum. The trees have shown signs of forest dieback since the 1980s. The soils range from acid brown soils mainly on the north-facing slope to podzolic ochre soils.
The climate is temperate oceanic with an average annual temperature of 6°C and an average pluviometry of 1400 mm/year (including 1 to 3 months of snow). The average water level at the outlet is 800 mm/year. Interannual variations are important with a factor of two between dry and wet years. Four springs for drinking water are collected on this site.
Major floods can occur at the end of winter when the snowpack melts or because of summer thunderstorms The low-water period is between late summer and mid-autumn.
Starting year: 1985
Localisation: Vosges, commune of Aubure (60 km southwest of Strasbourg).
Keywords: medium mountain, granite, temperate climate, exploited forest, acid rain, climate change.
Web site: http://ohge.unistra.fr/
The observatory was created in 1985 to study the link between acid rains and forest dieback. It has become a true natural laboratory whose scientific orientations focus on the issues of water and soil conservation and the sustainability of forests. It is a question of:
- To study and better évaluate the dynamics of water and chemical elements at the interface atmosphere/water/soil/plants;
- Understand the responses of this particular environment to local and global natural and anthropogenic perturbations (climate change, air pollution, logging, game growth);
- To model coupled bio-hydrogeochemical functioning at the scale of the watershed to predict/anticipate future changes in ecosystems.
Sites and measured variables
The different compartments (atmosphere, surface and ground water, soil, vegetation) are intensively instrumented and monitored over time. Deep drilling has allowed the exploration of up to 120 m of rock. The acquired variables are:
- Meteorological (rain, temperature, humidity, pressure, wind, radiation …) thanks to 2 complete weather stations. The spatialization of the rains is studied via a network of 7 rain gauges;
- Hydrological (flows at several points of the main stream and on different sources and creeks) and hydrogeological (piezometry) thanks to gauging stations, and level probes in a network of 6 boreholes, 12 piezometers including 4 in the wet zone at the bottom of the watershed;
- Geochemical (pH, conductivity, alkalinity, cations and major anions, silica, dissolved organic carbon, trace elements) in rain, rainfall, soil solutions, springs, groundwater, streams thanks to manual or automatic sampling (400 to 500 samples/year).
Two new equipments, a “RiverLab” (220 k€) and a superconducting gravimeter (350 k€) acquired within the framework of the equipment of excellence “CRITEX” – 2012-2022, (Parc national d’équipements innovants pour l’étude spatiale et temporelle de la Zone Critique des bassins versants), were installed on the site in June 2017 to allow a high frequency measurement of hydrogeochemical variables and the fine variation of gravity.
Partners and further information
The monitoring of the watershed is carried out by ITES, Institut Terre et Environnement de Strasbourg (UMR 7063 CNRS/University of Strasbourg – UNISTRA) part of the École et Observatoire des Sciences de la Terre (EOST). The OHGE has been labeled since 1997 by the EOST and since 2007 by the CNRS as a National Observatory Service (SNO). The site is distinguished by its close collaboration with the commune of Aubure, the inhabitants, the local community and the ONF. Numerous multidisciplinary research projects (geochemistry, hydrology, geophysics, ecology, geology, sociology, art, etc.) are carried out giving rise to collaborations with European and international laboratories and networks.