ObsErA observatory

ObsErA observatory

Scientific context

The Observatory of water and erosion in the Caribbean (ObsErA) is located in Guadeloupe, on the island of Basse-Terre into the tropical rainforest of the Basse Terre National Park. The island is mainly composed of andesitic rocks particularly reactive and vulnerable to chemical alteration and erosion, under a humid tropical climate,with a cyclonic season. ObsErA is composed of two contrasted watersheds in term of age of the geological formation, rainfall and relief. Capesterre watershed (16,4 km2) in the South of the island is established on relatively recent terrain (600 000 to 400 000 years), with poorly developed soils and steep slopes (average altitude of 1140 m). The average annual rainfall is 5200 mm/year. The Bras-David river drains a  11,9 km2 watershed in its older terrain (1 million years), with gentler relief and with very thick ferralitic soils. The average annual rainfall is 3300 mm/year. The two rivers are torrential and characterized by flows and exports of dissolved and solid matter among the highest in the world.  ObsErA studies a hot spot of alteration and erosion on Earth.

Summary

Starting year: 2005

Localisations: Guadeloupe

Keywords: erosion, tropical forest, tropical climate, volcanic lands, geological cycles

Database: http://webobsera.ipgp.fr/

Web site: http://www.ipgp.fr/en/obsera/obsera-observatory-of-water-and-erosion-in-the-caribbean

PIs : Éric Lajeunesse and Céline Dessert

Station hydrologique du site de la Digue, sur la rivière de Capesterre.
Hydrological station of the site of La Digue, on Capesterre river. © ObsEra

Scientific questions

The goal of the ObsERA observatory is to study and quantify mechanical and chemical erosion processes, their coupling and their influence on geological and carbon cycle as well as on the environment (soil development, river chemistry…) in the context of tropical volcanic island. The role of extreme hydrological events associated with cyclones and tropical waves, and their influence on erosion rates and relief morphology is also a main question of the observatory. 

To achieve its objectives, ObsErA developed new instruments (drone imagery) and new methods (in particular new isotopic tracers) for the study of sediment transport and the characterization of ecosystems.ObsErA collects long-term data to understand the establishment of soils and the development of the morphology of watersheds in a tropical climate, the residence time of materials in rivers and watersheds, the coupling between chemical and physical erosion processes (strongly active in the Caribbeans), and finally the role played by the primary tropical forest in sedimentary carbon sequestration.

Sites and measured variables

Both Capesterre and Bras David sites are instrumented with gauging and automatic sampling stations. The Quiock ravine is a sub-basin of Bras-David (0,78 km2), essentially on a thick lateritic area, highly instrumented. Variables measured are: rainfall and atmospheric depositions, flow, concentration of suspended matter, chemical composition of the river — pH, conductivity, main elements in solution, particulate and dissolved organic carbon —, chemical composition of soil solutions in the saprolite, as well as the morpho-sedimentary evolution of the riverbed (drone imagery and terrestrial Lidar).

Partners and further information

ObsErA is a Service national d’observation (SNO) from INSU-CNRS with the participation from Institut de Physique du Globe de Paris (IPGP) and the OSU of Lyon. It collaborates with the parc National de Guadeloupe and the DREAL. It is supported by the Observatoire volcanologique de la Soufrière de Guadeloupe (OHSG), from Université Antilles-Guyane, institut Universitaire européen de la Mer (UBO) and the Laboratoire de Sciences de la Terre de l’université Claude Bernard (Lyon I). It has active collaborations in particular with the observatory of the Critical Zone of Puerto Rico (Luquillo Forest Experimental Catchment).

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