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Water resources of small coral islands facing climate change and disasters

IWRA World Water Congress 2008 Montpellier France
3. Climate Change and Disasters
Author(s): Olivier Banton
Jean-Christophe Comte
Lionel Bigot
Antony Finizola
Jean-Paul Ambrosi
Pascale Chabanet
Hiroya Yamano
Yves Travi
1.Université de la Réunion, France 2.Institut de Recherche pour le Développement, New Caledonia 3.Université d'Avignon, France 4.UMR 161 CEREGE CNRS,Aix en Provence, France 5. National Institute for Environmental Studies, Japan

Keyword(s): groundwater, coral islands, climate change, modelling

AbstractLow lying coral islands are complex and vulnerable ecosystems that can be strongly affected by natural or human changes. The Tuvalu’s experience has shown that the global warming impact is a big issue for these islands that are very close to the sea level. However cause-effect relations remain difficult to highlight. In many places, the anthropic influences make the analysis more complex. The purpose of the project “INTERFACE” granted by the French research agency (ANR) is to predict the vulnerability of coral reef island ecosystems to climate change. To achieve this goal, the spatial and temporal variability of potential indicators (such as hydrogeological, sedimentary, biological and microbiological parameters) are studied with regard to climatic and oceanic variations. Two small islands without human influence were instrumented with observation wells for groundwater monitoring and with automatic measuring stations for climate and oceanic level monitoring. These islands are located in the same oceanic province (Indo-Pacific) but face different climatic regimes. One is located in the Glorieuses archipelago in the Mozambic Canal and the other in the New Caledonia lagoon. This communication focuses on the groundwater as the most important condition to maintain the terrestrial ecosystem. The freshwater lens is lying at the earth-sea and atmosphere interface. It may reflect any evolution of one of these boundary conditions. Hence monitoring of the groundwater lens may be a valuable indicator of the ecosystem state and evolution. For this, observation wells were drilled to monitor the water head, the temperature and the quality of groundwater. In parallel, geophysical investigations were carried out for the characterization of the geological structure and the freshwater/seawater distribution. By the way of numerical modelling, we study the response of freshwater lens to water recharge or to sea level variations related to periodic fluctuations (tides, seasons) and sudden or extreme events (storm, cyclone or drought). Variable-density flow modelling is used to compare the impact of different expected scenarios of climatic and oceanic evolutions. Results of this project will conduct in guidelines for future assessment of small island groundwater which constitutes an important societal challenge for low lying areas facing climate change. Associated with other environmental indicators, groundwater monitoring can be a valuable tool for the assessment of islands ecosystems vulnerability.
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