Modelling the response of fresh groundwater to climate and vegetation changes in coral islands

Writing up of the manuscript was partially supported by the Griffith Geoscience Research Award, Ireland. We are grateful to Météo France for access to the temperature and precipitation records, provided through the Climathèque agreement between Météo France and the University of Reunion Island. We are also grateful to P. Bauer-Gottwein for kindly providing the source code of the modified version of SEAWAT that was applied to carry out the phytotoxicity simulations in the last model scenario as well as our colleague R. Cassidy for assistance in code implementation. We thank the associate editor K. Hinsby as well as A. Vandenbohede and two anonymous reviewers for their valuable comments on the manuscript.International audienceIn coral islands, groundwater is a crucial freshwater resource for terrestrial life, including human water supply. Response of the freshwater lens to expected climate changes and subsequent vegetation alterations is quantified for Grande Glorieuse, a low-lying coral island in the Western Indian Ocean. Distributed models of recharge, evapotranspiration and saltwater phytotoxicity are integrated into a variable-density groundwater model to simulate the evolution of groundwater salinity. Model results are assessed against field observations including groundwater and geophys-ical measurements. Simulations show the major control currently exerted by the vegetation with regards to the lens morphology and the high sensitivity of the lens to climate alterations, impacting both quantity and salin-ity. Long-term changes in mean sea level and climatic conditions (rainfall and evapotranspiration) are predicted to be responsible for an average increase in salinity approaching 140 % (+8 kg m −3) when combined. In low-lying areas with high vegetation density, these changes top +300 % (+10 kg m −3). However, due to salinity increase and its phytotoxicity, it is shown that a corollary drop in vegetation activity can buffer the alteration of fresh groundwater. This illustrates the importance of accounting for vegetation dynamics to study groundwater in coral islands