Modelling the response of fresh groundwater to climate and vegetation changes in coral islands Jean-Christophe Comte & Jean-Lambert Join & Olivier Banton & Eric Nicolini Abstract In 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 pre- dicted 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. Keywords Small island hydrology . Freshwater lens . Climate change . Plant transpiration . Salt-water/ fresh-water relations Introduction Groundwater in small coral islands There are more than 50,000 small oceanic islands (typically defined as smaller than 2,000 km 2 ; Diaz Arenas and Falkland 1991), of which over 1,300 are populated. About 70 % of them are located in the Indo- Pacific area, the majority being coral islands with a humid tropical climate. Due to rapid rainfall infiltration, drinking- water supply is largely restricted to either rainfall harvest- ing or groundwater abstraction (Duncan 2012); freshwater lenses in small islands are fragile coastal hydrosystems that are highly vulnerable to natural or anthropogenic disturbances (Mimura et al. 2007; White et al. 2007; Terry and Chui 2012; Robins 2013). Coastal aquifer hydrogeology has been the focus of a large number of research studies, including characteriza- tion of resources, modelling, vulnerability assessment and prediction (Robins 2013; Werner et al. 2013). Coral islands present a particular case of small-scale, low- lying, naturally saline, coastal aquifers. They have not received as much attention as other types of aquifers despite natural specificities, putting them at particular risk of being impacted by changes (White and Falkland 2010). These specificities, which are responsible for considerable mixing between freshwater and saltwater, thus limiting freshwater availability (White and Falkland 2010), include their very small size (less than 100 km 2 ; Diaz Arenas and Falkland 1991) and the very high permeabilities of geological materials (coral sand and limestone having typical hydraulic conductivities of 10 -4 –10 -1 ms -1 ; Bud- demeier and Oberdorfer 1988; Oberdorfer et al. 1990; Underwood et al. 1992; Griggs and Peterson 1993; Ghassemi et al. 2000; Bailey et al. 2009). Other external factors also determine the mixing zone and resulting groundwater availability. These include short-term varia- tions in sea level such as tides (Ataie-Ashtiani et al. 1999; Robinson et al. 2007) and extreme events (Terry and Chui 2012), long-term changes such as rise in sea level (Bobba 2002; White and Falkland 2010; Terry and Chui 2012; Received: 12 March 2014 / Accepted: 8 June 2014 * Springer-Verlag Berlin Heidelberg 2014 J.-C. Comte ()) Queen’ s University Belfast, School of Planning, Architecture and Civil Engineering, Stranmillis Road, Belfast, BT9 5AQ, Northern Ireland, UK e-mail: j.comte@qub.ac.uk Tel.: +447728 9097 5633 J.-L. Join : E. Nicolini Laboratoire GéoSciences Réunion IPGP, CNRS, UMR 7154, University of Reunion Island, Reunion Island, France O. Banton Laboratoire d’Hydrogéologie, UMR 1114 EMMAH, University of Avignon-INRA, Avignon, France DOI 10.1007/s10040-014-1160-y Hydrogeology Journal (2014) 22: 1905–1920 Published online: 20 July 2014