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

Jean-Christophe Comte & Jean-Lambert Join &

Olivier Banton & Eric Nicolini

Abstract In coral islands, groundwater is a crucialfreshwater resource for terrestrial life, including humanwater supply. Response of the freshwater lens toexpected climate changes and subsequent vegetationalterations is quantified for Grande Glorieuse, a low-lying coral island in the Western Indian Ocean.Distributed models of recharge, evapotranspiration andsaltwater phytotoxicity are integrated into a variable-density groundwater model to simulate the evolution ofgroundwater salinity. Model results are assessed againstfield observations including groundwater and geophys-ical measurements. Simulations show the major controlcurrently exerted by the vegetation with regards to thelens morphology and the high sensitivity of the lens toclimate alterations, impacting both quantity and salin-ity. Long-term changes in mean sea level and climaticconditions (rainfall and evapotranspiration) are pre-dicted to be responsible for an average increase insalinity approaching 140 % (+8 kg m−3) whencombined. In low-lying areas with high vegetationdensity, 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 activitycan buffer the alteration of fresh groundwater. Thisillustrates the importance of accounting for vegetationdynamics to study groundwater in coral islands.

Keywords Small island hydrology . Freshwaterlens . Climate change . Plant transpiration . Salt-water/fresh-water relations

Introduction

Groundwater in small coral islandsThere are more than 50,000 small oceanic islands(typically defined as smaller than 2,000 km2; DiazArenas and Falkland 1991), of which over 1,300 arepopulated. About 70 % of them are located in the Indo-Pacific area, the majority being coral islands with a humidtropical climate. Due to rapid rainfall infiltration, drinking-water supply is largely restricted to either rainfall harvest-ing or groundwater abstraction (Duncan 2012); freshwaterlenses in small islands are fragile coastal hydrosystemsthat are highly vulnerable to natural or anthropogenicdisturbances (Mimura et al. 2007; White et al. 2007; Terryand Chui 2012; Robins 2013).

Coastal aquifer hydrogeology has been the focus of alarge number of research studies, including characteriza-tion of resources, modelling, vulnerability assessment andprediction (Robins 2013; Werner et al. 2013). Coralislands present a particular case of small-scale, low-lying, naturally saline, coastal aquifers. They have notreceived as much attention as other types of aquifersdespite natural specificities, putting them at particular riskof being impacted by changes (White and Falkland 2010).These specificities, which are responsible for considerablemixing between freshwater and saltwater, thus limitingfreshwater availability (White and Falkland 2010), includetheir very small size (less than 100 km2; Diaz Arenas andFalkland 1991) and the very high permeabilities ofgeological materials (coral sand and limestone havingtypical hydraulic conductivities of 10−4–10−1 m s−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 externalfactors also determine the mixing zone and resultinggroundwater 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 Chui2012), long-term changes such as rise in sea level (Bobba2002; 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 andCivil Engineering, Stranmillis Road, Belfast, BT9 5AQ,Northern Ireland, UKe-mail: j.comte@qub.ac.ukTel.: +447728 9097 5633

J.-L. Join : E. NicoliniLaboratoire GéoSciences Réunion IPGP,CNRS, UMR 7154, University of Reunion Island, Reunion Island,France

O. BantonLaboratoire d’Hydrogéologie,UMR 1114 EMMAH, University of Avignon-INRA, Avignon,France

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Gonneea et al. 2013), groundwater recharge (Jocson et al.2002; Van der Velde et al. 2006; Comte et al. 2010) andwater uptake by plants (White et al. 2007; White andFalkland 2011). The recharge can significantly vary inspace (Schneider and Kruse 2005; Comte et al. 2010) andtime (Van der Velde et al. 2006; Mollema and Antonellini2013) depending on soil/subsoil permeability, evaporationand plant uptake. A modification of these variables due toexpected climate change will result in major shifts in thedynamics of freshwater and its availability for ecosystems,including human water supply in populated islands(Mimura et al. 2007).

Climate change and coastal groundwaterIn 2007, the Intergovernmental Panel on Climate Change(IPCC) provided scenarios of climate changes for thecurrent century. Their impacts on the hydrology of smallislands in the tropical Indo-Pacific region are expected tobe significant (IPCC 2007; Christensen et al. 2007;Mimura et al. 2007; Nicholls and Cazenave 2010). Bythe end of the century, all models concur on a global sea-level rise (0.18–0.59 m) as well as an increase intemperature (+1.3 to +4.4 °C) and consequently inevaporation. Rainfall models suggest more variable trendsdepending on the region (−64 to +22 %). Global-scalemodels of groundwater recharge (Doll and Fiedler 2008)also suggest variable impacts of climate scenarios depend-ing on the region, but concur on a general decrease inrecharge of up to −30 % in the Western Indian Oceanregion (Doll 2009). Despite having been recently found torepresent 80–90 % of current global terrestrial evapotrans-piration (Jasechko et al. 2013), estimates of vegetationtranspiration from climate models remain poorlyconstrained.

Recent reviews (Holman et al. 2012; Green et al. 2011;Taylor et al. 2012) have pointed out that the impact ofclimate changes on groundwater systems has not beensufficiently quantified, but research has been accelerating.For coastal aquifers, numerous modelling studies haveinvestigated the impacts of climate or anthropogenicchanges on fresh groundwater availability (e.g. Bobba2002; Don et al. 2006; Ranjan et al. 2006; Vandenbohedeet al. 2008; Ferreira Carneiro et al. 2010; Payne 2010;Oude Essink et al. 2010; Rozell and Wong 2010;Ferguson and Gleeson 2012; Rasmussen et al. 2013;Sulzbacher et al. 2012). All studies show that: (1) sea-level rise, decrease in recharge and increase in abstractionhave negative impacts on freshwater availability (i.e.seawater intrusion), whereas (2) increase in recharge dueto climate change (Rozell and Wong 2010) or land usechange such as deforestation (Ranjan et al. 2006) havepositive impacts (freshwater replenishment).

Impact of climate change on freshwater lenseson small coral islandsRegarding coral island groundwater, quantitative studieson long-term climate impact are scarce (e.g. Bobba et al.

2000) and mainly focussed on the effect of rise in sealevel (e.g. Terry and Chui 2012; Ketabchi et al. 2013, andreferences therein). Only a few have considered realaquifer systems (Bailey et al. 2009) and there is a distinctlack of papers that have connected sea-level rise orchanges in vegetation to movement of saltwater wedgesusing observations. Moreover, few groundwater studieshave accounted for inner island depressions that common-ly occur in coral islands (Ayers and Vacher 1986;Schneider and Kruse 2003; Comte et al. 2010; Terry andChui 2012). The latter authors have modelled the impactof sea-level rise and cyclone-driven inundation for ahypothetical two-dimensional (2D) atoll island and havefound that increased sea level would result in a notablereduction of lens thickness and considerable salinizationin the central depression.

Werner et al. (2013) identify one current challenge,namely the assessment of the relative magnitude of theimpact of the various factors, and their combinations, onreal coral island systems. These factors include geologicalheterogeneity, spatial distribution of recharge and evapo-transpiration and coastal morphology. This gap in theresearch mainly originates in the lack of both field dataand numerical codes able to account for the complex andsensitive coupling of the various external forcings with thephysical structure of the island (Schneider and Kruse2003; Bailey et al. 2009).

In particular, the role of evapotranspiration, includingplant transpiration, on saline intrusion lacks referencestudies despite having been identified as potentiallycrucial (Ranjan et al. 2006; Bauer-Gottwein et al. 2008;Lubczynski 2009; Jasechko et al. 2013). From a hydro-geological point of view, evapotranspiration can bedivided into two categories: (1) direct evaporation ofwater from plant leaves and soil, and (2) planttranspiration. Plants uptake part of the water which hasinfiltrated into the soil and aquifer, depending on thedepth of the water table. These two processes arecontrolled by the spatial distribution of the vegetationas well as soil properties such as permeability andmoisture. For plants whose roots investigate groundwa-ter (phreatophytes), transpiration itself is also coupledwith water-table fluctuations (Bredehoeft et al. 1982;Lubczynski 2009). Moreover, transpiration is responsiblefor an increase in groundwater salinity through selectivefreshwater abstraction resulting in salt concentration.Recent works by Bauer-Gottwein et al. (2008) highlight-ed the major role of plant uptake on groundwaterdynamics and salinization. They also studied the toxicityeffect of saltwater on plants and found that groundwateruptake was significantly reduced with increasing salinity,which may induce a progressive death of vegetation anda subsequent deceleration of groundwater salinizationdue to increased recharge (Trapp et al. 2008; Bauer-Gottwein et al. 2008).

In this work, a full-scale variable-density groundwatermodel is coupled with a simple distributed water-balancemodel to a monitored coral island located in the WesternIndian Ocean. The water-balance model is distributed as a

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function of the tree density and includes the recharge andthe groundwater uptake by deep-rooted plants togetherwith its alterations resulting from the phytotoxic effect ofdissolved salt in groundwater. The reference model forcurrent (2009–2010) conditions is assessed against fieldobservations including both groundwater (heads andsalinity) and geophysical (subsurface resistivity) measure-ments. Independent and combined effects of both climateand vegetation changes on fresh groundwater are investi-gated. The climate changes considered are the projectionspublished in the IPCC 4th Assessment Report (IPCC2007) and include rise in sea level and alterations ofclimatic conditions (i.e. rainfall and potential evapotrans-piration with subsequent impacts on both rainfall infiltra-tion and plant groundwater uptake). Vegetation changesinclude anthropogenic land-use changes and vegetationregression due to the phytotoxic effect of dissolved salt ingroundwater.

Physical setting of Grande Glorieuse Island

The island of Grande Glorieuse (Fig. 1a) is part of theFrench Iles Eparses [Scattered Islands] archipelago locatedin the most northern part of the Mozambique Channel,Western Indian Ocean. It is a very small coral island witha rounded rectangular shape of 2.5 km×2 km (Fig. 1a, b).It forms a bench reef and its geology (Fig. 1b, c) iscomposed of a Pleistocene fractured/karstified reef lime-stone covered by Holocene sediments (BRGM 1967;Battistini and Cremers 1972; Guillaume et al. 2013). ThePleistocene reef is only found sub-outcropping on verylimited areas at two locations on the island (south-westerncape and immediately north of the airport runway; BRGM1967). Holocene sediments are characterized by variablethicknesses ranging in average from about 25 m belowground surface in the south-eastern half of the islandto more than 50 m in the north-western half. Thelower part of the Holocene unit, approximately 30 mthick, is made of poorly consolidated coral sands withfrequent beach-rock and coral limestone intercalations,as a result of Holocene glacial transgression–regressionevents. It outcrops as a relatively smooth plateau in thesouth and south-eastern part of the island, where thePleistocene reef is shallower. The upper Holocene unit,which can reach a thickness of 20 m, mainly occurs inthe north-western part of the island and is composedof fine-to-medium unconsolidated coral sands. Thenorthern and eastern parts of the island are coveredby contemporary aeolian sands, forming fixed (north)or active (east) dune systems. The highest dunesculminate at about 15 m above mean sea level (msl).A flat reef surrounds the island, which is largelyexposed at low tide and flooded at high tide. The neaptidal range is about 1 m and the spring tidal rangereaches up to 3 m. Grande Glorieuse has no permanentsurface water.

On land the vegetation is non-uniformly distributed(Fig. 1b) and consists mainly of Coco nucifera (coconut

palm), Ficus sp. (banyan), Casuarina sp. (casuarina),Tournefortia sp. (soldierbush), Scaevola sp., and Cordiasp. (Battistini and Cremers 1972; Gargominy 2003). Mostof the coconut trees are residual plantations dating back tothe late 19th century when the first colonists settled on theisland (1880–1882 according to Battistini and Cremers1972). A large-diameter well (still in existence, andlabelled Pts in Fig. 1) was used at the time to water avegetable garden located nearby. The garden, whose ruinsare still preserved today, was used to feed the colony (17workers were reported in 1921) from 1880–1882 until theplantation was abandoned in 1958 (Battistini and Cremers1972; see also Conservatoire Botanique National deMascarin 2014). About 6,000 coconut trees were reportedin 1921 and 15,000 in 1954 (Gargominy 2003). They havesince spread extensively throughout the central and north-western low-lying zones of the island (Fig. 1), where thewater table occurs at shallow depths (generally in the first5 m below ground surface).

In late June 1967, the French Geological Survey(BRGM 1967) undertook 20 shallow geological sound-ings (auger holes of depth <6 m). In the northern andcentral part of the island, when the sounding reached thewater table (mostly in low elevation areas, i.e.<5 m msl−1), they measured elevated groundwatersalinities, ranging between 13 and 17 kg m−3 of totaldissolved solids (TDS). Only the remaining well (Pts)showed a lower salinity of 4 to 5 kg m−3 TDS, and onesounding on the airport runway (southern plateau)provided a value of 7.6 kg m−3. These 20 holes no longerexist.

The climate of the Glorieuses Islands is representativeof the southern boundary of the equatorial low-pressurezone. It is typified by two seasons: (1) a cool and dryseason from May to October dominated by the east/south-east trade winds; (2) a hot and humid season fromNovember to April dominated by a monsoon regimeresponsible for high rainfall (100–210 mm per month,peaking in January). During the hot season, the island canbe affected by tropical cyclones.

Methods

Field instrumentationIn 2008–2009, the hydrogeologic and hydrologic con-ditions of Grande Glorieuse were investigated through theINTERFACE Project of the French ANR (AgenceNationale de la Recherche) research program. Six obser-vation wells, including five new hand-dug piezometers(GW) and one existing large-diameter well (Pts), weremonitored for groundwater heads and salinity (Fig. 1). Allelevation data (topography and groundwater heads) havebeen referred to the local mean sea-level value (0 m msl−1

by definition) as derived from pressure recordings (NKEprobe) on a local tidal gauge. The sensor was fixed on alevelled benchmark on the western flat reef and locatedbelow the level of the lowest tides. In addition, severalelectrical resistivity tomography (ERT) profiles were

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carried out, including a 1,500-m-long profile along theairport runway (Join et al. 2011). Daily meteorologicaldata, including rainfall and potential evaporation, werecollected from Météo France climatic database (MétéoFrance 2014) for subsequent calculation of recharge andevapotranspiration.

Groundwater modelling approachA numerical groundwater model was applied tosimulate the behaviour of the freshwater lens. Themodel was evaluated and calibrated against thegroundwater measurements carried out in 2009 andthe results of the geophysical investigations of April2008. After calibration, the model was first used tosimulate the expected past groundwater conditions(water table and salinity), taking into account

vegetation cover prior to the coconut plantation(Table 1). Indeed, the groundwater salinity of thecentral well (Pts) was much lower in the late 19thcentury when the first colony was established todevelop the coconut plantation. Because the well wasused to water a vegetable garden, it may be assumedthat the salinity (TDS) was in the range of ∼0.5–3 kg m−3 (see Grillot 1957; Pitman and Läuchli 2002).This contrasts with recent post-plantation conditions(BRGM 1967; and 2008–2009, fieldwork), in whichwater salinity measurements range between ∼4–8 kg m−3 for this well. Finally, the evolution of thefreshwater lens was simulated for four scenarios ofprojected changes in climate and plant transpiration(scenarios A–D, Table 1) using the published IPCCpredictions for the northern Mozambique Channel(scenario SRES/MMD A1B; IPCC 2007).

Fig. 1 a Geographic location and aerial photo, b physical map and c cross-section of Grande Glorieuse Island (11°34.7′ S, 47°17.9′ E)showing the main morphologic and geologic features, the vegetation cover and the location of the groundwater observation wells(elevations are in metres above the local mean sea level)

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Numerical modelSpatiotemporal resolution of groundwater heads and saltconcentrations was carried out using the three-dimensional(3D), variable-density, saturated flow groundwater codeSEAWAT ver. 4 (Langevin et al. 2007). This combines thewell-known finite-difference codes MODFLOW andMT3D to simultaneously solve Darcy’s Law (describingthe flow of fluid through porous media) and the generalequations of continuity (fluid mass conservation andsolute advection–diffusion). A coupling of flow andtransport is required to account for density effects relatedto the high salt concentrations characteristic of coastalgroundwater. The simulation of scenario D was carried outusing a previous version of SEAWAT (ver. 2; Guo andLangevin 2002) modified by Bauer-Gottwein et al. (2008)to account for a reduction in groundwater transpirationdue to the phytotoxic effect of saltwater. SEAWAT hasbeen widely applied to simulate salt transport in coastalaquifers (USGS 2013). Examples of specific application tosand/coral island aquifers are included in works bySchneider and Kruse (2003, 2005) in Florida; Mohdet al. (2010) and Praveena et al. (2012) in Malaysia;

Comte et al. (2010) in New Caledonia; Banerjee andSingh (2011) in India.

Domain, structure and hydrogeological parametersThe model has a total of 67,830 cells and includes the islandof Grande Glorieuse and its flat reef, which covers ahorizontal area of 4.2 km×3.5 km (Fig. 2). Model cells are50 m×50 m on the island’s emerged surface. Vertically, thedomain extends down to 100 m below the mean sea level,with layer thicknesses increasing from 3 m at the modelsurface to 10 m at the base. Geological mapping ofoutcropping Holocene sediments was carried out throughfield observations and photo interpretation. The 3D geome-tries of the upper/lower Holocene sediments and underlyingPleistocene limestone were obtained from joint interpretationof existing geomorphological maps (Battistini and Cremers1972), electrical resistivity tomography investigations (Joinet al. 2011), field observations, and aerial photography(Fig. 1). The aquifer’s hydrodynamic properties weredistributed according to both the geological structure (Joinet al. 2011; Guillaume et al. 2013) and typical values for

Table 1 Simulated scenarios and models used

Simulation Description Perturbation data

Recent state (reference model) Calibration on recent hydrogeologic and geophysicalobservations (2008–2009)

–

Past state Simulation of original coconut tree density prior to plantations Historic data on vegetation densitya

Scenario A Simulation of sea-level rise (SLR) IPCC predictionsb

Scenario B Simulation of change in climatic conditions (CCC) IPCC predictionsb

Scenario C Simulation of combined SLR and CCC IPCC predictionsb

Scenario D Simulation of combined SLR, CCC and transpiration reductiondue to salt phytotoxicity (TRP)

IPCC predictionsb

and phytotoxicity modelc

a From Battistini and Cremers (1972), Gargominy (2003)b From Christensen et al. (2007)c From Bauer-Gottwein et al. (2008)

Fig. 2 3D view of the model domain and the finite-difference mesh. The sea boundary condition is applied to the top face where cells arebelow the mean sea level. Recharge and water uptake are applied on emerged cells (island). All other faces are no flow boundaries. Verticalexaggeration ×10

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Holocene and Pleistocene coral sediments (Buddemeierand Oberdorfer 1988; Oberdorfer et al. 1990; Underwoodet al. 1992; Ghassemi et al. 2000; Jones and Banner 2003)and the applied values are summarized in Table 2. Notethat aquifer dispersivity values have been increasedtowards the shore to reflect the dispersive effect of thetides as found by Underwood et al. (1992). Indeed, semi-diurnal tidal fluctuations could not be taken into account inthe model because of the excessive induced increase of thesimulation time (×10). It was not necessary to considerthese high-frequency fluctuations as the objective of theresearch was to investigate the impact of much longer-termchanges (annual to multidecadal). Finally, to account forthe frequent occurrence of consolidated beds (beach rockand coral reef remnants) observed within the lowerHolocene unit, a reduced hydraulic conductance wasapplied to the seafloor on the south-eastern half of theisland.

Boundary conditions

Recharge and transpirationRecharge of the water table and plant transpiration fromgroundwater (freshwater output) have been consideredindependently (Fig. 3). Lens recharge was defined as theamount of rainfall reaching the water table, i.e. rainfallminus evaporation (from land surface, leaves and soil),minus the water uptake of vegetation within the soil andthe unsaturated zone. Runoff was excluded due to verypermeable sandy soils. The recharge was thereforeassumed to be controlled mainly by direct evaporationand the activity of shallow-rooted plants (bushes andsmall trees) taking up water above the water table. Planttranspiration from groundwater (here defined as ground-water uptake) was assumed to be controlled by the activityof deep-rooted trees below the water table (i.e. mainlyC. nucifera, but also some Ficus sp. and Casuarina sp.).Consequently, water uptake is determined by both thenature and density of vegetation and water-table depth.This model was defined to account for the typical two-layer forest observed in Grande Glorieuse (Fig. 4), whichis characterized by: (1) a forest of shrubs and young trees(mainly coconut) making up the lower layer, and (2) a tallforest dominated by mature coconut trees forming the

upper layer. Calculation of recharge and water uptake wasbased on the results of the study by Roupsard et al. (2006)in Vanuatu, South Pacific (15°26.6′ S, 167°11.5′ E),obtained in a well-maintained coconut plantation of 140trees per hectare. Vanuatu Island is characterized by ahumid tropical climate, where rainfall does not limit plantgrowth, i.e. monthly rainfall is always higher thanmonthly actual evapotranspiration. The study by Roupsardet al. (2006) provides very useful information regarding theneeds of vegetation in the absence of hydrologic stress.

On Grande Glorieuse, the model for computinggroundwater recharge R (mm) has been calculated as theresult of the water balance:

R ¼ P−ET when P > ETð Þ ð1Þ

where P (mm) is gross rainfall and ET (mm) actualevapotranspiration by shallow-rooted shrubs and youngtrees (Fig. 3a). Due to the coarse nature of sandy material,field capacity is very low and does not allow waterretention. Thus, the available water content of soil isneglected in Eq. (1). The actual evapotranspiration ETwasderived from Roupsard et al. (2006) who directlymeasured the actual stand evapotranspiration. Roupsardet al. (2006) gave results showing an excellent correlationbetween actual evapotranspiration ET and the Penman-Monteith potential evapotranspiration ET0:

ET ¼ 0:5086$ ET0 þ 0:6203 R2 ¼ 0:98! "

ð2Þ

Using Eqs. (1) and (2), the monthly recharge R wascalculated from the average monthly rainfall P (mm) andthe Penman-Monteith potential evapotranspiration ET0(mm) provided by Météo France (2014) for the period1964–1999. Recharge was then distributed and applied tothe surface of the groundwater model a function of theobserved tree density (Fig. 1).

The groundwater uptake from deep-rooted trees WUwas similarly derived from the observed potential evapo-ration ET0 based on the study of Roupsard et al. (2006),who also directly and independently measured coconuttranspiration via sapflow measurements. This studyassumes that the ratios between water uptake by coconut

Table 2 Groundwater model hydraulic parameter: Kh horizontal hydraulic conductivity; Kv vertical hydraulic conductivity; ne effectiveporosity; αL longitudinal dispersivity; αT transverse dispersivity; Cd hydraulic conductance at the seafloor

Parameter Unit Upper Holocene Lower Holocene Karstified Pleistocene Compact Pleistocene Source

Kh m s−1 1×10−3 1×10−4 2×10−3 1×10−6 Recalibrateda

Kv m s−1 8×10−4 2×10−5 2×10−3 1×10−7 Recalibrateda

ne – 0.25 0.20 0.15 0.01 Recalibratedb

αL m 3–50 3–50 10 10 CalibratedαT/αL – 0.01 0.01 0.01 0.01 CalibratedCd m2 s−1 NA 10−5 NA NA Calibrated

a From Buddemeier and Oberdorfer (1988), Ghassemi et al. (2000), Comte et al. (2010)b From Buddemeier and Oberdorfer (1988), Ghassemi et al. (2000), Jones and Banner (2003), Comte et al. (2010)

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trees WU and potential evapotranspiration ET0 are thesame in Grande Glorieuse and Vanuatu:

WUmax

ET0

# $Glorieuse

¼ WUmax

ET0

# $Vanuatu

ð3Þ

This enables distribution of the maximum water uptakeWUmax in Grande Glorieuse as a function of bothvegetation density (Fig. 1) and water-table depth andsubsequent application to the surface of the groundwatermodel. Computed WU equals WUmax when the water tableis at the surface and decreases to 0 at the root extinctiondepth. The depth-dependency of WU was computed usingthe MODFLOW evapotranspiration package. This consid-ers an extinction depth of the root zone of 3.5 m in areasdominated by smaller trees (typically Tournefortia sp.,Scaevola sp. and Cordia sp.), and 5 m in areas dominatedby larger trees (C. nucifera, Casuarina sp. and Ficus sp.),as expected in well-drained sandy soils (Ohler 1999; Chanand Elevitch 2006). Water uptake through transpiration isassumed at a salinity of 0, which leads to salt accumula-tion in groundwater. This model implies that zones with ashallow water table are highly transpiring zones, inducingboth salt accumulation in groundwater and seawaterintrusion.

The terms of the water balance for a vegetation density of100 mature trees per hectare are plotted in Fig. 3b. It is to benoted that in contrast to Vanuatu, Grande Glorieuse ischaracterized by a period of hydrologic stress with no rechargefrom May to December, whereas the maximum groundwateruptake shows low monthly variations over the year.

Sea levelThe local mean sea level msl was applied as a constanthead/constant-concentration boundary condition over theseafloor (Fig. 2). The mean sea-level value was obtainedthrough time average of the 5-min-frequency head records

Fig. 3 a Different components of the water balance model used: Ptotal precipitation, E direct evaporation at the plant and ground surfaceand in the soil, T plant transpiration from shallow roots in the soil andthe unsaturated zone, ET actual combined evapotranspiration atsurface, and in the soil and the unsaturated zone, WU water uptakefrom groundwater by deep roots (i.e. groundwater transpiration), Rdirect groundwater recharge, and Rn net (effective) groundwaterrecharge b 1964–1999 climatic data: monthly hydrologic budget,including rainfall P, actual evapotranspiration from soil and unsaturat-ed zone ET, maximum plant water uptake from groundwater WUmax(when water table is at surface) and direct groundwater recharge R (ET,R andWUmax correspond here to a reference vegetation density of 100equivalent mature coconut trees per hectare)

Fig. 4 Typical two-layered forest in the central depression ofGrande Glorieuse

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of the tidal gauge over the period 14 December 2008 to 16May 2009, and subsequently defined as the local reference0 m msl−1 for all field elevation data and model outputs.Dominant east to south-east trade winds and relatedseawater currents are considered to be responsible for anaverage sea level that is 0.2 m higher in the south-easternregion of the island than in the north-western sector(where it is applied at 0 m msl−1). Such a sea-leveldifference is common in coral islands as described byKench (1998) on a similar atoll in the Indian Ocean.Seawater salt concentration (TDS) was fixed at thestandard value of 35 kg m−3.

Climate and vegetation projectionsProjected changes in rainfall, evaporation, transpirationand sea level were derived from estimates provided bythe IPCC (Christensen et al. 2007) and used as modelinputs for climate impact scenarios. Sea level, rainfalland evaporation projections correspond to the regionalpredictions by the end of the century (2080–2099)provided by the 50 % scenario SRES/MMD A1B forthe Western Indian Ocean region (Christensen et al.2007). The sea level was raised to +0.35 m, whilerainfall P and potential evaporation ET0 were increasedto +3 % and +6 %, respectively. Projected monthlyrecharge R and groundwater uptake WU by trees wererecalculated from the changes in rainfall P andpotential evaporation ET0 using the model describedin section “Recharge and transpiration”, resulting inchanges of −4 % for R and +8 % for WU, for thereference tree density of 100 trees per hectare (Table 3).As a consequence of the +0.35 m rise in sea level, theinundation of the shore was accounted for by reducing theaveraged emerged surface of the island by about 64 ha(0.64 km2).

Model calibration and scenario simulationThe reference model was run in transient mode withthe hydrologic conditions of the year 2009 untilreaching near-steady state conditions, i.e. until changesin groundwater heads and salinities between twoconsecutive years were negligible. This near-steadystate was obtained after approximately 150 years ofsimulations from an initial state that considered waterheads as equal to mean sea level and the aquifer asfully saline (saturated with seawater at a salinity of35 kg m−3).

The reference model was then evaluated by com-paring simulation results to the hydrogeologic andgeophysical field observations carried out between2008 and 2010 (Fig. 5). On the observation wells,the calculated vs. observed water table displays acorrelation coefficient R2 of 0.71 and salinities displaya R2 of 0.85. The larger dispersion for the observedwater table in comparison to the calculated water tablemay be due to the effect of tidal fluctuations that arenot accounted for by the numerical model. The a

posteriori comparison between the recent calculatedsalinity with the salinity measurements of June 1967(BRGM 1967) suggests that since then it has contin-ued to increase in low-lying highly saline areas and todecrease slightly in fresher higher areas (Fig. 5b). Thiscan probably be attributed to the natural developmentof the forest following the abandonment of theplantation, with vegetation densification in low-lyingareas and clearing in higher areas. Changes in climaticfactors (e.g. decrease in rainfall as reported by Vincentet al. 2011) may have contributed too. Note that themodel boundary conditions are not those of 1967 butthe average for 1964–1999, meaning that a match obs./calc. is not expected for the 1967 data. In addition, thesubsurface apparent resistivities acquired through im-plementation of electrical resistivity tomography (ERT)surveys in April 2008 (Join et al. 2011) were alsocompared with the apparent resistivities calculatedfrom the groundwater model, using the forwardhydrogeophysical method of Comte and Banton(2007). Calculated vs. observed resistivities display anR2 of 0.88. This additional hydrogeophysical calibra-tion provides a complementary evaluation technique.Finally, a decisive evaluation (scenario ‘Past state’,Tables 1 and 3) was obtained by comparing thegroundwater salinity in the central well (Pts) in theabsence of coconut tree plantations, with the pastsalinity (pre-plantation) values known to be below thelimit of salinity for growing vegetables, i.e. below∼3 kg m−3.

Using this final calibrated model, three transientscenarios A, B and C (Tables 1 and 3) wereconsidered to simulate either independent or combinedeffects of the predicted changes in sea level, rechargeand water uptake, for 20 years after immediateapplication of changes. The last scenario (D) consid-ered a possible decrease in plant transpiration, whichmay result from a regression of the vegetation due tothe phytotoxic effect of saltwater (Tables 1 and 3). Thes-shape phytotoxicity model of Bauer-Gottwein et al.(2008) was used to compute the transpiration as afunction of a toxicity parameter τ:

RF ¼ 1−1

1þ τ=cð4Þ

where RF is the groundwater uptake reduction factor(comprised between 0 and 1; i.e. 0 = no reduction, and1 = full reduction = no groundwater uptake), τ is anempirical toxicity parameter in kg m−3 (Bauer-Gott-wein et al. 2008) and c is the groundwater TDSconcentration in kg m−3. Because no values have beenfound in the literature for coconut trees and other plantspecies of Grande Glorieuse, the toxicity parameter τwas set at 20 kg m−3 to reflect the decrease in coconutactivity beyond 10–15 kg m−3, as qualitatively reportedby studies such as Hassan and El-Samnoudi (1993)and Ohler (1999).

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Tab

le3

Parametersof

thedifferentclim

atic

andvegetatio

nscenariosappliedto

thereferencegrou

ndwater

mod

el(current

state),resulting

meanannu

alwater

tableandsalin

ityon

the

observationwells,andlens

volumeafter20

yearsof

simulation

Pastcond

ition

sand20

-year

projectio

nsPaststate

Scenario

AScenario

BScenario

CScenario

D

Scenario

description

Cocon

utplantatio

nsremov

ed

Sea-levelrise

Chang

ein

clim

atic

cond

ition

s

Scenario

A+B

Scenario

A+B+

phytotox

icity

App

liedaveragehy

drolog

icchanges

Sealevela

0+0

.35m

0+0

.35m

+0.35m

Rainfalla

00

+3%

+3%

+3%

Pot.evaporationa

00

+6%

+6%

+6%

Rechargeb

00

−4%

−4%

−4%

Max.plant

grou

ndwater

uptake

b−4

0%

inplaces

c0

+8%

+8%

+8%

functio

nof

water

salin

ityResultin

gwater-table

change

(m)

WellGW

1+0

.16

+0.24

−0.07

+0.20

+0.39

WellGW

2+0

.09

+0.27

−0.05

+0.25

+0.34

WellGW

3+0

.17

+0.19

−0.13

+0.12

+0.38

WellGW

4+0

.02

+0.23

−0.07

+0.20

+0.28

WellGW

5+0

.22

+0.19

−0.14

+0.11

+0.50

WellPts

+0.16

+0.22

−0.10

+0.16

+0.38

Average

+0.07

+0.19

−0.10

+0.18

+0.24

Resultin

ggrou

ndwater

salin

itychange

(kgm

−3)

WellGW

1−8

.4+4

.6+3

.2+7

.4−7

.4WellGW

2−5

.5+4

.7+2

.7+6

.9−6

.8WellGW

3−1

1.9

+10.1

+10.8

+15.7

−7.7

WellGW

4−0

.3+1

.6+1

.4+2

.3+0

.3WellGW

5−7

.4+5

.0+5

.3+7

.9−5

.3WellPts

−2.6

+6.5

+5.2

+8.3

d+1

.8Average

first10

mbelow

water

table

−1.3

+2.2

+1.5

+2.5

−0.5

Average

totallens

−0.6

+1.7

+1.3

+1.8

+0.6

Resultin

gchange

inlens

volume

Fresh-brackish

grou

ndwater

(TDS<30

kgm

−3)

+8%

−15%

−10%

−22%

+6%

Groun

dwater

availableformostsalt-tolerant

plantse

(TDS<10

kgm

−3)

+35%

−55%

−44%

−61%

+2%

Groun

dwater

availableforcommon

vegetables

e

(TDS<3kg

m−3)

+84%

−80%

−70%

−87%

−10%

Groun

dwater

availablefordrinking

f

(TDS<1kg

m−3)

+90%

−100

%(disappearance)

−100

%(disappearance)

−100

%(disappearance)

−30%

aFrom

region

alclim

atechanges(sea

level,rainfallandpo

tentialevaporation)

predictedby

theIPCC(Christensen

etal.20

07)

bFrom

applicationof

thewater

balancemod

eldescribedin

section“R

echargeandtranspiration”

(see

Eqs.1,

2and3)

cThe

transpirationup

take

incoconu

ttree

plantatio

nsandsurrou

ndingspreadingzonesisredu

cedas

aresultof

adecrease

intree

density

of40

%dThe

maxim

umsalin

itychange

atthecentralwellPtswou

ldreach+3

29%

whencomparedto

thecalculated

paststatebefore

thecoconu

tplantatio

neFrom

Grillo

t(195

7),HassanandEl-Samno

udi(199

3),Ohler

(199

9),Pitm

anandLäuchli(200

2)fLim

itof

1kg

m−3

TDSforpo

tablewater

asdefinedby

theWorld

Health

Organisation(2011)

DOI 10.1007/s10040-014-1160-yHydrogeology Journal (2014) 22: 1905–1920

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Results

Spatial impacts on the freshwater lensSimulated heads and salinity distributions generally showvery large 3D variability (Fig. 6). In recent conditions(2009), freshwater (salinity<1 kg m−3) is restricted to atotal volume of about 20,000 m3 (saturated pore watervolume) in the zones of high topography and sparsevegetation cover i.e., lower Holocene plateau and highestdunes of the upper Holocene. In this study, freshwatersalinity is defined according to the World Health Organi-sation (WHO) definition of potable water with regard tosalinity, i.e. when, based on taste considerations, TDSdoes not exceed 1 kg m−3 (World Heath Organisation2011). Both densely vegetated and low-lying zones showmajor seawater intrusions due to maximal transpirationpermitted by the shallow water table. The reconstitution ofthe past state (prior to coconut plantation) shows adecrease in salinity (−2.6 kg m−3 on the central well Ptsand −0.6 kg m−3 on average for the full lens volume,Table 3) with regression of seawater intrusion in the low-lying zones as well as development of the freshwater lensin zones of higher topography (the total volume offreshwater is almost doubled).

The scenarios of sea-level rise (scenario A; +0.35 m) andclimatic change (scenario B; +3 % in rainfall combined with+6 % of potential evaporation resulting in −4 % in rechargeand +8% in groundwater uptake) show fairly similar effects:a general increase in groundwater salinity resulting incomplete disappearance of lens volume available fordrinking (TDS<1 kg m−3) and significant amplification ofseawater intrusion in low-lying areas (+1.7 and +1.3 kg m−3

on average for scenarios A and B, respectively). Thecombination of sea-level rise with climate change (scenarioC = A + B) further increases general salinity and seawaterintrusions (+1.8 kg m−3 TDS on average). A possibleregression of vegetation due to salt toxicity (scenario D)would partially compensate for the effects of sea-level riseand climate change, resulting in a moderate increase insalinity (+0.6 kg m−3 on average) with a notable regressionof seawater intrusion in the depressive zones (between −5.3and −7.7 kg m−3 for wells GW1, GW2, GW3 and GW5).

Overall, it is shown that shallow (fresh) groundwater ismuch more reactive to changes than deeper brackishlevels (Table 3). This is illustrated by the rapid changesin volume of the shallow freshwater lens (TDS<1 kg m−3) generated by the different scenarios over the20-year simulation period, in contrast to the moderatechanges in volume for the total fresh-brackish lens (TDS<30 kg m−3).

Temporal impacts on the freshwater lensFigure 7 shows temporal evolutions of the water table andsalinity in the central well for a period of 20 years from thepresent day. All scenarios demonstrate a rapid impact onboth groundwater and salinity reaching a pseudo-steady stateafter 15–20 years. Changes (especially the rise in sea level)impact the water table more quickly than water salinitybecause of the speed of pressure transfers compared to masstransfers. The decrease in transpiration uptake due to localfelling of coconut trees (past state) induces a water-table riseof about +0.16 m in the centre of the island (+0.07 m onaverage over the island), combined with a salinity that isalmost halved (−2.6 kg m−3 for well Pts). Long-term salinityin well Pts would then reach an annual average of∼3.3 kg m−3 (∼2.7 kg m−3 during the recharge season).The impact of sea-level rise on groundwater (scenario A)

Fig. 5 Comparison of observed and simulated a groundwater heads and b salinities, complemented with c comparison between thesubsurface apparent resistivities measured during the ERT geophysical surveys of April 2008 (Join et al. 2011) and the apparent resistivitiescalculated from the groundwater model using the method of Comte and Banton (2007). The white circles are the groundwater datameasured by BRGM in 1967 (not included in model calibration)

Fig. 6 Mean annual water table and salinity distributions atgroundwater surface (top plot) and in cross-section (bottom plot):a recent conditions (2008–2010); b past state without coconut treeplantations; c–f after 20 years of different scenarios of sea level andchanges in climatic conditions and vegetation activity; c sea-levelrise (scenario A, see Tables 1 and 3); d change in climaticconditions (scenario B); e combined sea-level rise and climaticconditions (scenario C), and f combined sea-level rise, climaticchange and decrease in groundwater uptake due to phytotoxicity(scenario D). Colour range shows TDS salinity; plain black lines arewater-table isocontours; black dots are locations of observation wells(Fig. 1); X–Y dashed line on the map shows location of the cross-section; dotted line in cross-section is the iso-salinity contour 30 kgm−3 defined as the base of the lens; VOL is the pore volume of the lenswith a salinity of <30 kg m−3. Map units are metres UTM and cross-section elevations refer to the mean sea level. Lateral extension of themodel domain is cropped to focus on the island’s results

�

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appears slightly greater than the impact of change in climaticconditions (resulting in change in recharge and wateruptake—scenario B). Whilst in scenario A the water tablewould go up by approximately +0.22 m in the centre of theisland, scenario B would result in a reduction of the watertable of about −0.10 m. Salinity would increase in both cases(respectively +6.5 kg m−3 and +5.2 kg m−3). Scenario C,which combines both scenarios A and B, would logicallyresult in a rise in both the water table and salinity(respectively +0.16 m and +8.3 kg m−3). In the central well(Pts), when compared to the simulated past state prior to thecoconut plantation, the maximum long-term change ingroundwater salinity would reach +11.0 kg m−3, i.e. +329 %. A general decrease in plant transpiration due tosaltwater phytotoxicity (scenario D) would result in amaximal rise of the water table (+0.39 m) in the centralwell, but would tend to buffer the salinity increase (only +1.8 kg m−3 after 20 years). The first 2 years of simulationshow a slight decrease in salinity due to the relativeregression of seawater intrusion in the nearby depression,which is related to a sharp drop in water uptake caused byphytotoxicity. This scenario would lead to an averagewater-table rise of +0.26 m and a salinity increase of +0.6 kg m−3 across the total lens. The volumes ofgroundwater available for drinking (TDS<1 kg m−3)and for growing vegetables (TDS<3 kg m−3) woulddecrease by only −30 and −10 %, respectively.

Discussion

Modelling the response of the freshwater lens to expectedclimate and environmental changes emphasizes the highvulnerability of the lens to salinization, which is consistentwith previous studies (Bobba et al. 2000; White et al.2007; Rozell and Wong 2010; Sulzbacher et al. 2012;Rasmussen et al. 2013; Terry and Chui 2012; Bailey et al.2013). Out of the changes tested, the largest negativeimpact on groundwater quality stems from a +0.35 m risein sea level, but it is only slightly greater than projectedchanges in climatic conditions (rainfall, temperature andresultant groundwater recharge and water uptake). Vege-tation activity is also found to play a major role ingroundwater dynamics resulting in diffuse salinization ofgroundwater. Results show that lessened vegetationactivity owing to salt toxicity can potentially act as amajor spatio-temporal buffer to groundwater salinization.

This study is an illustration of freshwater’s vulnerabil-ity to changes in small coral islands. Climatic projectionsand subsequent recharge and transpiration can howevervary from one region to another and may result indifferent impacts. In the Western Indian Ocean inparticular, given the variability and uncertainty of rainfallscenarios (Christensen et al. 2007; Vincent et al. 2011),the fall in recharge could be greater than the −4 %computed in this study. The worst-case scenario estimated

Scenario C: sc. A + B (sea level rise + change in climatic conditions)

0.0

0.5

1.0

1.5

0 5 10 15 20

10

0 5 10 15 202

5

20

Years of simulation

Gro

undw

ater

salin

ity (g

L-1)

Wat

er ta

ble

elev

atio

n (m

/msl

)

Past state: coco plantations removedScenario A: sea level rise +35cmScenario B: change in climatic conditions (rainfall-temperature)

Scenario D: sc. A + B + decrease in water uptake due to salt phytotoxicity

Max. salinities formost salt-tolerantvegetables

a

b

Max. salinities forcoconut trees

Fig. 7 Temporal evolution of a the water table and b the salinity, in the central well (Pts) for 20 years of sea level, climate and vegetationchange scenarios; thin lines are the transient (monthly) fluctuations and thick lines are the respective yearly moving averages

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by Doll (2009) suggests a maximal decrease in recharge of−30 % for the region as a whole. This would result in amuch higher impact on groundwater salinity for scenariosB, C and D, and possibly higher than the impact of the risein sea level (Kundzewicz and Doll 2009).

In terms of structural uncertainties, the Pleistoceneaquifer only outcrops on small areas at two locations onthe island (BRGM 1967) and was never encountered inany of the 1967 holes nor in the current observation wells.The Pleistocene depth was extrapolated from both thesegeological data and the new ERT geophysical investiga-tions (Join et al. 2011). Therefore its actual depth and thelocation of possible karstic conduits are subject to relativeuncertainties, although geoelectrical modelling from thecalibrated groundwater model tallies well with measure-ments (Fig. 5c). Uncertainty also remains as to theposition of the mean sea level on the south-east andnorth-east of the island where the flat reef is particularlywide and no topographical or bathymetric information isavailable.

The water balance calculation used to compute therecharge has given independent consideration to evapo-transpiration in the soil/unsaturated zone and transpirationfrom the saturated zone (groundwater uptake by deep-rooted plants). This simplification was justified in thisinstance by the two-layer vegetation in Grande Glorieuse,which enabled differentiation between the mature coconut/banyan tree forest and the smaller sub-canopy forestformed mainly of bushes and young trees. A limitation topoint out, however, regards the validity of the applicationof the model of Roupsard et al. (2006) used to computethe evapotranspiration of the sub-canopy forest forvarying tree densities. The model of Roupsard et al.(2006) was established for a relatively homogeneouscoconut forest of about 140 trees per hectare and maynot be valid for significantly different tree densities.Future works would benefit from alternative approachessuch as specific partitioning between evaporation and thetranspiration of the shallow forest in the unsaturated zone.

Tidal fluctuations have not been considered in this workbecause of very high computational requirements. Asdemonstrated by Underwood et al. (1992) and others, tidalfluctuations are responsible for enhanced salt dispersion inthe aquifer, and can be avoided by introducing higherdispersivity values that reproduce the dispersion resultingfrom tidal effect. Their approach was applied in this work.More realistic simulations, however, could be obtained byincreasing computational times to model the tidal signal.

Short-term changes such as temporary island inunda-tion by seawater during cyclones have not been consid-ered. Because groundwater in low-lying areas of GrandeGlorieuse is already affected by seawater intrusion (TDSmeasured at up to 25 kg m−3), it has been assumed thattemporary seawater inundation would have a limitedimpact on the salinity of the freshwater lens, but anegative impact on vegetation activity. Terry and Chui(2012) have shown that seawater flooding in low-lyingareas can significantly and durably deteriorate lens quality.Such phenomena may have to be taken into account,

particularly on sandy islands where the freshwater lens ismore developed without central seawater intrusions.

Scenario D considers a probable decrease in vegetationactivity (transpiration by groundwater uptake) due to anincrease in groundwater salinity. The phytotoxicity modelwas based on a toxicity parameter that enables thegroundwater uptake by plants to be computed as aninverse function of the groundwater salinity (Bauer-Gottwein et al. 2008; Trapp et al. 2008). These authorshave developed a phytotoxicity model based on experi-mental results on glycophytes, i.e. plants with low salttolerance. No parameter values have been found in theliterature for halophytes such as coconut trees and otherisland species (banyan, casuarina, etc.). Therefore, thephytotoxicity parameter applied has been calibrated on thes-shaped model of Bauer-Gottwein et al. (2008) in order toreflect the high-to-moderate salt tolerance of the localhalophyte species. This tolerance results in a much highervalue, accounting for the salinity threshold qualitativelyobserved by other authors for coconut trees (Hassan andEl-Samnoudi 1993; Ohler 1999). Simulation results showthat groundwater salinity becomes excessively reduced(max. calculated TDS of 10 kg m−3) in low-lying areaswith high salinity, which in places does not tally well withthe field observations (TDS frequently measured at>20 kg m−3). It is therefore supposed that either: (1) thephytotoxic parameter applied underestimates the salttolerance of coconut trees and other island species in thisstudy or (2) as proposed by Trapp et al. (2008), differentenzymatic mechanisms opposed to glycophytes maycontrol transpiration and salt removal in halophyte roots.Moreover, with regards to scenario D, possible changes inevapotranspiration of the shallow forest (shallow rootedbushes and young trees) resulting from the regression ofthe deep-rooted mature forest have not been accounted for.This study has considered a decline of groundwater uptakeby coconut trees resulting from the decline in the growthof vegetation (not the removal of vegetation), but nosignificant decrease in the vegetation cover of coconuttrees. In case of significant decrease in mature tree cover,one may also expect changes in actual evapotranspirationin the surface/soil/unsaturated zone, i.e. an increase indirect evaporation due to increase in wind speed, surfaceenergy and water table rise especially in low-lying areas.Further research is needed on those aspects through e.g.more comprehensive modelling of the vegetation dynam-ics as well as the hydrologic processes in the unsaturatedzone.

The past state scenario that was intended to simulateconditions prior to coconut plantation clearly shows theimpact of forest management on lens salinity. In theabsence of climate change, restoring the forest to its stateprior to plantation would increase freshwater availabilitydue to a general decrease in root water uptake, especiallyin low-lying areas. Selective forest clearing could providean effective adaptative management strategy to climatechange in low-lying coral islands. For populated low-lyingislands where groundwater is extracted for domestic oragricultural purposes, the results of this study highlight the

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probable competition between vegetation uptake andborehole abstraction.

Model results have clearly shown that a time scale ofabout 20 years is enough for the shallowest parts of thelens to reach a new near-steady state after application ofchanges. The deeper levels, however, will not have doneso after two decades. The process is expected to take overa century in Grande Glorieuse, as evidenced by thesimulation time needed for the reference model to reachthe current near-steady state. This also suggests that thecurrent base of the lens may not yet be in equilibrium withthe current hydrogeological and climatic conditions.

Conclusions

The high sensitivity of freshwater lenses on small islandsregarding both climate and vegetation forcings has beenquantified. The observed increase in salinity of the freshwa-ter lens, especially in the low-lying zones, is mainlyexplained by the increase in freshwater uptake by vegetation.These findings suggest that neglecting the groundwateruptake by vegetation may result in overestimation of boththe amount and quality of freshwater resources andultimately in the implementation of unsustainable ground-water management strategies. The impacts of rising sea leveland falling recharge on groundwater are similar and theirrelative importance will depend on the magnitude of theirrespective changes. Despite its significant role, the vegeta-tion transpiration can drastically decrease when approachingthe maximum salt concentration tolerated by plants. In thiscase, the regression of transpiration can potentially bufferboth diffuse and local seawater intrusions resulting fromhydrologic changes alone, which confirms the need tocouple groundwater models with plant dynamics models tostudy groundwater resources in such islands.

Acknowledgements This research was carried out within the scope ofthe French ANR research program by the Project “INTERFACE -Vulnerability and Climate”. Writing up of the manuscript was partiallysupported by the Griffith Geoscience Research Award, Ireland. We aregrateful toMétéo France for access to the temperature and precipitationrecords, provided through the Climathèque agreement between MétéoFrance and the University of Reunion Island. We are also grateful to P.Bauer-Gottwein for kindly providing the source code of the modifiedversion of SEAWAT that was applied to carry out the phytotoxicitysimulations in the last model scenario as well as our colleague R.Cassidy for assistance in code implementation. We thank the associateeditor K. Hinsby as well as A. Vandenbohede and two anonymousreviewers for their valuable comments on the manuscript.

References

Ataie-Ashtiani B, Volker RE, Lockington DA (1999) Tidal effectson sea water intrusion in unconfined aquifers. J Hydrol 216:17–31. doi:10.1016/S0022-1694(98)00275-3

Ayers JF, Vacher HL (1986) Hydrogeology of an atoll island: aconceptual model from detailed study of a Micronesianexample. Ground Water 24:185–98. doi:10.1111/j.1745-6584.1986.tb00994.x

Bailey RT, Jenson JW, Olsen AE (2009) Numerical modeling ofatoll island hydrogeology. Ground Water 47:184–196.doi:10.1111/j.1745-6584.2008.00520.x

Bailey RT, Jenson JW, Taboroši D (2013) Estimating thefreshwater-lens thickness of atoll islands in the Federated Statesof Micronesia. Hydrogeol J 21:441–457. doi:10.1007/s10040-012-0923-6

Banerjee P, Singh VS (2011) Optimization of pumping rate andrecharge through numerical modeling with special reference tosmall coral island aquifer. Phys Chem Earth 36:1363–1372.doi:10.1016/j.pce.2011.04.014

Battistini R, Cremers G (1972) Geomorphology and vegetation ofIles Glorieuses. Atoll Res Bull 159:1–10

Bauer-Gottwein P, Rasmussen NF, Feificova D, Trapp S (2008)Phytotoxicity of salt and plant salt uptake: modeling ecohydro-logical feedback mechanisms. Water Resour Res 44, W04418.doi:10.1029/2007WR006067

Bobba AG (2002) Numerical modelling of saltwater intrusion dueto human activities and sea level change in the Godavari delta,India. Hydrol Sci J 47:S67–S80

Bobba AG, Singh VP, Berndtsson R, Bengtsson L (2000)Numerical simulation of salt-water intrusion into LaccadiveIsland aquifers due to climate change. J Geol Soc India 55:589–612

Bredehoeft JD, Papadopulos SS, Cooper HH Jr (1982) Groundwa-ter: the water-budget myth. In: Scientific basis of water resourcemanagement: studies in geophysics. National Academy Press,Washington, DC

BRGM (1967) Etude hydrogéologique de l’île Glorieuse, RapportTAN.67-A/11 [Hydrogeological study of Glorieuse Island,Report TAN.67-A/11]. Bureau de Recherches Géologiques etMinières, Madagascar

Buddemeier RW, Oberdorfer JA (1988) Hydrogeology and hydro-dynamics of coral reef pore waters. Paper presented at 6thInternational Coral Reef Symposium. James Cook University,Townsville, Australia

Carneiro JF, Boughriba M, Correia A, Zarhloule Z, Rimi A, ElHouadi B (2010) Evaluation of climate change effects in acoastal aquifer in Morocco using a density-dependent numericalmodel. Environ Earth Sci 61:241–252. doi:10.1007/s12665-009-0339-3

Chan E, Elevitch CR (2006) Cocos nucifera (coconut). In: ElevitchCR (ed) Species profiles for Pacific Island agroforestry. PAR,Holuahola, Hawai’i

Christensen JH, Hewitson B, Busuioc A (2007) Regional climateprojections. In: Solomon S, Qin D, Manning M, Chen Z, MarquisM, Averyt KB, Tignor M, Miller HL (eds) Climate change 2007:the physical science basis. Cambridge Univ. Press, Cambridge

Comte JC, Banton O (2007) Cross-validation of geo-electrical andhydrogeological models to evaluate seawater intrusion in coastalaquifers. Geophys Res Lett 34, L10402. doi:10.1029/2007GL029981

Comte JC, Banton O, Join JL, Cabioch G (2010) Evaluation ofeffective groundwater recharge of freshwater lens in smallislands by the combined modeling of geoelectrical data andwater heads. Water Resour Res 46, W06601. doi:10.1029/2009WR008058

Conservatoire Botanique National de Mascarin (2014) Flore etvégétations des îles Éparses [Flora and vegetations of theScattered Islands]. http://ileseparses.cbnm.org. Accessed 15May 2014

Diaz Arenas A, Falkland A (1991) Characteristics of small islands.In: Falkland A (ed) Hydrology and water resources of smallislands: a practical guide, UNESCO studies and reports inHydrology 49. UNESCO, Paris

Doll P (2009) Vulnerability to the impact of climate change onrenewable groundwater resources: a global-scale assessment.Environ Res Lett 4:035006. doi:10.1088/1748-9326/4/3/035006

Doll P, Fiedler K (2008) Global-scale modeling of groundwaterrecharge. Hydrol Earth Syst Sci 12:863–885. doi:10.5194/hess-12-863-2008

DOI 10.1007/s10040-014-1160-yHydrogeology Journal (2014) 22: 1905–1920

1918

Don NC, Hang NTM, Araki H, Yamanishi H, Koga K (2006)Salinization processes in an alluvial coastal lowland plain andeffect of sea water level rise. Environ Geol 49:743–751.doi:10.1007/s00254-005-0119-7

Duncan D (2012) Freshwater under threat: Pacific islands, vulner-ability assessment of freshwater resources to environmentalchange. UNEP/SPC, Bangkok

Ferguson G, Gleeson T (2012) Vulnerability of coastal aquifers togroundwater use and climate change. Nat Clim Chang 2:342–345. doi:10.1038/nclimate1413

Gargominy O (2003) Iles Eparses [Scattered Islands]. In: Gargo-miny O (ed) Biodiversité et conservation dans les collectivitésfrançaises d’Outre-Mer [Biodiversity and conservation inFrench overseas collectivities]. UICN, Paris

Ghassemi F, Alam K, Howard K (2000) Fresh-water lenses andpractical limitations of their three-dimensional simulation.Hydrogeol J 8:521–537. doi:10.1007/s100400000087

Gonneea ME, Mulligan AE, Charette MA (2013) Climate-driven sealevel anomalies modulate coastal groundwater dynamics anddischarge. Geophys Res Lett 40:2701–2706. doi:10.1002/grl.50192

Green TR, Taniguchi M, Kooi H, Gurdak JJ, Allen DM, HiscockKM, Treidel H, Aureli A (2011) Beneath the surface of globalchange: impacts of climate change on groundwater. J Hydrol405:532–560. doi:10.1016/j.jhydrol.2011.05.002

Griggs JE, Peterson FL (1993) Ground-water flow dynamics anddevelopment strategies at the atoll scale. Ground Water 31:209–220. doi:10.1111/j.1745-6584.1993.tb01813.x

Grillot G (1957) Les problèmes biologiques relatifs aux plantestolérant l’eau salée ou saumâtre, et à l’utilisation d’une telle eaupour l’irrigation [Biological problems related to plants toleratingsalt or brackish water, and the use of such water for irrigation].In: Grillot G, Hayward HE, Howe ED (eds) Utilisation des eauxsalines: compte rendu de recherche UNESCO, recherches sur lazone aride IV [Use of saline waters: UNESCO research report,researches on the arid zone IV]. UNESCO, Paris

Guillaume MMM, Reyss JL, Pirazzoli PA, Bruggemann JH (2013)Tectonic stability since the last interglacial offsets the Glori-euses Islands from the nearby Comoros archipelago. CoralReefs 32:719–726. doi:10.1007/s00338-012-1006-9

Guo W, Langevin CD (2002) User's guide to SEAWAT: a computerprogram for simulation of three-dimensional variable-densityground-water flow. US Geol Surv Tech Water Resour Invest 6-A7

Hassan MM, El-Samnoudi IM (1993) Effect of soil salinity on yieldand leaf mineral content of date palm trees. Egypt J Hortic20:315–322

Holman IP, Allen DM, Cuthbert MO, Goderniaux P (2012) Towardsbest practice for assessing the impacts of climate change ongroundwater. Hydrogeol J 20:1–4. doi:10.1007/s10040-011-0805-3

IPCC (2007) Causes of changes. In: Core Writing Team, PachauriRK, Reisinger A (eds) Climate change 2007: synthesis report.IPCC, Geneva

Jasechko S, Sharp ZD, Gibson JJ, Birks SJ, Yi Y, Fawcett PJ (2013)Terrestrial water fluxes dominated by transpiration. Nature496:347–350. doi:10.1038/nature11983

Jocson JMU, Jenson JW, Contractor DN (2002) Recharge andaquifer response: northern Guam lens aquifer, Guam, MarianaIslands. J Hydrol 260:231–254. doi:10.1016/S0022-1694(01)00617-5

Join JL, Banton O, Comte JC, Leze J, Massin F, Nicolini E (2011)Assessing spatio-temporal patterns of groundwater salinity insmall coral islands in the Western Indian Ocean. West IndianOcean J Mar Sci 10:1–12

Jones IC, Banner JL (2003) Hydrogeologic and climatic influenceson spatial and interannual variation of recharge to a tropicalkarst island aquifer. Water Resour Res 39:1253. doi:10.1029/2002WR001543

Kench PS (1998) Physical processes in an Indian Ocean atoll. CoralReefs 17:155–168. doi:10.1007/s003380050110

Ketabchi H, Mahmoodzadeh D, Ataie-Ashtiani B, Werner AD,Simmons CT (2013) Sea-level rise impact on a fresh

groundwater lens in two-layer small islands. Hydrol Process.doi:10.1002/hyp.10059

Kundzewicz ZW, Doll P (2009) Will groundwater ease freshwaterstress under climate change? Hydrol Sci J 54:665–675.doi:10.1623/hysj.54.4.665

Langevin CD, Thorne DT Jr, Dausman AM, Sukop MC, Guo W(2007) SEAWAT version 4: a computer program for simulationof multi-species solute and heat transport. US GeologicalSurvey Techniques and Methods, book, 6; chapter A22, USGS,Reston, VA

Lubczynski MW (2009) The hydrogeological role of trees in water-limited environments. Hydrogeol J 17:247–259. doi:10.1007/s10040-008-0357-3

Météo France (2014) Données publiques: données climatologiques[Public data: climatic data]. https://donneespubliques.meteo-france.fr/. Accessed 15 May 2014

Mimura N, Nurse L, McLean RF, Agard J, Briguglio L, Lefale P,Payet R, Sem G (2007) Small islands. In: Parry ML, CanzianiOF, Palutikof JP, van der Linden PJ, Hanson CE (eds) Climatechange 2007: impacts, adaptation and vulnerability. Contribu-tion of Working Group II to the Fourth Assessment Report ofthe Intergovernmental Panel on Climate Change. CambridgeUniversity Press, Cambridge

Mohd HA, Sarva MR, Aris AZ (2010) A numerical modelling ofseawater intrusion into an oceanic island aquifer, SipadanIsland, Malaysia. Sains Malaysiana 39:525–532

Mollema PN, Antonellini M (2013) Seasonal variation in naturalrecharge of coastal aquifers. Hydrogeol J 21:787–797.doi:10.1007/s10040-013-0960-9

Nicholls RJ, Cazenave A (2010) Sea-level rise and its impact oncoastal zones. Science 328:1517–1520. doi:10.1126/science.1185782

Oberdorfer JA, Hogan PJ, Buddemeier RW (1990) Atoll islandhydrogeology: flow and freshwater occurrence in a tidallydominated system. J Hydrol 120:327–340. doi:10.1016/0022-1694(90)90157-S

Ohler JG (1999) The coconut palm. In: Ohler JG (ed) Moderncoconut management: palm cultivation and products, chap 2.FAO, Rome

Oude Essink GHP, van Baaren ES, de Louw PGB (2010) Effects ofclimate change on coastal groundwater systems: a modelingstudy in the Netherlands. Water Resour Res 46, W00F04.doi:10.1029/2009WR008719

Payne DF (2010) Effects of climate change on saltwater intrusion atHilton Head Island, SC. U.S.A. Proceedings of SWIM21 - 21stSalt Water Intrusion Meeting, 21-26 June 2010, Azores,Portugal

Pitman MG, Läuchli A (2002) Global impact of salinity andagricultural ecosystems. In: Läuchli A, Lüttge U (eds) Salinity:environment – plants – molecules. Kluwer, Dordrecht, TheNetherlands

Praveena SM, Abdullah MH, Bindin K, Aris AZ (2012) Modelingof water balance components in a small island via a numericalmodel application. J Coast Res 28:202–209. doi:10.2112/JCOASTRES-D-10-00057.1

Ranjan SP, Kazama S, Sawamoto M (2006) Effects of climate andland use changes on groundwater resources in coastal aquifers. JEnviron Manag 80:25–35. doi:10.1016/j.jenvman.2005.08.008

Rasmussen P, Sonnenborg TO, Goncear G, Hinsby K (2013)Assessing impacts of climate change, sea level rise, anddrainage canals on saltwater intrusion to coastal aquifer. HydrolEarth Syst Sci 17:421–443. doi:10.5194/hess-17-421-2013

Robins NS (2013) A review of small island hydrogeology: progress(and setbacks) during the recent past. Q J Eng Geol Hydrogeol46:157–165. doi:10.1144/qjegh2012-063

Robinson C, Li L, Barry DA (2007) Effect of tidal forcing on asubterranean estuary. Adv Water Resour 30:851–865.doi:10.1016/j.advwatres.2006.07.006

Roupsard O et al (2006) Partitioning energy and evapo-transpiration above and below a tropical palm canopy.Agr ic For Meteoro l 139:252–268 . do i :10 .1016/j.agrformet.2006.07.006

DOI 10.1007/s10040-014-1160-yHydrogeology Journal (2014) 22: 1905–1920

1919

Rozell DJ, Wong T (2010) Effects of climate change on groundwaterresources at Shelter Island, New York State, USA. Hydrogeol J18:1657–1665. doi:10.1007/s10040-010-0615-z

Schneider JC, Kruse SE (2003) A comparison of controls onfreshwater lens morphology of small carbonate and siliciclasticislands: examples from barrier islands in Florida, USA. J Hydrol284:253–69. doi:10.1016/j.jhydrol.2003.08.002

Schneider JA, Kruse SE (2005) Assessing natural and anthropo-genic impacts on freshwater lens morphology on small barrierislands: Dog Island and St. George Island, Florida. Hydrogeol J14:131–145. doi:10.1007/s10040-005-0442-9

Sulzbacher H, Wiederhold H, Siemon B, Grinat M, Igel J,Burschil T, Günther T, Hinsby K (2012) Numericalmodelling of climate change impacts on freshwater lenseson the North Sea Island of Borkum using hydrological andgeophysical methods. Hydrol Earth Syst Sci 16:3621–3643.doi:10.5194/hess-16-3621-2012

Taylor R et al (2012) Ground water and climate change. Nat ClimChang 3:322–329. doi:10.1038/nclimate1744

Terry JP, Chui TFM (2012) Evaluating the fate of freshwater lenseson atoll islands after eustatic sea-level rise and cyclone-driveninundation: a modelling approach. Glob Planet Chang 88–89:76–84. doi:10.1016/j.gloplacha.2012.03.008

Trapp S, Feificova D, Rasmussen NF, Bauer-Gottwein P (2008)Plant uptake of NaCl in relation to enzyme kinetics and toxice f f ec t s . Env i ron Exp Bot 64 :1–7 . do i :10 .1016 /j.envexpbot.2008.05.001

Underwood MR, Peterson FL, Voss CI (1992) Groundwater lensdynamics of atoll islands. Water Resour Res 28:2889–2902.doi:10.1029/92WR01723

USGS (2013) Published reports and articles using the SEAWATcode. http://water.usgs.gov/ogw/seawat/pubs.html. Accessed 15May 2014

Van der Velde M, Javaux M, Vanclooster M, Clothier BE(2006) El Niño-Southern Oscillation determines the salinityof the freshwater lens under a coral atoll in the PacificOcean. Geophys Res Lett 33, L21403. doi:10.1029/2006GL027748

Vandenbohede A, Luyten K, Lebbe L (2008) Effects of globalchange on heterogeneous coastal aquifers: a case study inBelgium. J Coastal Res 24:160–170. doi:10.2112/05-0447.1

Vincent LA et al (2011) Observed trends in indices of daily andextreme temperature and precipitation for the countries of thewestern Indian Ocean, 1961–2008. J Geophys Res 116,D10108. doi:10.1029/2010JD015303

WernerAD,BakkerM, PostVAE,VandenbohedeA, LuC,Ataie-AshtianiB, Simmons CT, Barry DA (2013) Seawater intrusion processes,investigation andmanagement: recent advances and future challenges.Adv Water Resour 51:3–26. doi:10.1016/j.advwatres.2012.03.004

White I, Falkland T (2010) Management of freshwater lenses onsmall Pacific islands. Hydrogeol J 18:227–246. doi:10.1007/s10040-009-0525-0

White I, Falkland T (2011) Reducing groundwater vulnerability inCarbonate Island countries in the Pacific. In: Treidel H, Martin-Bordes JL, Gurdak JJ (eds) Climate change effects ongroundwater resources: a global synthesis of findings andrecommendations, IAH International Contributions to Hydro-geology 27. Taylor and Francis, London

White I, Falkland T, Metutera T, Metai E, Overmars M, Perez P,Dray A (2007) Climatic and human influences on groundwaterin low atolls. Vadose Zone J 6:581–590. doi:10.2136/vzj2006.0092

World Health Organisation (ed) (2011) Guidelines for drinking-water quality, 4th edn. WHO, Geneva, p 564. http://www.who.int/water_sanitation_health/publications/2011/dwq_guidelines/en/

DOI 10.1007/s10040-014-1160-yHydrogeology Journal (2014) 22: 1905–1920

1920