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

Abstract

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

Quantifying threats to groundwater resources in the Republic of Maldives Part I: Future rainfall patterns and sea‐level rise

“…Relatively few studies have been conducted for small coral islands. These studies (Bailey, Khalil, & Chatikavanij, ; Chui & Terry, ; Comte, Join, Banton, & Nicolini, ; Ketabchi et al, ; Morgan & Werner, ; Terry & Chui, ) are summarized in Table . Studies for other oceanic islands (Holding & Allen, ; Mahmoodzadeh et al, ; Sulzbacher et al, ) also are included to provide a broader range of methodologies.…”

“…Comte et al () performed a numerical modelling study for Grande Glorieuse Island, a small (5 km 2 ) coral island in the Western Indian Ocean with a maximum elevation of 15 m above mean sea level. They constructed a three‐dimensional groundwater model of the island subsurface using the SEAWAT modelling code (Langevin, Thorne, Dausman, Sukop, & Guo, ) and imposed one future climate scenario (SLR: 0.35‐cm increase; rainfall: +3% annual average from current) to assess impact on groundwater resources.…”

Simulations of freshwater lens recharge and salt/freshwater interfaces using the HYDRUS and SWI2 packages for MODFLOW

“…ET 0 was estimated using the method of Grabarczyk, developed in Poland (Grabarczyk and Żarski, 1992), which gives evapotranspiration values somewhat lower The total evapotranspiration flux includes both evaporation from the soil surface and transpiration of water taken up by plant roots from deeper soil layers. Several studies (e.g., Comte at al., 2014;Kamps et al, 2008) reported that water uptake by vegetation is an important factor influencing groundwater resources of freshwater lenses. In order to account for this, we performed two series of simulations.…”

“…In particular, in view of possible climate change, increased water demands and sea level rise, the development of freshwater lenses supplied with infiltrated rainwater on top of saline aquifers is a subject of intense research (e.g., Werner et al, 2013). A large number of contributions have focused on freshwater lenses in deltaic regions, as well as on islands, peninsulas, or sandbars located in tropical, arid, and temperate climatic zones (e.g., Comte et al, 2014;Contractor and Jenson, 2000;Chang et al, 2016;De Louw et al, 2011;Eeman et al, 2017;Holding and Allen, 2015;Houben and Post, 2016;Illangasekare et al, 2006;Jocson et al, 2002;Mahmoodzadeh et al, 2014;Sadurski et al, 1987;Sinclair et al, 2016;Stuyfzand, 2016;Sulzbacher et al, 2012).…”

“…Thus, recharge is inherently variable in space and time and this variability may have a significant influence on freshwater-saltwater dynamics in shallow coastal aquifers (Mollema and Antonelli, 2013;Trglavcnik et al, 2016;Vandenbohede et al, 2014). Other studies highlighted the importance of such factors as water uptake by tree roots (Comte et al, 2014) or hydraulic properties and water storage in the vadose zone (Contractor and Jenson, 2000;Jocson et al, 2002). There is thus a need to include seasonal fluctuations of recharge in groundwater models for coastal aquifers (Eeman et al, 2012;Mollema and Antonellini, 2013;Oude Essink et al, 2010) and to integrate vadose zone processes more fully into groundwater models.…”