Sea level rise and inundation of island interiors: assessing impacts of lake formation and evaporation on water resources in arid climates

Water resources of low-lying islands, which are threatened by sea-level rise, consist of freshwater lenses that float buoyantly on underlying saline water. Density contrasts between fresh and salt water cause the base of freshwater lenses to extend below sea level by a factor of 40 times the water table elevation above sea level. This 40:1 ratio means small decreases in water table elevation translate into substantial losses of fresh water. Potential losses of water resources from decreased water table elevations were previously evaluated for coastal inundation (Fig 1a) but not from flooding of island interiors and subsequent evaporation (Fig 1b). Here, we show that as sea level rise elevates water tables into low lying interior depressions to form lakes (Fig 1b), evaporation in arid climates, coupled with the dynamics of freshwater lenses, can cause the loss of water resources to be more than twice the magnitude that results from an identical area of coastal inundation. Our newly developed SEAWAT models demonstrate that water loss increases as a function of (in order of decreasing impact) lake width relative to island width, the deficit between rainfall onto and evaporation from the lake, and lake distance from the coast relative to the island center. The amplification of water loss results from lakes acting as large-diameter groundwater wells that discharge fresh water to the atmosphere through evaporation, lower water tables in the center of the lens, and reorganize groundwater flow paths toward the lakes (Fig 1b). Lake formation and evaporation also segment lenses, with multiple smaller lenses having less cumulative resistance to flow, contributing to lower water table elevations and thinner lenses. These results imply that low-lying island nations in arid climates, such as the Bahamian Archipelago, face greater threats to their water resources from sea-level rise than previously considered and that fresh water may be lost from islands even where sediment accretion allows island size to keep pace with rising sea level.