Availability of water resources is a limiting factor for economic and social development in small island states. Many such countries rely almost entirely on a single source of supply, such as groundwater (Barbados, Antigua, The Bahamas, Kiribati), rainwater (Tuvalu, northern atolls of Cook Islands, and the Maldives), surface reservoirs and imports (Singapore), or rivers and other surface flows (Seychelles, Dominica).

Water supply is most vulnerable in the atoll states of the Pacific, where water supply is sensitive to precipitation patterns and changes in storm tracks (Salinger et al., 1995). Captured rainwater is stored in cisterns; the only backup reserve for these islands is a thin wedge of fresh groundwater that sits on top of the saltwater lens. A reduction in precipitation coupled with sea-level rise would not only cause a diminution of the volume of potable water but would reduce the size of the narrow freshwater lens (Amadore et al., 1996).

The situation is equally critical in the low limestone islands of the eastern Caribbean, where seasonality of rainfall (a marked dry and wet season regime) is pronounced. On islands such as Anguilla, Antigua and Barbuda, Grenada, and Barbados, more than 65% of total annual rainfall may be recorded in the wet season, which spans the 6-month period of June to December. Moreover, most of the rainfall is strongly associated with the genesis and passage of easterly waves, tropical depressions, and storms (Gray, 1993; Nurse et al., 1998). Thus, changes in the occurrence of these heavy rainfall events will certainly impact the water supply of many Caribbean islands. The situation is further exacerbated in Barbados, where recent research has shown that groundwater recharge is restricted to the three wettest months of the year, and only 15-30 % of annual rainfall reaches the aquifer (Jones et al., 1998).

Reduced availability of adequate water supply in a changing climate also poses a potential threat to the Mediterranean islands of Cyprus and Malta. Because these two countries already experience water shortages (Nicholls and Hoozemans, 1996), and given a projected decrease in mean summer precipitation over the Mediterranean Sea region (IPCC, 1998), the water resources of these states could be placed under considerable pressure in the future (Tables 17-2 and 17-3). The threat is equally strong in some parts of the eastern Caribbean. Within the past few decades in Dominica, for instance, an apparent tendency toward more extended periods of drought is well correlated with reduced flows in the Castle Comfort, Roseau, Layou, and Geneva Rivers (Government of Commonwealth of Dominica, 2000). Because rivers are the main source of potable and irrigation water on the island and are also harnessed for power generation, declining flows have become a matter of serious national concern.

Climate change can present additional water management and related challenges. Such challenges may arise from a variety of sources, including increased flood risks and impeded drainage and the presence of elevated water tables—which may pose special engineering problems. It is projected that on Andros island, the Bahamas, where the water table presently is only 30 cm below the surface, high evaporation rates and increasing brackishness will eventuate with continued sea-level rise (Martin and Bruce, 1999). Similar projections also have been made for Cuba, where underground water supplies already are stressed (Planos and Barros, 1999). For many small island states, the prospect of salinity intrusion into the freshwater lens would be a matter of great concern. In many of these islands where salinization from overpumping of aquifers is already occurring (e.g., The Bahamas and Barbados), sea-level rise would compound the risk. In some cases, higher salinity would be experienced not only in coastal aquifers but also inland at freshwater pumping plants as the salty groundwater rises. Singh (1997a,b) has reported a recent increase in salinity levels for several coastal aquifers in Trinidad and Tobago in the southern Caribbean, attributable mainly to rapid drawdown exacerbated by sea-level rise.

There now is substantial evidence to support the view that precipitation variability in various parts of the world is linked to ENSO events. During the 1982-1983 ENSO event, rainfall in many parts of the western Pacific was a mere 10-30% of the long-term mean average (Falkland, 1992). More specifically, it is well established that ENSO has a strong influence on rainfall patterns in the tropics and low-latitude regions of the Southern Hemisphere (Shea, 1994; Whetton et al., 1996). In the Caribbean islands, droughts appear to be more frequent in El Niño years, whereas conditions tend to be wetter in La Niña years. The devastating drought in the region in 1998 coincided with what is believed to be the strongest El Niño signal on record. ENSO-related droughts also are known to occur in the low-lying atolls of the tropical Pacific. Thus, in countries such as the Federated States of Micronesia and the Marshall Islands where rainwater is the main source of supply, more frequent and intense ENSO events will impose further stress on already meager water resources (Meehl and Washington, 1996); other islands in the central and eastern tropical Pacific will experience heavy rains (Jones et al., 1999).

Realistically, the options available to many small islands for reducing the adverse effects of climate change on water availability are limited. This implies that greater urgency and emphasis will have to be placed on improving water resource management efforts, including inventorying of resources and rational and equitable allocation. Implementation of more efficient rainwater harvesting methods, efficient leak detection and repair, use of water-saving devices, and aggressive recycling efforts are strategies worth considering. Desalination also is becoming an increasingly attractive option, especially where the necessary technical and financial capacity is available and in cases in which more traditional strategies are inadequate or not feasible (e.g., in Singapore, Malta, Cyprus, Barbados, Antigua and Barbuda, St. Kitts and Nevis, and Grenada). As part of their long-term adaptation strategy, these water-scarce islands might wish to include "no-regrets" measures, which will promote sustainability with or without climate change. Some countries may even wish to consider application of market-based systems to allocate water supplies, which could result in less wasteful practices under current conditions and thus enable water users to more efficiently adapt to climate change (Amadore et al., 1996).

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