Impact of Climate Change on Water Resources
The following impact map shows how the projected changes may impact the supply and quality of water resources and demand on the Gilbert Islands in the manner described in Fig. 17.3.
Increase Water Supply
With the projected increase in mean annual rainfall and the decline in drought events, both surface water and groundwater will be frequently replenished. This will in turn provide a positive impact on domestic water availability through both
Fig. 17.3 Climate change impacts on water resources
rainwater tanks and groundwater abstraction. If average rainfall increased by 7%, the groundwater recharge is estimated to increase by 5.5% (in contrast, a 10% decline in rainfall may cause a 14% reduction in groundwater recharge) (World Bank 2000).
Reduced Water Supply
Climate change is expected to reduce water supplies in the Pacific to the point where supplies may be insufficient to meet demand during low rainfall periods (IPCC 2007). Under a warming climate it is likely that evapotranspiration would increase, but its effects on groundwater recharge would be much milder than the effect of changes in rainfall. The relatively large projected increases in rainfall that are projected for the islands near to the equator, exceed the smaller changes in potential evapotranspiration. A theoretical 10% increase in annual evapotranspira- tion could result in as much as a 6% decline in groundwater recharge (World Bank 2000). Higher projected temperatures will result in increased evaporation of surface water, but since freshwater lakes and dams are not a feature of water resource management in these islands, this should not be a concern.
A rise in sea level of 0.4 m (the worst-case scenario in 2090) would have little effect on the groundwater supply and could possibly even raise its volume, as the groundwater table (the top of the freshwater lens) would tend to rise while its base remained relatively unaffected. However, if the width of the islands were reduced by inundation and erosion, which is likely, the thickness of the groundwater could decline by as much as 29% (World Bank 2000, p. 24).
Reduced Water Quality
The effects of extreme events on the water supply could be significant. Currently, high sea levels during El Nino years can lead to seawater contamination of freshwater lenses. However, recovery is generally rapid due to the accompanying high rainfall. The higher overtopping and inundation that may occur in the future, however, could considerably increase the risks of saline contamination (World Bank 2000).
Increased Water Demand
The increase in average and extreme temperatures will likely increase the demand for water for drinking and cooling, although this is not considered to be significant. The increased availability of water due to the increased average annual rainfall, could result in an increase in the demand for water due to it’s perceived abundance, with competing demands over potable water for domestic uses versus agricultural initiatives.