Water, Water Everywhere, nor Any Drop to Drink? Options for Improving the Resilience of a Subtropical Island to Drought and the Sensitivity of Their Yield under a Projected Drier Future Climate

Norfolk Island, like many islands in the subtropical Pacific, is at increasing risk of drought. To test the hypothesis that desalination is the most cost-effective technology for providing potable water during 'water emergencies', this study calculated and compared the levelised cost on Norfolk Island of seven 'centralised' intervention options and two 'diffuse' options for increasing groundwater accessions. Gully dams were the 'centralised' intervention option found to have the lowest levelised cost but had the largest percentage reductions in yield (i.e., 39% for 1 ML storage) under a projected drier future climate, greater than twice the percentage reduction in rainfall (16%) but less than the percentage reduction in runoff (44%). Cluster-scale roof-harvested rainwater systems with 'turkey nest' earth embankment had the second lowest levelised cost but are probably socially unacceptable due to siting on the community's premier sporting oval. Desalination had the third lowest levelised cost, followed by the use of existing deep groundwater bores to pump and store water exclusively for use during a water emergency. Although desalination was not the most cost-effective technology, it is likely to be more socially acceptable than the two more cost-effective options. After gully dams, rainwater harvesting intervention options had the second highest proportional reductions in yield under a projected drier future climate (similar to 10%); however, the reductions were less than the percentage reductions in rainfall. The yield from desalination and groundwater were unaffected under a drier future climate projected for similar to 2060.