Dry-season freshwater scarcity is a recurrent hazard on small tropical islands, where seasonal rainfall, limited storage and tourism-driven demand coincide. We assess and reduce this risk for Si Chang Island, Thailand, with an integrated framework that resolves inter-month storage carry-over across coupled sources. Connected catchment is estimated from a land-use layer and satellite imagery, separating rooftop harvesting to household cisterns from watershed runoff to the reservoir–pond–bank system, while tourism demand enters through an equivalent-resident formulation. Supply reliability is quantified with reliability–resilience–vulnerability metrics under a downscaled CMIP5 ensemble, from which RCP4.5 is selected by bias screening. A mixed-integer optimisation, extended to a two-stage robust formulation, sizes capacity against stochastic dry years. Rooftop harvesting is the dominant source, and the present system is reliable but limited by small cistern capacity; under projected demand a structural annual deficit renders the island supply- rather than storage-limited, so additional storage alone cannot secure supply and harvestable catchment must expand. Critically, a design optimised on mean rainfall delivers markedly lower reliability under realistic variability, and drought-robust sizing requires substantially more catchment. The framework yields transferable, drought-robust design targets for small islands.