Assessment of Household Rainwater Harvesting Reliability and Limitations on Very Small Indonesian Islands During Wet and Dry Years

Very small islands face freshwater scarcity due to limited catchment areas, rainfall variability, and saline intrusion. In Indonesia, rainwater harvesting (RWH) can reduce household water stress, but its reliability as an independent source remains uncertain. This study used field surveys and a daily water-balance model to assess household RWH systems on the Belakang Padang and Mecan Islands. Representative wet (2023) and dry (2019) years were selected from a 30-year rainfall record to evaluate storage performance, rainwater supply reliability, and water saving efficiency (WSE). Simulations considered 200, 500, and 1000 L tanks under household-demand scenarios of 60–300 L/day. During the wet year, rainfall frequently refilled storage tanks and improved water availability, although overflow occurred when harvested water exceeded storage capacity. Larger tanks reduced overflow losses and extended water availability after rainfall events. In contrast, dry-year storage performance declined because rainfall was limited and concentrated within a short period. Even 1000 L tanks could not maintain sufficient storage under high demand during prolonged rainless periods. Rainwater supply reliability and water saving efficiency were higher during the wet year, ranging from 31 to 99.2% and 0.231 to 1.029 on Belakang Padang and 25.2 to 98.6% and 0.211 to 1.024 on Mecan, while dry-year reliability declined markedly as demand increased. Belakang Padang showed higher reliability than Mecan because its larger roof catchment area harvested more rainwater from the same rainfall event. Overall, household RWH is unlikely to provide a dependable independent domestic water supply on very small islands but can serve as an important supplementary source during disruptions to centralized systems. Reintroducing and maintaining household RWH, together with improvements in tank capacity, effective roof catchment area, and conveyance systems, could strengthen household water security on the Belakang Padang and Mecan Islands.