Island Water and Power Sizing: A Rainwater-Catchment and Diesel-Hybrid Reliability Model for a Small Island Community

Small island communities commonly size rainwater catchment, storage tanks and diesel-powered pumping by rules of thumb that treat catchment area and storage volume as interchangeable. This paper shows that they are not. A coupled daily water-balance and pumping-energy model for a community of 800 people demanding 64.0 m3/day under 1,200 mm/year seasonal rainfall shows that mean-year balance alone requires 22,902 m2 of catchment; below that area no tank reaches high reliability. The cheapest design meeting 95% monthly reliability is 30,000 m2 with 4,000 m3 of storage (62 days), at USD 1,582 per household per year. Pumping and treatment energy in the reference case is 0.150 kWh/m3 (USD 0.085/m3 in diesel); static lift dominates and transfer distance is negligible. The supported rule is conditional: secure about 1.3 times the mean-balance catchment area first, then about 60 days of storage for 95% reliability and about 100 days for 99%, retaining a priced shipped-water backup for multi-year drought. SIMULATED, NOT MEASURED.