Assessing rainwater harvesting potential based on required roof area per capita in subtropical urban areas.

Global water scarcity underscores the growing importance of rainwater harvesting systems (RWHS) as a sustainable supplement to conventional water supplies. This study proposes the 'required roof area per capita' as a new quantitative indicator for evaluating the long-term performance and economic feasibility of RWHS in subtropical metropolitan residential environments. Using 40 years of daily rainfall records from 12 representative meteorological stations across Taiwan, system performance was simulated through a water-balance model, and regional characteristics were further classified using K-means clustering. The results identify three distinct potential categories. (1) High-potential regions can maximize water savings by expanding storage capacity and improving substitute water yield ratios. (2) Mid-potential regions benefit from optimally scaled tank volumes - typically five to ten times the baseline demand - combined with adequate roof catchment areas to achieve cost-effective outcomes. (3) Low-potential regions should prioritize increasing roof collection area while limiting tank size to meet essential water-saving targets. By incorporating roof area as a cost component, the study emphasizes the need for region-specific RWHS design strategies that respond to local rainfall patterns and building conditions. These tailored approaches can enhance both system efficiency and overall economic viability, providing practical guidance for sustainable urban water management.