["Natural Resources Forum, Volume 50, Issue 1, Page 634-658, February 2026. ", "\nAbstract\nThe global water scarcity crisis demands innovative solutions, and rainwater harvesting systems (RWHS) offer a promising alternative. Focused on Taiwan's abundant rainfall, this study explores RWHS efficiency amidst changing precipitation patterns due to global climate change. Analyzing data from 1981 to 2020 across 20 stations, it scrutinizes variations in precipitation quantities, frequencies, seasonality indices, and dry period durations. Through rigorous analysis and simulation, it assesses RWHS operation efficiency metrics like potable water substitution and reliability. Utilizing principal component analysis, the study develops precipitation variation indicators to forecast RWHS efficiency, revealing significant variations in seasonality index and precipitation frequency. Despite declining long‐term efficiency, the study finds relatively modest variations in water‐savings percentage, suggesting avenues for mitigation. With a notable 78.04% explained variance, the study's credibility is reinforced, supported by high coefficients of determination (R2) for predicted water‐savings percentage (0.91) and reliability (0.90). Additionally, the study highlights factors impacting RWHS efficiency, advocating tailored approaches considering local precipitation patterns, building types, and population density. By optimizing RWHS design to diverse settings, effective water resource management and sustainability goals can be achieved. This research serves as a valuable guide in navigating RWHS implementation amidst a changing climate landscape.\n"]