Blue-green roofs (BGRs), compared to traditional green roofs, can provide enhanced water storage capacity for stormwater management and drought mitigation. However, the potential of utilizing stored water for external purposes, such as toilet flushing and lawn irrigation, remains underexplored. This study developed a daily time-step hydrological model based on the water balance equation to simulate the runoff reduction ratio, water supply reliability, and irrigation time fraction of BGRs with water supply functions. Using summer rainfall and evapotranspiration (ET) data (1980-2013) from six climatically different cities based on K & ouml;ppen climate zone classification in the United States (Atlanta, Flagstaff, Billings) and China (Guangzhou, Jinan, Lanzhou), the study revealed that: (a) BGRs with external water supply significantly improve runoff control and water supply benefits but increase irrigation demands, particularly in arid regions; (b) increasing storage layer capacity enhances BGRs' performance, but benefits diminish beyond 50 mm; (c) adaptive water supply strategies based on climate variations can improve both flood control benefit and irrigation reliability for BGRs; and (d) water deficits can be avoided in high-rainfall or low-ET regions. These findings provide valuable insights for optimizing BGR design and management in diverse climatic conditions.