Comparative Performance of Green Roof Systems with Smart Cisterns: Balancing Stormwater Capture and Irrigation Supply

Using five years of field monitoring data, this study compares two types of roof systems that combine green roofs, cisterns, and real-time control (RTC) strategies: one optimized to reduce stormwater runoff (a fully vegetated roof with cisterns operating under a “smart detention” [SD] logic that fully empties within 24 h), and one designed to balance architectural, economic, and structural tradeoffs (a half vegetated, half bare roof with cisterns operating under a “rainwater harvesting” [RWH] logic that partially drains in anticipation of rainfall while maintaining a reserve for green roof irrigation). Both configurations demonstrated strong stormwater performance, with cisterns improving roof retention by 10.2 to 13.0% over five years. For small to medium storms (under 25 mm), representing 71.2% of events, both strategies prevented more than 95% of runoff, while forecast accuracy primarily influenced larger events. Even with modest cistern sizing, the SD system captured 96.7% and the RWH system 95.8% of runoff from small to medium storms, approaching 100% assuming perfect weather forecasts. Irrigation analysis showed that RWH cisterns supplied ~51% of irrigation demand, increasing to ~70% under perfect forecasts. This study is among the first to compare stormwater and irrigation outcomes from side-by-side RTC-managed roof systems over multiple years. The results underscore that the mixed green/bare roof with RWH logic provides nearly equivalent stormwater benefits while offering added value through irrigation supply, reduced structural loading, and design flexibility.