Toward scalable green roofs: A critical review of hydrological design, modelling, monitoring, and future directions

This review addresses the gap between substantial research on extensive green roofs (GRs) and their limited large-scale implementation for urban stormwater management. Despite demonstrated benefits, widespread adoption remains rare due to fragmented knowledge across disciplines and practical challenges in monitoring, modelling, and performance evaluation. To support scalable deployment, the review synthesizes insights across four core areas. (I) Key design parameters are reviewed, including substrate depth, vegetation type, roof configuration, and the use of supplemental water storage modules, all of which influence retention capacity and peak flow reduction. (II) Existing hydrological models are evaluated for their ability to predict runoff across various spatial and temporal scales, highlighting opportunities for integrating process-based and data-driven approaches. (III) Monitoring strategies are examined, with an emphasis on the growing role of scalable technologies, such as remote sensing (RS) and spectral imaging, in enabling cost-effective, citywide diagnostics. (IV) Emerging directions are explored, including integrated blue green solar roofs (BGSRs) that combine vegetation, photovoltaics, and controlled drainage, alongside the development of RS-based hydrological indices for continuous performance assessment and flood risk reduction at the urban scale.