The benefits of green roofs for urban water management and thermal regulation: Insights from Catania (Sicily, Italy)

This study provides a comprehensive field-based assessment of the hydrological and thermal performance of three semi-intensive green roof (GR) technologies under Mediterranean climate conditions in Catania (Sicily, Italy). The experimental system, covering approximately 900 m2, was monitored over a two-year period (2024–2025) to evaluate stormwater runoff quantity and quality, substrate moisture dynamics, irrigation requirements, and thermal behavior, in comparison with a conventional roof. Results show that all GR technologies significantly reduced stormwater runoff volumes and delayed peak flows, with peak flow delays up to 21 hours under dry antecedent conditions. Runoff volume reduction ranged in all technologies from approximately 40 to 70%. However, performance strongly depended on antecedent dry weather periods and substrate moisture conditions. Runoff quality analysis revealed seasonal variability, with nutrient concentrations influenced by temperature-driven processes such as evapotranspiration and plant uptake dynamics. Thermal monitoring demonstrated a consistent cooling effect of vegetated surfaces, with substrate surface temperature reductions of 3–5 °C during warm periods compared to conventional roof. At the structural level, GRs markedly reduced thermal stress, limiting daily temperature fluctuations at the roof membrane to 1–2 °C, compared to >30 °C for the reference roof during summer. Differences among technologies were mainly evident at the surface and within the substrate profile, while all systems provided comparable protection at the drainage layer. Irrigation requirements were found to be closely aligned with reference evapotranspiration, highlighting the importance of climate-adaptive water management in Mediterranean environments. Overall, the results confirm that GRs represent effective nature-based solutions for integrated stormwater management and urban heat mitigation in warm climates, while emphasizing the need for site-specific design and management strategies to optimize performance.