Exploring the role of paedological and climatic aspects in the medium-term decline of green roofs hydrological performance: an experimental study in a Mediterranean environment

In the realm of sustainable strategies for urban flooding risk mitigation, green roofs (GRs) emerge as a key solution. The complex relationship between hydrological, paedological, and climatic aspects poses several challenges in the definition of GRs' medium-term behaviour, emphasizing the imperative for further research. Embedding paedological and climatological evidence, this study focuses on relevant observed changes in the hydrological performance and behaviour of two extensive GR test beds located in southern Italy over a 7-year monitoring period. Experimental rainfall and runoff data, at the event scale, point to a reduction of approximately 12% in the stormwater retention capacity (RC) of monitored GRs. Additionally, a comparative analysis of RC values in two specific time windows revealed how, in an early stage, it was controlled by soil moisture content whereas it is currently (aged state) mainly related to rainfall characteristics. After excluding climate variability as a potential driver for observed RC changes, a paedological experimental campaign highlights variations in the physical and hydraulic parameters of the peat substrate, which, in turn, is addressed to affect the retention and detention capabilities of the GRs. HIGHLIGHTS A reduction of 12% in green roof (GR) retention capacity was observed in 7 years, 32% if only considering large rainfall events. Cumulative depth is a good predictor of the GR retention capacity in an aged state. Data analysis excluded climate as a potential driver for observed hydrological changes. Peat substrates experienced an increase by an order of magnitude in hydraulic conductivity and a decrease in water repellency in 7 years