Analysis of potential benefits on flood mitigation of a CAM green roof in Mediterranean urban areas

In the last decades, green roofs have been proposed among several nature-based solutions, as promising and sustainable tools to mitigate urban flood risk and adapt to climate changes. Several vegetation types have been suggested as green roof top layers, depending on the region and the purpose of the roof. In a Mediterranean climate, the Crassulacean Acid Metabolism (CAM) vegetation represents a particularly advantageous choice for green roofs since it does not require artificial irrigation and maintenance. However, the flood mitigation performance of CAM green roofs has not been investigated as adequately as other vegetation types. In this framework, we aim to define the potential retention capacity of a no maintenance-cost spontaneous CAM green roof, located at the entrance of the University of Cagliari (Italy) and to compare it to C3 vegetation type. The structure has been equipped with gauges to measure the water fluxes in and out of the roof. Local observations are used to calibrate a conceptual ecohydrological model. A 51-year rainfall time series and corresponding potential evapotranspiration are then used to simulate and compare the relative performance of green roofs vegetated with spontaneous CAM and more common C3 plants. Results show the good performances of the CAM green roof in mitigating rainfall extremes, with an average retention capacity of 0.52 over the whole investigated period, while C3 presents an index of retention equal to 0.71, but it requires frequent irrigation. Moreover, this work highlights some potential economic and environmental benefits of CAM green roof implementation in Mediterranean areas.