Water retention and evapotranspiration of blue-green roofs relate to vegetation type, substrate depth and irrigation

This study draws attention to the role of vegetation cover type (VGT), beyond the substrate depth, on blue-green roofs (BGRs) performances in terms of water retention capacity and maintenance requirements. The one-year study was split in two Experiments (Experiment 1: July 2020-November 2020; Experiment 2: November 2020-July 2021) performed in mesocosms built to mimicking BGRs and evaluating the contribution of two different substrate depths (8 cm and 14 cm) and 4 different vegetation cover types (cool-season turf-CT, warm-season turf-WT, wildflower meadow-WM, and Sedum mixture-SM) on water retention capacity, vegetation canopy cover of the surface and required management effort. In Experiment 1 (19 July 2020 - 12 November 2020), an overall higher retention capacity (range: 75-98%) was registered compared to Experiment 2 (13 November 2020 - 2 July 2021; range: 42-55%). In both experiments, the highest retention capacity was observed with deeper substrate layer and when WM was used as vegetation cover type, thanks to its higher evapotranspiration. This latter guaranteed a very fast and consistent surface cover during Experiment 1 while in Experiment 2, SM performed better. About management, the least demanding cover type was SM, since it did not require neither irrigation intervention nor regular harvesting. This highlights the trade-off among ecosystem services that BGRs can provide, which should be taken into consideration when selecting species or mixtures for BGRs.