Hydrological and plant performance of green roofs in the climate context of the Venetian Plain (northeastern Italy): preliminary results

Twelve different extensive green roof configurations has been analyzed to evaluate how the different components affect hydrological and plant performance. The research was conducted at the Agripolis Campus of the University of Padova (Italy). The green roof microcosms were obtained from a factorial combination of two drainage/storage layers, two substrates, and three vegetation treatments. The experimental design was the randomized complete block with three replications. The drainage/storage layers tested were a plastic preformed layer and a mineral layer (expanded perlite). The substrates tested were a volcanic medium and a recycled crushed bricks medium. Three types of vegetation were tested: a sedum mix (S. album, S. reflexum, S. sexangolare, S. acre), an herbaceous perennial mix (Bromus erectus, Melica ciliata, Campanula spicata, Potentilla pusilla, Euphorbia cyparissias), and a suffruticose mix (Dianthus spp., Thymus serpillum, Clinopodium nepeta, Artemisia alba, Anthemis tinctoria). In addition, as a control, gravel was considered to mimic a conventional roof with storage layer. The experiment was set on June 2014. The runoff volumes were evaluated by weighing, after each rainstorm, the water collected in tanks placed under the trays. Runoff values after each rainfall events, from July 2014 to June 2015 (981 mm), were measured. Plant closure was evaluated elaborating digital image analysis taken on monthly basis. The preliminary results indicate seasonal effects on the hydrological performance of green roof components; in fact, probably because of the low losses by evapotranspiration, the reduction of rainfall runoff in winter was low (on average, about 20%), with little differences between treatments. Considering the entire period, the green roofs configuration that appeared to offer the best performance in terms of water retention in the climate context of the Venetian Plane seems to be composed by the suffruticose mix, the recycled crushed bricks substrate and the mineral drainage layer of expanded perlite, which allowed to reduce more than 60% of the total rainfall runoff compared to 10% of the conventional roof with gravel.