Hydrological and plant performance of green roofs in the climate context of the Veneto Plain (north-eastern Italy): preliminary results

Abstract

A research was conducted at the Agripolis Campus of the University of Padova (Italy) to evaluate in the climate context of the Veneto plain the hydrological and plant performance of twelve different extensive green roof configurations in plastic trays (green roof microcosms), obtained from a factorial combination of two drainage/ storage layers, two substrates, and three vegetation treatments. 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. acre, S. album, S. reflexum, S. sexangolare), 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 in May 2014. The runoff volumes were evaluated by weighing, after each rainstorm, the water collected in tanks placed under the trays. Plant closure was evaluated elaborating digital image analysis taken on monthly basis. The results related to the preliminary phase from July 2014 to June 2015 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 fall and winter was quite low (on average, about 31.5%), with little differences among the different microcosm configurations. Considering the entire period, the best performance in terms of water retention in our climate context was shown by the green roof configuration composed by suffruticose mix, recycled crushed bricks substrate and expanded perlite drainage/storage layer, which allowed a reduction of 62.7% of the total rainfall runoff compared to 14.1% of the conventional roof with gravel. Canopy closure of suffruticose and herbaceous mixes were higher than that of sedum mix throughout the experiment, and values were increased by crushed bricks medium and perlite drainage/storage layer.