Green roofs of young age (≤ 5 years old) have boomed in China since the Sponge City Construction initiative was implemented. To use green roofs for better urban stormwater management, it is necessary to investigate the runoff quality of field-scale young green roofs as well as to examine common plant-media combination in green roof projects of China. The influence of two Sedum-vegetated extensive green roofs of different designs at the early stage of operation on runoff water quality was investigated by a field-scale study in Chengdu, southwest China. The water quality parameters of pH, suspended solids (SS), chemical oxygen demand (COD), total phosphorus (TP), and total nitrogen (TN) of rainwater (that is, input water for roofs), runoff from the two green roofs, and runoff from a conventional concrete control roof were compared. The results indicate that both green roofs mainly act as pollutant sources with greater concentrations of SS, COD, and TP when compared with rainwater quality. When compared with runoff quality from the control roof, greater TP concentrations in runoff from one green roof with commercially available substrate were observed. Attention should be paid to TP leaching in runoff for retrofitted green roofs with imported commercial substrates in that region. Adoption of pre-cultivated S. lineare mats of low fertility and localized soils may reduce nutrient leaching in green roof runoff. A nitrogen-rich substrate is not recommended for a plant community of a single species. Investigation of the effect of green roofs on water quality involving various pollutants in the long run is recommended.
Keywords: Green roofs; Nutrients; Runoff quality; Sedum; Sponge City; Stormwater.