Roof greening, one of the major control indicators of “The Sponge City” in China, has practically been proved to have the capability of retaining rain water. However, the water retentions are quite different between experiments, even in the same place. The data are poorly repeatable and comparable between the reported cases. Here we summarize and analyze the reported data in roof greening.
This study summarize 17 factors affecting the rainwater retention according to spatio-temporal variables, culture medium, structure of green roof system, roof and rainfall. These factors could function positively or negatively, and influence directly or indirectly. Based on their correlations, we propose a formula of roof greening runoff: Q = q - ((Qmmax- Wi+ E + Vmax - Vi)×(1 + a -s) + Qvmax+T), in which Q ~ volume of runoff; q ~ rainfall; Qmmax ~ maximum volume of rainwater stored in the medium; Wi ~ initial water content in the medium; E ~ evaporation capacity; Vmax ~ maximum volume of rainwater stored in the structure layers of greenroof; Vi ~ initial water content in the reservoir layer; a ~ roof area coefficient; s ~ roof slope coefficient; Qvmax ~ maximum volume of rainwater stored in vegetation; T ~ the amount of transpiration. Taken together, this study proposes a method to improve repeatability and comparability in the field experiments, i.e. to control key indicators such as content of organic matter, medium depth, initial moisture of medium, biomass, water-storage board and rainfall intensity. Future research reports should provide clear data of key indicators.