Factors affecting the hydrological response of substrate material for green roofs and bioretention

Green roof and bioretention have been frequently used as stormwater management practices to address urban hydrology issues, and researches regarding these technologies are increasing lately. However, a knowledge gap remains in characterizing the hydraulic properties of these practices. The hydrological process of green roof and bioretention is mainly determined by the substrate media of the practices. Based on current studies, this paper discussed factors affecting the hydrological response of substrate material for green roofs and bioretention. Most researches provide qualitative information on water movement within substrate, but few have further investigated the theoretical explanations on the hydrological process of substrate and give quantitative data. This study proposed that following factors affecting water flow dynamics of substrate media: type of materials and composition of the materials; depth of the substrate media; pore structure/network of the substrate materials. Many investigations focus on the effect of a single factor and generalizing the other, but the hydrological process of the substrate material is mostly under the effect of multiple factors simultaneously. And under different study conditions and different designs of green roof or bioretention, the importance of these factors to the hydrological performance of substrate media is different. In addition, interrelationships exist among the factors, changes in one factor could influence other factors. Current research on the factors influencing the hydrological process of the substrate material needs to be related. Future study needs to consider the effect of multiple factors and their interrelations, to create integrated approaches for a better understanding of the hydrological process of the substrate material, and for modeling the hydrological performance of the green roof and bioretention more accurately and designing the practices more efficient.