Investigation of spatial variability of soil hydraulic properties for application in intensive green roofs

Intensive green roofs have been suggested as one infrastructure to manage rainstorm in many countries like Malaysia and China. Generally, Shrubs and herbs were mixed in intensive green roofs. Root of shrubs is found to denser and thicker than herbs, which significantly affect the substrate properties (e.g., hydraulic properties, physico-chemical properties and cracks development), leading spatial variability of hydraulic properties of soil around the shrubs stem. Such spatial variability in hydraulic properties critically affects the stormwater management (i.e., runoff delay, water retention and rainfall infiltration) of intensive green roof. The objective of this study is to explore the spatial variability of hydraulic properties (e.g., soil water retention, hydraulic conductivity and evapotranspiration-induced suction.). Two intensive green roof model boxes were built to monitor the hydraulic properties between near-root zone and far-root zone. The physico-chemical properties and crack development of substrate in intensive green roofs were quantitatively investigated. Evapotranspiration-induced suction of near-root zone is higher than far-root zone. Crack intensity factor (CIF) of near-root zone is found to 11–18 times lower than far-root zone. The hydraulic conductivity of near-root zone is 1.05–2.02 times higher than far-root zone in both of intensive green roofs. Moreover, asymmetric water flow is found around the soil of shrub stem. The results of this study can promote designer to consider reasonably the spatial layout of shrubs to improve hydrological performance of intensive green roofs.