Green roof, as an important measure of sponge city construction, is considered as a win-win alternative for alleviating rainwater runoff and urban heat island. The ecological benefits of green roofs are highly dependent on the quality of substrates. Biochar (BC) prepared from agricultural waste biomass has the potential to be used as a substrate amendment for green roofs. However, the influences of BC properties on hydrothermal properties of green roofs remain unclear. We evaluated the effects of natural soils incorporated with two kinds of BCs (particle size and dosage) on runoff retention capacity and roof thermal performance. Results indicated that the runoff reduction benefit of green roofs declines with the increase of rainfall. When the rainfall is less than 10 mm, the green roofs with different substrates hardly generate runoff, otherwise runoff reduction rates of all green roofs reduce below 75%. BC particles have abundant micro-pores and higher specific surface area, significantly improving the water holding-capacity of roof substrate and playing a critical role in the runoff regulation and cooling effect of green roofs. Application of 20% finer BC particles is the optimal for stormwater retention in all BC addition substrates. Moreover, it could reduce the roof upper surface temperature by 3-5 °C and reduced the daily heat gain of the green roof by at least 0.06 MJ/m2 compared with BC-free ones. Overall, adding BC into the substrates of green roofs can achieve better hydrothermal properties, which is beneficial to the design optimization of green roofs.
Keywords: Biochar; Cooling effect; Green roofs; Runoff; Substrate amendment.
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