Evaluating multifunctional performance of green roofs in rainstorm events: The role of modifiers in substrate layer

Green roofs (GRs) often show unsatisfactory functional performance in rainstorm events, especially rainwater management performance. Evaluating and guiding the GR component design from a multifunctional perspective may enable GRs to achieve promising outcomes in actual engineering scenarios. Moreover, whether modifiers can still play a positive role in improving the multifunctional performance of GRs in rainstorm events needs to be further clarified. In this study, four different addition ratios of biochar, polymeric ferric sulfate (PFS), and Trichoderma harzianum inoculum (THI) were respectively added to the substrate mixture composed of 40 % peat soil and 60 % vermiculite to install twelve modified GR modules. A control group without any modifiers was also installed at the same time. During a two-month experimental period, ten rainstorm events were conducted by simulated rainfall experiments based on the local rainfall data over the past decade. In this case, the water-holding performance, nutrient retention performance, rainwater detention performance, and pollutant interception performance of the GR modules were compared and evaluated, as well as their multifunctional performance based on an assessment model established by analytic hierarchy process (AHP). Results indicated that all the modifiers with any addition ratios could improve the multifunctional performance of GRs. Most notably, GRB-15 (biochar at the ratio of 15 %) obtained the highest multifunction index among the GR modules under the context that only its rainwater detention performance with a low weight coefficient in the assessment model was the best. Specifically, GRB-15 could still exhibit an average delayed outflow time of 77.87 min and an average rainwater reduction rate of 62.38 % in continuous rainstorm events. We proposed that more ecological service benefits that GRs can provide should be incorporated into the framework for evaluating their multifunctional performance to clarify and minimize the existing function trade-offs in GRs.