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As illustrated in Figure 2, total rainfall depth of the six types of simulated rains (R1–R6) ranged from 43.2 to 87 mm, while the corresponding runoff depth varied between 4.73 and 55.2 mm for the investigated green roofs. Runoff quantity from the green roofs was obviously much lower than that from the conventional bare roof (39.3–81.5 mm). Under R1–R6, the green roofs of G1–G8 delayed the initiation of runoff by an average of 3–21 minutes (Table 6), while the bare roof produced runoff almost as soon as it began to rain. Compared to the average RC of G1–G8 roofs (0.35–0.78, Table 6), the average RC of the bare roof under R1–R6 (0.92) was much higher. Calculated from the product of rainfall intensity and time of runoff initiation, the average maximum cumulative rainfall depth with 100% retention varied from 4.8 mm for G1 to 27.9 mm for G8 (Table 6), showing large fluctuations in stormwater retention capacity among the different green roofs.

Table 6

Characteristic hydrological parameters of the green roofs (G1–G8) under the six types of simulated rains (R1–R6)

No.Average time for delaying the initiation of runoff (min)Average time for discharging runoff longer than the rainfall duration (min)Average maximum cumulative rainfall depth with 100% retention (mm)Average RC
G1 4.8 0.78 
G2 12.6 0.61 
G3 6.5 0.74 
G4 8.9 0.70 
G5 14 18.7 0.40 
G6 12 17.3 0.51 
G7 10 15.2 0.55 
G8 21 27.9 0.35 
No.Average time for delaying the initiation of runoff (min)Average time for discharging runoff longer than the rainfall duration (min)Average maximum cumulative rainfall depth with 100% retention (mm)Average RC
G1 4.8 0.78 
G2 12.6 0.61 
G3 6.5 0.74 
G4 8.9 0.70 
G5 14 18.7 0.40 
G6 12 17.3 0.51 
G7 10 15.2 0.55 
G8 21 27.9 0.35 

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