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A total number of 505 runoff episodes during SCPs were identified. Variable statistics for the episodes are summarized in Table 4. The statistical analyses indicated that some of the independent variables were correlated. After rejecting the highly correlated variables, the total number of variables in the model was reduced from 17 to 10. The stepwise regression resulted in the following linear relationship:
formula
(1)
where Qmax is the maximum runoff (mm/h), RT is the roof type (0 for REF and 1 for green roof), Pmax is the maximum precipitation intensity (mm/h), and is the maximum and minimum air temperatures, respectively (°C), is the minimum snow depth (cm), and dh is the reduction in snow depth snow during the episode (cm). The and for Equation (1) were both 0.72 and all variable coefficients were significant.
Table 4

Variables used to predict and explain the maximum runoff from the roofs

VariableSymbolUnitMin.MedianMeanMax.
Maximum runoff intensity Qmax mm/h 0.1 0.6 1.0 6.5 
Episode duration  17 37 331 
Total precipitation  mm 0.0 0.5 7.4 72.3 
Precipitation as snow  mm 0.0 0.0 1.2 14.6 
Precipitation as rain  mm 0.0 0.4 6.1 69.7 
Maximum precipitation intensity  mm/h 0.0 0.3 1.0 6.8 
Mean precipitation intensity  mm/h 0.1 0.6 0.6 1.6 
Precipitation duration  11 139 
Mean air temperature  °C −6.8 2.6 2.8 14.5 
Minimum air temperature  °C −8.0 0.0 0.5 12.0 
Maximum air temperature  °C −6.0 4.7 4.9 16.1 
Mean air temperature 24 h before  °C −7.8 0.3 0.4 8.3 
Mean air temperature 48 h before  °C −6.4 0.1 0.1 8.1 
Mean snow depth  cm 14 64 
Minimum snow depth  cm 12 64 
Maximum snow depth  cm 12 15 64 
Reduction in snow depth  cm 34 
VariableSymbolUnitMin.MedianMeanMax.
Maximum runoff intensity Qmax mm/h 0.1 0.6 1.0 6.5 
Episode duration  17 37 331 
Total precipitation  mm 0.0 0.5 7.4 72.3 
Precipitation as snow  mm 0.0 0.0 1.2 14.6 
Precipitation as rain  mm 0.0 0.4 6.1 69.7 
Maximum precipitation intensity  mm/h 0.0 0.3 1.0 6.8 
Mean precipitation intensity  mm/h 0.1 0.6 0.6 1.6 
Precipitation duration  11 139 
Mean air temperature  °C −6.8 2.6 2.8 14.5 
Minimum air temperature  °C −8.0 0.0 0.5 12.0 
Maximum air temperature  °C −6.0 4.7 4.9 16.1 
Mean air temperature 24 h before  °C −7.8 0.3 0.4 8.3 
Mean air temperature 48 h before  °C −6.4 0.1 0.1 8.1 
Mean snow depth  cm 14 64 
Minimum snow depth  cm 12 64 
Maximum snow depth  cm 12 15 64 
Reduction in snow depth  cm 34 

Note: Observations are from the 505 episodes. Details on each variable is presented in ‘Methods’. Equation (1) is valid for the observations in the table.

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