Improving water quality modelling for green roof runoff using Storm Water Management Model

Modelling of green roof is important for assessing its effectiveness as a low impact development (LID) practice. This study evaluated the performance of Storm Water Management Model (SWMM)’s LID Controls for modelling runoff water quality load from a green roof in Portland, Oregon, USA, for possible improvement. Such an attempt has been made rarely, although plenty of studies have evaluated SWMM's LID Controls for predicting green roof runoff. From the evaluation, the applicability of the SWMM for evaluating water quality such as total phosphorous (TP) and total nitrogen (TN) was “not satisfactory”. Nash-Sutcliffe Efficiency (NSE) value was 0.27 for TN and −1.45 for TP when using “rain concentration”, the only parameter available for water quality simulation of green roof runoff. The applicability was improved to “very good” level for TP with an NSE value of 0.66 when buildup and washoff were made possible by creating an imaginary twin subcatchment of the same size as the original one. However, NSE value for TN was not improved. The value of the work is that it could lead to a more accurate green roof design by minimizing pollutant release with improved prediction technology for TP. •SWMM's runoff module performance for green roof was found to be very good.•SWMM's water quality module performance for green roof was not satisfactory.•This poor performance for green roof was improved to above satisfactory level.•In so doing, buildup and washoff were utilized through an imaginary subcatchment.•Further consideration on internal constituent processes deemed necessary.