Water retention performance of green roof technology: A comparison of canadian climates

In recent decades, urbanization has led to more frequent flood events in urban areas. Several studies have shown the promise of green roof technology for storm water reduction. However, limited studies have assessed water retention performance of green roofs in a range of climates. To investigate the retention performance of a green roof in various Canadian climates, several cities across the country (Vancouver, Calgary, Regina, Toronto, London, and Halifax) were selected. The water balance equation along with evapotranspiration (ET) models were employed to simulate runoff produced by green roofs in these six Canadian cities for two different vegetation types. A seven year simulation period was selected running from 2000 to 2006, with modeling conducted for eight months of the year using hourly time steps. The results showed that the performance of a green roof in runoff reduction depends on location, varying from 17% to 50% for low water use plants. The best performance in storm water reduction was predicted to occur in Regina and Calgary, while the poorest performance was predicted for Halifax and Vancouver. However, the average annual water retention (mm) was the highest in Toronto and London and the lowest in Vancouver, with Calgary, Regina and Halifax showing intermediate performance. Sensitivity analyses were performed for substrate depth, porosity, and wilting point to determine the importance of each for green roof performance for the different cities. The results revealed that the vegetation type had a greater impact on the water retention performance of green roofs than increases in substrate storage capacity associated with different substrate depth, porosity and wilting point over the range assumed in this study.