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| [[File:YorkU2.jpg|thumb|Detail showing the early root development through the mat to the granular planting medium.]] | | [[File:YorkU2.jpg|thumb|Detail showing the early root development through the mat to the granular planting medium.]] |
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− | Green roof performance has not been reported to reduce over time. Controlled studies have instead indicated that maturing green roofs may have improved water retention properties <ref>Simon De-Ville, Manoj Menon, Xiaodong Jia, George Reed, Virginia Stovin, The impact of green roof ageing on substrate characteristics and hydrological performance, In Journal of Hydrology, Volume 547, 2017, Pages 332-344, ISSN 0022-1694, https://doi.org/10.1016/j.jhydrol.2017.02.006.</ref>.
| + | Controlled studies have indicated that maturing green roofs may have improved water retention properties <ref>Simon De-Ville, Manoj Menon, Xiaodong Jia, George Reed, Virginia Stovin, The impact of green roof ageing on substrate characteristics and hydrological performance, In Journal of Hydrology, Volume 547, 2017, Pages 332-344, ISSN 0022-1694, https://doi.org/10.1016/j.jhydrol.2017.02.006.</ref>. |
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| The key hydrologic benefit which green roofs have over other forms of LID is the proportion of the water returned to the atmosphere through evapotranspiration. | | The key hydrologic benefit which green roofs have over other forms of LID is the proportion of the water returned to the atmosphere through evapotranspiration. |
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| *Including winter periods with snow accumulation and thaw, the annual retention of extensive green roofs is around 50% <ref name=VS/><ref name=Hill/>. | | *Including winter periods with snow accumulation and thaw, the annual retention of extensive green roofs is around 50% <ref name=VS/><ref name=Hill/>. |
| *Using a compost based planting medium improves retention by around 10% i.e. 60 % for compost compared to 50% for granular. | | *Using a compost based planting medium improves retention by around 10% i.e. 60 % for compost compared to 50% for granular. |
− | *Daily irrigation can reduce the annual retention by 20% compared to a roof without irrigation. i.e. 40% for irrigated compared to 60% without irrigation<ref name=Hill/>. However, recirculating rainwater from a cistern was estimated to double the annual retention in Florida<ref>http://www.mdpi.com/2073-4441/4/4/914</ref>. The research team modeled 87% retention for a green roof coupled with a cistern, compared to 43% for the green roof alone. | + | *Daily irrigation can reduce the annual retention by 20% compared to a roof without irrigation. i.e. 40% for irrigated compared to 60% without irrigation<ref name=Hill/>. However, recirculating rainwater from a cistern was estimated to double the annual retention in Florida<ref>Hardin, M.; Wanielista, M.; Chopra, M. A Mass Balance Model for Designing Green Roof Systems that Incorporate a Cistern for Re-Use. Water 2012, 4, 914-931. http://www.mdpi.com/2073-4441/4/4/914</ref>. The research team modeled 87% retention for a green roof coupled with a cistern, compared to 43% for the green roof alone. |
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| An appropriate NRCS curve numbers for green roofs without irrigation in Southern Ontario is 90 <ref>Curve Number and Runoff Coefficients for Extensive Living Roofs | | An appropriate NRCS curve numbers for green roofs without irrigation in Southern Ontario is 90 <ref>Curve Number and Runoff Coefficients for Extensive Living Roofs |