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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. 2017. The impact of green roof ageing on substrate characteristics and hydrological performance, In Journal of Hydrology, Volume 547, 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. 2017. The impact of green roof ageing on substrate characteristics and hydrological performance, In Journal of Hydrology, Volume 547, Pages 332-344, ISSN 0022-1694, https://doi.org/10.1016/j.jhydrol.2017.02.006.</ref>.
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| evapotranspiration]].
*In Southern Ontario rainwater retention of extensive green roofs without irrigation is between 60% and 70%<ref>Liu, M. Minor, J. 2005. Performance evaluation of an extensive green roof. National Research Council of Canada. NRCC-48204 https://sustainabletechnologies.ca/app/uploads/2013/03/NRC_EastviewGRrept.pdf</ref> <ref name=VS>Van Seters, T., Rocha, L., Smith, D., MacMillan, G. 2009. Evaluation of Green Roofs for Runoff Retention, Runoff Quality, and Leachability, Vol. 44 (1): 33 - 47 (2009). Innovative Approaches to Stormwater Management in Canada</ref> <ref name=Hill>Hill J, Drake J, Sleep B, Margolis L. Influences of Four Extensive Green Roof Design Variables on Stormwater Hydrology. J Hydrol Eng. 2017;22(8):04017019. doi:10.1061/(ASCE)HE.1943-5584.0001534</ref>.
*In Southern Ontario rainwater retention of extensive green roofs without irrigation is between 60% and 70%<ref>Liu, M. Minor, J. 2005. Performance evaluation of an extensive green roof. National Research Council of Canada. NRCC-48204 https://sustainabletechnologies.ca/app/uploads/2013/03/NRC_EastviewGRrept.pdf</ref> <ref name=VS>Van Seters, T., Rocha, L., Smith, D., MacMillan, G. 2009. Evaluation of Green Roofs for Runoff Retention, Runoff Quality, and Leachability, Vol. 44 (1): 33 - 47 (2009). Innovative Approaches to Stormwater Management in Canada</ref> <ref name=Hill>Hill J, Drake J, Sleep B, Margolis L. Influences of Four Extensive Green Roof Design Variables on Stormwater Hydrology. J Hydrol Eng. 2017;22(8):04017019. doi:10.1061/(ASCE)HE.1943-5584.0001534</ref>.
*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/>.