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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.
*In Southern Ontario rainwater retention of extensive green roofs without irrigation is between 60% and 70%<ref>http://www.sustainabletechnologies.ca/wp/wp-content/uploads/2013/03/NRC_EastviewGRrept.pdf</ref><ref>http://www.sustainabletechnologies.ca/wp/wp-content/uploads/2013/03/GR_york_fullreport.pdf</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>http://www.sustainabletechnologies.ca/wp/wp-content/uploads/2013/03/NRC_EastviewGRrept.pdf</ref><ref name=VS>T. Van Seters, L. Rocha, D. Smith, G. MacMillan; 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>
*Including winter periods with snow accumulation and thaw, the annual retention of extensive green roofs is around 50% <ref>http://www.sustainabletechnologies.ca/wp/wp-content/uploads/2013/03/GR_york_fullreport.pdf</ref><ref name=Hill/>.
<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/>.
*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>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.