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Line 117: − + − + − + − Recirculating water from a rainwater harvesting cistern was estimated to double the annual stormwater retention in Florida[http://www.mdpi.com/2073-4441/4/4/914]. The research team modeled 87% retention for a green roof coupled with a cistern, compared to 43% for the green roof alone. + +
→Performance
<h4>Water Quantity</h4>
<h4>Water Quantity</h4>
<h5>Water Balance</h5>
<h5>Water Balance</h5>
<p>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. </p>
<p>In Southern Ontario rainwater retention of extensive green roofs without irrigation is between 60% and 70%[http://www.sustainabletechnologies.ca/wp/wp-content/uploads/2013/03/NRC_EastviewGRrept.pdf][http://www.sustainabletechnologies.ca/wp/wp-content/uploads/2013/03/GR_york_fullreport.pdf][http://ascelibrary.org/doi/abs/10.1061/(ASCE)HE.1943-5584.0001534]. </p>
<p>Including winter periods with snow accumulation and thaw, the annual retention of extensive green roofs is around 50% [http://www.sustainabletechnologies.ca/wp/wp-content/uploads/2013/03/GR_york_fullreport.pdf][http://ascelibrary.org/doi/abs/10.1061/(ASCE)HE.1943-5584.0001534]. </p>
<p>Using a compost based planting medium improves retention by around 10% i.e. 60 % for compost compared to 50% for granular.</p>
<p>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[http://ascelibrary.org/doi/abs/10.1061/(ASCE)HE.1943-5584.0001534]. However, recirculating rainwater from a cistern was estimated to double the annual retention in Florida[http://www.mdpi.com/2073-4441/4/4/914]. The research team modeled 87% retention for a green roof coupled with a cistern, compared to 43% for the green roof alone.
<h5>Peak Flow Control</h5>
<h5>Peak Flow Control</h5>