Changes

===Water quality===

===Water quality===

Many green roofs receive only rainwater, which is relatively free of contaminants like nutrients, salt and heavy metals. As such green roofs can contribute contamination, most notably in leaching of [[Nutrients|nutrients]] from the growing medium during early establishment. Reported values of total [[phosphorus]] in green roof runoff vary from less than 0.1 ppm to over 10 ppm.   

Many green roofs receive only rainwater, which is relatively free of contaminants like nutrients, salt and heavy metals. As such green roofs can contribute contamination, most notably in leaching of [[Nutrients|nutrients]] from the growing medium during early establishment. Reported values of total [[phosphorus]] in green roof runoff vary from less than 0.1 ppm to over 10 ppm.   

But, in dense urban centres, green roofs are increasingly being used to receive irrigation from harvested rainwater. Current Ontario Building Code permits the use of rooftop runoff to be reused in this manner, so long as it is 'free of solids'.

However in dense urban centres, green roofs are increasingly being used to receive irrigation from harvested rainwater. Current Ontario Building Code permits the use of rooftop runoff for landscape irrigation purposes, so long as it is 'free of solids'.

A 'closed loop' system can be created by coupling a rainwater harvesting system to a green roof by catching and reusing green runoff as a source of water for irrigation during dry periods, so the only water leaving the system is through evapotranspiration. This can prevent runoff from leaving the site and thereby prevent contaminant loading to the environment.

A 'closed loop' system can be created by coupling a rainwater harvesting system with a green roof and harvesting and reusing roof runoff as a source of water for irrigation during dry periods, so the only water leaving the system is through evapotranspiration. This can prevent runoff from leaving the site and thereby prevent contaminant loading to the environment.

[[File:TP.PNG|thumb|Total phosphorous concentrations reported in green roof runoff(abstracted from Hill 2017)]]

[[File:TP.PNG|thumb|Total phosphorous concentrations reported in green roof runoff(abstracted from Hill 2017)]]