Evapotranspiration Rates of Representative Green Roof Systems and the Implications for Green Roof Policies

Urban intensification, population growth and climate change are creating multiple issues within cities on a global scale. As more buildings are constructed to support increasing urban populations, cities are having a more difficult time managing sporadic weather events; especially managing stormwater from intense rainfall. Toronto currently has in place a mandatory green roof bylaw, that is designed to utilize shallow green roofs (GRs) on all new buildings as a measure to intercept rainfall and transpire the water back into the atmosphere. Four green roof modules with varying system buildups were constructed and instrumented on top of a downtown building to analyze how the amounts of retained rainfall and evapotranspiration varied amongst the systems. It was determined that the total evapotranspiration (ET) volume of blue/green roofs and vegetable producing green roofs were considerably higher than a traditional extensive green roof; 236% higher in the blue/green roof and 356% higher in the vegetable roof. It was also determined that unvegetated blue roof systems are also a viable method of managing stormwater on rooftops, with the blue roof module showcasing a total evaporation volume of 134 mm over the duration of the study. The results from this research exhibit that higher hydrological benefit can be derived from green roofs using blue-green technology or vegetable producing assemblies.