Hydrologic responses to urbanization: Towards a holistic approach for maximizing green roofs' performance in controlling urban precipitations

Urbanization alters the cities' natural hydrology by downsizing permeable surfaces and through climate change which leads to changes in precipitation properties. Runoffs resulting from cities' built environment create enormous environmental challenges including floods, water pollution, and degrading water quality. Green roofs have significant potential for mitigating urban runoffs that have not been adequately utilized due to a lack of research offering appropriate guidelines. This review paper provides a holistic analysis of the application of green roofs for maximizing their runoff control capacity. These factors are discussed in six categories 1) design properties including the thickness of the layers, the surface area of the roof, and the slope of the roof; 2) substrate and growing media properties; 3) climatic factors including seasonal variability, weather temperature, and rain characteristics; 4) vegetation properties including type, phytoremediation potential, the need for fertilizers and pesticides, and plants diversity; 5) roof age, and 6) quality of maintenance. For each factor, research findings are reviewed, and the possible inconsistencies in them are discussed. The research also aims at highlighting the existing gaps in the research about the hydrologic performance of green roofs in urban areas.