A Framework for optimizing hydrologic performance of green roof media
One of the primary functions of green roofs in urban areas is to moderate rainwater runoff, and one of the major impediments to the survival of plants on an extensive green roof (EGR) is a lack of available water during dry periods. Runoff moderation and water storage are both influenced by the composition of the growing media. In this three-phase study, we: 1) measured hydrologic attributes of individual EGR growing media constituents, 2) predicted hydrologic performance of media mixtures using data for the individual constituents, and 3) tested the seven top-ranking mixtures. Hydrologic attributes included wet weight (WW), water held at saturation (WH), water retained after 14 days of drying (WR), and hydraulic conductivity (HC). We fixed the amount of organic matter (peat moss) at 20% by volume for media mixtures and varied the composition of the inorganic fraction. Because perlite was light in weight, yet held higher amounts of water both when saturated and after 14 days, media mixtures dominated by perlite were predicted to have the best overall hydrologic performance. Mixtures dominated by pumice were also predicted to perform relatively well, but were heavier. Although vermiculite retained the most water after its first wetting, its water retention sharply decreased after undergoing a drying and re-wetting cycle due to a decrease in volume of about 50%. Performance of tested mixtures reflected predictions for WW, WH, and WR; indicating performance of mixtures can be adequately predicted using performance of individual constituents for these attributes. Hydraulic conductivity (HC) was less predictable, possibly because the pore volume in mixtures may be unrelated to the pore volume of the individual constituents. Our approach is intended to provide a framework to test other media mixtures. The results of this study can be used to assist in formulating green roof media mixtures for specific applications, but also underscore the need for standardization of methods for more effective comparisons of EGR substrates as well as other attributes of EGRs, and reinforce the need to evaluate EGR components using real-world settings or scenarios.