Green roof seasonal variation: comparison of the hydrologic behavior of a thick and a thin extensive system in New York City

Abstract

Green roofs have been utilized for urban stormwater management due to their ability to capture rainwater locally. Studies of the most common type, extensive green roofs, have demonstrated that green roofs can retain significant amounts of stormwater, but have also shown variation in seasonal performance. The purpose of this study is to determine how time of year impacts the hydrologic performance of extensive green roofs considering the covariates of antecedent dry weather period (ADWP), potential evapotranspiration (ET 0 ) and storm event size. To do this, nearly four years of monitoring data from two full-scale extensive green roofs (with differing substrate depths of 100 mm and 31 mm) are analyzed. The annual performance is then modeled using a common empirical relationship between rainfall and green roof runoff, with the addition of Julian day in one approach, ET 0 in another, and both ADWP and ET 0 in a third approach. Together the monitoring and modeling results confirm that stormwater retention is highest in warmer months, the green roofs retain more rainfall with longer ADWPs, and the seasonal variations in behavior are more pronounced for the roof with the thinner media than the roof with the deeper media. Overall, the ability of seasonal accounting to improve stormwater retention modeling is demonstrated; modification of the empirical model to include ADWP, and ET 0 improves the model R 2 from 0.944 to 0.975 for the thinner roof, and from 0.866 to 0.870 for the deeper roof. Furthermore, estimating the runoff with the empirical approach was shown to be more accurate then using a water balance model, with model R 2 of 0.944 and 0.866 compared to 0.975 and 0.866 for the thinner and deeper roof, respectively. This finding is attributed to the difficulty of accurately parameterizing the water balance model.

Detention and Retention Behavior of Four Extensive Green Roofs in Three Nordic Climate Zones

“…Few actual field studies, and especially long duration studies, are available from cold and wet climates comparable to the Nordic climates, and design estimates for retention are needed. Most studies from locations with large seasonal variation in evapotranspiration report lower retention in winter and higher in summer [27,31,[38][39][40]. Some studies does also explain part of the observed seasonal variations as a result of seasonal variations in precipitation patterns [34,41].…”

Role of Substrates Used for Green Roofs in Limiting Rainwater Runoff

“…Such roofs have a high potential of stopping runoff, as they can comprise as much as half of the non-permeable area of cities (Mentens et al 2006). It is estimated that yearly rainfall retention on green roofs can amount to anywhere from 5 to 85% (Li and Babcock, 2014, Cipolla et al 2016, Elliott et al 2016, Sims et al 2016. The variability in rainfall retention can be attributed to, among others, the configuration of the green roof (Berndtsson 2010, VanWoert et al 2005a, Speak et al 2013.…”

Laboratory Tests of Substrate Physical Properties May Not Represent the Retention Capacity of Green Roof Substrates In Situ

“…Our simulation was skewed towards larger events as compared with the typical rainfall distribution pattern in Melbourne, such that we could focus on retention performance for significant events [26]. Further, rainfall retention in our experiment was likely lower than would be expected on an annual basis, as it mainly took place in late winter and early spring [48].…”