Elsevier

Journal of Environmental Management

Volume 232, 15 February 2019, Pages 404-412
Journal of Environmental Management

Research article
Green roof storage capacity can be more important than evapotranspiration for retention performance

https://doi.org/10.1016/j.jenvman.2018.11.070Get rights and content

Highlights

  • We evaluated plant effects on rainfall retention.

  • Rainfall retention depends more on substrate storage capacity than on ET.

  • Some species reduce substrate storage through preferential flow paths.

  • These root-induced preferential flow paths can overwhelm plant ET benefits.

Abstract

Green roofs can significantly reduce stormwater runoff volumes. Plant selection is crucial to retention performance, as it is influenced by how well plants dry out substrates between rainfall events. While the role of plants in evapotranspiration (ET) on green roofs is well-studied, their potential influence on retention via their impacts on water movement through substrates is poorly understood. We used a simulated rainfall experiment with plant species with different water use strategies to determine the key drivers of green roof retention performance. Overall per-event retention was very high (89–95%) and similar for all plant species and unplanted modules for small events. However, for larger events, some species showed lower retention than unplanted modules or low-water using succulent species. Despite the fact that these species were more effective at replenishing storage between rainfall events due to their higher ET, they reduced the maximum storage capacity of the substrate, likely due to their root systems creating preferential flow paths. This finding has important implications for green roofs, as although ET represents the primary means by which the storage capacity of green roofs can be regenerated, if species with high ET also reduce the maximum storage capacity, effective retention performance is reduced. Therefore, we suggest that species selection must first focus on how plants affect storage capacity in the first instance and consider water use strategies as a secondary objective.

Introduction

The creation of impervious areas as part of urbanisation increases surface runoff and disturbs the natural flow regime of streams (Poff et al., 1997), resulting in the degradation of receiving ecosystems (Madsen et al., 2009; Walsh et al., 2005) and causing flooding. To return towards a more natural flow regime, surface runoff should be reduced by ∼60–80%, equivalent to pre-development losses via evapotranspiration (ET) (Burns et al., 2012; Zhang et al., 2001). Green roofs are among the more promising technologies employed to reduce surface runoff, as roofs constitute a large proportion of impervious area in dense urban areas and do not compete with ground-level development for space (Arnold Jr and Gibbons, 1996). Green roofs can help compensate for the loss of natural landscapes and hydrological processes by intercepting and retaining rainfall which is used to sustain plant growth, restore ET and therefore reduce runoff (Stovin et al., 2012; Viola et al., 2017).

Green roofs have been shown worldwide to be an effective stormwater management tool, although their performance varies. Previous studies have shown annual rainfall retention from 5 to 90% (Carter and Rasmussen, 2006; Cipolla et al., 2016; Locatelli et al., 2014; Sims et al., 2016; Szota et al., 2017b). Per-event retention varies from 5 to 100% (Bliss et al., 2009; Carson et al., 2013; Locatelli et al., 2014; Sims et al., 2016; Stovin et al., 2015). Climatic conditions and differences in green roof configuration, i.e., substrate type and depth, as well as plant species selection, drive this variation in performance (Berndtsson, 2010). Rainfall characteristics (size, intensity and temporal distribution) and reference evapotranspiration (ETo) dynamics are particularly important drivers of green roof rainfall retention and runoff across Mediterranean (Fioretti et al., 2010; Viola et al., 2017), temperate (Fassman-Beck et al., 2015; Stovin et al., 2012) and tropical climates (Voyde et al., 2010). Climates with a high frequency of small rainfall events combined with high evaporative demand tend to have higher rainfall retention (Carpenter and Kaluvakolanu, 2011; Elliott et al., 2016; Sims et al., 2016; Speak et al., 2013). The variation in retention performance has also been explained by green roof substrate water holding capacity, which is dependent upon substrate characteristics such as the proportion of different materials and depth (De-Ville et al., 2017; Farrell et al., 2013a; Hilten et al., 2008; Morgan et al., 2013; VanWoert et al., 2005; Voyde et al., 2010). ET is influenced by substrate characteristics, depth and species selection (Farrell et al., 2013b; Nagase and Dunnett, 2012; Szota et al., 2017a; Wolf and Lundholm, 2008). Therefore, on an event basis, retention will be determined by substrate water content before the rainfall event, the size of the rainfall event and the amount of water available for ET after the event.

As such, retention performance will ultimately depend on the interactions between climatic conditions (rainfall and ETo), green roof configuration (substrate depth and properties, plant species, protection and drainage layers) and available storage conditions before rainfall events (Sims et al., 2016; Stovin et al., 2015; Viola et al., 2017). However, few studies have investigated the relative importance of these different parameters on retention and how they interact with each other. Locatelli et al. (2014) identified that the most sensitive parameters for long-term retention performance were crop coefficients (ratio between actual ET and ETo which depends on plant type) and sub-surface storage (water retention in the drainage layer). For single events, the most sensitive parameters were sub-surface storage and initial substrate moisture conditions. However, Locatelli et al. (2014) primarily investigated Sedum species, which are low water-using species (Starry et al., 2014), and it is possible that other species with different water-use strategies might change which parameters are the most important for event-based retention.

Several studies have explored the impact of alternative species to Sedum on retention (Buccola and Spolek, 2011; Graceson et al., 2013; Szota et al., 2017b), but their results were not conclusive or consistent. Succulent species such as Sedum outperform non-succulent species during drought due to their conservative water use. Although they have relatively high biomass, succulents show low canopy interception and ET and generally show lower rainfall retention (Azeñas et al., 2018; Li et al., 2018). In contrast, non-succulent species show significant potential for runoff mitigation (Soulis et al., 2017a). Considering that plants can significantly influence green roof retention performance via ET (Li et al., 2018), several authors have suggested that selecting plants with high ET should improve retention performance (MacIvor et al., 2011; Poë et al., 2015; Szota et al., 2017a; Whittinghill et al., 2015).

However, the influence of plants on green roof retention is potentially due to factors other than ET (Fassman-Beck et al., 2013; Morgan et al., 2013; Ouldboukhitine et al., 2012). These additional factors include leaf succulence, which adds extra usable water (Farrell et al., 2012; Wadzuk et al., 2013), and plant roots, which can fill large substrate pores to increase substrate water capacity (Nagase and Dunnett, 2011; Poë et al., 2015). In our previous study (Zhang et al., 2018), we found that the root systems of some plant species reduced substrate water holding capacity by introducing preferential flow paths. Therefore, while previous studies have started to pay attention to non-ET plant effects on retention, there are only few studies looking at how these factors might interact to influence retention performance. Consequently, it may mean that the importance placed on selecting plants solely with high ET to achieve higher rainfall retention may lead to inappropriate plant selection for stormwater mitigation on green roofs.

To assist with green roof plant selection, we therefore need to better understand the relative importance of plant-related factors on green roof hydrological processes and therefore retention performance. In this study, we determined to what extent non-ET plant effects, i.e., the reduction in substrate storage capacity by plant roots, affected retention relative to how efficiently plants replenished storage capacity in the substrate via ET.

Section snippets

Experimental design

This study used a simulated rainfall setup with green roof modules (Fig. S2), where components of the water balance could be measured, including runoff after each rainfall event, substrate moisture before and after rainfall events and ET between rainfall events. This experiment was set up at the Burnley Campus of The University of Melbourne (−37.828472, 145.020883). The experiment ran for 468 days, from 14 July 2015 to 25 October 2016.

Eighteen green roof modules (1.15 × 1.15 m) were arranged in

Per-event retention performance

Under simulated Melbourne rainfall conditions, 89–95% of rainfall was retained on an event basis for the 65 rainfall events measured (Table 1). Modules planted with L. longifolia showed 5.5% greater retention than modules planted with S. glauca modules, which had the lowest retention. For 92 applied rainfall events, modules planted with L. longifolia generated runoff in 20% of applied rainfall events and S. glauca modules had 29% of events generating runoff.

Relationship between rainfall depth, runoff and retention

Runoff was significantly related to

Per-event retention performance

In this study, mean per-event retention ranged from 89 to 95%, which is relatively high compared with values reported in the literature (Locatelli et al., 2014; Loiola et al., 2018; Sims et al., 2016; Stovin et al., 2012), which most likely reflects the influence of different climates and green roof configurations (Berndtsson, 2010). Interestingly, although retention was high overall, the differences among plant treatments for the whole period were minor. Even the unplanted bare treatment had a

Conclusions

This study suggests that the maximum storage capacity defines available, pre-event storage, which in turn limits the water availability for plants (i.e., ET/ETo). Therefore, although S. glauca and the mixture had the physiological potential to increase water use when substrate water content increased, their retention performance was limited by storage capacity, which was always less than L. longifolia and S. pachyphyllum. The plant-induced reduction in the storage capacity of the substrate had

Acknowledgements

Thanks to Ariane Lenhardt, Andrea Pianella, Pei-wen Chung, Joerg Werdin and Jessica Kurylo for assisting with data collection. This study was funded by Australian Research Council Linkage grant LP130100731 supported by Melbourne Water and the Inner Melbourne Action Plan group of local governments). Zheng Zhang was supported by the Melbourne International Research Scholarship (MIRS) and Melbourne International Fee Remission Scholarship (MIFRS). Fletcher was supported by an ARC Future Fellowship (

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