The role of plants in green roof rainfall retention
AffiliationSchool of Ecosystem and Forest Sciences
Document TypePhD thesis
Access StatusOpen Access
© 2019 Dr. Zheng Zhang
Increasing urbanisation and associated impervious surfaces have negative impacts on humans and the environment. Therefore, more resilient and sustainable stormwater management that mimics the natural predevelopment hydrology cycle to protect urban water sources and reduce urban flooding is required. Contributing to this approach, green roofs can partly compensate the loss of natural landscapes and mitigate runoff. Green roof plant selection has mostly been limited to species with the ability to survive extreme environmental conditions, typically Sedum species. However, green roofs with these species planted in monoculture may not be efficient in terms of rainfall retention. Selecting plants with water use plasticity and increasing substrate depth may improve green roof hydrological performance in these highly variable environments, without substantially introducing drought risk. To examine this, two experiments were conducted. The first, a 17-month controlled rainfall experiment, was examined in two studies: 1) to understand the overall effect of plant water use strategies and of the mixture of plants within water use plasticity on retention, and 2) to understand the mechanisms driving rainfall retention. The second experiment, a 27-month green roof field study, examined the effect of increasing substrate depth on rainfall retention and plant drought stress. Contrary to my initial hypotheses, plant species with water use plasticity did not always provide greater retention performance, compared with conservative succulent species. Retention performance of a plant mixture was dominated by key species and was not always greater than that of monocultures. The effects of plant treatment on retention performance were significant for medium and large rainfall events, but minor for small rainfall events. Rainfall characteristics and available storage of substrates before rainfall events strongly drove retention performance. Although the effects of ET were well understood and shown to be as the key factor for green roof retention, ‘Non-ET’ plant effects (i.e., the reduction in storage capacity, either via preferential flow, or roots occupying storage volume) can change the relative importance of ET. That is, the maximum storage capacity becomes a more important driver of available storage in the substrate compared with the ability of plants to deplete that stored moisture. Increasing substrate depth only marginally improved retention performance, however, deeper substrates significantly reduced the period of time plants were exposed to drought stress. The results showed that green roofs perform well for retention (average ~73% in total study period; average ~91% in per-event), particularly in warm and dry climates with a large proportion of small rainfall events. Selection of plants with water use plasticity and the use of mixtures was less important for improving retention than climate. However, when planting treatments were compared, root-induced preferential flow paths were shown to reduce the water storage capacity of substrates, reducing the water available for evapotranspiration and therefore reducing retention. Therefore, future plant selection needs to consider root traits in addition to water use strategies. In Melbourne, green roof substrates did not need to be much deeper than 150 mm, as the increase in retention was minimal beyond this depth and it was sufficient for maintaining plant performance.
Keywordsgreen roof; retention; evapotranspiration; plant water use; substrate depth; drought stress
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