Author: Sylvie Kunz
PhD candidate, Green Infrastructure Research Labs (GIRLS), Cities Research Institute, Griffith University, Queensland, Australia
Green roofs are a nature-based approach to help cities overcome the negative effects of urbanisation. By greening the cities, stormwater can be retained, the urban environment cooled, and habitats can be created. Monitoring of green roof water balance can assist in understanding these improvements.
For green roofs to deliver these desired ecosystem services, a suitable selection of substrates and plant species needs to be chosen. By trialing native plant species under real life conditions, their performance to provide green coverage and retain rainwater can be quantified.
One trial of the Green Infrastructure Research Labs (GIRLS) at Griffith University investigates extensive green roof water balance and native plant performance. The study is currently trialing different underlay designs (preformed water retention layer (Fig. 1); granular drainage layer) and their effect on plant health and rainwater retention. A compromise between water retention for urban stormwater control and drainage performance for plant health needs to be reached.
Figure 1: Classic green roof layers (waterproofing, water retention layer and geofabric) located underneath the engineered substrate (not shown).
Key outcomes will be knowledge on underlay design and plant selection for extensive roofs. The study is set up on a roof space on campus with full sun exposure in the humid-subtropical climate of southern Brisbane. The experiments are aimed to run for at least one full year and will capture weather variability over all seasons in Southeast Queensland.
To monitor the moisture held in the substrate, each module was fitted with a soil moisture sensor in the centre of the substrate layer. Tipping buckets connected to the lower end of the modules collect data on runoff water volumes (Fig. 2).
Figure 2: Green roof modules on Griffith University's Nathan campus (QLD, Australia).
The experiment looks at a range of different native plant forms including groundcovers, small shrubs, and strappy-leaved plants, selected from different heathland and coastal habitats. As a control to distinguish the effect of planting on the water balance of extensive green roofs, each underlay treatment is set up with a bare substrate module to quantify the water balance without planting.
Weather data is collected in the monitoring location using an all-in-one weather station (ATMOS 41) connected to a data logger. The insight on the growing conditions helps scientists to design suitable green roof layouts for the local climatic conditions.
Runoff water samples are being analysed for nutrients that can be washed out from the substrates. A minimum amount of slow-release fertiliser should be applied to keep water quality leaving the green roof acceptable. Australian native plants are typically adapted to low-nutrient soils and are therefore expected to have a positive influence on runoff water quality compared to exotic species.
Beyond the effect of the drainage layers, this research also investigates the need for irrigation of the different plant communities. After plant establishment, the supplemental irrigation will be reduced to identify the minimum irrigation demand. This demand might vary for the different underlay treatments and plant species chosen. Symptoms of plant stress will be monitored to select the best suited irrigation for each green roof design.
Plant health is monitored to assess species suitability for the exposed conditions and limited substrate depths. Individual plant health is recorded using a Chlorophyll content index meter, visual health ratings, and height measurements. On a plot basis, green coverage of the vegetation communities is compared using digital image analysis from photographs (Fig.3). High green coverage is an indication of a healthy green roof.
Figure 3: Example of green coverage analysis at experiment establishment.
This research will help to provide climate-appropriate design guidance for green roofs in the subtropics.
For further information on this experiment, please contact Sylvie Kunz (sylvie.kunz(Replace this parenthesis with the @ sign)griffithuni.edu.au).
Green infrastructure research labs (GIRLS) are available for design consultation and industry collaborations in the fields of green infrastructure (green roofs, green walls, phytocapping and related topics) – please contact Dr. Ruby Michael (ruby.michael(Replace this parenthesis with the @ sign)griffith.edu.au) to enquire.