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ACADEMIC CORNER DROUGHT TOLERANCE FOR A CHANGING CLIMATE – PART 2 By Dr Tom Young, Blue-Green Infrastructure Associate at TEP. When I spoke about water on green roofs in my last article, I mentioned in passing the role of irrigation on green roofs. However, it really deserves a full article to itself. Irrigation can be a contentious issue on green roofs, but very often it is not thought through properly. In many situations irrigation may not be appropriate for a green roof. However, due to our changing climate some form of irrigation, either as a backup or an establishment method, is recommended in some situations. As a caveat, this article is designed to speak to a UK design audience. Other countries have much more extreme climates than us and therefore irrigation requirements are very different. When and how to irrigate You need to be very careful when deciding to specify an irrigation system on a green roof. An irrigation system shouldn’t be used to make up for a poor or inadequate design. It should be used as a backup system to cope with extreme events. With extreme events becoming more common, the need for additional irrigation is likely to increase in the future. However, green roof design should also adapt to changing climate i.e. different plant species, landscape design or substrate depth. There should be a hierarchy of water sources used for any irrigation system. Potable mains water should always be used as a last resort as there are plenty of more sustainable – and cheaper in the long term – sources of water. This list provides a rough water source hierarchy that should be followed: Left: Dr Tom Young. Above: Figure 1 – Illustration shows a potential fog net and solar panel dew harvesting green roof. See Fog water section on next page. 1. Rainwater direct or reuse (this could also include fog/dew water) 2. Recycled greywater or air-conditioning condensate 3. Treated wastewater (hopefully to be made more available in the UK in the future) 4. Saline water (very much location dependent) 5. Potable water. Rainwater harvesting The most obvious way of irrigating a green roof is to collect all the water that falls on it. A degree of water is retained anyway but green roofs will drain like a normal roof when at field capacity. A variety of methods exist to capture drainage water. These range from large tanks at ground level, to more innovative blue-green roofs which store water on the roof (see subsurface irrigation section). Water can then be moved from these storage areas with either pumps, or passively wicked into the substrate on demand. Air conditioning condensate Air conditioning (AC) units produce condensate water as part of their mechanical process. In UK cities the use of air conditioning units is becoming much more prevalent. An AC unit designed for a single room or apartment on a green roof in Beirut provided between 0.2-1.2 l hour -1 depending on the humidity (Sisco et al. 2017). This wouldn’t be enough to support a large green roof but could be used to help supplement other water sources. The advantage of this water source is that as temperatures increase and water demand rises, the amount of AC water available is also likely to increase. 14 GREENSCAPE WINTER <strong>2022</strong> • www.greenscapemag.co.uk
Greywater Greywater can be defined as wastewater from domestic and industrial uses. The easiest and safer to use sources of greywater include kitchen drains, hand washing from sinks and domestic clothes washing. This can be reused on green roofs with minimal treatment, with the green roof actually providing the treatment in some cases. Due to its regular production, large amounts of storage are not needed as it is a very predictable supply (Mahmoudi et al. 2021). Fog water In coastal or humid environments, a surprising amount of water can be collected from fog/dew. Fog collectors can range from giant vertical mesh nets which capture fog, to adapted solar panels which collect dew. A study of different warm climates found that between 1.2-15.6 l m -2 day -1 could be harvested from fog water and 0.2- 0.7 l m -2 day -1 from dew. This could potentially supply 100% of the irrigation need of green roofs in these climates (Pirouz et al. 2021). Figure 1 (left) shows a potential fog net and solar panel dew harvesting green roof by Pirouz et al. 2021. Saline water Saline water is another option to use on green roofs. Saline water can come directly from the sea, or even from brackish groundwater or treated wastewater. In an extreme example, Ntoulas & Varsamos 2021 irrigated warm season turfgrass in a green roof setting with seawater from Athens’s harbour. The idea was to see if the turfgrass could be kept alive under extreme conditions i.e. during a long summer drought when no other water was available. They showed that although turf quality declined during the irrigation period, turf still survived. An effective method was to over irrigate with salty water in order to Above: Figure 2 shows an inappropriate irrigation system on a green roof in the UK. prevent salt build-up in the substrate. The extra water, despite being salty helped to keep salt in solution and pass out of the substrate as the excess drained (Ntoulas & Varsamos 2021). Benefits of irrigation It sounds obvious, but in the harsh green roof environment a small amount of extra water, specifically during extended dry spells, can significantly increase plant survival. This is particularly true in the first few establishment years of a green roof. It also applies to green roofs that have a wider range of plants other than very hardy succulents. Irrigation provides a greater amount of plant available water which prevents plant death, lowers substrate temperature, and reduces stress on plants. A study in the US showed that irrigating green roof modules could reduce their average summer substrate temperature by between 2-4ºC, as well as reducing the level of extreme substrate temperatures (Guckenberger et al. 2011). This can also then result in greater ecosystem service benefits, for example a greater cooling effect on the urban environment (Van Mechelen 2015). Reduced plant mortality also works out to be much cheaper in the long term as plants do not need to be regularly replaced! Negatives of irrigation Irrigation, when used carefully, is very effective at helping plants establish and survive extended dry spells. However, when used excessively, irrigation can lead to poor quality green roofs. Green roofs are designed to be harsh environments to prevent excessive plant growth and ingress of weed species. Excessive irrigation leads to certain plant species (grasses, tree seedlings) dominating and pushing other more desirable species out. See Figure 2 (above) for an example of an inappropriate irrigation system on a green roof in the UK. This system was too small to cover the whole roof and was set on a regular timer, which had over-irrigated the roof for a number of months. Species diversity was significantly less in this area and had Continued on page 16 >>> www.greenscapemag.co.uk • GREENSCAPE WINTER <strong>2022</strong> 15
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Winter 2022

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