By Dr Tom Young, Blue-Green Infrastructure Associate at The Environmental Partnership (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:
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.
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.
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).
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 (above) shows a potential fog net and solar panel dew harvesting green roof by Pirouz et al. 2021.
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 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 promoted a much less species-diverse habitat.
Another example is provided in Figure 3 (below) which shows the massive effect a dripping pipe can have on vegetation. In this instance a tap had been leaking for a couple of years on an extensive roof, leading to a very different vegetation composition in this area.
Methods of irrigation
Water can be applied in a number of ways to a green roof. A number of these were compared in an excellent trial by Rowe et al. 2014. The authors compared overhead sprinklers, drip lines and sub-surface emitters in a variety of green roof settings. In this trial, overhead irrigation seemed to be the most effective at encouraging plant growth. Due to the coarse nature of green roof substrate, the wetting front of sub-surface and surface drip lines did not cover the whole green roof, resulting in patchy growth. However, the addition of a water retentive fabric did help to improve the effectiveness of both sub-surface and surface drip lines by retaining more water and helping to more evenly distribute it (Rowe et al. 2014).
Blue-green roof subsurface
A more efficient sub-surface irrigation method combines the drainage layer of a green roof and the water storage of a blue roof. This means that drainage water from the green roof is retained and can be easily reused. An additional benefit is the much greater amount of stormwater retention, reducing potential flooding downstream. Generally, this involves passive capillary wicking of water from the storage tank into the substrate via a water retentive fabric which helps to evenly distribute water. This type of system has been shown to be very effective at irrigating extensive green roofs, allowing a reduced substrate depth and providing large amounts of evapotranspiration, without the need to use additional water sources (Cirkel et al. 2018), although this type of system can also be topped up with other water sources if captured rainwater runs out.
Irrigation controls are also becoming much smarter. There are plenty of systems now that irrigate according to weather conditions, predicted evapotranspiration and green roof moisture levels (Bandara et al. 2016). Care still needs to be taken when using these to ensure they are set up correctly and calibrated on a regular basis. I have seen plenty of examples where they have gone rogue and irrigated nonstop for months on end!
• Irrigation is sometimes required on green roofs in the UK.
• Requirement is likely to increase in some areas due to climate change.
• Requirement can be reduced by good green roof design (substrate depth, type, drainage layer, moisture retention) and appropriate plant selection.
• Irrigation system is important – some are much more efficient at providing irrigation direct to plant, reducing losses.
• Water source should always be considered. Use non potable, captured or recycled water when possible.
• Excessive irrigation should be avoided at all costs. Irrigation should be viewed as a backup and only used to keep roofs alive. This advice is specific to UK climate.
• Irrigation system can be combined with stormwater storage and passively used to irrigate the roof.
• The green roof system needs to be thought of carefully as a whole system. Irrigation should not be considered at the end as a passing thought to tack onto the design.
Contact Dr Tom Young – email: firstname.lastname@example.org
Find out more about The Environmental Partnership: www.tep.uk.com