# Adapting green roof irrigation practices for a sustainable future: A review

Carmen van Mechelen, Thierry Dutoit, Martin Hermy

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Carmen van Mechelen, Thierry Dutoit, Martin Hermy. Adapting green roof irrigation practices for a sustainable future: A review. Sustainable Cities and Society, 2015, 19, pp.74-90. 10.1016/j.scs.2015.07.007. hal-01445205

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# Adapting green roof irrigation practices for a sustainable future:
A review

Carmen Van Mechelen<sup>a,*</sup>, Thierry Dutoit<sup>b</sup>, Martin Hermy<sup>a</sup>

## ABSTRACT

Sustainable water management is an important global action to alleviate the growing pressure on our water resources and a way to balance both socio-economic and environmental water needs. Irrigation of green roofs and other urban green infrastructures often has a negative connotation in a water management context. This is especially the case in semi-arid regions where both water scarcity during summer and other water needs compromise the public support of urban irrigation systems. However, irrigating green roofs is also beneficial as it improves plant survival and temperature control. Information on green roof irrigation is still very scarce, and this review attempts to draw attention to alternative water sources and water conservation strategies in order to make green roof irrigation practices more sustainable. Suggestions for more sustainable green roof irrigation showed that water can be conserved by adapting irrigation requirements (i.e. by targeting the vegetation or materials), by using alternative irrigation sources (e.g. gray water and rainwater harvesting) or by controlling and monitoring irrigation regimes. Based on the deficit irrigation strategy, irrigation specifications for green roofs in different climates were proposed. It became clear that irrigation is essential during establishment and the first growing season on all types of green roof and in all climates here considered. Afterwards, irrigation is necessary on green roofs in (semi)-arid climates, and advised in small amounts in the other climates. If public awareness of sustainable water consumption is promoted and if green roofs are designed carefully, then irrigation practices can be sustainable in the long term and contribute to better urban life quality.

Keywords:

Green roofs

Sustainability

Climate

Water management

Deficit irrigation

## Contents

<table><tr><td>1.</td><td>Introduction</td><td>75</td></tr><tr><td>2.</td><td>Methods</td><td>75</td></tr><tr><td>2.1.</td><td>Publication search and selection</td><td>75</td></tr><tr><td>2.2.</td><td>Data exploration</td><td>75</td></tr><tr><td>3.</td><td>Green roof irrigation practices and stormwater management regulations</td><td>76</td></tr><tr><td>3.1.</td><td>Current green roof irrigation practices</td><td>76</td></tr><tr><td>3.2.</td><td>Optimizing irrigation practices</td><td>77</td></tr><tr><td>3.3.</td><td>Examples of stormwater management regulations</td><td>79</td></tr><tr><td>4.</td><td>Irrigation specifications for green roofs in different climatic contexts</td><td>79</td></tr><tr><td>4.1.</td><td>Deficit irrigation theory in a green roof context</td><td>79</td></tr><tr><td>4.2.</td><td>Specifications for arid climates</td><td>79</td></tr><tr><td>4.3.</td><td>Specifications for temperate climates: Hot or warm summers without dry season</td><td>82</td></tr><tr><td>4.4.</td><td>Specifications for temperate climates: Hot or warm and dry summer</td><td>82</td></tr><tr><td>4.5.</td><td>Specifications for temperate climates: Hot summer and dry winter</td><td>83</td></tr><tr><td>4.6.</td><td>Specifications for cold climates</td><td>83</td></tr></table>

E-mail addresses: carmen.vanmechelen@ees.kuleuven.be, vanmechelencarmen@gmail.com (C. Van Mechelen), Thierry.dutoit@imbe.fr (T. Dutoit), Martin.hermy@ees.kuleuven.be (M. Hermy).

<footnote><sup>a</sup> KU Leuven—University of Leuven, Department of Earth and Environmental Sciences, Division of Forest, Nature and Landscape, 3001 Heverlee, Belgium</footnote>

<footnote><sup>b</sup> IMBE, CNRS-IRD, Avignon University, Aix-Marseille University, IUT Avignon, Site Agroparc, BP 61207, 84911 Avignon Cedex 9, France</footnote>

<footnote>* Corresponding author. Tel.: +32 16 329 760.</footnote>
<table><tr><td>5. Concluding remarks</td><td>83</td></tr><tr><td>Acknowledgements</td><td>83</td></tr><tr><td>Appendix A</td><td>84</td></tr><tr><td>Appendix B</td><td>86</td></tr><tr><td>References</td><td>87</td></tr></table>

# 1. Introduction

Next to land use changes and habitat fragmentation, the urbanization process has a pronounced effect on the natural water cycle (Schmidt, 2010). As urban green gets scarcer, evapotranspiration (ET) is reduced, thereby reducing precipitation which reduces ET even further. This ultimately leads to dehydration of the urban environment (Schmidt, 2010). In many semi-arid regions, water is amongst the most limiting natural resources (Kotsiris, Nektarios, Ntoulas, & Kargas, 2013). The problem is becoming increasingly acknowledged and incentives arise to stimulate sustainable water management practices, which aim to balance social, economic and environmental water needs (Bean & Pitt, 2012; Blanco-Gutiérrez, Varela-Ortega, & Purkey, 2013; Islam, Lefsrud, Adamowski, Bissonnette, & Busgang, 2013; Schmidt, 2010). Urban green infrastructures (e.g. parks, gardens, green roofs) are a vital part of water management (Gill, Handley, Ennos, & Pauleit, 2007; Schmidt, 2010). Green roofs provide many ecosystem processes and services, including stormwater management, cooling and insulation of buildings, mitigation of the urban heat island effect, contribution to human health and habitat provisioning (Oberndorfer et al., 2007). They can be classified into two main categories of which extensive green roofs represent the most shallow (2-ca. 20 cm substrate depth) and light weight (70–170 kg/m²) ones. The vegetation of those green roofs generally consists of succulents and other drought-tolerant plants (Bousselot, Klett, & Koski, 2011). Within this green roof category, semi-extensive green roofs have a slightly deeper substrate (12-ca. 20 cm) and the associated higher water holding capacities allow a more diverse range of plant species (Theodosiou, 2009). Finally, intensive green roofs have substrate depths above 20 cm and are also denoted as roof gardens because a more intensive management is needed (Oberndorfer et al., 2007).

Traditionally, extensive green roofs are designed so that natural precipitation is sufficient to ensure plant survival (Durhman, Rowe, & Rugh, 2006). However, the harsh environmental conditions on extensive green roofs pose severe constraints on vegetation development and plant diversity, particularly during summer when high temperatures prevail and water is scarce (Butler & Orians, 2011; Coutts, Tapper, Beringer, Loughnan, & Demuzere, 2012; Maclvor, Margolis, Puncher, & Carver Matthews, 2013; Oberndorfer et al., 2007). Furthermore, climate change will lead to a shift in seasonal precipitation (IPCC, 2014) and it may be expected that irrigation on extensive green roofs even will become a necessity in summer in order to keep the vegetation vital (Gross, 2012; Maclvor et al., 2013; Vanuytrecht et al., 2014).

Despite the eventual necessity of irrigation on green roofs, there are certain drawbacks in terms of the installation of irrigation systems. Globally, water consumption has tripled over the last 50 years, and an increase in water demand has been registered in almost every country (WWAP, 2012). It is expected that by 2030, half of the world's population will live in water stressed areas (UNEP, 2007). The growing consensus on the urban water scarcity problem converted many cities to restrict irrigation rates on green roofs (Rowe, Graber, & Durhman, 2012), especially in regions with frequent droughts like southern Europe and Australia (Ascione, Bianco, de Rossi, Turni, & Vanoli, 2013; Coutts et al., 2012). The additional installation and maintenance costs pose another constraint to irrigation installation (Price, Watts, Wright, Peters, & Kirby, 2011).

Looking for alternative water sources and water conservation practices is essential to obtain more sustainable green roof irrigation practices (Moritani et al., 2013). In essence, a sustainable irrigation system should (i) be able to function properly and deliver benefits over its lifetime; (ii) not compromise other water needs; and (iii) be publicly supported (Abernethy, 1994). Knowledge on irrigation practices and specifications for green roofs, especially in relation to different climates, is still limited. This paper reviews the available information from scientific publications involving both green roofs and irrigation. First, current irrigation techniques will be discussed and an overview of suggestions to make irrigation practices more sustainable will be given. Also public incentives regarding stormwater management will be highlighted. Thereafter, we discuss the deficit irrigation strategy in a green roof context and compare the calculated irrigation requirements for different climates with irrigation regimes from studies in which outdoor green roof experiments were conducted.

# 2. Methods

## 2.1. Publication search and selection

The review is based on scientific studies found through the electronic journal database ISI Web of Knowledge (consulted on 07/07/2014) using the specific search terms 'green roofs' and 'irrigation'. All publication years were considered and papers had to be written in English. Both peer-reviewed scientific publications and conference proceedings were taken into account. The list of studies was further completed with publications, found through Google Scholar using the same search terms. The database for the review comprised 109 publications of which 94 peer-reviewed publications and 15 conference proceedings.

## 2.2. Data exploration

Essential information (study type, study location, topic, green roof type, irrigation method and regime) was extracted to construct a database. The study type categorized the documents into general information, greenhouse experiments, model simulations and outdoor experiments. For outdoor green roof experiment studies, the study location was linked to the respective climate using the Köppen climate classification (KCC, Table 1; Peel, Finlayson, & McMahon, 2007). Both warm temperate climates (KCC: C) and snow climates (KCC: D) are represented by multiple studies, as well as the arid climate (KCC: B) in Colorado and Spain. In Table 1, the $P_{th}$ is a threshold for dryness (in mm, Eq. (1)) which has been introduced for the arid climate (KCC: B). It depends on the annual temperature and precipitation cycle (Peel et al., 2007).

$$P_{threshold} = \begin{cases} 2 \times \{\text{annual } T\} & \text{if } 70\% \text{ of annual } P \text{ falls in winter} \\ 2 \times \{\text{annual } T\} + 28 & \text{if } 70\% \text{ of annual } P \text{ falls in summer} \\ 2 \times \{\text{annual } T\} + 14 & \text{if } \text{otherwise} \end{cases} \quad (1)$$

Irrigation methods and regimes were summarized for the outdoor experiments (Appendix A) and greenhouse and model
<table><caption>**Table 1**
Summary of Köppen Climate Classification codes (KCC) covered by the studies collected in this review. Adapted from Peel et al. (2007). (T: temperature in °C; P: precipitation in mm).</caption><thead><tr><th>KCC</th><th>Description</th><th>Region examples</th><th>No. of studies</th></tr></thead><tbody><tr><td>Bsk</td><td>Arid climate: cold steppe<br>Annual T &lt; 18 °C; annual P ≥ 5 × P<sub>th</sub></td><td>USA: Colorado (Denver),<br>Spain (Lleida)</td><td>2</td></tr><tr><td>Bsh</td><td>Arid climate: hot steppe<br>Annual T ≥ 18 °C; annual P ≥ 5 × P<sub>th</sub></td><td>Israel (Beersheba)</td><td>1</td></tr><tr><td>Cfa</td><td>Temperate climate: hot summer but no dry season<br>0 °C &lt; T<sub>min</sub> &lt; 18 °C; T<sub>max</sub> ≥ 22 °C</td><td>USA: Texas, Alabama,<br>Florida, Japan (Kobe)</td><td>6</td></tr><tr><td>Cfb</td><td>Temperate climate: warm summer but no dry season<br>0 °C &lt; T<sub>min</sub> &lt; 18 °C; T<sub>max</sub> &lt; 22 °C; at least 4 months with T ≥ 10 °C</td><td>UK (Sheffield), Germany<br>(Berlin), Australia<br>(Melbourne)</td><td>6</td></tr><tr><td>Csa</td><td>Temperate climate: hot and dry summer<br>0 °C &lt; T<sub>min</sub> &lt; 18 °C; T<sub>max</sub> ≥ 22 °C; P<sub>dry, summer</sub> &lt; 40 mm;<br>P<sub>dry, summer</sub> &lt; P<sub>wet, winter</sub>/3</td><td>Greece (Athens), Israel (Tel<br>Aviv), Italy (Pisa), Australia<br>(Adelaide)</td><td>12</td></tr><tr><td>Csb</td><td>Temperate climate: warm and dry summer<br>0 °C &lt; T<sub>min</sub> &lt; 18 °C; T<sub>max</sub> &lt; 22 °C; at least 4 months with<br>T ≥ 10 °C; P<sub>dry, summer</sub> &lt; 40 mm; P<sub>dry, summer</sub> &lt; P<sub>wet, winter</sub>/3</td><td>USA: Oregon</td><td>3</td></tr><tr><td>Cwa</td><td>Temperate climate: hot summer and dry winter<br>0 °C &lt; T<sub>min</sub> &lt; 18 °C; T<sub>max</sub> ≥ 22 °C; P<sub>dry, winter</sub> &lt; P<sub>wet, summer</sub>/10</td><td>Hong Kong (China)</td><td>3</td></tr><tr><td>Dfb</td><td>Cold climate: warm summer but no dry season<br>T<sub>min</sub> ≤ 0 °C; T<sub>max</sub> &lt; 22 °C; at least 4 months with T ≥ 0 °C</td><td>USA: Massachusetts,<br>Michigan, Canada: Nova<br>St. Petersburg (Beijing)</td><td>16</td></tr><tr><td>Dwa</td><td>Cold climate: hot summer and dry winter<br>T<sub>min</sub> ≤ 0 °C; T<sub>max</sub> ≥ 22 °C; P<sub>dry, winter</sub> &lt; P<sub>wet, summer</sub>/10</td><td></td><td>2</td></tr></tbody></table>

simulation studies (Appendix B). If relevant, the minimum daily summer irrigation requirement (I<sub>day</sub>, in mm) was calculated. This allowed comparison of water requirements specified in different studies.

Fig. 1 suggests that research regarding green roofs and irrigation is growing. Before 1999, no studies were found with the search terms 'green roofs' and 'irrigation'. The first four publications, used in this review (Argue & Pezzaniti, 1999; Kolb, 2004; Monterusso, Rowe, Rugh, & Russell, 2004; Velazquez, 2003), were all conference proceedings, after which more publications emerged. Although the modern green roof technology was founded in Northern and Central Europe, only 21% of the studies used in this review originate from this continent (Table 2). The majority of the studies were conducted in North America (44%), especially in the USA. In Asia and Oceania, respectively China and Australia are the countries in which green roof research is progressing. In South America, only

<figure><img src="image_2.png" /><figcaption>**Fig. 1.** Stacked bar chart showing the evolution in time of the number of publications on green roofs and irrigation. Both publications used in the review (n = 109) and excluded publications (n = 33) after screening (because not useful or irrelevant) are shown.</figcaption></figure>

one study could be found (Vieira, De Queiroz, Fagundes, & Dallacort, 2013) and no studies were found in Africa. As regards the main topics, only five publications (5%) just consider the subject irrigation (Fig. 2). The largest part of the publications involved the topics: vegetation performance (36%), energy and thermal performance (27%) and stormwater management (20%). This suggests that the subjects green roofs and irrigation have a wide support base and are of particular interest to ecologists, engineers, designers and policy makers.

## 3. Green roof irrigation practices and stormwater management regulations

### 3.1. Current green roof irrigation practices

A green roof comprises multiple layers that protect the underlying roof and support the vegetation, but the water storage capacity

<table><caption>**Table 2**
Overview of the continents and countries in which the studies used in the review were conducted. Number of publications and relative frequency indicate where research regarding green roofs and irrigation is situated. (NA: not applicable).</caption><thead><tr><th>Continent</th><th>Country</th><th>Number of publications</th><th>Relative frequency (%)</th></tr></thead><tbody><tr><td rowspan="3">Oceania</td><td>Australia</td><td>6</td><td rowspan="3">7.34</td></tr><tr><td>New Zealand</td><td>2</td></tr><tr><td><strong>Asia</strong></td><td><strong>13</strong></td></tr><tr><td rowspan="10">Europe</td><td>China</td><td>7</td><td rowspan="10">11.93</td></tr><tr><td>Japan</td><td>3</td></tr><tr><td>Korea</td><td>1</td></tr><tr><td>Israel</td><td>2</td></tr><tr><td><strong>UK</strong></td><td><strong>23</strong></td></tr><tr><td>France</td><td>8</td></tr><tr><td>Germany</td><td>2</td></tr><tr><td>Greece</td><td>2</td></tr><tr><td>Italy</td><td>2</td></tr><tr><td>Spain</td><td>1</td></tr><tr><td rowspan="2">South America</td><td>Brazil</td><td>1</td><td rowspan="2">0.92</td></tr><tr><td><strong>North America</strong></td><td><strong>48</strong></td><td></td></tr><tr><td></td><td>Unknown</td><td>3</td><td rowspan="2">44.04</td></tr><tr><td></td><td>Canada</td><td>5</td></tr><tr><td></td><td><strong>USA</strong></td><td><strong>40</strong></td><td></td></tr><tr><td>NA</td><td></td><td><strong>16</strong></td><td>14.68</td></tr></tbody></table>
<figure><img src="image_3.png" /><figcaption>**Fig. 2.** Diagram showing the main topics of the 109 publications used in this review.</figcaption></figure>

is limited due to physical and economic factors (FLL, 2008). The substrate is the main water storage layer, but water can also be retained in the drainage layer (Theodosiou, 2009) or if present, the water retention layer (WRL), which is a mat from mineral wool or polymeric fibers (1–6.5 cm thick) under or above the drainage layer (Grant, 2007). For extensive green roofs, additional irrigation is often suggested during plant establishment as early drought can be very harmful for further plant establishment, performance and survival (Getter & Rowe, 2006; Nagase & Dunnett, 2010; Thuring, Berghage, & Beattie, 2010). Permanent irrigation would not be necessary after establishment if the extensive green roof is properly designed in terms of plant selection, substrate type and depth, and if sufficient water from precipitation is available (Getter & Rowe, 2006; VanWoert, Rowe, Andresen, Rugh, & Xiao, 2005a). Next to improved plant growth and survival under dry conditions (Butler & Orians, 2011; Price et al., 2011), irrigation can be beneficial in other ways as well. Less drought stress implies that more variation in plant species is possible (Köhler, 2006). Considering ecosystem services, irrigation is generally believed to improve thermal performance of a green roof, as water has a high thermal mass, or heat capacity, and can therefore be more effective in insulating the building from temperature extremes compared to dry, non-irrigated green roofs (Jim & Peng, 2012a,b; Kolokotsa, Santamouris, & Zerefos, 2013). Also, many studies indicate positive effects of irrigation on ET and therefore mitigation of the urban heat island effect (e.g. Santamouris, 2014; Sun, Bou-Zeid, & Ni, 2014). The model of Bass, Krayenhoff, Martilli, Stull, and Auld (2003) for example, indicated that irrigated green roofs can reduce the temperature of a large part of the city of Toronto with 1 °C. However, some studies point at the climate-dependency of the both the insulating capacity and evaporative cooling potential of irrigated green roofs (Jim & Peng, 2012a,b; Sun et al., 2014).

From an economic perspective, considering an irrigation system during the green roof design phase is less expensive compared to the need of replanting dead plants during the annual maintenance period (Maclvor et al., 2013; Price et al., 2011), an action which should be added to the average maintenance cost (e.g. 1.28 €/m² per year; Eastside Sustainability, 2004). It is also less expensive compared to an air conditioning installation. Sun et al. (2014) calculated the costs of different irrigation strategies under the current prizes of water and electricity in Beijing (respectively 0.44 €/m³ and 0.06 €/kWh). An irrigated green roof of 100 m² with an irrigation control limit of 0.3 m³/m³ can save 14 € over a 3-month period compared to a non-irrigated but air conditioned building with a moderate performance coefficient (Sun et al., 2014). Less positive effects of irrigation include a decrease in stormwater

performance as moist substrates can retain less water during a precipitation event (Schroll, Lambrinos, Righetti, & Sandrock, 2011a; Spolek, 2008). There is also an increased probability of weed establishment on irrigated green roofs, especially when the proportion of bare ground is still high (Schroll, Lambrinos, & Sandrock, 2011b). Furthermore, under saline irrigation (e.g. when secondarily treated municipal wastewater is used) salt accumulation may lead to salinity stress and reduced ET (Moritani et al., 2013). Irrigation is also considered unsustainable in regions with water scarcity or when potable water is used (Moritani et al., 2013; Rowe, Kolp, Greer, & Graber, 2014).

In Northern and Central Europe, the green roof technology follows the guidelines of 'The Landscape Development and Landscaping Research Society', or FLL (FLL, 2008). These however, provide only little information regarding irrigation specifications. The main irrigation methods suggested in the FLL guidelines are (1) a (spray) hose or sprinkler system; (2) drip lines; (3) overhead irrigation systems and (4) automated irrigation systems with in-built reservoirs (FLL, 2008). In the context of vegetable cultivation on green roofs (i.e. lettuce, chicory and endive), Cho, Choi, and Lee (2010) studied the following irrigation methods: water retained in the drainage layer, irrigation through a wick, and drip irrigation. The water holding capacity (WHC) of the substrate was highest when using drip irrigation, but wick irrigation proved to be more efficient for plant growth because the water content in the substrate remained constant for a longer time (Cho et al., 2010). Rowe et al. (2014) suggested that wick irrigation and drip irrigation are good options for green roofs with plug plants, as these types of irrigation mainly provide water at the plant root zone in close proximity to the wick or drippers. However, they might not be the ideal choice for more traditional extensive green roofs where a large vegetated surface consisting of pre-vegetated mats, seeded species or succulents, needs to be irrigated. Combining drip irrigation with an additional WRL could improve irrigation efficiency. Overhead irrigation may be a better choice, as it distributes water more uniformly and leads to higher substrate WHC, less runoff, and better plant growth and health compared to drip irrigation (Rowe et al., 2014).

## 3.2. Optimizing irrigation practices

Different water conservation strategies have been proposed that enable sustainable irrigation of green areas (Chang, Rivera, & Wanielista, 2011). These strategies can be categorized into (1) adaptation of irrigation requirements; (2) alternative irrigation sources and (3) control and monitoring of irrigation regimes. All strategies can be implemented separately or simultaneously, depending on both design and financial project considerations. First of all, water can be conserved by targeting the vegetation. Extensive green roofs are mainly designed with succulent, drought tolerant Sedum spp. (Carter & Keeler, 2008). These can also act as nurse plants for other less drought tolerant plants through facilitation, i.e. by reducing the substrate temperature and evaporation rate (Butler & Orians, 2011). Also many mosses can act as facilitators, and they are very well suited to survive harsh environmental conditions, can store a considerable amount of rainwater and only need a thin substrate (Anderson, Lambrinos, & Schroll, 2010). Alternatively, one could think of green roofs with wetland species as these species have an excellent WHC and can effectively decrease surface temperatures (Song et al., 2013). Over an entire growing season, wetland plants only need 20% of the water needed to irrigate traditional extensive green roofs, which makes them more sustainable and less costly. Wetland plants are easy to propagate and they are effective in cooling buildings because of their high transpiration rates. The two macrophyte species tested by Song et al. (2013), Iris laevigata Fisch. and Iris pseudoacorus L., proved to be rather stress tolerant, as they could survive both after three weeks of flooding and two
<caption>**Table 3**</caption>

Summary of water holding capacity (WHC) values advised by the FLL guidelines and of alternative substrate materials tested in different green roof studies. The WHC indicates the ability of a material, in compacted condition, to hold water, and is generally expressed as the water content of a substance after initial water saturation followed by a 2-h dripping period (FLL, 2008).

<table><thead><tr><th>Reference</th><th>Materials</th><th>WHC</th><th>Unit</th></tr></thead><tbody><tr><td rowspan="2">FLL (2008)</td><td>General intensive GR substrate</td><td>≥45 (max 65)</td><td>% vol.</td></tr><tr><td>General extensive GR substrate</td><td>≥35 (max 65)</td><td>% vol.</td></tr><tr><td rowspan="2">Elstein et al. (2008)</td><td>Potting soil</td><td>394<sup>a</sup></td><td>% DW</td></tr><tr><td>Petrochemical-based hydrophilic foam sheets</td><td>1994<sup>a</sup></td><td>% DW</td></tr><tr><td rowspan="6">Harp et al. (2008)</td><td>Fiberglass mats</td><td>1804<sup>a</sup></td><td>% DW</td></tr><tr><td>Open cell foam</td><td>56.5</td><td>% vol.</td></tr><tr><td>Polyurethane foam</td><td>71.9</td><td>% vol.</td></tr><tr><td>Synthetic sponge</td><td>65.9</td><td>% vol.</td></tr><tr><td>Sand</td><td>36.3</td><td>% vol.</td></tr><tr><td>Peat moss-bark-sand 3:1:1</td><td>51.9</td><td>% vol.</td></tr><tr><td rowspan="3">Nektarios et al. (2011)</td><td>Green roof substrate</td><td>30.3</td><td>% vol.</td></tr><tr><td>Soilless substrate</td><td>37<sup>b</sup></td><td>% vol.</td></tr><tr><td>Substrate with sandy loam soil</td><td>38<sup>b</sup></td><td>% vol.</td></tr><tr><td rowspan="4">Kotsiris et al. (2013)</td><td>Pumice-peat-zeolite 65:30:5</td><td>25<sup>b</sup></td><td>% vol.</td></tr><tr><td>Pumice-compost-zeolite 65:30:5</td><td>30<sup>b</sup></td><td>% vol.</td></tr><tr><td>Sandy loam soil-perlite-zeolite 30:65:5</td><td>35<sup>b</sup></td><td>% vol.</td></tr><tr><td>Grape marc compost-soil-perlite 2:3:5</td><td>32<sup>b</sup></td><td>% vol.</td></tr><tr><td rowspan="3">Papafotiou et al. (2013)</td><td>Peat:soil:perlite 2:3:5</td><td>30<sup>b</sup></td><td>% vol.</td></tr><tr><td>Amended soil</td><td>58.7</td><td>% vol.</td></tr><tr><td>Furnace bottom ash</td><td>43.8</td><td>% vol.</td></tr></tbody></table>

weeks of drought. The main problem that withholds wetland green roofs from being implemented is their increased weight compared to traditional extensive green roofs, together with a potential pest problem associated with standing water (Song et al., 2013).

Another strategy to adapt the irrigation requirement is by optimal design of green roof materials, by for example developing green roof substrates with higher WHC. Addition of sandy loam soil and use of amended soils (i.e. mix of red gravel, vermiculite and bark compost), perlite based substrates, foam sheets and fiberglass can all improve the WHC of the green roof system (Table 3). Some water holding additives, like hydrophilic gels, are also currently explored (Sutton et al., 2012; Williams, Rayner, & Raynor, 2010). Alternatively, a WRL retains additional runoff and hence available water for green roof vegetation (Guo, Zhang, & Liu, 2014; VanWoert et al., 2005a, 2005b). This layer has been shown to be crucial for plant survival in semi-arid climates (Benvenuti & Bacci, 2010; Guo et al., 2014; Santamouris et al., 2007) but also in other climates, the irrigation requirements can be significantly reduced. In Vancouver (Canada), summer irrigation decreased from 54 to 9 mm by adding a WRL, and in subtropical Shanghai (China), the WRL suppressed the need of additional irrigation completely (Roehr & Kong, 2010).

A second way to conserve water is by finding alternative irrigation sources. For example gray water, which is the waste water from in and around the house (including bathroom sinks, showers and washing machines, but excluding water originating from toilet flushing, dishwashers and kitchen sinks), could be reused for irrigation purposes (e.g. Chang et al., 2011; Gill et al., 2007; Ouldboukhitine, Spolek, & Belarbi, 2014). In Sweden for example, gray water, together with black water, currently accounts for 58%

of the yearly water discharge into wastewater treatment plants (Semadeni-Davies, Hernebring, Svensson, & Gustafsson, 2008). By making use of this gray water, potable water consumption of a household can be reduced by 31%, and there is less pressure on the city's sewage system (Chang et al., 2011). However, if the gray water is clean enough to sustain the green roof vegetation and related ecosystem services (e.g. thermal regulation, stormwater retention, aesthetic quality, habitat provisioning) in the long term is still unclear. Ouldboukhitine et al. (2014) showed that the thermal performance of green roofs decreased with approximately 30% when irrigated with gray water instead of clean water, as the chemical products in the gray water, like lactic acid, cellulose and glycerol, affected the plant's activity and hence transpiration processes.

Another possibility is rainwater harvesting (RWH), in which runoff is collected and stored (Coutts et al., 2012; Gill et al., 2007). Runoff harvested from green roofs themselves has been shown to be sufficiently clean to be reused for urban irrigation (Argue & Pezzaniti, 1999; Razzaghmanesh, Beecham, & Kazemi, 2014a). By passing through the substrate, the green roof runoff pH increases from approximately 5.5 to 7.5 and slightly mitigates acid rain (Czemiel Berndtsson, 2010). The green roof runoff also has an elevated amount of suspended solids and a higher turbidity, which will decrease over time (Morgan, Alyaseri, & Retzlaff, 2011). Besides being an irrigation source, RWH helps to control floods and reduces erosion and stormwater pollution (Hardin, Wanielista, & Chopra, 2012; Islam et al., 2013). There are many ways to harvest rainwater, including rainwater tanks or cisterns, constructed wetlands and treatment trains (Coutts et al., 2012; Hardin et al., 2012). In the latter method, the rainwater passes a bio-filtration system and is stored in a tank. Sometimes, underground aquifers can be recharged with the bio-filtrated rainwater, a process also called Aquifer Storage and Recovery (Argue & Pezzaniti, 1999; Coutts et al., 2012). By installing a rainwater tank on a green roof, the total annual stormwater retention increased from 43 to 87% in Florida (Hardin et al., 2012). In Brasil, the potential of RWH has also been acknowledged, although the installation of rainwater tanks still causes some concern as a considerable area is required (reservoir volume 16–102 m³ for a 100 m² green roof), which is expensive in urban areas (Vieira et al., 2013).

In the third category, irrigation quantity can be minimized through monitoring and control of irrigation regimes (Sutton et al., 2012; Guo et al., 2014). Meteorological factors, mainly relative humidity and number of sunshine hours as they affect the water consumption the most, are important to consider for green roof irrigation systems (Feng, Yao, & Zhang, 2012). Water requirements could be calculated by using ET data and precipitation information (Bean & Pitt, 2012; Feng et al., 2012; Stovin, Poë, & Berretta, 2013), or by studying substrate moisture with sensors (Jim & Peng, 2012a,b). Depending on the local climate, it is often possible to use irrigation only during establishment and during prolonged drought (Sutton et al., 2012). Otherwise, irrigation should be turned on when the substrate moisture drops below a specified level, like the stress point (transition between readily available water in the substrate's larger pores and less available water in the small pores; Jim & Peng, 2012b). Irrigation can be controlled using a smart controller, which turns on when necessary (at night or when soil moisture drops below the stress point) but deactivates when rainfall is registered (Hardin et al., 2012; Sun et al., 2014). Ascione et al. (2013) are currently investigating a multivariable controller for irrigation and ET management, which takes both solar radiation and relative air humidity into account. Furthermore, best management practices for monitoring and managing irrigation are examined by Lea-Cox et al. (2013). Their work includes the development of sensor networks and software to control irrigation at the species level. An overview of the scope, vision and future challenges of their work is summarized in Lea-Cox et al. (2013).

<footnote><sup>a</sup> WHC on a dry weight (DW) basis = [(saturated weight - DW)/DW] × 100.</footnote>

<footnote><sup>b</sup> WHC at 50 cm tension.</footnote>
Next to water conservation strategies, many cities also aim at saving energy, as the energy required for heating and cooling buildings is expensive (Chang et al., 2011). Solar shading and evaporative cooling are two ways to mitigate urban temperatures and hence the need for heating or air conditioning (Blanco-Gutiérrez et al., 2013; Santamouris et al., 2007). Green roofs play an important role in improving thermal comfort, especially during the day (Gross, 2012; Kolokotsa et al., 2013) and when the climate is not very hot (KCC: Cfb or Dfb) (Ascione et al., 2013; Sailor, 2008; Sailor, Elley, & Gibson, 2011). But in order to maintain the reduced energy demand during summer, irrigation is needed to keep the substrate moist (Kolokotsa et al., 2013; Li, Bou-Zeid, & Oppenheimer, 2014; Sailor et al., 2011). Therefore, other solutions to save energy are investigated in warm to hot climates. Cool roofs (also referred to as reflective or white roofs) are considered a good alternative (Ascione et al., 2013; Kolokotsa et al., 2013; Santamouris, 2014). This roof type can decrease the total daily heat flux by around 80% compared to a traditional bitumen roof, while green roofs decrease the daily heat flux by 52% (Scherba, Sailor, Rosenstiel, & Wamser, 2011). However, the performance of cool roofs is strongly hindered by factors like dirt accumulation, acid rain and UV radiation (Sailor et al., 2011; Santamouris, 2014), and compared to green roofs, cool roofs do not contribute to stormwater management, aesthetics or habitat value.

### 3.3. Examples of stormwater management regulations

Next to the water conservation and energy saving strategies described above, public authorities can develop regulations involving stormwater management (Carter & Keeler, 2008). For example, in several countries, building owners need to pay a fee for the quantity of runoff that runs into the sewer system. If green infrastructure elements like green roofs are installed, the owner can be exempt from paying this fee (Carter & Keeler, 2008). In Portland (Oregon, USA), building height restrictions are attenuated if a building owner has a floor-area ratio bonus. By definition, this ratio is the ratio of the total floor area of the building to the area of its zoning lot, which is the basic unit for zoning regulations (NOAA, 2012). The bonus can only be obtained if a green roof is installed and irrigation regulations (≤12.7 mm every 10 days during establishment; ≤6.35 mm every 10 days after establishment) are met (Schroll et al., 2011a). In the USA, it is also possible for building owners to obtain a Leadership in Energy and Environmental Design (LEED) certificate if green roofs without irrigation or using gray water or recycled water for irrigation are installed (Carter & Keeler, 2008; Grant, 2007). A University in Atlanta (Georgia, USA) for example, obtained a LEED certificate by applying multiple sustainable practices, including controlled irrigation, RWH, and gray water recycling for irrigation, toilet flushing, sinks and showers. This way, the total water consumption was already reduced by 12% from 2007 to 2009 (Lynch & Dietsch, 2010). A last incentive worth mentioning is the Water Sensitive Urban Design (WSUD), which is applied in amongst others Australia to restrict water use and retain water in the urban landscape (Coutts et al., 2012). Technologies that fall within the WSUD include RWH for irrigation, bio-retention systems, treatment wetlands and green roofs and green walls (Coutts et al., 2012; Razzaghmanesh et al., 2014a).

## 4. Irrigation specifications for green roofs in different climatic contexts

### 4.1. Deficit irrigation theory in a green roof context

Precipitation is usually unpredictable and sporadic in (semi)-arid regions, and is quickly lost to evaporation and runoff.

Therefore, the contribution of rainwater to crop yield is rather low (Oweis & Hachum, 2006). Irrigation is used to supplement rainfall in order to achieve maximum and more stable crop yields (Fereres & Soriano, 2007; Oweis & Hachum, 2006; Zhang & Oweis, 1999). Due to water scarcity, this supplemental irrigation is often extracted from groundwater aquifers, leading to a groundwater table decline and further encouraging the water scarcity problem (Zhang & Oweis, 1999). A better strategy would be to maximize the production per unit of water consumed (the water productivity) instead of maximizing production per unit area (Fereres & Soriano, 2007; Oweis & Hachum, 2006). This 'deficit irrigation' strategy is increasingly used in agriculture and horticulture, especially in regions where water is scarce. In the deficit irrigation strategy, irrigation is applied at quantities below the full ET requirement of the crop, thereby increasing the water potential while only marginally decreasing the crop yield (Fereres & Soriano, 2007; Oweis & Hachum, 2006; Pereira, Oweis, & Zairi, 2002). It has been shown that the maximum water potential can be reached when only 1/3 to 2/3 of the full irrigation requirements are applied to the crop (Pereira et al., 2002). This optimization strategy could be useful in a green roof context as well, as by limiting irrigation to drought-sensitive growth periods (i.e. establishment phase and growing seasons) and by only adding a fraction of the green roof ET rate, the water productivity can be maximized. For green roofs specifically, not the yield is stabilized by using the deficit irrigation strategy, but rather plant growth and survival which ensures the contribution to urban ecosystem services. Cameron, Harrison-Murray, Atkinson, and Judd (2006) applied the deficit irrigation strategy to some ornamental woody plants and conclude that moderate water deficit (i.e. 50% of the crop ET (ETc)) has the potential to improve the quality of the crop. Also moderate water stress during early life stages of some ornamental species (e.g. *Silene vulgaris* and *Rosmarinus officinalis*) resulted in better stomatal regulation and higher survival rates of the species during dry periods in later life stages (Sánchez-Blanco, Ferrández, Navarro, Bañon, & Alarcón, 2004). More severe water deficits (i.e. 25% or less of ETc) resulted in reduced quality of the ornamental plants (Cameron et al., 2006). We therefore hypothesized that a deficit irrigation regime of 50% of the green roof ET (ETgreen roof) would be sufficient to ensure green roof vegetation survival while also limiting the amount of water used for irrigation. In Table 4, ETgreen roof is the reference ET multiplied by a stress factor $K_c$, which is set on 0.4, 0.7 or 1 for a vegetation consisting of *Sedum*-moss, *Sedum*-herbs and grass-herbs, respectively (cf. GreenRoof model; Raes, Timmerman, Mentens, & Hermy, 2005).

### 4.2. Specifications for arid climates

For green roofs in arid climates (KCC: Bsk and Bsh) it is strongly advised to install an irrigation system. In Colorado and Israel, all months of the year can be considered as dry, as they have an aridity index below the threshold value 0.65 (cf. UNEP, 2007; Table 4). Coutts, Daly, Beringer, and Tapper (2013) conducted a study in Melbourne (Australia) with unirrigated extensive green roofs and concluded that the transpiration potential of the succulent vegetation was low and those green roofs only slightly offered any thermal benefit. Some studies show that irrigation is necessary in order to stimulate evaporative cooling during hot summer days (Coutts et al., 2012, 2013; Voyde, Fassman, Simcock, & Wells, 2010), while others indicate that the evaporative cooling potential is not very significant in hot and dry climates (Ascione et al., 2013; Sailor, 2008; Sailor et al., 2011). As the cooling potential of irrigated green roofs in this climate is still questionable, an irrigation system should rather be considered to keep the vegetation alive. Bousselot, Klett, and Koski (2010) concluded that an irrigation regime of 0.9 mm/day for an extensive green roof with substrate depth of 10 cm can be sufficient to support a variety of plant species,
<table><caption>**Table 4**
Precipitation, reference evapotranspiration (ET₀) and Aridity index of cities with different Köppen climate classes (data derived from New_LocClim v1.1, FAO 2005). Only dry months (Aridity index <0.65; UNEP, 2007) are listed. ET绿 roof (stressfactor K_c × ET₀) is calculated for three green roof vegetation types: Sedum-moss (K_c 0.4), Sedum-grasses (K_c 0.7), grasses-herbs (K_c 1). During the dry periods, the green roof requires a minimum irrigation regime of 0.5 × ET绿 roof (Deficit irrigation in mm/day). The highest irrigation requirements are framed. For comparison with the deficit irrigation regime, the daily applied irrigation regimes found in literature sources are provided (cf. Appendix A).</caption><thead><tr><th rowspan="2">Climate</th><th rowspan="2">Location</th><th rowspan="2">Precipitation (mm/month)</th><th rowspan="2">ET₀ (mm/month)</th><th rowspan="2">Aridity index (P/ET₀)</th><th colspan="3">ETgreen roof (mm/month)</th><th colspan="3">Deficit irrigation (mm/day)</th><th rowspan="2">Daily irrigation literature (mm/day)</th></tr><tr><th>Kc 0.4</th><th>Kc 0.7</th><th>Kc 1</th><th>Kc 0.4</th><th>Kc 0.7</th><th>Kc 1</th></tr></thead><tbody><tr><td rowspan="12">Bsk</td><td colspan="12"><strong>Colorado (US)</strong></td></tr><tr><td>Jan</td><td>7</td><td>43</td><td>0.16</td><td>17</td><td>30</td><td>43</td><td>0.3</td><td>0.5</td><td>0.7</td><td rowspan="12">0.9 (Bousselot et al., 2010)</td></tr><tr><td>Feb</td><td>5</td><td>57</td><td>0.09</td><td>23</td><td>40</td><td>57</td><td>0.4</td><td>0.7</td><td>0.9</td></tr><tr><td>Mar</td><td>18</td><td>95</td><td>0.19</td><td>38</td><td>67</td><td>95</td><td>0.6</td><td>1.1</td><td>1.6</td></tr><tr><td>Apr</td><td>25</td><td>144</td><td>0.17</td><td>57</td><td>101</td><td>144</td><td>1.0</td><td>1.7</td><td>2.4</td></tr><tr><td>May</td><td>54</td><td>186</td><td>0.29</td><td>74</td><td>130</td><td>186</td><td>1.2</td><td>2.2</td><td>3.1</td></tr><tr><td>Jun</td><td>50</td><td>222</td><td>0.22</td><td>89</td><td>156</td><td>222</td><td>1.5</td><td>2.6</td><td>3.7</td></tr><tr><td>Jul</td><td>68</td><td>237</td><td>0.29</td><td>95</td><td>166</td><td>237</td><td>1.6</td><td>2.8</td><td>4.0</td></tr><tr><td>Aug</td><td>59</td><td>208</td><td>0.28</td><td>83</td><td>146</td><td>208</td><td>1.4</td><td>2.4</td><td>3.5</td></tr><tr><td>Sep</td><td>29</td><td>163</td><td>0.18</td><td>65</td><td>114</td><td>163</td><td>1.1</td><td>1.9</td><td>2.7</td></tr><tr><td>Oct</td><td>13</td><td>116</td><td>0.11</td><td>47</td><td>81</td><td>116</td><td>0.8</td><td>1.4</td><td>1.9</td></tr><tr><td>Nov</td><td>10</td><td>62</td><td>0.16</td><td>25</td><td>44</td><td>62</td><td>0.4</td><td>0.7</td><td>1.0</td></tr><tr><td>Dec</td><td>8</td><td>44</td><td>0.18</td><td>17</td><td>30</td><td>44</td><td>0.3</td><td>0.5</td><td>0.7</td></tr><tr><td rowspan="12">Bsh</td><td colspan="12"><strong>Israel</strong></td></tr><tr><td>Jan</td><td>28</td><td>64</td><td>0.44</td><td>26</td><td>45</td><td>64</td><td>0.4</td><td>0.7</td><td>1.1</td></tr><tr><td>Feb</td><td>16</td><td>69</td><td>0.23</td><td>28</td><td>48</td><td>69</td><td>0.5</td><td>0.8</td><td>1.1</td></tr><tr><td>Mar</td><td>13</td><td>99</td><td>0.13</td><td>40</td><td>69</td><td>99</td><td>0.7</td><td>1.2</td><td>1.7</td></tr><tr><td>Apr</td><td>11</td><td>135</td><td>0.08</td><td>54</td><td>94</td><td>135</td><td>0.9</td><td>1.6</td><td>2.2</td></tr><tr><td>May</td><td>1</td><td>183</td><td>0.01</td><td>73</td><td>128</td><td>183</td><td>1.2</td><td>2.1</td><td>3.1</td></tr><tr><td>Jun</td><td>0</td><td>182</td><td>0.00</td><td>73</td><td>128</td><td>182</td><td>1.2</td><td>2.1</td><td>3.0</td></tr><tr><td>Jul</td><td>0</td><td>188</td><td>0.00</td><td>75</td><td>131</td><td>188</td><td>1.3</td><td>2.2</td><td>3.1</td></tr><tr><td>Aug</td><td>0</td><td>171</td><td>0.00</td><td>69</td><td>120</td><td>171</td><td>1.1</td><td>2.0</td><td>2.9</td></tr><tr><td>Sep</td><td>0</td><td>138</td><td>0.00</td><td>55</td><td>96</td><td>138</td><td>0.9</td><td>1.6</td><td>2.3</td></tr><tr><td>Oct</td><td>6</td><td>115</td><td>0.05</td><td>46</td><td>81</td><td>115</td><td>0.8</td><td>1.3</td><td>1.9</td></tr><tr><td>Nov</td><td>9</td><td>87</td><td>0.10</td><td>35</td><td>61</td><td>87</td><td>0.6</td><td>1.0</td><td>1.4</td></tr><tr><td>Dec</td><td>22</td><td>66</td><td>0.34</td><td>26</td><td>46</td><td>66</td><td>0.4</td><td>0.8</td><td>1.1</td></tr><tr><td rowspan="4">Cfa</td><td colspan="12"><strong>Texas (US)</strong></td></tr><tr><td>Apr</td><td>80</td><td>126</td><td>0.63</td><td>50</td><td>88</td><td>126</td><td>0.8</td><td>1.5</td><td>2.1</td><td rowspan="4">0.6 (Kanechi et al., 2014)<br>6 (Sendo et al., 2010)<br>13.4 (Price et al., 2011)</td></tr><tr><td>Jun</td><td>101</td><td>170</td><td>0.60</td><td>68</td><td>119</td><td>170</td><td>1.1</td><td>2.0</td><td>2.8</td></tr><tr><td>Jul</td><td>62</td><td>180</td><td>0.34</td><td>72</td><td>126</td><td>180</td><td>1.2</td><td>2.1</td><td>3.0</td></tr><tr><td>Aug</td><td>65</td><td>167</td><td>0.39</td><td>67</td><td>117</td><td>167</td><td>1.1</td><td>1.9</td><td>2.8</td></tr><tr><td rowspan="3">Cfb</td><td colspan="12"><strong>Sheffield (UK)</strong></td></tr><tr><td>Jun</td><td>59</td><td>92</td><td>0.64</td><td>37</td><td>64</td><td>91.8</td><td>0.6</td><td>1.1</td><td>1.5</td><td rowspan="3"></td></tr><tr><td>Jul</td><td>54</td><td>93</td><td>0.58</td><td>37</td><td>65</td><td>92.8</td><td>0.6</td><td>1.1</td><td>1.5</td></tr><tr><td colspan="12"></td></tr><tr><td rowspan="3">Csa</td><td colspan="12"><strong>Adelaide</strong></td></tr><tr><td>Jan</td><td>14</td><td>212</td><td>0.07</td><td>85</td><td>148</td><td>212</td><td>1.4</td><td>2.5</td><td>3.5</td><td rowspan="3">1 (Ntoulas et al., 2012)<br>1.1 (Razzaghmanesh et al., 2014b)<br>1.2 (Santamouris et al., 2007)</td></tr><tr><td>Feb</td><td>22</td><td>176</td><td>0.13</td><td>70</td><td>123</td><td>176</td><td>1.2</td><td>2.0</td><td>2.9</td></tr><tr><td>Mar</td><td>18</td><td>152</td><td>0.12</td><td>61</td><td>106</td><td>152</td><td>1.0</td><td>1.8</td><td>2.5</td></tr></tbody></table>
<table><caption>Table 4 (Continued)</caption><thead><tr><th rowspan="2">Climate</th><th rowspan="2">Location</th><th rowspan="2">Precipitation (mm/month)</th><th rowspan="2">ET<sub>0</sub> (mm/month)</th><th rowspan="2">Aridity index (P/ET<sub>0</sub>)</th><th colspan="3">ET<sub>green</sub> roof (mm/month)</th><th colspan="3">Deficit irrigation (mm/day)</th><th rowspan="2">Daily irrigation literature (mm/day)</th></tr><tr><th>K<sub>c</sub> 0.4</th><th>K<sub>c</sub> 0.7</th><th>K<sub>c</sub> 1</th><th>K<sub>c</sub> 0.4</th><th>K<sub>c</sub> 0.7</th><th>K<sub>c</sub> 1</th></tr></thead><tbody><tr><td rowspan="5">Csb</td><td>Apr</td><td>30</td><td>94</td><td>0.32</td><td>37</td><td>66</td><td>94</td><td>0.6</td><td>1.1</td><td>1.6</td><td>2 (Papafotiou et al., 2013)</td></tr><tr><td>Sep</td><td>37</td><td>81</td><td>0.46</td><td>32</td><td>56</td><td>81</td><td>0.5</td><td>0.9</td><td>1.3</td><td>2 (Kotsiris et al., 2013)</td></tr><tr><td>Oct</td><td>29</td><td>126</td><td>0.23</td><td>50</td><td>88</td><td>126</td><td>0.8</td><td>1.5</td><td>2.1</td><td>8.9 (Schweitzer & Erell, 2014)</td></tr><tr><td>Nov</td><td>22</td><td>168</td><td>0.13</td><td>67</td><td>118</td><td>168</td><td>1.1</td><td>2.0</td><td>2.8</td><td></td></tr><tr><td>Dec</td><td>16</td><td>196</td><td>0.08</td><td>78</td><td>137</td><td>196</td><td>1.3</td><td>2.3</td><td>3.3</td><td></td></tr><tr><td rowspan="2">Cwa</td><td>Oregon (US)</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Jun</td><td>108</td><td>165</td><td>0.65</td><td>66</td><td>116</td><td>165</td><td>1.1</td><td>1.9</td><td>2.8</td><td>2.3 (Schroll et al., 2011a, 2011b)</td></tr><tr><td></td><td>Jul</td><td>110</td><td>171</td><td>0.64</td><td>68</td><td>120</td><td>171</td><td>1.1</td><td>2.0</td><td>2.9</td><td>6.4 (Anderson et al., 2010)</td></tr><tr><td rowspan="4">Dfb</td><td>Hong Kong</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Jan</td><td>23</td><td>79</td><td>0.29</td><td>32</td><td>55</td><td>79</td><td>0.5</td><td>0.9</td><td>1.3</td><td>5 (Jim & Peng, 2012a,b)</td></tr><tr><td>Nov</td><td>35</td><td>104</td><td>0.34</td><td>42</td><td>73</td><td>104</td><td>0.7</td><td>1.2</td><td>1.7</td><td></td></tr><tr><td>Dec</td><td>27</td><td>87</td><td>0.31</td><td>35</td><td>61</td><td>87</td><td>0.6</td><td>1.0</td><td>1.5</td><td></td></tr><tr><td rowspan="2">Dwa</td><td>Michigan (US)</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Jun</td><td>88</td><td>139</td><td>0.64</td><td>55</td><td>97</td><td>139</td><td>0.9</td><td>1.6</td><td>2.3</td><td>2.5 (Maclvor et al., 2013; Lundholm et al., 2010)</td></tr><tr><td></td><td>Jul</td><td>89</td><td>152</td><td>0.59</td><td>61</td><td>106</td><td>152</td><td>1.0</td><td>1.8</td><td>2.5</td><td>3.8 (Monterusso et al., 2004, 2005; Rowe et al., 2006)</td></tr><tr><td rowspan="10">Dwa</td><td>Beijing (China)</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Jan</td><td>6</td><td>35</td><td>0.17</td><td>14</td><td>24</td><td>35</td><td>0.2</td><td>0.4</td><td>0.6</td><td>0.7 (Feng et al., 2012)</td></tr><tr><td>Feb</td><td>3</td><td>39</td><td>0.08</td><td>15</td><td>27</td><td>39</td><td>0.3</td><td>0.4</td><td>0.6</td><td>2.5 (Feng et al., 2012)</td></tr><tr><td>Mar</td><td>7</td><td>73</td><td>0.10</td><td>29</td><td>51</td><td>73</td><td>0.5</td><td>0.8</td><td>1.2</td><td></td></tr><tr><td>Apr</td><td>10</td><td>116</td><td>0.09</td><td>46</td><td>81</td><td>116</td><td>0.8</td><td>1.4</td><td>1.9</td><td></td></tr><tr><td>May</td><td>27</td><td>151</td><td>0.18</td><td>60</td><td>106</td><td>151</td><td>1.0</td><td>1.8</td><td>2.5</td><td></td></tr><tr><td>Jun</td><td>62</td><td>156</td><td>0.40</td><td>62</td><td>109</td><td>156</td><td>1.0</td><td>1.8</td><td>2.6</td><td></td></tr><tr><td>Sep</td><td>44</td><td>101</td><td>0.44</td><td>40</td><td>71</td><td>101</td><td>0.7</td><td>1.2</td><td>1.7</td><td></td></tr><tr><td>Oct</td><td>17</td><td>72</td><td>0.24</td><td>29</td><td>51</td><td>72</td><td>0.5</td><td>0.8</td><td>1.2</td><td></td></tr><tr><td>Nov</td><td>4</td><td>45</td><td>0.09</td><td>18</td><td>32</td><td>45</td><td>0.3</td><td>0.5</td><td>0.8</td><td></td></tr><tr><td>Dec</td><td>5</td><td>34</td><td>0.15</td><td>14</td><td>24</td><td>34</td><td>0.2</td><td>0.4</td><td>0.6</td><td></td></tr></tbody></table>
including *Antennaria parvifolia* and the grass *Bouteloua gracilis*, two species native to the Rocky Mountains region. However, according to the deficit irrigation strategy, it is advised to provide a little more irrigation. Daily irrigation requirements during the driest month in Colorado (KCC: Bsk) should range between 1.6 mm/day for *Sedum*-moss vegetation and 4 mm/day for grasses and herbs (Table 4). In Israel, the irrigation requirement in summer ranges between 1.3 and 3.1 mm/day for different green roof vegetation types. Another possibility relevant in arid climates during summer is to leave the green roof unvegetated and only wetting the substrate, which is efficient in terms of evaporative cooling of the roof surface (Pearlmutter & Rosenfeld, 2008). Although plants provide cooling through transpiration and substrate shading, they can also prevent soil evaporation and thus reduce the potential of the green roof to cool buildings. Without the vegetation layer, the plant-specific irrigation requirement is solved, and their shading function can be replaced by other covering materials (Pearlmutter & Rosenfeld, 2008). However, other beneficial aspects of unvegetated roofs, including habitat provision and aesthetic value, are eliminated (cf. Lundholm, MacIvor, MacDougall, & Ranalli, 2010; Oberndorfer et al., 2007).

### 4.3. Specifications for temperate climates: Hot or warm summers without dry season

In temperate regions with hot summers and sufficient annual rainfall (KCC: Cfa), (un)irrigated green roofs are very beneficial in terms of improving thermal comfort during summer (Dvorak & Volder, 2013; Sonne, 2006). Unirrigated green roofs are the most sustainable option but the vegetation is limited to succulents. By providing a minimum irrigation regime, the range of potential plant species can be widened, while decreasing soil temperature and hence the thermal benefit (Price et al., 2011; Provenzano, Cardarelli, Saccardo, Battistelli, & Proietti, 2010). During summer and after establishment, daily watering with 0.62 mm for a 10 cm extensive green roof favored the growth and flowering of *Evolvulus pilosus* (a non-native but common ornamental groundcover plant in Japan) compared to watering every other day (Kanechi, Fujiwara, Shintani, Suzuki, & Uno, 2014). To prevent waste of water, irrigation should be turned off on days when more than 5 mm of rain is predicted (Aitkenhead-Peterson, Dvorak, Volder, & Stanley, 2011; Sendo, Kanechi, Uno, & Inagaki, 2010). To keep the green roof alive in summer, a daily irrigation regime of 1.2 mm for *Sedum* and mosses and 3 mm for grasses and herbs is advised according to the deficit irrigation theory (Table 4), which is more than double of the irrigation quantity used by Kanechi et al. (2014). Other ornamental species like *Thymus serphyllum* and *Petunia × hybrida* can be used as green roof plants when daily irrigated with 6 mm of water (Sendo et al., 2010), which is the double of the advised deficit irrigation. Price et al. (2011) even use 13.4 mm of irrigation daily, which is almost five times higher than the advised daily 3 mm for grasses and herb vegetation.

When the climate is rather marine, with warm summers and fully humid (KCC: Cfb), typical for the UK, irrigation is generally unnecessary (Nagase & Dunnett, 2010). Even on unirrigated extensive green roofs, various plant species and vegetation type options exist, like an annual meadow on 7 cm substrate depth (Nagase & Dunnett, 2013a). Also geophytes like *Iris bucharica*, *Muscari azureum* and *Tulipa humilis*, non-native to the UK but suitable for green roof purposes, can survive without irrigation in only 5 cm substrate depth (Nagase & Dunnett, 2013b). However, plant growth can be improved by increasing substrate depth (Fassman & Simcock, 2012), or with a limited amount of irrigation once a week in summer. The advised amount of irrigation during these periods is 0.6 mm/day for *Sedum* and mosses and 1.5 mm for grasses and herbs.

### 4.4. Specifications for temperate climates: Hot or warm and dry summer

In Mediterranean regions with dry, hot summers (KCC: Csa), green roofs could provide great thermal benefits (Jaffal, Ouldboukhitine, & Belarbi, 2012), but irrigation is necessary in these regions for establishment, survival and success of extensive green roof plants especially during dry summers (cf. Kotsiris et al., 2013; Ntoulas, Nektarios, Charalambous, & Psaroulis, 2013; Schweitzer & Erell, 2014; Williams et al., 2010). Succulent species are generally considered as the best vegetation option, but even these stress tolerant species have difficulties to survive the hot dry summers in Australia on extensive green roofs if no irrigation system is provided (Williams et al., 2010). Sometimes, increasing substrate depth is sufficient to maintain the plant's growth and physiological status during drought (Ntoulas et al., 2013). Nektarios, Amountzias, Kokkinou, and Ntoulas (2011), Nektarios, Ntoulas, and Nydriioti (2012) found that non-native turfgrass *Zoysia matrella* 'Zeon' performed best in semi-extensive green roofs with 15 cm substrate depth compared to 6 cm deep extensive green roofs. The water requirement of turfgrass was based on the pan evaporation ($E_{pan}$), and ranged from 15% $E_{pan}$ every 10 days to 80% every 3 days (Appendix A; Nektarios et al., 2011, 2012). Growth and physiological status of turfgrass were only slightly influenced by irrigation (Ntoulas et al., 2013), although irrigation greatly improved the plant's color (Nektarios et al., 2012). Likewise for *Dianthus fruticosus*, native to the Mediterranean region, irrigation during drought did not result in better growth (Nektarios et al., 2011). Other studies point at a considerable water quantity needed to ensure plant survival in these Mediterranean regions. In a study of Schweitzer and Erell (2014) in Israel, the water requirement of the tested species in 5 cm deep extensive green roofs ranged from 3 to 6 mm/day for the South-African Mediterranean succulent species *Aptenia cordifolia* to 7-9 mm/day for the tropical East-African drought-tolerant grass species *Pennisetum clandestinum*. According to the more sustainable deficit irrigation strategy, the green roof requires a maximum daily watering regime of 1.4 mm for *Sedum* and mosses, and 3.5 mm for grasses and herbs (Table 4). The irrigation regimes used by Kotsiris et al. (2013), Ntoulas, Nektarios, Spaneas, and Kadoglou (2012), Papafotiou, Pergialioti, Tassoula, Massas, and Kargas (2013), Razzaghmanesh, Beecham, and Kazemi (2014b), and Santamouris et al. (2007) are in agreement with the deficit irrigation regime, but the regime applied by Schweitzer and Erell (2014) is too high in this respect.

Coastal Mediterranean areas like for example Oregon (KCC: Csb) have dry summers but are slightly cooler than interior Mediterranean regions. It is possible to install unirrigated green roofs in this climate, especially if only succulent species are applied (Nagase & Dunnett, 2010). Next to succulents, also geophytes and rhizomatous forbs can be used on unirrigated extensive green roofs in Oregon (Schroll et al., 2011a). In addition, the adapted life cycle dynamics of annual species make them useful for unirrigated green roof application in this climate, as they will contribute to green roof ecosystem services during periods with sufficient rainfall (Van Mechelen, Dutoit, & Hermy, 2014a; Van Mechelen, Dutoit, Kattge, & Hermy, 2014b; Van Mechelen, Dutoit, & Hermy, 2015). In other studies, irrigation is advised throughout the summer to ensure survival of non-succulent species with different life forms (Monterusso, Rowe, & Rugh, 2005; Schroll et al., 2011a, 2011b). The discussion whether or not to irrigate green roofs in coastal Mediterranean climates depends on the main goal for which the green roof was designed. A more diverse vegetation is preferred in terms of habitat- and biodiversity value (MacIvor & Lundholm, 2011; Nagase & Dunnett, 2010), but summer irrigation is required, which in turn reduces the green roof's stormwater retention potential and also increases weed pressure (Schroll et al., 2011a, 2011b).
If irrigation is opted for, a watering regime of 1.1 mm/day for Sedum and moss vegetation and 2.9 mm/day for grasses and herbs is advised (Table 4). The irrigation regime of 2.3 mm/day used by Schroll et al. (2011a, 2011b) is in line with the proposed deficit irrigation regime, but in Anderson et al. (2010), the irrigation regime of 6.4 mm/day is too high. For Portland specifically, where the FAR bonus regime is implemented, the maximum summer irrigation is 9 mm/day (Table 4; Schroll et al., 2011a, 2011b), which is in essence too high if the green roof vegetation only needs to be kept alive.

### 4.5. Specifications for temperate climates: Hot summer and dry winter

Finally, in temperate climates characterized by wet, hot summers and dry winters (KCC: Cwa) like Hong Kong (China), irrigation is not essential, but it can mitigate the building's temperature fluctuations. According to Jim and Tsang (2011) and Jim and Peng (2012a,b), an optimal extensive green roof irrigation regime would be 5 mm/day (Table 4). According to the deficit irrigation strategy, green roofs in this climate should only require a maximum of 0.7 mm/day for a Sedum and moss vegetation, and 1.7 mm/day for grasses and herbs (Table 4).

### 4.6. Specifications for cold climates

Multiple studies have been conducted in fully humid cold climates with warm summers (KCC: Dfb). Unirrigated extensive green roofs are possible if succulent species are used, but geophytes (e.g. Allium cernuum) and other perennial plants, all native to Michigan, like Coreopsis lanceolata and Tradescantia ohiensis are useful as well (Maclvor & Lundholm, 2011; Monterusso et al., 2005; Rowe, Monterusso, & Rugh, 2006). In this climate, Sedum species can suffer from freezing injury in winter (Benvenuti & Bacci, 2010; Durhman et al., 2006), which is a reason to diversify the vegetation. Other non-succulent drought tolerant plants can be applied if some irrigation is provided, when substrate depth is increased, organic material is added to the substrate, or when the green roof is situated in the shade (cf. Durhman et al., 2006; Dvorak & Volder, 2010; Sutton et al., 2012; Thuring et al., 2010). Maclvor et al. (2013) showed that by irrigating extensive green roofs, plant diversity is maintained, although no effect on cover and biomass was found. So in the case that the irrigation system has to be turned off, an extensive green roof with high plant diversity will not undergo any significant visual changes. Except for the establishment phase and the first growing season, irrigation is not provided in most studies (Getter & Rowe, 2008, 2009; Maclvor & Lundholm, 2011; Monterusso et al., 2004, 2005; Rowe et al., 2006, 2012; VanWoert et al., 2005b). In other studies, green roofs are watered when necessary, eventually controlled with a moisture sensor (Carpenter & Kaluvakolanu, 2011; Durhman, Rowe, & Rugh, 2007;Getter, Rowe, Andresen, & Wichman, 2011; Lundholm et al., 2010; Maclvor et al., 2013). Maclvor et al. (2013) and Lundholm et al. (2010) use an irrigation regime of 2.5 mm/day, which is in line with the deficit irrigation regime (maximum of 2.5 mm/day for grasses and herbs vegetation, Table 4). The irrigation quantity of 3.8 mm/day used by Monterusso et al. (2004, 2005) and Rowe et al. (2006) is higher than necessary to keep the green roof vegetation alive in summer.

Very little information was found from studies conducted in cold climates with hot summers and dry winters (KCC: Dwa). One study tested the potential of wetland green roofs in Korea (Song et al., 2013) and concluded that less irrigation is necessary than on traditional green roofs. Feng et al. (2012) investigated the most important meteorological factors to determine green roof irrigation regimes in Beijing (China). In this context, relative humidity and number of sunshine hours appear to be more important than rainfall and temperature. Their experimental green roofs received

a minimum additional water quantity of 0.7 mm/day in July for an extensive green roof and 2.5 mm/day for an intensive green roof. Those values approach the amount of irrigation advised by the deficit irrigation theory, which is a maximum of 1 mm/day for Sedum and moss vegetation and 2.6 mm/day for grasses and herbs.

## 5. Concluding remarks

Irrigating green roofs and other urban landscapes is often considered an unsustainable practice, mainly because of the installation and water cost, but also due to competition with other water needs, e.g. for agriculture or consumption (Abernethy, 1994). Watering urban vegetation however, also provides socio-economic and environmental benefits, by ensuring plant survival and related aesthetic aspects of the green roof, as well as by increasing ET and hence thermal comfort, especially in climates which are not too hot and dry (KCC: Cfb or Dfb; Ascione et al., 2013; Sailor, 2008; Sailor et al., 2011). To ensure sustainable irrigation in the long term, there should be public awareness of sustainable water consumption, and green roofs should be designed carefully, without wasting any water and limiting the use of potable water. Potential solutions are gray water re-use, RWH, controlled irrigation in space and time (using for example a smart controller) and the addition of a WRL (Coutts et al., 2012; Razzaghmanesh et al., 2014a; Sun et al., 2014). Also an appropriate selection of vegetation is needed, with a diverse mix of life forms including succulents, geophytes, mosses, annual species and drought-tolerant herbs (Coutts et al., 2012). Ideally, a deeper substrate should be opted for to increase the plant water availability, but as this is not always possible due to building weight limitations and colonization of undesirable weeds, shrubs and invasive plants, efficient irrigation methods should be considered as an alternative. By considering substrate type, depth, vegetation and irrigation regime during the design process, an adaptive approach is created in which green roof establishment and sustainability can be facilitated (Kotsiris et al., 2013; Ntoulas et al., 2012, 2013).

During the establishment phase and the first growing season throughout summer, it is advised to use irrigation on all green roof types and climates. Afterwards, irrigation is only necessary on extensive green roofs in arid climates (KCC: B) and temperate climates with dry period (KCC: Csa). Installation of extensive green roofs with a substrate depth below 10 cm is discouraged in these climates, as both a deeper substrate and irrigation are necessary to support plant survival during summer. In the other climates, a minimum amount of irrigation, preferably based on the deficit irrigation strategy, is advised as well to ensure plant growth and survival, diversify the vegetation and improve thermal comfort during summer (Dunnett, Nagase, & Hallam, 2008).

Future research should further elaborate on green roof materials with increased WHC, in order to develop green roofs with high biodiversity value and with no or only limited irrigation requirements, also in (semi)-arid climates (Dvorak & Volder, 2010). Also studies selecting appropriate drought tolerant plant species should be supported, as well as studies quantifying the contribution of green roofs to urban ecosystem services, weighed against the cost of adaptive green roof design (Schroll et al., 2011a; Schweitzer & Erell, 2014). This review pointed at sustainable irrigation possibilities that need to be elaborated further with scientific experiments and examples of local implementations. Different irrigation regimes are needed according to climatic context, and guidelines for irrigation specifications in each climate need to be determined.

## Acknowledgements

This work could be done thanks to a scholarship of the University of Leuven.
# Appendix A.

<caption>Table A1.</caption>

<table><caption>**Table A1**

Irrigation method and regime from outdoor green roof experiments conducted in different climatic conditions. Green roof dimensions (length × width) were used to calculate the minimum daily water requirements ($I_{day}$) during the growing season and summer. Green roof type: extensive (EGR), semi-extensive (SEGR) or intensive (IGR). NA: not applicable.</caption><thead><tr><th colspan="10">Outdoor experiments</th></tr><tr><th>KCC</th><th>Type</th><th>Irrigation method</th><th>Irrigation regime</th><th>Dimensions (m)</th><th>I<sub>day</sub> (mm/day)</th><th>Reference</th><th>Study location</th></tr></thead><tbody><tr><td rowspan="2">Bsk</td><td rowspan="2">EGR</td><td>Drip irrigation; Overhead rotator system</td><td>Growing season 1: 18.7 mm/week, later 8 mm/week; Growing season 2: 6.4 mm/week</td><td>NA</td><td>0.9</td><td>Bousselot et al. (2010)</td><td>USA: Colorado (Denver)</td></tr><tr><td>EGR</td><td>Drip irrigation</td><td>In summer</td><td>-</td><td>-</td><td>Pérez, Coma, Solé, Castell, and Cabeza (2012)</td><td>Spain (Lleida)</td></tr><tr><td>Bsh</td><td>SEGR</td><td>Drip irrigation</td><td>Daily until saturation</td><td>-</td><td>-</td><td>Pearlmutter and Rosenfeld (2008)</td><td>Israel (Beershera)</td></tr><tr><td>Cfa</td><td>EGR</td><td>No irrigation</td><td>-</td><td>-</td><td>-</td><td>Dvorak and Volder (2013)</td><td>USA: Texas (College Station)</td></tr><tr><td></td><td>EGR</td><td>Hand watering; Subsurface capillary irrigation</td><td>After planting: every 2 days. Later: 51 two times/day for 30 min. Winter: no irrigation</td><td>0.61 × 1.22</td><td>13.4</td><td>Price et al. (2011)</td><td>USA: Alabama</td></tr><tr><td></td><td>EGR</td><td>Drip irrigation</td><td>Month 1: 500 ml daily. Later: daily or every two days (except if >5 mm precipitation event occurred)</td><td>0.9 × 0.9</td><td>0.6</td><td>Kanechi et al. (2014)</td><td>Japan (Kobe)</td></tr><tr><td></td><td>EGR</td><td>unknown</td><td>Month 1: daily. Later: 51 every two days (except if >5 mm precipitation event occurred)</td><td>0.9 × 0.45</td><td>6</td><td>Sendo et al. (2010)</td><td>Japan (Kobe)</td></tr><tr><td></td><td>EGR</td><td>Unknown</td><td>1 time/week with tap water (except if >5 mm precipitation event occurred or if predicted chance of rain is >50% next days)</td><td>-</td><td>-</td><td>Aitkenhead-Peterson et al., 2011</td><td>USA: Texas (College Station)</td></tr><tr><td></td><td>SEGR</td><td>Unknown</td><td>2 Times/week for 15 min with collected rainwater</td><td>-</td><td>-</td><td>Sonne, 2006</td><td>USA: Florida (Orlando)</td></tr><tr><td rowspan="5">Cfb</td><td rowspan="5">EGR</td><td rowspan="5">No irrigation</td><td rowspan="5">1 Time/week in summer</td><td rowspan="5">-</td><td rowspan="5">-</td><td>Nagase and Dunnett (2013a)</td><td>UK (Sheffield)</td></tr><tr><td>Nagase, Dunnett, and Choi (2013)</td><td>UK (Sheffield)</td></tr><tr><td>Nagase and Dunnett (2013b)</td><td>UK (Sheffield)</td></tr><tr><td>Köhler (2006)</td><td>Germany (Berlin)</td></tr><tr><td>Dunnett et al. (2008)</td><td>UK (Sheffield)</td></tr><tr><td rowspan="2">EGR + SEGR</td><td>Handheld hose</td><td>When substrate moisture < 15%, until saturation</td><td>-</td><td>-</td><td>-</td><td>Coutts et al. (2013)</td><td>Australia (Melbourne)</td></tr><tr><td>EGR</td><td>unknown</td><td>Year 1–7: irrigation. Later: no irrigation</td><td>-</td><td>-</td><td>Schweitzer and Erell (2014)</td><td>Israel (Tel Aviv)</td></tr><tr><td></td><td>EGR</td><td>Handheld hose</td><td>1 Time/week</td><td>-</td><td>-</td><td>Nektarios et al. (2012)</td><td>Greece (Athens)</td></tr><tr><td></td><td>EGR</td><td>Handheld hose</td><td>Establishment: daily. Later: no irrigation. Growing season 1: daily until saturation. Drought stress treatment: 3 or 6 mm every 3 days (=12.5 or 25% of E<sub>pan</sub>)</td><td>0.5 × 0.5 × π</td><td>8.9</td><td>Ntoulas et al. (2012)</td><td>Greece (Athens)</td></tr><tr><td></td><td>EGR</td><td>Handheld hose</td><td>Establishment: daily. Later: no irrigation. Growing season 1: daily until saturation. Drought stress treatment: 3 or 6 mm every 3 days (=12.5 or 25% of E<sub>pan</sub>)</td><td>0.5 × 0.5 × π</td><td>8.9</td><td>Nektarios et al. (2012)</td><td>Greece (Athens)</td></tr><tr><td></td><td>EGR</td><td>Handheld hose</td><td>Establishment: daily. Later: no irrigation. Growing season 1: daily until saturation. Drought stress treatment: 3 or 6 mm every 3 days (=12.5 or 25% of E<sub>pan</sub>)</td><td>0.5 × 0.5 × π</td><td>8.9</td><td>Nektarios et al. (2012)</td><td>Greece (Athens)</td></tr></tbody></table>
<table><caption>Table A1 (Continued)</caption><thead><tr><th colspan="8">Outdoor experiments</th></tr><tr><th>KCC</th><th>Type</th><th>Irrigation method</th><th>Irrigation regime</th><th>Dimensions (m)</th><th>I<sub>day</sub> (mm/day)</th><th>Reference</th><th>Study location</th></tr></thead><tbody><tr><td></td><td>EGR + SEGR</td><td>Handheld hose</td><td>Water deficit cycles (15 days): 25% of E<sub>pan</sub> every 3 days. In between: daily until saturation</td><td>–</td><td>–</td><td>Ntoulas et al. (2013)</td><td>Greece (Athens)</td></tr><tr><td></td><td>EGR + SEGR</td><td>Handheld hose</td><td>15 or 30% of E<sub>pan</sub> every 10 days</td><td>–</td><td>–</td><td>Nektarios et al. (2011)</td><td>Greece (Athens)</td></tr><tr><td></td><td>EGR + SEGR</td><td>Automatic drip irrigation</td><td>Month 1: 3.3 l/h daily for 25 min (EGR) or 45 min (SEGR). Month 2–5: Every 3, 5 or 7 days</td><td>0.6 × 0.4</td><td>2</td><td>Papafotiou et al. (2013)</td><td>Greece (Athens)</td></tr><tr><td></td><td>SEGR</td><td>No irrigation</td><td>–</td><td>–</td><td>–</td><td>Benvenuti and Bacci (2010)</td><td>Italy (Pisa)</td></tr><tr><td></td><td>SEGR</td><td>unknown</td><td>0.4–0.6 E<sub>pan</sub> every 6 days</td><td>–</td><td>–</td><td>Nektarios et al. (2012)</td><td>Greece (Athens)</td></tr><tr><td></td><td>IGR</td><td>Automatic drip irrigation</td><td>91 per Plot every 3 days for 30 min (except if >3 mm precipitation event occurred Daily</td><td>1.2 × 1.2</td><td>2</td><td>Kotsiris et al. (2013)</td><td>Greece (Athens)</td></tr><tr><td></td><td>Unknown</td><td>Unknown</td><td>1.2 kg/m<sup>2</sup></td><td>NA</td><td>1.2</td><td>Santamouris et al. (2007)</td><td>Greece (Athens)</td></tr><tr><td></td><td>EGR + IGR</td><td>No irrigation</td><td>–</td><td>–</td><td>–</td><td>Razzaghmanesh et al. (2014a)</td><td>Australia (Adelaide)</td></tr><tr><td></td><td>EGR + IGR</td><td>Unknown</td><td>201 Every 10 days</td><td>1.5 × 1.2</td><td>1.1</td><td>Razzaghmanesh et al. (2014b)</td><td>Australia (Adelaide)</td></tr><tr><td>Csb</td><td>EGR</td><td>Sprinkler system</td><td>Growing season 1: 2.3 mm daily. Growing season 2: no irrigation; Portland floor-area ratio bonus regime, 45.7 mm every 5 days; or horticultural regime, irrigation when necessary + 0.3 cm every 2 days</td><td>NA</td><td>2.3</td><td>Schroll et al. (2011a, 2011b)</td><td>USA: Oregon</td></tr><tr><td></td><td>SEGR</td><td>Unknown</td><td>0.64 cm in 6 min</td><td>NA</td><td>6.4</td><td>Anderson et al. (2010)</td><td>USA: Oregon</td></tr><tr><td>Cwa</td><td>EGR + IGR</td><td>Automatic sprinkler system</td><td>Daily 5 l/m<sup>2</sup>. Regulated by a rainfall detector to stop the pump when antecedent rainfall >10 mm</td><td>NA</td><td>5</td><td>Jim and Peng (2012a,b), Jim and Tsang (2011)</td><td>China (Hong Kong)</td></tr><tr><td>Dfb</td><td>EGR</td><td>No irrigation</td><td>–</td><td>–</td><td>–</td><td>Maclvor and Lundholm (2011)</td><td>Canada: Nova Scotia (Halifax)</td></tr><tr><td></td><td>EGR</td><td>Overhead irrigation</td><td>Day 1–20: 30 ml 5 times/day for 5 min, until saturation. Day 21–41: 2 min. Later: irrigation when necessary. Growing season 2: no irrigation</td><td>–</td><td>–</td><td>Durhman et al. (2007)</td><td>USA: Michigan (Lansing)</td></tr><tr><td></td><td>EGR</td><td>Hand watering</td><td>Only during planting. No further irrigation</td><td>–</td><td>–</td><td>Getter and Rowe (2008, 2009)</td><td>USA: Michigan (Lansing)</td></tr><tr><td></td><td>EGR</td><td>Unknown</td><td>Month 1: 3 times/day for 20 min; Month 2: daily. Later: no irrigation</td><td>–</td><td>–</td><td>Getter, Rowe, and Cregg (2009a)</td><td>USA: Michigan (Lansing)</td></tr><tr><td></td><td>EGR</td><td>Automated overhead irrigation</td><td>Month 1: 0.38 cm 3 times/day for 15 min. Month 2: 2 times/day. Month 3: daily. Later: no irrigation. Year 2 June: daily. Later: no irrigation</td><td>NA</td><td>3.8</td><td>Monterusso et al. (2004, 2005), Rowe et al. (2006)</td><td>USA: Michigan (Lansing)</td></tr><tr><td></td><td>EGR</td><td>Overhead irrigation</td><td>Year 1: when necessary. Year 2–7: no irrigation</td><td>–</td><td>–</td><td>Rowe et al. (2012)</td><td>USA: Michigan (Lansing)</td></tr><tr><td></td><td>EGR</td><td>Handheld hose + sprinkler; Drip irrigation</td><td>Establishment: 7.12 l 3 times/day for 1 min. Later: no irrigation; daily for 5 min or when moisture is low</td><td>2.4 × 1.2</td><td>2.5</td><td>Maclvor et al. (2013)</td><td>USA: Michigan (Lansing)</td></tr></tbody></table>
<table><caption>Table A1 (Continued)</caption><caption>Outdoor experiments</caption><thead><tr><th>KCC</th><th>Type</th><th>Irrigation method</th><th>Irrigation regime</th><th>Dimensions (m)</th><th>I<sub>day</sub> (mm/day)</th><th>Reference</th><th>Study location</th></tr></thead><tbody><tr><td></td><td>EGR</td><td>Hand watering</td><td>when necessary</td><td>–</td><td>–</td><td>Getter et al. (2011)</td><td>USA: Michigan (Lansing)</td></tr><tr><td></td><td>EGR</td><td>unknown</td><td>Month 1: 3–6 times/week. Later: only when necessary in summer (3 times 750 ml in 1 week)</td><td>0.36 × 0.36</td><td>2.5</td><td>Lundholm et al. (2010)</td><td>Canada: Nova Scotia (Halifax)</td></tr><tr><td></td><td>EGR</td><td>unknown</td><td>Year 1–2: irrigation. Later: no irrigation</td><td>–</td><td>–</td><td>Carpenter and Kaluvakolanu (2011)</td><td>USA: Michigan (Lansing)</td></tr><tr><td></td><td>EGR</td><td>Automated overhead irrigation</td><td>Establishment: 6 times/day for 10 min. July: 4 times/day. August: no irrigation</td><td>–</td><td>–</td><td>VanWoert et al. (2005b)</td><td>USA: Michigan (Lansing)</td></tr><tr><td></td><td>EGR</td><td>unknown</td><td>Months 1–3: 2.8 mm 3 times/day for 20 min. Growing season 2: No irrigation</td><td>–</td><td>–</td><td>Getter, Rowe, Robertson, Cregg, and Andresen (2009b)</td><td>USA: Michigan (Lansing)</td></tr><tr><td></td><td>SEGR</td><td>unknown</td><td>Week 1: every 2 days. Week 2–4: decrease. Later: once every 5 days (if two weeks without rain)</td><td>–</td><td>–</td><td>Butler and Orians (2011)</td><td>USA: Massachusetts (Medford)</td></tr><tr><td>Dwa</td><td>EGR</td><td>Unknown</td><td>0.71 l/m<sup>2</sup> Daily in July</td><td>NA</td><td>0.7</td><td>Feng et al. (2012)</td><td>China (Beijing)</td></tr><tr><td></td><td>IGR</td><td>Unknown</td><td>2.48 l/m<sup>2</sup> Daily in July</td><td>NA</td><td>2.5</td><td>Feng et al. (2012)</td><td>China (Beijing)</td></tr></tbody></table>

## Appendix B.

<caption>Table B1.</caption>

<caption>Table B1</caption>

Irrigation method and regime used in greenhouse experiments and model simulations. Studies are classified according to main topic and green roof type: extensive (EGR), semi-extensive (SEGR) or intensive (IGR).

<table><caption>Greenhouse studies</caption><thead><tr><th>Topic</th><th>Type</th><th>Irrigation method</th><th>Irrigation regime</th><th>Reference</th></tr></thead><tbody><tr><td rowspan="2">Irrigation</td><td>EGR</td><td>Reservoir-drainage, wick irrigation, drip irrigation</td><td>unknown</td><td>Cho et al. (2010)</td></tr><tr><td>EGR</td><td>Unknown</td><td>Frequent (1–3 days) and intermittent irrigation (5–17 days); Quantity: 1.2 × observed evapotranspiration during each irrigation interval</td><td>Moritani et al. (2013)</td></tr><tr><td rowspan="7">Vegetation performance</td><td>EGR + SEGR</td><td>Unknown</td><td>3 or 6 mm every 3 days</td><td>Ntoulas et al. (2013)</td></tr><tr><td>EGR</td><td>Unknown</td><td>Every 2 days until saturation; Later: every 3 days, 4 days, then no irrigation</td><td>Bousselot et al. (2011)</td></tr><tr><td>EGR</td><td>Unknown</td><td>13 mm every 2, 7, 14, 28, or 88 days</td><td>Durhman et al. (2006), VanWoert et al. (2005a)</td></tr><tr><td>EGR</td><td>Unknown</td><td>1 time/week or every 2 or 3 weeks, until saturation</td><td>Nagase and Dunnett (2010)</td></tr><tr><td>EGR</td><td>Drip irrigation</td><td>when substrate moisture ≤ 90 or 45%</td><td>Provenzano et al. (2010)</td></tr><tr><td>EGR</td><td>Hand watering</td><td>No drought: 2 times/week until saturation. Drought treatments: no water for the first 2 weeks after planting, or from week 9 to 11</td><td>Thuring et al. (2010)</td></tr><tr><td>EGR</td><td>Overhead, drip, and sub-irrigation</td><td>4.2, 9.6, and 8.5 l For overhead, drip, and sub-irrigation, respectively, for 30 min</td><td>Rowe et al. (2014)</td></tr><tr><td>Energy and temperature performance</td><td>SEGR</td><td>Unknown</td><td>Establishment: potable water, 2 times/week. Later: gray or clean water, weekly to saturation</td><td>Ouldboukhitine et al. (2014)</td></tr></tbody></table>
<table><caption>Table B1 (Continued)</caption><caption>Greenhouse studies</caption><thead><tr><th>Topic</th><th>Type</th><th>Irrigation method</th><th>Irrigation regime</th><th>Reference</th></tr></thead><tbody><tr><td rowspan="4">Stormwater management</td><td>EGR</td><td>Handheld sprinkler</td><td>11 Every 2 weeks for 2 min with distilled water</td><td>Morgan et al. (2011)</td></tr><tr><td>EGR</td><td>Sub-irrigation</td><td>Daily for 40 min. Test day: 2 h to saturation</td><td>Voyde et al. (2010)</td></tr><tr><td>EGR + SEGR</td><td>Unknown</td><td>Establishment: regular irrigation. Later: when summer dry periods ≥3 weeks</td><td>Fassman and Simcock (2012)</td></tr><tr><td>EGR</td><td>Unknown</td><td>When necessary</td><td>Harp, Cabrera, Mackay, Sloan, and Arnold (2008)</td></tr><tr><td rowspan="3">Green roof materials</td><td>EGR</td><td>Gravity-fed drip irrigation (water tank 6 m above plot)</td><td>When necessary</td><td>Elstein, Welbaum, Stewart, and Borys (2008)</td></tr><tr><td>EGR</td><td>Automatic sprinkler system</td><td>Unknown</td><td>He and Jim (2010)</td></tr><tr><td>SEGR</td><td>Unknown</td><td>When soil moisture &lt; threshold value</td><td>Scherba et al. (2011)</td></tr><tr><td rowspan="5">Model simulation studies<br>Energy and temperature performance</td><td>EGR</td><td>Automatic sprinkler system, timer controlled</td><td>10 mm 2 times/day</td><td>Jim (2012)</td></tr><tr><td>EGR + IGR</td><td>Unknown</td><td>0, 0.1 and 0.3 W/m²</td><td>Kolokotsa et al. (2013)</td></tr><tr><td>EGR + SEGR + IGR</td><td>Unknown</td><td>When soil moisture &lt; threshold value. 0, 1, 2 cm/week</td><td>Sailor (2008), Sailor et al. (2011)</td></tr><tr><td>SEGR</td><td>Unknown</td><td>Nighttime irrigation 2 mm/h</td><td>Sun et al. (2014)</td></tr><tr><td>IGR</td><td>Unknown</td><td>When soil moisture content &lt; threshold value (0.35 and 0.45 m³/m³)</td><td>Li et al. (2014)</td></tr><tr><td rowspan="4">Stormwater management</td><td>EGR</td><td>No irrigation</td><td>-</td><td>Stovin et al. (2013)</td></tr><tr><td>EGR</td><td>Rainwater harvesting</td><td>Unknown</td><td>Vieira et al. (2013)</td></tr><tr><td>SEGR</td><td>Unknown</td><td>When soil moisture &lt; wilting point</td><td>Roehr and Kong (2010)</td></tr><tr><td>SEGR</td><td>Rainwater harvesting, cistern of 127 mm</td><td>Unknown</td><td>Hardin et al. (2012)</td></tr></tbody></table>

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