Sustainable Water Management in Green Roofs

Abstract

In this chapter, the contribution of green roofs in management of the urban water cycle is addressed. Primarily, proper water management strategies are presented, with specific regard to the sustainable practice of irrigation and the definition of water quality standards. We reference the application of alternative water sources, such as rainwater harvesting and gray water regeneration. Then, the environmental, ecological, and financial benefits associated with rooftop greening are described, including reference to life cycle cost assessment. Ecosystem service provision is analyzed in specific relation to the role played by water in improving urban microclimate and air quality and promoting resilience to climate change.

1 Introduction

The past few decades are characterized by a continuous, intense, and complex process of urbanization; today almost 54 % of the world population inhabits urban areas, and in Europe, three quarters of citizens live in metropolitan regions [1]. Consistently, reconciliation between the development of our cities with respect and protection of the environment is becoming an important challenge. Cities are composed of structures and extensive interventions of anthropogenic origin, which make them poles of environmental problems [2]. In many cases, a significant percentage of a city’s soil surface is sealed by impervious materials, surfaces that do not absorb water and increase the occurrence of runoff. Furthermore, most structural materials used in such environments are generally characterized by low albedo (a measure of the reflectivity of the surface), a fact that intensifies the conversion and storage of the incident thermal radiation to sensible heat when compared to the surrounding countryside. Therefore, the urban surface layer tends to be hotter than the rural one [2, 3]. This effect is exacerbated in cities where green infrastructures are scarcely present. In other words, as green transpiring surfaces are replaced by impermeable soil cover, the water available for evaporation is reduced, affecting the flow of latent heat. Therefore, especially in the absence of precipitation, the value of Bowen ratio (sensible heat flux/flow latent heat) becomes quite high [4].

When isothermal curves are plotted on a surface weather map, the result is a profile that looks like the topographic contours of an island (Fig. 1) – the reason why the urban surface layer is also called “heat island” (urban heat island or UHI) [2]. In highly populated cities, the higher temperature is related to the higher emissivity of surface materials, an increased energy consumption for building air conditioning, and an effect of the pollution associated with road traffic (including sulfur dioxide, carbon monoxide, nitrous oxides, and suspended particulates) [5]. Pollution effects may be exacerbated in climates with a distinctively hot season [6, 7].

Fig. 1

Graphical representation of the heat island effect on the skyline of a city (top) showing the differences of temperature between rural and urban areas during the afternoon. The temperature in downtown may exceed in measure of 8–10 °C the surrounding countryside. The image below is a simulation of the typical surface temperature map from which it is possible to observe the urban heat island (UHI) effect (Image by second author)

A number of recent studies [8, 9] point out that increase of green infrastructure in urban environments contributes to mitigation of microclimate problems and also to a wide range of ecosystem services, such as improving air and water quality [10, 11], mitigating stormwater runoff [12], providing resilience to exceptional meteorological events [13, 14], or improving urban biodiversity [15]. Furthermore, functions of green infrastructure may also be social (e.g., aesthetic, recreational, educational, etc.) as well as financial ones (e.g., by increasing the property values). All such aspects are strongly related to the water cycle. Although the diffusion of urban green infrastructure is being promoted by many governmental and nongovernmental agencies for increasing city resilience, the architectural and urbanistic features of most cities make prohibitive the construction of new gardens or green parks, where soil and vegetation work as buffers and filters. Consistently, the conversion of concrete surfaces (e.g., walls or rooftops) into green areas is becoming a commonly diffused strategy. In this chapter, the environmental functions provided by green roofs will be introduced, with a particular view on the role they can play on the urban water cycle and, overall, on ecosystem services. This chapter presents and analyzes information on many environmental benefits provided by green roofs and addresses information on how water is managed in green roofs, discussing specific design and management elements, and identifies water quality standards and potential for using alternative water sources (e.g., rainwater or regenerated water) for irrigation.

2 Water Management in Green Roofs

2.1 Water Management

Many challenges and impacts require consideration when addressing water needs for green roofs. The increasing risk of climate change unpredictability determines the need for adapting water management strategies to both more resilient green covers and the inclusion of irrigation efficient methods, which is particularly true in dryer climates (e.g., in the Mediterranean basin), characterized by extreme weather events as droughts or scattered but intense rainfalls. Moreover, water management should utilize interdisciplinary approaches, allowing a better understanding of crosscutting water resource issues [16], which are crucial to assess the connection between soil-plant-atmosphere continuum and irrigation options and the implementation of successful and sustainable solutions.

Irrigation systems for green roofs need to account for the special characteristics unique to these kinds of projects. Consistently, the main elements to be considered when planning irrigation of green roofs are described in Table 1, i.e., various environmental and policy features for consideration when identifying practical recommendations. The number and nature of topics to be considered, with respect to a specific project, should be linked to the different scenarios of parameters and detail levels. Beyond the scope of this section, which is to present basic information, other items and more comprehensive checklists and inventories of information are available in irrigation handbooks [17–19]. For example, more elements could be provided to promote a better knowledge about conservation techniques, local atmospheric conditions, habitats, solar radiation, historical hydrological variability and budget, water reservoirs, and reuse facilities, among others. Another important point is the use of gray water concerning different types of farming as described in the World Health Organization (WHO) guidelines [20], identifying practices and standards for the treatment, control, and use of wastewater, including several considerations and restrictions (a topic which is discussed in more detail in chapter “Urban Wastewater for Sustainable Urban Agriculture and Water Management in Developing Countries”).

Table 1 Irrigation development framework to green roof planning. Linkages between irrigation management and environmental factors (Sources: irrigation [18, 19]; soil and water [24, 25]; climate [23, 26]; agro-environmental indicators and methodological tools [27, 28]; urban agriculture and green roofs [29, 30]; socioeconomic and governance [31, 32])

However, we note that water management issues, dealing with the water cycle and the soil-plant-atmosphere continuum, are not “exact sciences.” Factors involved in irrigation projects are sometimes uncertain, incomplete, or unreliable, thus leading to suboptimal precision levels [21], and achieving a high efficiency of urban irrigation systems is not straightforward, since its performance is affected by many constraints, e.g., high variability (spatial and temporal) of soils and microclimates, variable water/hydraulic supply and operating conditions, vegetation quality and architectural patterns, etc. [22]. For these reasons, in order to improve green roof planning, innovative skills and technical ability must be developed. Furthermore, advanced irrigation management strategies should include the formulation and ranking of suitable project alternatives, which may be supported by application of statistical and modeling tools and techniques based on updated information from data platforms and resource evaluation. These procedures also contribute to increasing public awareness about the main benefits and disadvantages associated with each option, integrating their technical, ecological, economic, and social components. Then, as large-scale implementation of green roofs is being promoted and observed across the world, it must be ensured that they improve the quality of urban life and help adapt areas to expect hydrological variability and climate change [23].

2.1.1 Irrigation System Selection

Evaluation and selection of irrigation systems should consider a number of factors and criteria as related to design layout, scheduling, performance, resource efficiency, and socioeconomic issues and enable the user to establish decisions, comparing the adaptability of installation options to site-specific conditions. Making use of proper tools (e.g., a decision support or expert system) to classify and rank the feasible irrigation systems according to their suitability to input factors, the selection process will consist in evaluation stages, while meeting needs, constraints, and beneficial procedures [21], preferably based on case studies provided in experimental plots. Main topics and factors needed to develop a selection procedure are presented in Table 2. Using micro-irrigation systems as a comparison, this table was compiled with detailed information from several handbooks and commercial catalogues available in most world markets for irrigation equipment.

Table 2 Guidelines for irrigation system selection in GR plots [18, 19, 21]

Notes

The brief analysis of the factors presented in Table 3 (focusing evaluation of topics 2–6 with main comparative limitations of systems, given by factors with level 3) resulted on the basis of the following considerations:

• 1. Operational

  • Micro-irrigation systems provide low flow rates. Typically, values in drip irrigation are close to 2 L h⁻¹. Water applied through micro-spray heads will range from 20 to 100 L h⁻¹, but flow is still classified as low

  • Required spacing between emitters depends on soil/substrate properties and plant type/density, generally ranging from 1.5 to 4 m in microsprinklers and less than 1 m in drip irrigation. In microsprinkler irrigation, it is important to achieve an overlap of wetted areas, meaning the spacing of outlets must be close to 100 % of the wetted radius. In drip irrigation projects shall propose only a slight overlapping between the wetted areas of emitters along the lateral. A dry area between crop rows is usually expected in drip systems

  • The application rate determination is based on flow rates and wetted areas (Box 1) and shall be lower than final infiltration rate (explanation is provided in Sect. 3.1.2). The micro-irrigation emitters usually provide rates ranging from 5 to 40 mm h⁻¹, in agreement with expected infiltration rates increasing (and soil wetting areas decreasing) from fine to coarse textured soils, respectively

  • An important parameter for such systems is pressure, and typically emitters operate under low pressure (typically up to 1.5 bar)

• 2. Soil

  • “Tape”: volume of wetted soil is very limited, “subsurface”: not suitable unless soil is nonsaline

• 3. Climate/weather

  • “Microsprinkler”: problems with windy conditions, “subsurface”: soil surface remains dry reducing the plot cooling effects

• 4. Plant

  • “Microsprinkler”: not suitable to many plants as spray effects may damage canopy, “subsurface”: more difficulties with seed germination and transplants, requiring other solutions to initial water requirements of some plants

• 5. Water

  • “Subsurface”: water supply problems with high sediments content

• 6. Practices and Maintenance

  • “Subsurface”: difficulties in detecting clogged emitters or leakages from buried laterals

• 7. Cost

  • “Subsurface”: highest investment costs

Table 3 Hydrological properties of soil/substrate texture classes [34]

Proper selection of an irrigation system must be carried out, taking also into account the substrate’s physical properties. Table 3 presents most typical substrate properties and moisture conditions from 11 texture classes. Effective (or saturated) hydraulic conductivity (mm h⁻¹) is the parameter associated with the infiltration capacity limit of the substrate, and the water application rate (also in mm h⁻¹) of irrigation supply shall never be above that value.

Other important parameters are the AWC (available water capacity) and the MAD (management allowed deficit) used to estimate the water deficit in the root zone. This deficit is considered in order to compute water application amount and to determine when irrigation is needed. Improved skills to scheduling practices, based on the substrate water balance, must be developed to answer “when” and “how much” to irrigate. Such practices shall point out solutions to prevent substrate water shortages or waterlogging taking into account specific urban environmental conditions. Several systems may be implemented to check and monitor substrate water levels and deficits. A common method for checking substrate moisture comprises appearance observation and hands feel. Consistently, knowing substrate texture can enable a green roof manager to understand the substrate moisture status based on the substrate features described in Table 4. Other methods with better accuracy may be used to control moisture level, at different substrate depths, along the crop stages. Substrate moisture sensors are more expensive means and require some training for proper installation and calibration. Using moisture meters to aid in irrigation scheduling may be relatively easy, but there are keys to success that need to be considered, as comparative procedures with visual inspection of the substrate, surface wilt, and response to irrigation inputs for a length of time before irrigation [33].

Table 4 Guide for estimating soil moisture [35]

2.1.2 Irrigation and Drainage Configuration

The design process for a suitable pressurized irrigation system leads to a set of technical specifications comprising (1) system capacity (flow), (2) irrigation layout and selection of outlets (flow, distance, discharge rates, and pressures), and (3) water supply (water pumping, according to pressure and flow determinations).

Site-specific studies regarding a soil-plant-atmosphere continuum are a key component to ensure reliable irrigation design and management [36]. Rather than concentrating on analytical detail in an abstract sense, sequential sample calculations of a design process are extensively used [37]. Heavy irrigation or rainfall events may lead to substrate profile saturation, if the pore space of the substrate is filled with water [18], or to substrate surface ponding conditions if the event intensity is higher than the infiltration capacity. Related to these mechanisms, the occurrence of waterlogging and surface runoff in plots will cause damage to both plants (i.e., root asphyxia, diseases, etc.) and substrate (i.e., erosion, lack of aeration, etc.). Substrates of clay texture classes, with low infiltration capacity (see column in Table 3 – effective hydraulic conductivity), are more influenced by intensity-infiltration mechanisms, but sandy substrates, with low water storage capacity (see column in Table 3 – available water capacity), are more commonly affected by sudden saturation conditions. These problems may be controlled by drainage methods, which allow water to be efficiently removed from the substrate surface and mass, as it moves out (due to hydraulic potential gradients) through drain systems and materials to the lower point of water removal [38]. Drainage layers, drain holes, perforated drain pipes, and systems of channels are currently available technologies, and some of them may also provide the possibility for diverting water to storage infrastructures. Additionally, a proper micro-irrigation design and management will also contribute to the control of excess water. In light substrates, with shallow root systems, irrigation is scheduled with small and frequent irrigation events (even twice a day) and the application rates of irrigation systems may be increased. In heavy and deep substrates, water application amounts may be increased (and frequency is reduced) and the application rate of emitters should be lower.

2.1.3 Green Roof (GR) Irrigation: Sample Calculation

In this section, the main topics and steps to be considered when implementing and managing an irrigation system in a green roof are presented. Sample calculations are provided following a simulation procedure concerning a green roof plot with some site-specific characteristics (Fig. 2). Data originate from direct measurements or estimated from reference figures or tables. Values obtained are then applied in the formulation of descriptive, qualitative, or quantitative indicators, enabling to complete the final irrigation management strategy:

Fig. 2

Generic layout of a micro-irrigation system (Image by third and fourth authors)

• 1. Plot – an area of 50 m² is considered.

• 2. Substrate water evaluation – an inventory of resources is completed which provides information about conditions of plot viability. Their quality is approached to identify water availability, infiltration capacity, and substrate and water chemistry (e.g., salinity/sodicity and pH). Resource characterizations considered are (a) sandy loam texture providing an available water capacity (AWC) of 120 mm m⁻¹ and an infiltration capacity of 25 mm h⁻¹ (Table 3), (b) plot structure with a substrate depth of 25 cm, and (c) substrate and water pH equal to 6.0 and electrical conductivity of substrate (EC_e) and water (EC_w), less than 1.0 dS m⁻¹. Maximum net depth of water application (MWAn), defined by substrate water deficit for MAD value of 25 %, is 30 mm m⁻¹ (Table 3); thus, the substrate depth under consideration will reach 7.5 mm. Substrate physical and hydrodynamics characterization ensures a good infiltration. Substrate chemical characteristics, with very low salinity and almost neutral pH, will allow a nutrient cycling without problems of deficiency and toxicity [27] and are good indicators to the plant selection without restrictions [24].

• 3. Climate – the highest reference crop evapotranspiration (ETo) values, expected in summer, vary considerably in different climate zones. For instance, in Europe, the monthly ET₀ predicted can reach 200–250 mm in southern countries and 150–200 in the north depending on regions and years.

• 4. Plant – the main plant selection and cultivation factors that should be considered are (a) growth stages with effects on water demand and on allowable depletion; (b) crop evapotranspiration (ET_c) along growth stages, reaching 5–10 mm day⁻¹ in summer in Europe; (c) root depth (substrate depth in this example) to determine the water application amount; (d) the spacing between plants and rows; (e) the tolerance to substrate and water quality; and (f) the adaptation to climatic factors. Many tables from irrigation handbooks may be used to access information from most plants [18, 19].

• 5. Water use – the physical layout of an irrigation system must be adjusted to the green roof plot conditions. Whenever water shortage occurs, the irrigation system must be able to deliver and apply the amount of water needed to meet the crop-water requirement [19]. In this example, MWAn is 7.5 mm and for system application efficiency attainable of 90 %, the gross water application will be 8.3 mm. During the peak consumptive use period, it is assumed that the needed water depth is 5 mm/day. Thus, the irrigation scheduling could be consistent with two irrigation events each three (3) days (2 × 7.5 = 3 × 5). The selected kit, of 15 microsprinklers (43 L h⁻¹ each), applies a total rate of 13 mm h⁻¹ (645 L h⁻¹/50 m²), lower than the infiltration capacity (25 mm h⁻¹), and will operate for 40 min for each irrigation event (or 8.3/13 = 0.64 h).

  Energy saving must also be a goal. Considering Table 2 guidelines, a low-pressure system (below 3 bars) with emitters discharging lowest flow rates (43 L h⁻¹; system capacity: 645 L h⁻¹) and operating with an optimized/efficient water pump will require an installation of lower power.

• 6. Energy – in green roofs the selection of pressurized and preferably automated irrigation systems will result in high initial costs. Thus, a technical-economic approach, considering design alternatives of a system, must be made. For instance, in our sample, the investment may increase due to larger diameters of irrigation pipes, but energy costs (regarding a pump station) will be reduced (less pressure loss due to pipe friction). On the other hand, reducing pipe size will result in larger annual energy costs. In this economic method, with a hydraulic basis for selecting pipe diameters, the velocity of flow in main pipe shall be close to 1.5–2 m s⁻¹ [37]. In this example an adequate option is to select an available commercial size pipe with a diameter of 16 mm. Following this procedure, the laterals with outlets and emitters may use reduced sizes. The impact of the number of emitters, spacing, and other parameters must also be properly evaluated, regarding suitable agro-environmental and economic options.

• 7. Economics – a final cost-benefit analysis is developed for the system, design, and management options, considering several engineering, operational, and maintenance expenses and economic, social, environmental, and marketing values [19, 39].

2.2 Irrigation Water Quality

In order to allow plant growth, water with certain quality standards should be used. While the need of providing a sufficient amount of water is always recognized, water quality issues are frequently overlooked. Although drinking water may present microbiologically acceptable features, its chemical composition (especially as a consequence of added chloride) may not be suitable for plant needs. Furthermore, due to its high cost and the overall need to save water, using alternative sources (e.g., rainwater, regenerated water, etc.) should be assessed in urban environments. When unconventional water is used for irrigation, appropriate and periodic tests should be conducted in order to verify its chemical and microbiological properties. When hydroponic cultivation systems are used, periodic water pH and EC measurements should be performed [40].

Analyses of water quality should be performed in order to avoid plant phytotoxicity, to rationalize plant nutrition, and to decide whether or not a water treatment unit is needed. If rainwater is used, seasonal variations may be encountered and should be taken into consideration. If municipal potable water is adopted, analyses are generally periodically provided by the public institution responsible for the water supply. However, interpretation of an analysis certificate can appear complex to those not in the business, for a number of reasons. The first difficulty is identification of the “threshold values,” i.e., the concentrations beyond which a certain substance can become harmful. Plant species have different levels of tolerance and the growing techniques affect these thresholds. Furthermore, irrigation water quality must be assessed by examining the relationships between various quality parameters. Consistently, the opinion of an expert, having a thorough knowledge of the green roof in question, will certainly be more accurate as compared to fixed thresholds. Lastly, the units of measurement used to express the results may differ, making it difficult to compare different analyses or an analysis and a series of threshold values. The purpose of this section is therein to enable the reader to understand which parameters should be considered when choosing water for green roof irrigation. These parameters can be classified in the following categories:

• 1. Physical (temperature, suspended solids)

• 2. Chemical (gaseous substances, pH, alkalinity, soluble salts, element concentration)

2.2.1 Physical Features

Water temperature should be as close as possible to that of the substrate explored by the roots. Cold water (below 75 % of the air temperature) should be avoided as it can cause plant stress. Therefore, adopting reservoirs where temperature can adapt to the environmental conditions is recommended. Warm water is alternatively useful in order to provide supplemental heat in coldest seasons, but when the temperature exceeds 35 °C, it may damage aesthetic properties (e.g., leaf spotting) and overall plant physiological functions. Suspended solids in the water may consist of substrate particles but also particulates contained in non-purified municipal wastewater. Although, generally, they do not directly affect plant growth, they may reduce aesthetic plant properties (e.g., by staining leaf tissues) or may clog irrigation nozzle and damage the water distribution system. This results in higher maintenance costs, as well as in possible occurrence of health and hygiene hazards.

2.2.2 Chemical Features

Gaseous substances dissolved in the water may vary upon the presence of biodegradable substances which is a function of temperature. Indeed, given the low solubility of air in water, rainwater and surface water are generally preferred. Water use for irrigation may be restricted due to the presence of CO₂, H₂S, SO₂, and CH₄. Furthermore, chlorine (highly present in municipal water as a purifying agent) may be present in gaseous form; it becomes volatile when the water is exposed to both light and air.

Another important parameter is pH, which defines the water acidity or basicity (below 7 acid; 7 neutral; above 7 basic or alkaline). Water pH (together with the growing substrate) affects nutrient availability, with optimal values between 6.0 and 8.0. However, sometimes rainwater may present acidic pH (below 5), whereas saline well or regenerated water may be basic (pH above 8.5). In these cases, correction is needed prior application. While pH defines water acidity or basicity, alkalinity is a relative measurement of water’s capacity to resist a change in pH or to alter the pH of the growing substrate. It increases together with concentrations of carbonates and bicarbonates (generally expressed as ppm of calcium carbonate equivalents). When alkalinity is high, pH of the growing media will likely rise over time, therein requiring acid applications.

Another important parameter affecting water quality is the content of soluble salts, generally expressed as salinity of the water. Both groundwater and regenerated water may present high salinity, which will affect plant functions and, to the extreme, survival. Among dissolved salts, some are of greater concern, due to their toxic effect on plants, resulting in lower root water uptake, phytotoxicity, and alteration of substrate properties. The most frequently found dissolved salts are nitrates, chlorides, sulfates, carbonates, and bicarbonates of sodium, potassium, magnesium, and calcium. Measure of salinity may be performed analytically or by electrical conductivity methods. While analytical methods provide direct measurement of dissolved salts (e.g., expressed by g l⁻¹ or mg l⁻¹ or as concentration in ppm), electrical conductivity is linked to the osmotic pressure that a given saline concentration creates in the solution which, in turn, directly affects plant capability to absorb water. EC is expressed by millisiemens (mS cm⁻¹) or microsiemens (μS cm⁻¹) per centimeter or decisiemens per meter (dS m⁻¹) as measured by a conductivity meter at 25 °C (where 1 dS m⁻¹ = 1mS cm⁻¹ = 1000 μS cm⁻¹), and water is defined as brackish whenever the EC is 3.0 dS m⁻¹ or more. Whenever dealing with salty water, agronomical practices can help to minimize losses, for instance, by satisfying the leaching requirement (e.g., by application of exceeding water in order to flow away excessive salts from the root zone), by applying frequent irrigations (enabling the plant to absorb water upon needs), or by localizing (e.g., by using drip irrigation) water nearby roots. Leaching fraction calculation integrates a number of key attributes, including substrate porosity, gravitational potential (influenced by the substrate layer height), and especially irrigation volume (how much water is applied in each irrigation). A high percentage of leachate (over-irrigation) from containers removes salts and results in a large volume of runoff. In contrast, a reduction in leaching leads to more salt remaining in the container and becoming available to the plant.

The presence of toxic ions in the irrigation water may lead to phytotoxicity problems. Symptoms become observable whenever these ions build up in the plant tissue. Visible symptoms are strictly related to the ions that generated the toxicity phenomena, which are generally chloride, sulfur, boron, and sodium, or, at lower concentrations, trace elements (e.g., heavy metals derived from human activities, such as industry or traffic). In any case, as for salinity problems, toxicity problems are also increased during the period of greatest environmental evapotranspiration demand, meaning that where good quality water is available, it is best to use it during the hottest period of the irrigation season.

2.3 Rainfall, Runoff, and Green Roofs as Rainwater Harvesting Systems

The development of infrastructure of central water supply systems and the evolution of relevant technology in urban areas of developed nations created a belief in their populations that water is an inexhaustible natural resource. Without getting in the climate change debate, from time to time, and unfortunately more frequently during the last decade, periods of water shortages oblige authorities to take precautions and apply watering bans. However, even if the mass media and the environmental campaigns provide information regarding the fragility of ecosystems and the crucial point at which they stand, there is not yet a wide and strong sense for adopting sustainable solutions.

Rainwater harvesting (RWH) by constructing public and/or home reservoirs has a long tradition to provide water for irrigation purposes. Pipes (mainly by ceramic) and canals (mainly by stone) drive water to pools or underground cisterns [41]. Leaks are generally avoided by using waterproof internal coating. Rainwater runoff refers to rainwater which flows off a surface. In case of an impervious surface, runoff occurs almost immediately. For a pervious surface, such as a green roof, runoff will not occur until one of the following conditions is identified: (1) rainfall intensity exceeds the surface intake rate, or (2) the water storage capacity of the profile is lower than the water amount of the rainfall event. Runoff can be harvested (captured) and used immediately to irrigate plants or can be stored for later use. Rainwater has an advantage – when compared to other alternative water sources like gray and recycled water – in general it contains less contaminant, it is easily collected, and there are no legal limitations regarding its use for irrigation of nonedible crops. Probably the only disadvantage of such systems is the uncertainty of replenishment of the reserve.

In order to develop a sustainable rainwater harvesting system, aiming to satisfy irrigation water demand, a holistic approach should be applied and thus the system must be combined with appropriate substrate, native plants, mulching techniques, and an efficient irrigation system (regarding the design, quality of equipment, and operational performance). These systems can also be coupled with a number of other solutions like rain gardens, green roofs, and other bioretention systems. Rainwater harvesting systems range from simple to complex and are considered as low-impact development (LID) practices for an urban environment and a way to lower the urban “footprint.” Whether the landscape is large or small, a rainwater harvesting system is composed of the following basic components (Fig. 3): the supply (rainfall), the rainfall catchment (precipitation surface and conveyance pipes), the irrigation/distribution system that discharges water to the plants, and the demand system (substrate water holding capacity and landscape water requirement). Storage (Fig. 3) is an additional element which may be optionally integrated. Alternatively, rainwater is distributed immediately to the planted areas.

Fig. 3

Basic elements of a rainwater harvest system (Image by third and first authors)

Green roofs are good examples of rainwater harvesting systems. They can keep an amount of water in their drainage layer and provide storm water retention (63 % on average in a variety of climates) [12]. Once maximum storage capacity is reached, runoff water can be channeled into a gray water system and returned to the roof as irrigation [42]. If the rainwater harvested at the rooftop level exceeds the green roof requirements, it can be also used for irrigation of landscapes on lower floors or ground level, given its latent pressure which is very useful in case driplines are used (every 10 m of height difference corresponds to about 1 bar).

Regarding irrigation methods, the selection of a high effective type, like pressurized micro-irrigation systems (e.g., driplines or microsprinklers), is warmly suggested. In this category, subsurface dripline systems are also included. The water application efficiency (and uniformity) of such systems ranges between 80 % and 95 % [43]. When big green areas are to be irrigated, sprinkler systems are also a good solution. Their application efficiency is between 70 % and 80 % [44]. Proper zoning should also be applied during the design phase [45]. It is not clear by current legislation whether in case of rainwater, the system’s components should be of contrasting color in order to signify that they do not deliver potable water, but it could be applied as a safety measure. For example, purple color is used in many cases [46, 47] for regenerated water distribution systems (pipes, valves, valve box caps, driplines, nozzles, etc.). British Standard BS8515:2009 for rainwater harvesting [48] indicate that all pipework should be in contrasting color (not blue but green or black with green stripes), or material, to mains pipework and properly labeled.

Finally, an appropriate irrigation scheduling method should be provided and adjusted to the variability of water needs. An irrigation timer is suggested. In addition, a rain sensor attached to the timer is a must, as it would be ironic to irrigate from a rainwater reservoir while it rains. The use of other kinds of sensors like ET multisensor systems, soil moisture sensors, wind sensors, etc., could also contribute to higher irrigation efficiency. A simple and clear written plan containing information about irrigation scheduling, timer and sensor settings, system audit, and maintenance would contribute to the overall system efficiency.

2.4 Regenerated Water for Green Roof Irrigation

The pressure on water resources in Europe has encouraged more active consideration of using alternative water sources. Typical regenerated alternative sources of freshwater are recycled gray water and saline water. In a very recent European Commission’s JRC Science and Policy Report [49], the need to find sustainable solutions to water challenges in urban, industrial, and agriculture sector was highlighted. In the same publication, a model for wastewater reuse potential in European countries up to 2025 was presented. The estimates suggest a wastewater reuse potential of 3222 Mm³ year⁻¹ and among the EU countries; Spain shows the highest reuse potential as the calculations result in a value of over 1200 Mm³ year⁻¹.

Recycled water may be primary, secondary, or advanced (tertiary) treated municipal or industrial wastewater [50]. The characterization “recycled” refers in general to any water that has undergone one cycle of (human) use and then received sufficient treatment at a sewage treatment system in order to become suitable for various reuse purposes, including irrigation. Gray water refers to soft-treated or even untreated water that has gone through one cycle of use, usually in households or office buildings. Gray water by definition does not include the discharge from toilets or other uses that may contain human waste or food residues (which make up the sewage or blackwater). Gray water usually passes through appropriate filters before it can be used. As it contains many fewer pathogens than blackwater, it is more easily treated and recycled on-site for a number of purposes among which is landscape irrigation [51].

Saline or salt water refers to water with high salt content. If the salt content stands below a critical level, it can be used for irrigation purposes [52]. In the case of landscapes, its use can be broader as yield could not be among the goals and a variety of saline tolerant plants is available [53].

2.4.1 Water Reuse Application Risks

2.4.1.1 Agronomic Concerns

Reusable water for irrigation poses the risk of toxicity to plants because of dissolved salts. Some soluble salts are nutrients and therefore beneficial to plant growth but others are phytotoxic. Even the first category can be harmful if it is present in high concentrations. Sodium when accumulated or applied directly on the leaves of specific plants can cause injury. Recycled waters are prone to high bicarbonate (HCO₃) levels. HCO₃ is connected with increase of pH and SAR in circulating solution and adversely affects substrate permeability. Municipal recycled water may contain excessive residual chlorine, a potential plant toxin. Chlorine toxicity is almost always associated with recycled waters that have been disinfected with Cl-containing compounds. Boron, although is an essential micronutrient for plant growth, when applied in concentrations even as low as 1–2 mg L⁻¹, can be phytotoxic. Periodical monitoring of the applied water with chemical water analysis is a key component of good irrigation management [50].

2.4.1.2 Human Health and Environmental Concerns

Sources of reusable water may also contain a wide array of hazards including microbial, chemical, physical, and radiological agents that could pose a risk to human health and environmental matrices. In order to implement irrigation with alternative water sources, these risks must be mitigated. The most significant health and environmental hazards of using reclaimed water are due to pathogen microorganisms and chemical contaminants. Many microbial pathogens found in reclaimed water are enteric in origin. The numbers of pathogens will vary depending on rates of illness in the humans and animals that contribute to fecal waste [54–56]. Regenerated water may also contain elevated chemical pollutants that not only need to be considered from environmental aspect but also entail considerable long- or short-term risks to human health. These agents have cumulative effects that most often are not assessed [56, 57]. There are several treatment practices that can be applied to such an irrigation system in order to ensure safety. They include disinfection, filtration with either sand or activated carbon filters, aerobic biological treatment, ultraviolet radiation, or membrane bioreactor treatment [58].

2.4.2 Water Regeneration Systems and Green Roof Irrigation

It is common for water regeneration systems to directly distribute water to plants – after the completion of the treatment – without storing any amount. Nevertheless, regenerated water can be stored for a period, depending on the level of its treatment. The use of such water sources would be probably subjected to legal limitations and relevant permissions. Only few published studies are available regarding the use of regenerated water in a green roof context [42, 59, 60]. A number of system layouts provide water treatment in various levels before it can be used for irrigation purposes [61].

In the case of green roofs, irrigation systems provide a number of advantages including the reduced demand for growth media depth, the augmentation of plant palette, and the protection of plant capital in case of severely hot weather [62]. In the case of regenerated water use, indicative signs regarding the water source should be placed, special care for filtering should be applied, and water should not be sprayed. Furthermore, as a general rule, the various components of the system should be colored purple. The use of purple color for the distinction of water type was first used in California more than 50 years ago. History says that it was an available yet easy to remember color (in the USA blue is for potable water; green is for sewers; yellow signifies natural gas, oil, petroleum, or something else that’s potentially flammable; orange is for telecommunications; red is for power lines; and white is for marking where excavations and new pipe routes will go). Many standards around the world have adopted this color code (e.g., California Health Laws Related to Recycled Water June 2001 Edition the Purple Book, [46], the Greek legislative framework, relevant to irrigation using treated water [47]). Consistently, purple is not a universal standard but a practical selection that is expanding mainly through irrigation industry practice, since most major manufacturers now produce purple pipes to be used for regenerated water irrigation.

Where there is the possibility that regenerated water will enter the potable water system, a backflow prevention device should be installed. Drainage should also be taken into account. Diverting runoff from green roofs into a gray water system is another approach in minimizing the impact of green roof irrigation on regional water demand. Rooftop gardens as public concentration places, for the sake of aesthetics, oblige a more “hidden” irrigation system, which is not the case for green roofs. As it was noted for rainwater, in the overall concept of preserving water, all the precautions for developing and operating an efficient irrigation system should be considered.

2.4.3 Standards, Guidelines, and Handbooks

Hundreds of national organizations or federal governments around the world refer to water reuse applications (like irrigation), treatment processes, water quality criteria, water monitoring, on-site preventive measures, and environmental monitoring and communication strategies [49]. Regarding gray water capture and reuse, there is a significant lack of legislative pieces in many countries, but there are many regulations and standards in the USA and Australia that set the framework for its application [58]. In European Union, the Urban Wastewater Treatment Directive [63, 64] requires that “treated wastewater shall be reused whenever appropriate” and “disposal routes shall minimize the adverse effects on the environment” [57], with the objective of the protection of the environment from the adverse effects of wastewater discharge. Several member states and autonomous regions have developed their own legislative frameworks, regulations, or guidelines for water reuse applications. In Greece, for instance, a legislative act (Joint Ministerial Decision (JMD) 145116/2011, Governmental Gazette (GG) Β 354 8/3/2011) and its amendment (JMD 191002/2013, GG Β' 2220 9/9/2013), both in Greek, are based on 91/271/EEC to define the terms and procedures for the reuse of reclaimed water [47]. In Portugal, criteria for the adoption of urban wastewater for irrigation are defined in specific standards [65], which define limits to microbiological and physical-chemical parameters and also include irrigation system restrictions. In the UK, the application of wastewater in agriculture is quite common, mainly in golf courses, parks, and urban green infrastructures [66]. In Italy, based on the national regulation DM 185/2003, wastewater may be used for irrigation given that certain sanitary standards are met and that water-saving techniques are adopted [67]. In New Zealand, the “Guidelines for Sewerage Systems – Use of Reclaimed Water” provides information regarding irrigation using regenerated water [68]. The Purple Book of the State of California (Titles 17 and 22 of CCR/2001), which promotes and regulates the use of recycled water for various purposes (including irrigation), should be referred to as it was used as a basis for several relevant codes around the world. The United States Environmental Protection Agency (USEPA) published the “Guidelines for Water Reuse” [46]. The “WHO Guidelines for the Safe Use of Wastewater, Excreta and Greywater” refers also to the safe application of recycled water for irrigation [20]. Despite the water reuse applications already developed in many countries, a number of barriers still prevent the widespread implementation of water reuse. These barriers will have to be mitigated if wastewater reuse strategies are to be adopted on a larger and more effective scale than at present, developing the potential in terms of technologies and services related to water recycling in industry, agriculture, and urban sectors [49].

3 Green Roofs for More Efficient Cities

Green roofs are increasing in cities all around the world. Vegetated covers make use of a particular technology that combines living and dynamically evolving vegetation with static and long-lasting building structures. While building architecture roots on the concept of forecast capability and stability, nature is opposite, being autonomous and responsive to changes. As a consequence, adapting technical elements for protecting the building structure to host vegetation may result to being, at the same time, risky and intriguing. Where will the water go? How can plants survive seasonal climatic variations across the years? Which depth will the root system explore, and consistently, which substrates and technical solutions should be used to minimize drought and over-watering stresses? In order to address these questions, a first classification shall be made among the most represented green cover solutions, which directly consider their required maintenance and therein their installation and running costs. The most common classification is between extensive (EGRs) and intensive (IGRs) green roofs. EGRs are those featuring shallow substrate depth, plants characterized by low water and nutritional needs and low need for maintenance. Typically, in EGRs a high percentage of the total roof area is covered by vegetation (in most cases hardy grasses, succulents, wild indigenous species, etc.), with almost no space for recreational activities. On the other hand, IGRs have deeper substrate layers, often hosting planter boxes and sometimes trees. This type of roof requires higher maintenance and is often accessible to residents and visitors. Hosted floras include walkable lawn, ornamental species with high aesthetic value, and edible crops. Changes in crop intensification are linearly correlated with installation and maintenance costs (Fig. 4). However, when designing and implementing a green roof, the evaluation of its financial viability shall consider a number of functions (reduced costs, improved building efficiency, etc.). In the following paragraphs, the main benefits associated with the building/city integration of green roofs will be explored.

Fig. 4

Classification of green roofs according to installation/maintenance costs and grown plant species (Image by first author)

3.1 Green Roofs and Ecosystem Service Provision

Green roofs can improve sustainability of the urban environment by providing a range of ecosystem services, each of them connected with the city water cycle. As efficiently summarized in Fig. 5, water affects all stages of plant growth, from seed germination to all the physiological functions that lead to plant growth and green biomass accumulation. Consistently, water availability will directly affect the whole green roof ecosystem, and this will be reflected in the magnitude of the many ecosystem services provided. Biodiversity is a function of seasonal variations in flora. Air filtration is associated with plant photosynthesis and canopy size. Thermal regulation reflects both transpiration and the related effects on wind canyoning. Water captured (and transpired) is associated with the whole canopy coverage and determines the potential for flood control. Finally, as water affects plant growth, the size of the plants and the water content of the substrate will affect the noise reduction function of the green roof.

Fig. 5

Relationship between water and main ecosystem services provided by green roofs (Image by first author)

The quantification of the ecosystem services provided is a complex procedure that must be adapted to local environmental and climatic conditions, as well as the technological level of the green roof solution adopted. Preliminary studies have addressed the quantification of these ecosystem services as summarized in Table 5.

Table 5 Ecosystem services provided by green roofs (Source: First and second authors)

3.1.1 Water Regulation

One of the main functions provided by a green roof is the reduction of stormwater runoff from commercial, industrial, and residential buildings. As compared with traditional asphalt or metal roofing, green roofs absorb, store, and restitute the rainfall to the atmosphere through evapotranspiration. Consistently, they efficiently act as a stormwater management system, overall reducing peak flow to the storm sewer system [14]. Furthermore, conventional roofing may generally lead to the enrichment of rainwater with a number of pollutants, e.g., lead, zinc, pyrene, and chrysene [74]. Moreover, green roofs have the potential for reducing discharge of pollutants (e.g., nitrogen and phosphorous) due to both substrate microbial processes and plant nutrient uptake. Consistently, when implemented on a city scale, green roofs will efficiently reduce the volume of stormwater entering local waterways resulting in lower volumes, lower water temperatures, and better water quality. This is particularly true in cities where combined sewer systems are adopted: in these conditions, stormwater and untreated human and industrial waste are collected within the same pipes. As a consequence, during rainy periods or snow melting, these systems can become overwhelmed by the volume of water and overflow into nearby waterbodies. This risk, generally referred to as combined sewer overflow (CSO), can efficiently be mitigated by urban green infrastructures, including green roofs [75].

3.1.2 Thermal Regulation

In many cities, the adoption of greened infrastructures for their energy and ecological functions is an already established governance policy. By placing a vegetated canopy over and around built structures, the first observed effects are temperature mitigation and reduction of the energy cost associated with air conditioning, especially during summer (Fig. 6).

Fig. 6

Analysis conducted with a thermal imaging camera in Bologna (Italy) showing temperature differences between a green and a concrete wall cover (Source: Second author)

The indirect cooling effect provided by vegetated structures is determined by a great protective capacity against thermal radiation, lowering the temperature of the buildings’ surface [76]. This benefit is a direct consequence of the albedo modification of walls and roofs. Buildings with dark impervious roofs have generally a low albedo (Fig. 7), which means higher absorption of solar radiation. This translates into a more intense surface heating, especially when compared to a vegetated canopy. During summertime, this leads to an increase of the day-night heat island effect, energy consumption for indoor artificial cooling, and pollution emission. In European cities, more than 90 % of roofs are dark in color, and the surface of the cover under the sunlight reaches temperatures around 80 °C, with a negative impact on the duration of waterproof insulation [77].

Fig. 7

Different albedo effects from building surfaces. Top image – albedo values for different elements of the urban landscape. Image below – surface temperatures of conventional and green roofs, measured during an experimental trial at the Department of Agricultural Sciences at the University of Bologna, Italy (Source: Second author)

Alternatively, the adoption of greened roofs promotes the conversion of solar energy to transpiration (cooling), as well as the growth of plants. This is particularly the case during summer, given the direct relationship between plant transpiration and solar radiation and temperature (Fig. 8). As a consequence, from both the vegetated cover and the adopted substrate, a thermal insulation is provided.

Fig. 8

Graphical representation of the relationship between solar radiation, temperature, and plant transpiration (Source: First and second authors)

3.1.3 Air Filtering

Beyond the previously described effects, the presence of urban structures has a physical modification on the distribution of airborne pollutants – they act as obstacles that exert a frictional force on the atmosphere [3]. Within the urban air profile, the urban canopy, or roughness layer, is the layer of air closest to the surface in cities, extending upward approximately to mean building height (Fig. 9). The mechanical impact of channeling and recirculation of the air turbulence, when combined with emissions of pollutants, leads to a high pollution risk within urban canyons [78, 79]. Vortex recirculation creates an accumulation of pollutants inside the canyon profile. Only a little leakage of flow allows air renewal, and these particular atmospheric conditions cause concerns related to health of the inhabiting population [79].

Fig. 9

Graphical representation of the urban profile effects on friction induced in the lower troposphere [3, 80] (Image by second author)

Air pollutants are naturally present in the atmosphere, although in densely urbanized areas their concentration could be very high. The main air pollutants are represented by gases such as NO_x, a wide class of binary molecule compounds of oxygen and nitrogen; SO_x, in particular sulfur dioxide; and carbon monoxide (CO). In addition, there is a wide amount of airborne aerosols indicated as particulate matter 10 or 2.5 (PM₁₀ and PM_2.5) constituted by dust of diameter lower than 10 and 2.5 μm, respectively, as well as dissolved substances. These pollutants can be removed by urban forests, parks, and green covering such as green roofs through different mechanical and biochemical processes. In plants, aerial pollutant absorption mainly takes place through their entrance from the stomata openings [81] and occurs during the physiological processes of plant photosynthesis and transpiration. These are passive processes, by which gases dispersed in the atmosphere enter into the plant. Once into the plant tissues, some of the dissolved air pollutants such as NO_x and SO_x are absorbed due to active biochemical reaction and used for plant metabolic processes [82]. Dust components of the airborne aerosol (PM_10–2.5) are removed from the atmosphere via electrostatic deposition on the leaf cuticle [83] and successively partially absorbed, washed through runoff, or resuspended in air. Recent studies show that installation of green roofs on buildings’ surfaces in urban areas significantly reduces airborne pollutants, contributing indirectly to the increase of the environmental health and well-being of citizens [72]. The qualitative benefit to the low atmosphere is principally associated with alteration of the roughness provided by buildings’ facades. Nevertheless, city architecture exacerbates accumulation of particulates within the canyon, and plants on roof gardens and walls reduce only in part the presence of pollutants. As shown in Fig. 10, particulate removal efficiency is higher when plants are placed along vertical surfaces of the canyon (green walls), whereas it is lower on flat surfaces (green roofs), although the latter is also dependent on the height of the plants grown [84].

Fig. 10

Graphical representation of the particulate matter removing capacity of green walls and rooftop gardens [84] (Image by second author)

The capacity for reducing dissolved gases and PM is attributed to the increased impact surfaces provided by plant canopy that results in increased depuration effects for turbulence impact and interception [85]. This, however, is a relatively new area of study and clearer understanding of the air filtering capacity of such green infrastructures will likely come in the near future [10].

3.1.4 Reduction of Noise Pollution

Noise pollution is described as “the introduction of noise in indoor or outdoor environment, such as to cause nuisance or health hazard to humans or the ecosystem” [86]. In urban areas, the level of sound intensity is generally high, because of many combining factors – car traffic, trains, airplanes, public transport, roadwork sites, production activities, etc. Consistently, elevated noise is considered a real source of pollution that causes disturbances as well as changes in social behavior. The reduction of urban noise is an argument of great scientific interest. Different studies have shown that green covers provide an insulating sound barrier because of their capacity to attenuate sound waves. This benefit is determined by a double combination of plants and growing substrate below the canopy level [69]. In general, plant covering can be used in urban areas to control and attenuate noise [87]. Many studies have shown a great potential of the different association of plants in dissipation of noise, especially regarding walls [88] and roofs [69, 89, 90]. According to these results, green infrastructures placed on walls and roofs may result in a more efficient noise barrier as compared to a traditional surface. In fact, the heterogeneity of plant covering and substrates leads to a greater sound absorption and scattering coefficient, especially lower frequencies. The noise attenuation given by green cover is calculated taking in consideration absorption and scattering effects. In plant covering, the ratio between sound radiation absorbed and total incident one is greater than brick and cement coverings. This effect is principally given by the presence of a stratigraphy able to absorb most of the low frequency waves. The reduction of noise is more effective (−5 to −13 dB) for low-mid frequencies comprised between 50 and 2000 Hz, while just a moderate impact for frequencies higher than 2000 Hz (−2 to −8 dB) [69]. In addition, the stratigraphy of green covering includes growing substrate and other layers needed for the functional anchorage of the system on the surface: this heterogeneous stratigraphy determines absorption of a wide range of waves and scattering of sound [69].

3.1.5 Energy Saving

Most energy consumption research concludes that when modifying the albedo of buildings, energy demand is significantly lowered, especially during warmer periods of the year. The mitigation effect has been analyzed with both green covering and white-painted roofs, producing similar results in terms of general benefits for the entire year [91]. Another study conducted specifically on the thermal isolation given by plants on building [92] showed that green roofs provide a reduction of energy required for cooling interior climates. These positive effects were observed on roofs with both extensive and intensive production of edible and ornamental plants. However, similar benefits were observed when building surfaces were painted white, due to the increase in albedo as compared to darker roof colors [93]. Quantification of the energy saved may be obtained by using the equation describing the energy balance. Simplifying the equations of the system, the energy balance (qE) has been described as the ratio between energy gained and lost, allowing calculation of energy savings from 32 % up to 100 % in commercial buildings and homes with vegetated covering [91].

Various studies, especially in the sectors of planning and building design, apply the energy equation to estimate energy savings by the utilization of covering surfaces. An analysis conducted on the City of Toronto estimated an energy saving from completely covering the city’s buildings. The hypothetical energy balance determined that the annual cost due to energy consumption could be reduced by 58 %, resulting in saving about 20 million USD per year [8]. Another study conducted in New York City addressed the estimation of the energy balance of roofs with low albedo (e.g., dark or black covering) as compared with white and living roofs [93]. Authors estimated the annual cost of energy used to cool the buildings of New York City as 8.5 billion USD. If the surfaces were, instead, painted white (bringing the albedo from 0.1 to 0.7), an economic saving, of around 2.34 billion USD [9], could be obtained. Finally, as the whole city balance is improved by the presence of green infrastructures, also the global water cycle will benefit, resulting in a lower city water footprint [94].

3.2 Environmental Assessment

In recent years, the adoption of environmental assessment tools for evaluation of city sustainability has been spreading among municipal administrations across the world. Environmental assessment tools attribute an economic value to the benefits connecting the city, environment, and citizenship. These evaluations should take in account various issues such as engineering knowledge, urban design, physics of the troposphere, and biological and social factors of urban life. In order to combine multiple scientific areas, currently, public administrations and research institutions make use of instrumentations and predictive models that allow estimating and planning sustainable land use. Therefore, the mitigation of the adverse environmental effects of urbanization and generation of urban resilience can be planned by implementing appropriate policies to improve the metabolism of the city. In addition it is also possible to reduce, at least partially, adverse effects on the population, food waste production, and energy consumption. Physical models of the atmosphere allow prediction of particulate matter dispersion in the lower layer of the atmosphere and within the urban canyon, whereas geographical information systems (GIS) allow mapping of descriptive data in the urban area. In addition, life cycle analysis (LCA) is widely utilized to study and describe the half-life of a product or process in order to calculate total energy cost, improve environmental performance, and consider multiple factors simultaneously [95]. These analytical tools provided useful information enabling holistic urban environment evaluation that integrates many ecological and social aspects.

One of the main reasons for conducting this analysis is to improve urban management with particular attention to water, energy, and material consumption, microclimate quality, and the effects on the health of citizens. Tools of environmental analysis can provide quantification in terms of energy savings. This, in the long term, may be used to assess the return on investment required for implementation of green infrastructures (e.g., parks, green walls, rooftop gardens, etc.). However, to date, commercial or residential building owners are often reluctant in accepting or selecting greened infrastructures as a solution to many climate/environmental issues, mainly due to elevated start-up costs and uncertainties in maintenance requirements.

Taking into consideration the whole life cycle of a green roof, different initial and maintenance costs have been recently addressed. It was shown that the cost of extensive green roof (EGR), i.e., shallow substrate roofs, is lower than conventional roofs. On the other hand, intensive green roof (IGR), or deep substrate roof system, presents a higher life cycle cost (LCC) than conventional roofs [95]. The mere analysis of economic figures for installation and maintenance should be further integrated with a series of benefits that a roof garden offers in order to support the decision for its construction. Throughout the life of a more ecological building, many environmental costs are taken into consideration.

LCA offer a very powerful tool for this evaluation. It examines most of the environmental aspects correlated with the construction of a rooftop garden on a building, taking in consideration primarily initial and running costs but also all the benefits that are brought to the environment, including ecosystem services (e.g., microclimate, air filtering, water regulation, etc.) provided [96]. Many LCA studies, conducted on roof gardens, showed positive environmental performances, including the improvement of food system sustainability whenever agricultural activities were included on the rooftop [15]. Within these kinds of studies, a particular focus was reduction of long-range transport and benefits on urban resilience provided by local food production. Current food supply systems are highly reliant on the global transport/energetic system [97]. LCA tools can be applied in many studies, in order to identify the different streams of energy and matter within the urban system. As the studies have scarcely explored the subject of these innovative green infrastructures, the preliminary steps include data acquisition and creation of the life cycle inventory (LCI), which includes all the factors involved in the cycle. Successively, analysis proceeds with characterization of overall impact, through multiplication of each factor’s impacts with the category of impact [98].

3.2.1 Analysis for the Energy Balance

Urban resilience is strongly affected by the energy use efficiency of its components [9, 91]. A predictive analytic model for environmental sustainability study allows planning construction and/or renovation of buildings in order to improve the energy class and reduce emissions. These kinds of tools are currently used to calculate the benefits related to energy savings offered by plant covers on building. One of these tools is the energy balance model, an instrument of relatively easy application that allows calculation of the energy flow from inside to outside, or vice versa, in building roof system taking in consideration the different fluxes of energy within the system [99]. Application of the energy balance can help to assess the effect of a roof with vegetated cover in an urban context, comparing it with buildings with conventional roofs, estimating the effect of the heat island effect and energy consumption in buildings, as described by the energy balance model (Fig. 11). The predictive model (Eq. 1) uses seven streams of energy – shortwave radiation downward and upward, longwave radiation downward, longwave radiation emitted upward, sensible heat loss or gain, latent heat loss, and heat conduction downward or upward. The left side of the equation indicates the fluxes of energy into the building from the roof; meanwhile the right side indicates the thermal changing of the roof canopy:

$$ {\mathrm{SW}}_{\mathrm{down}}-{\mathrm{SW}}_{\mathrm{up}}+{\mathrm{LW}}_{\mathrm{down}}-{\mathrm{LW}}_{\mathrm{up}}-{\mathrm{Q}}_{\mathrm{conv}}-{\mathrm{Q}}_{\mathrm{cond}}-{\mathrm{Q}}_{\mathrm{lat}}={\mathrm{C}}_{\mathrm{roof}}\cdot \frac{d}{dt}\left[\frac{{\mathrm{T}}_{\mathrm{roof}}+{\mathrm{T}}_{\mathrm{ceiling}}}{2}\right] $$

(1)

where SW and LW refer, respectively, to shortwave and longwave radiation. The subscripts indicate direction (downward or upward). Latent heat transport (Q) is divided between convective (conv), conductive (cond), and latent (lat) terms. On the right-hand side of the equation is the heat capacity coefficient of the roof (C_roof), the roof temperature (T_roof), and ceiling temperature (T_ceiling).

Fig. 11

Seven fluxes of energy that influence the energy balance model [91] (Image by second author)

This model has been applied in several studies to monitor the effects of green roofs or roofs with high albedo [9], showing that both solutions reduce the effects of the UHI by reducing city warming induced by thermal solar radiation. The economic investments and maintenance for high-tech and living roofs obviously must be considered, keeping in mind that despite high installation costs, living roofs offer a number of ecosystem services (e.g., water regulation, esthetic value) not provided by white-painted roofs [99].

3.2.2 Analysis of the Comfort

The bioclimatic comfort analysis is a statistical procedure used to correlate micro-climatic and meteorological parameters with the sensations of comfort or discomfort felt by citizens. A wide range of bioclimatic indices can be adopted for this kind of assessment. Many are based on empiric estimation and can be applied in a range of situations. Some indices fit better in hot or cold conditions, and others are applied in presence of high humidity or wind. The choice is often purely operational and linked to the availability of measures of specific atmospheric parameters [100]. The thermo-hygrometric index (THI) [101] is one of the most utilized indexes in analyses of bioclimatic comfort (Eq. 2):

$$ \mathrm{T}\mathrm{H}\mathrm{I}=\mathrm{A}\mathrm{T}-\left(0.55-0.0055\cdot \mathrm{R}\mathrm{H}\right)\cdot \left(\mathrm{A}\mathrm{T}-14.5\right) $$

(2)

where THI is the thermo-hygrometric index, AT is air temperature (°C), and RH is relative humidity (%).

Utilizing this equation, a diagram can be produced correlating relative humidity and air temperature. The resulting patches indicate the comfort physiologic classes for human life within the studied environment (Fig. 12). From the analysis of the comfort classes, it is possible to observe that a THI between 15 and 20 determines a condition of optimal comfort, while when THI rises over 20, different classes of physiological stress are encountered. In conclusion, the tools of environmental analysis, such as the bioclimatic comfort, allow modeling of urban microenvironmental characteristics. These possibilities are very useful during the design phases of the city and help public administrations to maximize the economical, physical, and climatic conditions of people that live and work within urban areas.

Fig. 12

Comfort classes in the urban environment [101] (Image by second author)

4 Conclusions

Installing and maintaining green roofs contribute to many aspects of urban sustainability, especially in urban water management strategies. Furthermore, urban water consumption is increasingly linked to resource conservation (soil, water, energy, air). Achieving better efficiencies in green roofs irrigation is a challenge that authorities, municipalities, and city communities are facing. Therein, rules and interactions to the improvement of governance must take into account natural and socioeconomic resources.

Optimal irrigation management is obtained by combining interdisciplinary issues. Table 6 provides a brief presentation of suitable agronomic, engineering, management, and policy solutions [102]. Suitable agronomic solutions include some important decisions helpful in improving urban irrigation efficiency. A basic agronomy management action is to establish crop rotations. This option is a proper measure to reduce problems in the substrate-plant system related to pest, diseases, and nutrients. Constraints in urban crop management may be due to shallow root systems and varying plant species and water requirements in close proximity [22]. Many tables from irrigation handbooks may be used to access information concerning most plants. More specific indications may come from general urban cultivation guidelines and manuals (directly addressing adoption of grass, shrubs, trees, annual crops, ornamental horticulture, vegetables, etc.). The substrate physical characterization is a crucial action needed to develop adequate irrigation and drainage systems, which shall ensure the infiltration limitations are controlled. In this way, the potential for waterlogging or runoff/erosion problems, mainly related to heavy rainfall or water application, is reduced. For a green roof design with a microsprinkler irrigation system, it is also important that the water losses caused by climatic factors, as wind drift or soil evaporation, are efficiently controlled. If pressurized irrigation systems are properly designed and operated, the application efficiency and uniformity must reach 80–90 %. Many regions around the globe present climate changes, leading to rainfall decrease and/or seasonal anomalies and to temperatures increase. Those phenomena reinforce the need for better management guidelines and application of smart technologies in irrigation, like soil moisture and rain sensors, automation switches, or wireless control, which will likely improve water use efficiency [22]. Currently, smart controllers equipped with ET sensors or connected to ET information providers constitute a cutting-edge technology.

Table 6 Available options for the improvement of irrigation efficiency [102]

Regenerated water irrigation can make a significant contribution to reducing water demand, recycling nutrients, improving soil health, and cutting the amount of pollutants discharged into the waterways. Another advantage of this resource – when compared to rainwater – is that it can be available in almost stable quantity through the year and specifically during summer period where there is need for irrigation. However, relevant systems must be carefully managed to protect the environment and public health.