Water Scarcity and Sustainable Urban Green

30 Water Scarcity and Sustainable Urban Green Landscape 30.1 Introduction ......................................................................................558 30.2 Urban Green Landscape: Principles and Benefits .......................559 Public Health Improvement • Reducing Air Pollution • Sound Pollution Amelioration • Improving the Social Well-Being and Psychological Health • Providing Thermal Comfort, Reducing Energy Use • Economy: Adding Value to the City • Other Benefits

30.3 How Much Urban Green Space Is Enough? .................................565

Soleyman Dayani Payame Noor University

Mohammad R. Sabzalian Isfahan University of Technology

Mahdi Hadipour Sari Agricultural and Natural Resources University

Saeid Eslamian Isfahan University of Technology

Opportunistic Model • Shape-Related Models • Space Standards • Park System • Garden City

30.4 Sustainability and Resiliency Concepts ........................................567 30.5 Eco-City: A Sustainable City ..........................................................568 30.6 Municipal Water Sources ................................................................569 Natural Water Bodies • Reuse Water

30.7 Water Demand Estimation in Urban Green Spaces ....................573 30.8 Irrigation Systems for Urban Landscapes .....................................575 30.9 Multidisciplinary Solutions for Sustaining a Water-Wise Landscape ..........................................................................................576 Plant Species Selection • Soil and Substrate Condition • Maintenance Strategies

30.10 Summary and Conclusions .............................................................588 Authors...........................................................................................................589 References.......................................................................................................590

Abstract The urban green landscape is considered one of the pivotal elements of a sustainable city. The green spaces significantly promote the city dwellers’ physical and psychological health and social well-being and the city economy and preserve the surrounding natural environment. Although scientific papers have widely investigated the concepts of sustainable cities and individual elements of urban landscapes, the general concept of an urban landscape that encompasses all the known influencing factors of urban greenery with respect to integrated sustainability and water scarcity planning and development has not yet been explored. This chapter reviews a plethora of publications from various study fields, including urban planning, hydrology, water management, ecology, plant physiology, and sustainability, to present a general picture of a sustainable water-wise green landscape. An overview of the current state of findings on urban green space benefits is provided. Then, the concepts of sustainability in general and the eco-city in particular are presented. The proposed modern strategies and solutions for urban green space planning and 557

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Handbook of Drought and Water Scarcity

its water demand estimations are discussed. Finally, a number of multidisciplinary approaches and present irrigation systems in urban areas under drought stress for water-efficient and enduring urban greenery are examined.

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30.1 Introduction The term landscape refers to any visible natural or human-made features of an area of land. The Earth’s geography exhibits a vast range of natural landscapes, including mountains, deserts, the icy polar regions, islands and coastal landscapes, dense forests, tundra, lakes, wetlands, and agricultural lands scattered throughout different climatic regions. Human society’s rapid development has led to unprecedented changes to the planet’s face. Urbanization is a consequence of the human lifestyle shift toward modernity and inevitably occurs in the context of the planet’s natural landscapes. Urban lands are expected to triple by 2030, which will result in about 1.4% of land cover being occupied by urban areas [206]. Urban development introduces major problems for nature and humans. That is, the balance of various natural phenomena is disturbed and consequently humans are forced to undertake huge amounts of time and investments to hopelessly compensate for it. Water shortage is another problem that has become a critical issue in many areas of the world, and extensive agricultural irrigation usually worsens the conditions. Although the planet Earth is rich in respect to water, however, not only the water of the oceans, but also most inland water sources are highly saline [55], which makes them readily unsuitable for human bio-applications. Water scarcity and drought are interchangeable terms in most situations. Water scarcity could refer to any condition under which access to useable water supplies for human living purposes is restricted. Here, the problem is mainly due to lack of the minimum necessary industrial and human health safety standards for available water sources. In the twentieth century, due to unprecedented human interventions, new forces have emerged that threaten to destroy valuable water sources. It is not surprising that the most important water supplies are more prone to such damages since humans first established their towns and industries on or nearby these sources. Drought, however, refers to a situation caused by water scarcity due to natural or human-made causes. Drought may result from short or long periods of unusual local temperature increase and precipitation decrease over or nearby a geographical region. Climate change is a major cause for drought in many regions. Human activities also could cause or worsen the water availability condition in a region. The term drought, from a biological viewpoint, similarly refers to the condition caused by a considerably long period of dry weather and water shortage that could result in injury to living organisms [77]. Over-harvesting from natural under- or surface water reservoirs, mainly for irrigation, is the common cause for water stress in developing societies. The green urban landscapes are hybrid infrastructures of green spaces and synthetic elements, which range from large-scale sustainable plans to small-scale green streets, parking lots, and private yards. In general, they contribute to ecosystem resilience and offer various benefits to human societies [124,129,239,256]. Although the human-made green spaces may not be able to fully compensate for the vast destructive interventions of human activities in nature, it is still regarded necessary and beneficial for preserving the remaining natural ecosystems [93] and promoting urban life standards by carbon sequestering, cleaning the air and water, increasing energy efficiency, restoring natural habitats, and creating value in the context of economic, social, and environmental benefits [105]. Sustaining a thriving green space in urban areas especially with semiarid or water-stressed climates will not be a cheap and easy task. Among the many important elements for developing a landscape, water supply is considered as the first priority. Water is the ultimate and defining element for the development of any community, be it a small village or a modern city. Climate change and global warming have already made the task of satisfying water demand of urban populations a major concern for policymakers and stakeholders. The result of studies on water consumption in cities shows that a majority of municipal water supplies are used for irrigation and maintaining urban public or private green spaces,


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which could be up to 50% of the total domestic consumption in some cities [30,133,159]. In this regard, implementing water-wise strategies as the main component of a sustainable urban green landscape is vital. A sustainable green landscape refers to a green space that is flexible and adaptive to its local climate and environment and is naturally and economically regenerative. In other words, the sustainable green landscape could be defined as a nondestructive integration of urban expansion into an existing natural environment or introducing flexible and local climate adaptive green spaces inside cities. To this end, a plethora of parameters and strategies should be considered and applied, including understanding plant physiology, identifying various factors that affect plant growth and development, and water and energy efficiency principles. This chapter reviews the known influencing factors of planning, developing, and maintaining an urban green landscape with respect to the sustainability concept and the prevailing global water scarcity condition. Since water is the most important element for any living organism on Earth, including those involved in a green space project, the main scope of all the discussion fields mainly circles around the water issue from different perspectives. The first section of the chapter summarizes the latest findings regarding the advantages of urban green space, varying from the benefits for individuals to public criteria. The second section discusses the demand estimation of the concept of green space in cities, followed by a discussion on sustainability and eco-city concepts. The urban water sources and water demand estimation models will be presented in Section 30.7. Finally, a brief discussion on candidate irrigation systems and multidisciplinary approaches, including proper plant species selection, soil and substrate preparation, and suggestions for better green space maintenance strategies, is presented.

30.2 Urban Green Landscape: Principles and Benefits The urban green landscapes are mixed infrastructures of green spaces and synthetic elements inside cities that are run by public authorities. The most commonly known urban green space elements in cities include parks and reserves, sporting fields, stream and river banks, greenways and trails, street trees, and nature conservation areas [256] and private gardens. The green landscape could vary in size, vegetation cover and type, biota species varieties, natural quality, proximity to public transport, facilities, and services [56,79,113,206,207]. The urban green space provides a wide range of ecosystem services that could improve the living standards for citizens. Green spaces may buffer high temperatures, reduce city noises, alleviate air pollution, infiltrate storms, and sustain groundwaters. In some cities, people could grow their own vegetables in private city gardens [71,256]. However, green spaces are not essentially planned on the ground. Today, in many urban settings, there is little space to plant trees or cultivate an urban forest within the land use chart of the city because of the plethora of impervious surfaces such as streets, parking lots, and rooftops [188]. The rooftops are defined as any kind of land occupation by synthetic elements and often comprise 40%–50% of the impermeable area in an urban land [127]. Today, the idea of green roofs is gaining more attention as a way to compensate for the natural lands lost by rooftops. Accordingly, rooftops are generally categorized as either “intensive” or “extensive” with respect to the feasibility and kind of green elements that could be embedded in these literally dead spaces. By definition, intensive green roofs are frequently designed as public places and may include trees, shrubs, and hardscapes similar to ground-based landscaping [145]. This kind of green cover is mainly suitable for roofs of underground city facilities or steady lowlight buildings. In contrast, extensive green roofs, often never seen, require minimal maintenance and are generally built with substrate depths less than 15 cm. Because of the shallower depth of the substrate in this model due to the buildings’ technical limitations and safety standards, the plant choices are limited to grasses, herbaceous perennials, annuals, and drought-tolerant succulents such as Sedum [189]. The discussion regarding the techniques and design principles of implementing rooftop greenery is out of the scope of this work. In this chapter, we consider all types of ground- or rooftop-based vegetation as urban green spaces.

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The presence of green spaces in cities of any type and to any extent offers numerous benefits for the city and its citizens. In the next part of this section, a summary of the most notable urban green space contributions to urban life quality and the natural environment will be presented.


30.2.1 Public Health Improvement The contribution of an urban green landscape to human health in cities could be considered as its most important and visible advantage. The health benefits of green spaces inside or nearby cities are mainly described under two major categories, including a physical aspect like reducing air and sound pollutions in cities, and improving citizens’ psychological and social life standards. Numerous empirical studies have revealed a positive correlation between urban green spaces and various urban well-being and life quality parameters in cities as well. Green spaces contribute to cities’ air pollution reduction, sound pollution amelioration, improving the social well-being and psychological health of the citizens, and providing thermal comfort, which are the most sought-after living standards in healthy cities.

30.2.2 Reducing Air Pollution Air pollution is a major environmental concern in most major cities across the world. As the levels of industrialization increase in cities, coupled with high densities of vehicular movement, the exposure of urban dwellers to air pollutants is anticipated to rise exponentially [173]. Air pollution is mainly referred to as the accumulation of high concentrations of toxic gases and aerosols, including carbon monoxide (CO), ozone (O3), nitrogen dioxide (NO2), sulfur dioxide (SO2), and suspended particulate matter (SPM) smaller than 10 PM (PMlo), at low atmospheric levels above cities due to human and industrial activities inside or nearby urban areas. Natural environments vary in their capacity to remove air pollution. Comprehensive and detailed data on green space types and their relative action on air pollution in cities are still not available [149]. Both the common green landscapes and green roofs in urban areas may reduce air pollution by absorbing certain airborne pollutants from the atmosphere through leaf stomata and by dry deposition of SPM on leaf surfaces [99,161,267]. Trees filter gaseous air pollution primarily by uptaking the surrounding air via their leaf stomata, and some larger gaseous particles are collected on the plant surface. The intercepted particles often are resuspended to the atmosphere, washed off by rain, or dropped on the ground with leaf and twig fall. Consequently, some authors believe that the urban vegetation is only a temporary retention site for many atmospheric particles [161]. The presence of vegetation in cities can indirectly further contribute to the reduction of air pollution by reducing the need for air-conditioning in warm semiarid regions, leading to a meaningful decrease in the fossil-fuel-generated air pollution [35,71,236].

30.2.3 Sound Pollution Amelioration Sound pollution is defined as noise or soundscape. Noise is understood as a sound that is loud, unpleasant, unexpected, or undesired, while the quality soundscape can be defined as a sound or the combination of sounds that forms or arises from an immersive environment, referring to both the natural acoustic environment and sounds generated from human activities [25,157,239]. Although important improvements have been made in the transport industry during the last decades through continued car tire reengineering, producing more silent engines, and developing new types of road surface top layers, the sound pressure levels are still too high for dwellings at a limited distance from roads [238]. There is a growing body of evidence that the urban vegetation by itself could affect noise perception positively [75,128,264]. Cities aiming to create a new sustainable urban lifestyle have found that greenery is a key element in addressing the noise pollution problem. Therefore, there is an increased focus on



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vegetation-based noise abatement measures, such as shrubs, trees and bushes, green barriers, and green facades and roofs that absorb, scatter, and affect the reflection of sound [238,259]. Urban vegetation in general may consist of two main components: the plant structures (leaf, stem, and root) and the growing media (soil or substrate) [112]. The combination of the substrate, plant s pecies, and the trapped layer of air between plants and the facade surface can act as an insulating barrier or noise filter against abnormal sound levels through sound absorption, reflection, and deflection mechanisms [259]. Green belts are found to be good initiatives for filtering high levels of traffic noise when planted on urban roads. The structure and layout of tree belts as the most common types of green belts were studied to evaluate their noise reduction effect [76,175,266]. The acoustic absorption capacity of grass surfaces was also examined and found to depend on frequency [132]. A few negative and positive logarithmic relationships were found between the visibility, width, and length or height of tree belts when compared with their relative attenuation effects, respectively [76,143]. The result of one research showed that a combination of earthen berm with a variety of plants can reduce the noise level by 6–15 dB and increasing the width of the plant belt can lower the noise level [259]. The green roofs and green walls also provide an environment that reduces the noise level and, on the other hand, amplifies natural and artificial pleasant sounds. In addition, they can create an exciting urban space of higher quality [106]. The green walls yield additional aesthetic or amenity qualities compared to traditional noise-attenuation measures, like synthetic wooden or glass noise barriers [239]. One easily tangible acoustical application of green roofs is the increased sound insulation of the roof system [238]. Ground effect is the result of interference between direct sound and sound reflected from the ground. It depends on the acoustic properties of the ground, as well as the positions of the source and the receiver [266]. The plants and their substrates could block sounds with higher and lower frequencies, respectively [262]. Both types of substrates including the thin layer of soil for low-growing plants like turfs and grasses and larger soil depths for growing shrubs and trees can be categorized as acoustically effective mediums [265].

30.2.4 Improving the Social Well-Being and Psychological Health The issue of people’s mental health and well-being especially in modern and crowded cities is of increasing concern to policymakers and public-health officials [64]. The World Health Organization in 2008 reported that unipolar depressive disorders were the leading cause of disability in middle- to highincome countries [254]. It is suggested that this rise might, in part, be associated with the increased level of urbanization and detachment from the kinds of natural environments because humans naturally have evolved in nature and are thus best adapted to it [251]. The physical and social features of the environment may affect the citizens’ behavior [181]. Studies among various groups, such as students, ordinary city dwellers, and workers, suggest that there were associations between green spaces with a variety of psychological, emotional, and mental health benefits [135,136,165]. The provision and access to green spaces also positively affects common stresses and improves the quality of city life [221,237]. Urban green spaces could also influence the social capital by providing a gathering place for inhabitants to interact with nature [118], contact with each other [98], congregate and build social ties [110,192], and further develop and maintain neighborhood social ties [135]. The social interaction in the context of natural settings could enhance the personal and social communication skills of citizens [13]. The presence of green vegetation and its consequent formation of neighborhood social ties in urban areas significantly contribute to residents’ sense of safety and social comfort [124]. Urban green spaces contribute to the improvement in the health and well-being of people [235] by facilitating physical exercise [46] and promoting mental wellness [233,252]. Moreover, they offer recreational benefits including active and passive activities [234]. Furthermore, the green spaces provide aesthetic contributions to cities’ images by expressing values, beliefs, and cultural trends in urban societies [138].

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30.2.5 Providing Thermal Comfort, Reducing Energy Use The majority of global population now resides in urban areas. In 2013, urban populations made up approximately 52.5% of the world’s population, and it is expected that by 2050 the urbanization share will rise to over 67%. During the same period, the global average temperatures are projected to rise between 1.3°C and 1.8°C in response to anthropogenic atmospheric warming [1]. The excess warmth is an emerging problem [222] and is known to have serious implications for human health [240]. The World Health Organization estimated that on average an additional 141,000 people died each year from elevated temperatures during the last 30 years of the twentieth century [1], and this rate could rise in cities that are more prone to climate change–motivated global warming [147]. The urban heat island (UHI) refers to an atmospheric and surface phenomenon of higher temperatures in urban areas than in the surrounding rural areas [48,139,168,183]. This phenomenon is widely observed in cities, and as measured by the UHI intensity or the temperature difference between the heat center in a city and that in its less- or un-urbanized surroundings, its magnitude has been estimated at 0.1–12 [50,54,94,129,231]. The UHI is mainly caused by the modification of land surfaces in the course of urbanization processes, which involves the application of synthetic materials that effectively absorb short-wave radiation of the light spectrum [218]. As a result, land surface temperature (LST) increases due to the UHI effect, which may disrupt the living species’ composition and distribution in an area due to abnormal increase in the length of growing seasons and decrease in air quality levels [78,120,200,250], introducing higher risks to human health [176]. Shahmohamadi et al. [209] investigated the impact of anthropogenic heat on the formation of UHI and proposed three important strategies to minimize the impact of UHI on urban energy consumption, including green landscaping, using albedo or lightreflecting materials on external surfaces of buildings and urban areas, and promoting natural ventilation. It clearly showed that green spaces could significantly influence the energy use indices by directly creating cool islands and indirectly through enhanced air circulation in cities. The green space areas play a critical role in moderating and dampening the warming effects of impervious land covers in urban environments [249,260]. The urban green zones are generally cooler than their surrounding built-up areas and can cause an air temperature difference of up to 1°C–7°C. This phenomenon is known as “park cool island” [23,38,42,66,167,210]. Therefore, natural parks in the heart of urban areas can act as temperature buffers that moderate the sharp temperature rises in overpopulated urban areas with heavy traffic and heating sources. The urban vegetation cools down the local climate through two major processes: shading and evapotranspiration [114,115,220,236]. The shading has a direct effect on the temperature regulation by reducing the amount of radiation received by any heat-absorbing urban surfaces, whereas the evapotranspiration indirectly reduces the amount of sensible heat by the transformation of heat-generating energy to latent heat [34]. The amount of plants’ leaves that cover in an area, as indicated by the leaf area index, was found to account for 62% of the variation in surface temperature [88,198]. Investigations in Australia demonstrated that tree shade could reduce wall surface temperatures by up to 9°C and external air temperatures by up to 1°C. The relationship between surface temperature and vegetation cover indices for tree, mixed vegetation, and non-tree vegetation in the urbanized parts of Sydney was examined, and it was concluded that increasing the tree cover might reduce the average surface temperatures more dramatically than mixed vegetation cover [1,14]. From another perspective, the indirect effect of evapotranspiration is the sum of evaporation and plant transpiration, which accumulatively reduces air temperature because it removes the heat energy from its surroundings and consumes it for transpiration [270]. There are numerous empirical studies that support the positive effect of urban greenery on temperature regulation. In Greece, urban green areas were reported to reduce air temperature by about 3.1°C, mainly through evapotranspiration [80]. The cooling effect of natural and human-made urban vegetation was estimated to reduce the air-conditioning demand in Beijing by 60% as expressed in net cooling energy usage. Zhang et al. [270] reported that annual reduction in CO2 emissions from power plants associated with electricity savings might reach 243,000 tons with an average of 61 kg/(ha/day).



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The large number of studies focusing on the relationship between LST and the green space composition that have been published over the last two decades indicates the importance of the issue [5,23,38,50,231,246]. A few analytical modeling approaches [81,177] and empirical analyses [23] were also developed to investigate the cooling effect of urban green spaces. The empirical studies have investigated the minimum distance over which the urban green space “oasis effect” occurs [215]. UHIs and the oasis effect of the green spaces on urban temperature vary among cities and largely depend on a range of general factors, including aspect, elevation, shape, the population size, and size of the urban area, and the changing or dominant local weather conditions like wind speed, cloud cover, relative humidity, and time, latitude, and location of measurements [90,100]. However, despite all the variations in measurements, the result of various modeling studies consistently demonstrated a negative relationship between the vegetation cover and the city overall temperature. The thermal amelioration effect of green landscapes inside cities has been the subject of many studies in many major cities in Brazil [220], Switzerland [141], England [81], the United States [188], Portugal [167], Argentina [51], and Hong Kong [153]. One adaptation strategy that has been suggested was to increase the amount and efficiency of planning, design, and installation of urban green spaces [23] such as parks, gardens, street tree plantings, and green roofs [215]. Cities such as Chicago, New York, and London have major tree planting programs underway. Copenhagen’s Climate Plan specifies additional green areas, pocket parks, and green roofs and walls to manage heat and slow down rainfall runoff. London is planning to increase the green element coverage in its city center by 5% by 2030, and a further 5% increase by 2050. Chicago has a similar provision of 20% overall increase in plant cover by 2020 [1]. The French government passed a law in 2015 to urge all new rooftops in urban areas to be covered with plants or solar panels.

30.2.6 Economy: Adding Value to the City Different forms of urban green landscapes have positive impacts on the surrounding land and properties values. Urban areas can contain public parks, protected forests, unprotected forest areas, and trees growing around a house or in the neighborhood surrounding the house. Each type of forest cover, if present in or nearby an urban area, may provide different amenities to the homeowners and to the society. It affects the overall physical and built environment of the city and makes the cities attractive places not only to their own citizens, but also to external visitors. In this respect, the urban green space may have a role to play in economic development in terms of improving the quality of urban life, building a desirable “city image,” and enhancing the position of the city in respect to its competitiveness [8]. The restoration of derelict lands into green spaces increases the value of such areas and enhances their uses for recreational purposes in urban areas [170]. Urban green landscapes also improve workplace environments and commuting routes. The empirical results imply that increasing the street landscape planting ratio improves the quality of dwelling environment, in terms of both the magnitude and saturation level of the planting ratio [96]. Many modern cities around the world have already invested in increasing the green space ratios inside municipal districts. For example, the rate of growth of street landscape planting in Japan has increased threefold from 2.8% in 1977 to 9.7% in 2005 [96]. The value-added effect of greeneries on urban private lands with respect to economical and pricing models has been examined. The green spaces inside cities showed a positive correlation with the price of nearby residential and other urban lands [41,101,116,117,155,261]. The presence of trees inside residential units has been found to increase the selling price of the building unit as well [65,228]. Hilaire et al. [89] reported that urban landscapes contributed as much as 20% of the fair market value of a residential property in some parts of the United States. Therefore, the loss of some landscape elements such as trees and shrubs because of ill-conceived water restrictions or unmitigated drought could severely depress the property value. Sander and Zhao [197] found that the values of the case-studied blue and green space amenities varied significantly based on their location. However, the influence of tree cover on home sale price was always positive. Other studies also showed that the location of green spaces from human communities was a


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key factor on the land pricing. The housing values usually decrease rapidly as the distance from urban parks increases [232]. The opportunity of having a natural view for a residential area influenced the housing prices as varying from 4.9% with a forest view [232] to 8% with a park view [134]. Paterson and Boyle [174] found that the amount and type of land cover modification near a residential unit and the visible portion of the modifications from the home could have different impacts on the property value. Mansfield et al. [142] explored different definitions of forest covers inside or in the city outskirts and assessed the relative value of these various types of forest cover to homeowners. The economy is an integral, pivotal, and moving force of any sustainable concept. Therefore, when there are opportunities for economic benefits from nature, it is expected to be evaluated and presented through the language of economy, which is mathematics. To this end, various mathematical approaches based on the hedonic property price model were developed to evaluate the green space value-added qualities on urban lands and residential areas [117,140,194–196,199]. As an example, Tajima [226] examined the economic effects of a green landscape project in the United States based on hedonic pricing methods. Using Boston’s land use and assessed property price data, it was determined that the proximity to urban open space has positive impacts on the property value, while proximity to highways has negative impacts on the property price. Overall, the theoretical and empirical studies have well documented the positive and meaningful contribution of greeneries in cities to properties’ price and attraction. Therefore, it is not surprising that the economic motivation of gaining higher interest from urban lands might more effectively encourage the expansion of green spaces in urban areas. In addition, increasing the citizens’ knowledge about the many benefits of the natural elements in their neighborhoods, including gaining more profit, could lead to the promotion of less destructive approaches for urbanization of neighboring natural environments in many developing cities.

30.2.7 Other Benefits Urban blue and green spaces are key resources for developing a sustainable, resilient, and adaptive urban system. These spaces promote numerous social and ecological qualities, including promoting diverse learning streams, environmental stewardship, social–ecological memory [11,47], and supporting wildlife habitat [197]. One of the major environmental challenges facing humankind—also in cities—is climate change. Climate change affects both biotic and abiotic elements of a city. Therefore, the mitigation and conception of adaptation measures to climate change are also an issue for the governance of cities [156]. The green urban infrastructures have shown to be promising elements for reducing the adverse effects of climate change in urban areas [61]. They balance the water flows to alleviate flooding [62], reduce runoff peak flows [212], provide thermal comfort by shading vegetation (see Section 30.2.5), increase soil infiltration [95], and support coping capacities in some areas by providing people with opportunities to grow cheap food in their small backyards [31]. The biomass that is produced by the green urban vegetation can function as carbon storage [57,162] and influence the carbon cycle in cities as well. Research for shedding more light on the unexplored characteristics of integrating modern urban lives with natural settings is still underway. Every day, more benefits of green spaces and peaceful coexistence with nature in urban areas are revealed. These findings hopefully further encourage the development of more adaptive urbanization models with the natural environments and more effectively persuade the city authorities to grant higher priorities for urban green landscape initiatives. However, a key question that somehow has received less scrutiny from both the scientific communities and stakeholders is, “how much greenery in a city is really needed?” One obstacle to developing solutions for responding to this issue might be the prioritization of the synthetic built-up expansion programs in cities over letting more urban lands free for green landscaping. In the following section, we will review the literature, although limited, to evaluating the required landscape quantity and ratios in cities.


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30.3 How Much Urban Green Space Is Enough? The history of open green space planning is relatively young when compared to that of the city lifestyle. Open spaces have been easily accessible to most city dwellers since the eighteenth century, when agricultural lands used to surround cities, and the technological barriers for transportation and development of urban amenity facilities limited the city borders’ expansion into the surrounding natural setting. However, since the mid-twentieth century, the industrial revelation and economy boom have greatly accelerated the process of urbanization and metropolitan expansion. The foundation of many of today’s big cities was laid during that period when the destructive consequences of such unprincipled disturbances of the nature by human activities were not yet felt, or at least were largely ignored. The need for natural elements in our cities is no longer a hollow gesture of nature conservation. Today, it is noticeably gaining importance as a tool for health and survival in our societies. The b alance of natural phenomena in urbanized areas is seriously disturbed and has led to the emergence of a plethora of human health problems, economic damages, and above all is undermining the planet’s resilience and capacity to sustain its viability against these dramatic human-made changes. On the other hand, the lack of a general understanding of contemporary human–environment interaction issues in urban green space is still incomplete [105], which reduces the effectiveness of urban planners’ attempts to devise universal methodologies and standards for supporting the least possible ratio of natural elements, like green and blue spaces in cities to help return the nature–urban balance. The ratio of biologically vital areas (RBVAs) is defined as the minimum proportion of green space required for good environmental performance in cities and is considered an important factor in modern urban planning and development in any city [225]. Open spaces are regarded as an integral part of land use planning decisions. However, it is not surprising that approaches to open space planning vary among different cities. The geographical, climatic, economy, culture, and history could influence the urbanization trends in different regions. Therefore, there may be no universal agreement for all cities with regard to the desirable planning criteria for the amount of urban green landscape, the appropriate location, and the preferred mode of services that it should offer. Various methods and concepts of open space planning have emerged over the years to respond to such an important demand [144]. Among the models suggested for green space planning for urban open areas, five models seem more promising with respect to the general developing modes of cities and the feasibility of their implementation [225]. These models are briefly described here.

30.3.1 Opportunistic Model The first provision of an urban land lot has been traditionally known for providing the necessary and most profitable service to its citizens. Therefore, in most cases, devoting a free lot of land in a good location inside the city for green landscaping would not be among the first priorities for stakeholders and traditional city planning strategists. However, the trend of devoting pieces of urban land for public recreation or pleasure purposes began in the 1960s, when urban land price started its climb in many developing nations [82]. At the time, planning and introducing a natural setting in the heart of a city was a matter of luck, in the sense of taking advantage of opportunities such as reassigning a piece of newly freed public land after demolishing an old building or receiving donated land by rich families for greening purposes. The “opportunistic” model in this context refers to urban open green space development through taking advantage of the occasional opportunities rather than of a systematic planning process. Even in the earlier times when cities still had ample land and environmental resources to share, many of the first major parks that were opened to the public inside cities were established on lands donated by kings or wealthy families for their citizens’ or peasants’ benefit. Other opportunities for establishing urban greeneries emerged from the changes and modification that were made into the predefined applications of an urban land lot in the course of time. For example, by removing slum areas in the course of city development and growth, the freed land would be available for green landscaping. The opportunistic

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model, nevertheless, is basically casual and mainly depends on realizing and seizing the occasional opportunities if and when they are present. Therefore, it is not a systematic outcome of any specific revolutionary methodology in urban planning and does not serve as an appropriate response to the modern cities’ needs, or efficient protection for the remaining natural resources.


30.3.2 Shape-Related Models The methodology of this model is quite straightforward in which the green space in urban open areas is defined with respect to the shape of the land devoted for this purpose and the existing elements on the land lot. The shape-related models might be implemented alone or in combination with other models, for instance, as a complement to the opportunistic model where the main goal is to make the most out of the available space. Among the many casual urban green plans implemented based on standards of this model, some examples gained in more popularity like the greenbelts, green hearts, green fingers, and greenways. Overall, the principle of this model is to design based on what you have got on the maps or aerial photos of available land. In addition, no understanding of social or ecological processes is necessarily required in this model. Therefore, it is quite clear why this model is widely prioritized by urban and green space planners around the world.

30.3.3 Space Standards Urban planning based on the evaluation of the quantitative ratio between human population and required public services is common practice worldwide. The space standard model in urban open and green space design is based on this strategy. This model basically claims that adequate response to the city dwellers’ natural amenity needs involves a predefined minimum open space land for a given population. Today, the space standard methods are the popular and common planning tools for various urban public services, and are usually expressed in terms of land units per person. Implementing the protocols and methods of this model is easy because it is solely based on quantitative data and does not require complex considerations of ecological systems or human–nature interactions.

30.3.4 Park System Offering an expanding variety of experiences and nature-friendly activity is the basic idea of developing a park system. By definition, a park system is a set of functionally interrelated open spaces that sometimes are physically interconnected in a given geographical area. In urban settings, such planning is usually foreseen in the context of urban green spaces. This model follows the path of the quantitative model that focuses on the needs per capita. Meanwhile, the natural, ecological, and environmental considerations might not be fully observed under this model since it follows rather systematic human preferences. Another issue regarding utilizing this strategy is that the implementation of interrelated parks and garden systems is easy in new developing areas, but there will be fewer chances for its success in the built-up urban areas due to the constraints imposed by the spatial distribution of existing developments. There are successful examples of park systems around the world like Prospect Park in New York, United States [204]. This park in addition to its natural aesthetic value offers a wide range of other environmental and amenity services to its visitors, including baseball fields, bicycling and greenways, jungle areas, fishing, fitness equipment, hiking trails, horseback riding trails, ice-skating rinks, playgrounds, tennis courts, zoos, and aquariums.

30.3.5 Garden City The garden city is basically a planned settlement that integrates the natural environment into the urban living style to provide high-quality social housing and local jobs in a beautiful and healthy place with diverse communities. The idea was first devised based on a work by the British writer Ebenezer Howard



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in 1898 [248]. The major principles of this model include providing fair and long-term opportunities for citizens’ housing, employment, and social and cultural integration, interlaced and accessible transport systems, and imaginative design and planning of urban elements with greeneries to enhance and support the natural environment. The city is intended to be planned, self-contained, with proportionate areas of residences, industry, and agriculture. The idea is among the first manifestations of urban sustainable development that provides not only individual opportunities for local food or energy production but also the fair distribution of community assets. The garden city principles are designed as an indivisible and interlocking framework for the delivery of high-quality places for citizens. The open or green spaces are considered as the integral parts of modern development plans that connect their spatial array to the overall configuration of developed zones. Today, the garden city model is regarded as a cornerstone of modern urban planning in general, and open (green) space planning in particular. Overall, none of the methods mentioned earlier could independently offer a truly balanced approach between the human demands and natural values and environmentally sound urban green space planning. Therefore, it is suggested that different aspects of urban green landscape models must be integrated. Such strategic integrated method should “simultaneously optimize benefits to biodiversity value, human well-being, and economic output” [79]. Schilling [202] argued that ecosystem services should be used as criteria within a multicriteria decision-making methodology in urban green space planning. The application of such methodology could help to increase public stakeholder involvement, reach a uniform vision within the different municipal divisions, and increase the green space quality—both for nature and for humans. An example of integrated implantation of different models and positive methodologies was presented by Szulczewska et al. [225], who estimated a threshold of 45% RBVA as the minimum green space required for some Polish urban neighborhoods under certain assumptions.

30.4 Sustainability and Resiliency Concepts The term sustainability can be defined as the ability to “maintain,” “uphold,” or “defend” at a certain rate or level [169]. The term first entered the political and environmental literatures in the 1980s, and when humans had noticed that the time of serious planetary resource depletion has already come and that nature’s ability to maintain and fix the humans’ unprecedented, undisciplined disturbances in the environment has become critical and increasingly vulnerable, the term began to be widely recognized in every community. Since the 1980s, the sustainability concept has been used more in the sense of human sustainability on planet Earth, and this has resulted in the most widely quoted definition of sustainability as a component of the discipline of sustainable development. The applied meaning of the sustainable development was presented at the Brundtland Commission of the United Nations on March 20, 1987: “Sustainable development is development that meets the needs of the present without compromising the ability of future generations to meet their own needs” [27]. In the period of 2 years that followed, around 140 alternative and variously modified definitions of “sustainable development” were proposed. Currently, it has been estimated that some 300 definitions for “sustainability” and “sustainable development” concepts exist broadly within the domain of environmental management and associated disciplines, either directly or indirectly [102]. The sustainable development goals, regardless of what the definitions might elaborate, such as economic development, social development, and environmental protection, were initially established on three overlapping ellipses: economy, politics or society, and environment. The three pillars of sustainability are not mutually exclusive and can be mutually reinforcing [33]. Recently, culture is considered as the fourth domain of sustainable development, since it influences a variety of social and economic factors in a society (Figure 30.1). Resiliency in ecology is a homologous term for sustainability, which means the capacity of an ecosystem to absorb destructive disturbances and still retain its basic structure and viability. It rather focuses on the management of human–nature interactions and commitment of human society to promote and manage the essential planetary ecological resources to promote resiliency and achieve sustainability of

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Culture Art, history, religion, ethics, ethnics

Society

Economy

Politics

Sustainability

Environment


Ecology

FIGURE 30.1 The main pillars of the concept of sustainability.

these essential resources for supporting the future generations’ demands [243]. Urban environments are mixed ecosystems involving humans, animals, plants, and nonliving elements that have evolved over time and space as the outcome of dynamic interactions between socioeconomic and biophysical processes that operate over multiple scales. The ecological resiliency of urban ecosystems means the degree to which they tolerate alteration before reorganizing around a new set of structures and processes. In cities, the fragmentation of natural habitats, simplification and homogenization of species’ composition, disruption of hydrological systems, and alteration of energy flow and nutrient cycling reduce the cross-scale resilience capacity of nature. In addition, various urban development models affect the patterns of built and natural land cover. The demands of human societies also differ across the world and consequently reflect on ecosystems in different ways [4]. All these phenomena could potentially alter the resilience level of nature in any human-occupied land. The word sustainability is a more commonly used term across many fields of science and managerial disciplines, while resiliency is rather an ecological phenomenon and terminology. Both terms, nevertheless, can be used interchangeably as they emphasize the same principles. Therefore, in the context of urban green space, which encompasses a plethora of management and scientific fields, the term sustainability could offer a better common understanding. Today, almost every human-made phenomenon could be reprogrammed with respect to the principles of sustainability. In the following section, we will first define a sustainable city from a theoretical viewpoint, and then we will discuss the role of urban green spaces in a sustainable city.

30.5 Eco-City: A Sustainable City The rapid pace of worldwide urbanization has raised many discussions and ambiguities about the concept of sustainable cities. Moreover, before defining any criteria for a sustainable city, a comprehensive and generally agreed upon definition for a city or urban area should be presented. The term urban has various meanings with respect to a variety of conditions, like the population density, land cover type and layout, and social and cultural practices. In terms of a city shape, urbanization phenomenon creates an environment, which is compositionally more heterogeneous, geometrically more complex, and ecologically more fragmented [7]. However, whatever the definition might be, the phenomenon of urbanization is something quite tangible since it drastically influences the surrounding environment of human communities. It does this by disturbing the natural setting in local weather due to increasing air



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temperature, changing water cycles, and altering ecological processes. Hence, the concept of sustainability in a city becomes an issue. There is still no unanimous agreement on the definition of a sustainable city and its components. There is a universal consensus on the goal of a sustainable city—that the layout of a sustainable city should efficiently meet the present-time needs of citizens without sacrificing its capacity to satisfy the future generation’s needs with the same natural and supporting renewable resources. However, the ambiguity and disagreement arises when the standards, principles, and procedures of how cities might carry out their attempts to become sustainable are on the agenda. Some cities have been developing their own sustainability indicator indices to attempt to measure the components of the quality of city life. Environmental issues such as water and energy supply/saving, municipal services, transportation, and security and citizens’ life qualities are important elements according to all the various definitions of a sustainable city. Therefore, a multidisciplinary and integrated task should be implemented to develop a universal theme for a sustainable city. A sustainable city, or eco-city, accordingly could be literally defined as a city that is ideally designed to facilitate a supportive and everlasting mode of life across the four pivotal domains of ecology, economics, politics, and culture [97]. In addition, considering environmental impacts, such a city is inhabited by people determined to minimize the required inputs of energy, water and food, waste recycling, and various environmental pollutions. It is strongly believed that developing a more sustainable city is not just about improving the abiotic and biotic aspects of urban life; it is also about the social aspects of city life, that is—among other factors—about the people’s satisfaction, experiences, and perceptions of the quality of their everyday urban environments [40]. Among the fundamental factors of sustainability, the environmental issue or urban green spaces and landscapes, because of their numerous advantages, are more noteworthy. The amount of public green spaces to support city dwellers is often mentioned as an important factor for a livable, pleasant, and attractive city and ultimately contributes to a sustainable community. Moreover, to support the principles of sustainability, any green landscape inside city borders should be self-enduring enough to help support the overall sustainability of its hosting urban community. Developing such supportive green space requires the implementation of multidisciplinary approaches across water management and irrigation techniques, plant biology, soil and chemical engineering, and maintenance strategies. Water is the most important factor in developing and maintaining a living green space under any environment. There is no landscaping solution that promotes a nonwatering regime for sustaining its living plants. Therefore, various methodologies and discussions for keeping a lively green space under water scarcity are aimed at reducing the water demands through devising an effective and integrated system of interwoven solutions for any related aspect of green space components. The outcome of the system will act as a funnel that collects every drop of water from various urban, industrial, and agricultural origins and efficiently manages them to support the city’s water demands, especially the urban green landscaping (Figure 30.2). Therefore, it will be wise as a first step to survey all the possible water resources inside and nearby urban areas because water is inevitably the most important factor in planning or introducing new urban green elements or expanding the existing vegetation cover.

30.6 Municipal Water Sources Throughout human history, civilizations and cities have been established near drinking water sources. Today, the technological advancements in liquid transport piping through long distances and water purification and desalination systems for utilizing recycled or sea waters have contributed immensely to the wider geographical distribution of big cities, even into arid and semiarid parts of the world. However, regarding the drastic global climate change effect on various local climates, many of the earlier predictions on economic supply of water with respect to earlier available effluent water sources now require serious revisions.

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Rain

waters ound Gr

Municipal waste

s iple ep

rinc

Proper plant species

n la

nds

cap

Efficient irrigation

c Agri

ac ral ultu

it tiv

fl

rba se u

rn etu yr

Local climate adapted design Maintenance

Support nearby industries

Nearby agri-animal farms

la nd sc e ap irr ig at io n

Municipal general uses

an

ire rf fo tion up ec ck ot Ba pr

Wa terwi

ow s

Proper soil

rb U


ret ur n

s

ay Gr

Industries

rs wate

FIGURE 30.2 A schematic presentation of the relationship between water inputs, water management principles, and some water-demanding activities in an urban setting. The small and scattered water inputs are collected and managed according to a water-wise water management package, which symbolically acts as a funnel. The efficient interaction among different components of the system could eventually provide sufficient water for supporting various water demands, including small-scale agriculture, green space, metropolitan services, and nearby industrial activities.



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The urban landscape water use, as mentioned earlier, is ranked second to third in terms of the total urban water consumption, only after drinking and household applications. Therefore, finding reliable water sources for supporting such huge demand has become a serious challenge in urban management in the arid and semiarid regions of the world. There are numerous examples of once prosperous and big cities in temperate climates that used to host rivers passing through them or in their neighborhood but now are suffering serious water stress due to the climate change and global warming phenomena. Therefore, the need for developing water-wise strategies for urban green landscapes in any city is no longer a matter of fantasy. To this end, the critical task is to define the urban water sources and develop reliable strategies to manage them. Municipal water supplies generally originate from nearby wells, rivers, lakes, and dams. The untreated water is first transferred to cities’ purification facilities and the purified water is distributed across the urban sections through urban water distribution network. The distance from water reservoirs and the quality of water directly influence the operational costs of municipal water supply and distribution. Although, the irrigation of urban greenery requires a considerable amount of water, the water quality is not as critical as that of drinking water. The geography, local annual weather condition, regional climate, distance from water sources, available water-related technologies, economic status, and cities’ water purification and distribution infrastructures are influencing factors for planning water supply strategies in a city. Today, with the advancement of various technologies, the hope for better and more efferent water treatment and distribution is high. However, access to a reliable water source is still a pivotal issue in any water-related program. In the following section, the common water sources in urbanized zones will be briefly discussed.

30.6.1 Natural Water Bodies Cities near lakes and dams or on river banks could reliably satisfy their general nondrinking urban water demands, like green space irrigation. Since the natural water in rivers and in-land lakes is generally considered safe for supporting their surrounding wildlife and there is no immediate need for applying high water sanitation standards in this context, there will be an economic opportunity to utilize such reservoirs for nondrinking purposes in cities. However, this is not the case for most of the cities that are located in water-stressed regions. On the other hand, there are increasing environmental concerns due to the introduction of new industrial pollutants into natural water reservoirs and the legal amendments demanding improvement of the minimum standards for natural water applications in cities that limit the availability of such resources [272]. Saline natural waters like seawater cannot be considered a readily harvestable source. Due to their high mineral concentrations, mainly chloride and sodium ions, they require considerable capital investment and processing costs. Despite progress in desalination technologies, seawater desalination is still far more energy intensive compared to conventional technologies for treatment of freshwater. There are also concerns about the potential environmental impacts of large-scale seawater desalination plants [68]. Rainwater is another natural but temporary source for urban general greenery applications. The availability and quantity of this source widely vary among local climates, and during different seasons of the year. However, it is noteworthy to develop methods for the utilization of rainwater harvesting systems in the context of sustainable development [166].

30.6.2 Reuse Water Urban development also had a significant impact on water cycles in nature. The water body that is used for urban water supply reduces the natural surface and underground water sources because a large portion of the used water cannot safely return to nature. The application of wastewaters after


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different degrees of desalination or disinfection is one way that an urban community can extend the usefulness of its existing water resources and improve water-use efficiency. In fact, water reuse in urban areas is now the first and the most sought-after strategy in water sustainability. The advantages of water reuse strategy include overall cost-savings in water processing, securing groundwater resources, reducing the natural water harvests, and long -term water supply [20,72,158]. Therefore, the reutilizing of any off-the-grid water from any municipal, industrial, and agricultural activity that is located at a logical distance from a city is gaining attention worldwide. O’Connor et al. [163] described any type of water that has lost its natural biochemical and physical attributes due to human activities as “degraded waters” and discussed their specific characteristics. The common sources of degraded waters in urban areas may include the cooling water sink in thermoelectric power plants, agricultural irrigation return flows, large volumes of wastewater derived from various animal husbandry activities, municipal wastewater effluents, storm water runoff, and gray waters. Cities vary in respect to the types and harvest capacities of such sources. The reclaimed water utilization based on its type-specific characteristics might include urban and residential landscape irrigation, industrial uses, air-conditioner chiller water demand, and a backup source for fire protection. However, the irrigation of urban landscapes with reclaimed water has become one of the highlighted applications that could significantly contribute to the sustainability of existing urban water resources and urban green spaces [6,16]. Some types of recycled waters may reduce nutrient discharges back into natural water bodies like rivers and improve the soil N, P, Ca, K, and many micronutrient contents [32,122]. It was also observed that irrigation by reclaimed water could significantly improve the soil microorganism activity [37]. The ultimate goal of recycled water management in green landscapes is to ensure that the recycled water is of sufficient quality to maintain healthy plants and is hazard-free for city dwellers. Various influencing factors should be considered to achieve a healthy landscape using recycled water, including local climate conditions, plants and soils’ characteristics, site evaluation, and water quality based on the plant species’ sensitivity, soil texture, and soil drainage irrigation practices [52]. Navarro et al. [152] reported a number of successful cases of wastewater reuse plans for irrigation purposes and described the main health and management factors that should be exercised on recycled waters. Moreover, from a sustainability viewpoint, the overall process of collecting, reconditioning, and redistributing urban wastewaters should be economical [70]. However, there are some disadvantages regarding the application of recycled waters in or nearby urban areas, like chemical pollutants’ buildup in the soil, the probable contamination of groundwater pools, and possible human health risks due to the possible occurrence of pathogenic microorganisms in reclaimed urban or industrial waters [87]. With respect to the latter—probable microbial activities in wastewaters—the health standard consideration of recycled water is an important issue for the human and the ecosystem that survive on or are exposed to this type of water sources. A few field investigations have revealed that the reclaimed water source and different lengths of irrigation times might influence the soils’ heavy metal pollution indices [213,263], salinity level [60,89,244], microbial activities [2,230], and vegetation health. Chen et al. [37] reported that health conditions of the soils under study were improved with reclaimed water irrigation, and soil quality increased when the reclaimed water irrigation period became longer. The response of landscape plants and soils to irrigation with recycled water depends on the degree to which the soil might be affected over time, and the tolerance of plant species to salts and specific ions. The evaluation of sites, which are being irrigated with recycled water, should include an assessment of soil conditions, plant species’ salt tolerance, water quality, and irrigation management program. There are several factors to be considered when evaluating a site’s suitability for irrigation with a particular type of recycled water, including the average water quality, soil current salinity and pH, soil texture, soil profile, soil drainage, salt sensitivity of plants in the landscape, and implemented irrigation system and program [52].


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30.7 Water Demand Estimation in Urban Green Spaces Historically, water has been recognized as an economic good in human societies [186]. In 1992, under the Dublin Water Principles issued by the United Nations, it was again reiterated “water as an economic good” [217]. Therefore, this economic good just like any other commodity requires necessary safeguards and logical management, especially when it is becoming scarce. Green landscapes are often associated with high water requirements, particularly during periods of long dry weather. This is the case for many municipalities in semiarid zones, as in the western United States, where irrigation consumes 50% of the total municipal water supplies [133]; in the west of Australia, it reaches 56% of domestic consumption [224]; in Texas, this is the third largest use [30]; and about 60% of potable municipal water in Utah, United States [223]. Urban green spaces are mostly considered as mixed-vegetation greeneries. It makes the estimation of demand for irrigation water an even more challenging task because different plant species require different or conflicting growth conditions. In addition, other environmental factors and metabolic aspects of plants should be taken into account [193,212]. The ability to estimate water demand under multiple climates, population growth, and conservation scenarios is inevitably related to the urban hydrological processes and modeling. Many urban areas face similar stresses and consequently need to expand their water supplies and distribution facilities. The accurate estimation of the quantity and operation capacity of reservoirs, pumping stations, and distribution pipe systems relies on existence of cost-effective and reliable infrastructures that are managed under localclimate-adapted and reliable water demand forecast models, especially for assessing peak demands [91]. There are two types of forecasting procedures for urban general water demand. The first type includes short-term forecast models, which are basically utilized for operation and management. The second type is the long-term forecasts that are required for planning and infrastructure design [22]. Currently, water managers produce demand estimates by considering long-term climate trends and the principle of natural systems’ fluctuation within an unchanging setting of variability known as “stationarity” [148]. Climate change also introduces serious uncertainties that may limit the accuracy of this method, because historical trends will no longer be reliable for predicting future climate-sensitive water demand [91,148]. However, there are still no suitable specialized methods to estimate the water demands of urban landscape vegetation [160]. It is logical to suggest that increasing water-use efficiency in urban landscapes is achievable by supplying plants with the minimum amount of water that is required to maintain their healthiness and aesthetic appearance. But how could the minimum level be defined? Landscape designers, water science experts, and policymakers definitely require the type of information that could facilitate the estimation of minimum plant water demand in order to balance their atmospheric evaporative pressure with visual, functional, and health performance expectations [211,253]. Despite the similar physiological characteristics between landscape plant species and forestry or agricultural crops, technically, it is not feasible to apply agricultural methodologies for estimating the water demand of urban landscapes since they are quite different due to the specific conditions in urban green spaces [53,160]. Unlike the agricultural crops and turfs, landscape plantings are usually composed of a mixture of trees, shrubs, and turfs with different species having quite different water demands. The vegetation planting density varies seasonally for different species resulting in different evapotranspiration rates [257]. In addition, a wide range of microclimates are possible in an urban landscape planting through protected, shaded, or paved areas that might affect solar radiation and reflection, wind, or other weather parameters. These variables all need to be taken into consideration when developing efficient irrigation systems under urban settings [159]. Agricultural water management modeling and planning have been used as a base for developing landscape water requirement (LWR) prediction models. Basically, various key factors in a green landscape are incorporated into the existing agri-models. Methodological approaches have been devised to utilize a modified version of the agricultural ET0 model. This model is mainly related to the amount of water that is lost through the evapotranspiration process through different growth and developmental

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stages in plants. Equation 30.1 [83] represents the crop evaporation (ET0), which is a relationship between weather data and crop characteristics:

ETcrop = K crop ´ ET0

(30.1)


where ETcrop is the crop evapotranspiration (mm/day) Kcrop is the crop coefficient ET0 is the reference evapotranspiration (mm/day) The reference evapotranspiration can be derived from an installed weather station. The crop evapotranspiration is estimated by monitoring the water inputs and outputs of the test plants under irrigation regimes in research fields [83]. The appropriate coefficients for crop characteristics then could be calculated for the specific crop under particular environmental and weather conditions [21]. Nouri et al. [160] have reviewed different approaches for ET measurement for estimating the water requirements of urban landscape vegetation. The first methodology based on this model puts the LWRs at the level of ET0 values [86]. This comparison is logical if most of the landscape area is turf. The second approach is based on the direct estimation of LWRs through the use of instruments such as volumetric soil water sensors [253] or weighing lysimeters [26]. The water use classification of landscape species (WUCOLS) method is a hybrid methodology that relies on expert evaluations to estimate the water requirements of mixed urban vegetation to meet acceptable aesthetic expectations, health, and reasonable growth for the plant species [53]. The model applies an ad hoc procedure to estimate the coefficients that replace the crop coefficient by a landscape coefficient [193] and is derived from the following equation: ETL = K L ´ ET0

(30.2)

where ETL is the landscape evapotranspiration K L is a landscape coefficient K L is based on empirical field observations and is calculated as [159]

K L = K s ´ K d ´ K mc

(30.3)

where K s is the plant species factor Kd is the density factor Kmc is the microclimate factor that may vary between and within different landscape vegetation types On the other hand, since the crop coefficients for many landscape plants are still not available to define their irrigation requirements, some authors have proposed other concepts to facilitate more realistic plant water estimation methods in urban green spaces like plant factor [179], IPOS method [219], and gross irrigation water requirement [86]. Elaboration on the theoretical assumptions and principles of these approaches is out of the scope of this text, and interested readers are encouraged to read the literature reviews in this field (e.g., House-Peters and Chang [91]). The effectiveness of these models is practically influenced by varying climatic and site-specific factors, so nominating one model as a comprehensive approach for all climates may not be feasible. Recently, a study comparing three observation-based approaches was conducted in order to develop a practical and more realistic method for estimating the water demand of urban landscape vegetation in Australia. It was concluded that the


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WUCOLS method produced the best estimation of urban vegetation water requirements for the studied area [159]. However, more comparative and site-specific studies are required to devise reliable and more realistic approaches for water estimation in urban green spaces. The water demand estimation under any method and to any level of certainty is more demanding, and will be more effective when applied at the planning and developing stages in an urban green landscape. The information revealed by such analysis could shed light on many aspects of the long-term development and maintenance of urban green landscapes, as all the stakeholders already have a general and almost realistic understanding about the future water crisis. This also further facilitates the adoption of proper irrigation and water management strategies.


30.8 Irrigation Systems for Urban Landscapes Water management in the context of sustainability could be defined as a steady state of water supply and distribution and is achieved when water is abstracted from natural or human-made resources at a rate lower or equal to the rate of recharge. Therefore, it could be assumed that other natural competitors like microorganisms, plants, and animals, in addition to various physicochemical conversion processes in the surrounding environment, would get their share of the locally and globally available water resources. One strategy to achieve this goal in human occupied areas is applying the science of irrigation. The irrigation science and strategies were basically developed for implementing more efficient water management. Landscape irrigation is the systematic application of water to land areas that supply the water needs of ornamental and landscape plants. The irrigation methods applied in agricultural practices around the globe are affected by accessibility to water reservoirs and land topology. The increasing knowledge of plant physiology and modern plant cultivation techniques has contributed to improving the response to plant water need and has led to devising various technologies for water and land management. Wise or fine-tuning irrigation has shown to improve plants’ water-use efficiency, allowed more precise use of water, and enhanced the quality of vegetation [36]. Many modern irrigation systems and strategies are being developed and widely used under different climatic and geographical regions in agriculture and ornamental greenery projects around the world. However, conventional irrigation methods like flooding irrigation are still practiced in many parts of the world due to the various limitations of modern instruments and new skills or ease of access to great water resources. A few commonly used irrigation methods in agriculture were examined for their efficiency and flexibility in urban green space irrigation plans under standard conditions [10,29,74]. However, empirical data show that under different circumstances, by considering influencing factors including weather status, soil, and vegetation type, the optimum irrigation strategy might differ. Technology advances have also played an important role in improving the efficiency and performance of current systems, as well as introducing more specific watering techniques for particular purposes. The irrigation efficiency from a topology viewpoint in landscapes could be the result of uniform or even watering in order to reduce the difference between the minimum and maximum wetted areas [89]. Despite the application of different irrigation systems, the watering quantity and timing is still a matter of concern because the actual amount of water used by plants widely varies among plant species and different plant growth stages. Moreover, to limit this assumption to only plant water demand, one theoretically needs to rule out the climatic and other environmental factors that influence the plant water consumption. The incorporation of new technologies into the common manual or conventional systems does not always render useful. This has been the case for standard or time-based irrigation system controllers in which a human supervisor should set up the irrigation schedule. This procedure has shown to increase the irrigation water volume when compared with a manual irrigation regime. This fact seems to be a result of the prioritization of time saving over saving the irrigation water. Adjusting the irrigation controller to changes in water requirements is a time-consuming task, and many users perceive it as too complicated [193]. One way to overcome this problem is to link the real-time reading of the landscaped environmental conditions with the plants’ actual water demand, in order to identify the


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exact moments when watering is appropriate. The application of advanced irrigation controllers, which are equipped and programmed by environmental sensors like soil humidity sensors, rainfall sensors, surface heat sensors, and thermometers with the capacity of obtaining ET0 estimates [58,63,67,85], has resulted in considerable reductions in water-use volume. In addition to conserving the irrigation water, some reports suggest that the application of such real-time atmospheric and hydrologic reporters has led to an increase in the visual quality of landscape [193]. Although very few studies have been conducted under controlled conditions to evaluate the waterconserving performance of various smart controllers, it has been demonstrated that the advanced irrigation controllers permit reduction of water use by 11%–75% as compared with the manual irrigation for commercial landscapes and reduce runoff by 64%–71% [89]. Automatic irrigation using moisture sensors has improved the efficiency of the process [43,187]. These sensors may reduce water consumption up to 75% when compared to manual irrigation. In theory, with smart controllers, the burden of decision making in irrigation management is removed from the hands of the human operator. However, studies indicate that the use of such controllers still does not completely rule out the human interaction with the system, because they require a technician with horticulture science and landscape irrigation management to set up and maintain [89]. Every day, new environment sensing and surveillance technologies are being introduced in the markets that offer various advantages and options. The incorporation of such solutions with the knowledge of plant water requirements would definitely enhance the plant watering efficiency and will support a more enduring green space in terms of both landscaping and agriculture.

30.9 Multidisciplinary Solutions for Sustaining a Water-Wise Landscape Water-wise and efficient landscaping is the consequence of a comprehensive understanding of the local environmental conditions, innovative and flexible design, efficient irrigation systems, appropriate plant species choices, improved soil, and strategic maintenance [89]. Water-efficient green space planning can meaningfully lower water consumption without compromising the landscape functionality or aesthetics [223]. However, in addition to the water issue, there are other factors that influence a sustainable urban landscaping. These factors are mainly plant-related issues and green space management. In this section, the key factors that contribute to a sustainable urban green space setting will be briefly discussed. It should be noted, however, that our knowledge about plant–environment–human interactions is still expanding, and there are yet other parameters that might not be listed in this text.

30.9.1 Plant Species Selection The plant varieties’ composition and species richness are the distinguishing factors that affect all the developmental and management practices in urban landscapes. Almost all water-efficient landscaping principles apart from plant material could similarly be applied to any landscape, while plant selection strategies drastically vary among climatic and geographic regions [223]. Despite the designing, implementation, supplying water, and maintenance limitations in urban areas, the overall plant species richness might be greater in cities than in rural areas [146]. This is mostly due to the city dwellers’ preferences for higher aesthetics and the acclimatization of introduced exotic species into urban areas [255]. However, higher vegetation variation in a region especially when exotic species are involved might hinder the efficient implementation of sustainable and water-wise landscaping under water-stressed climates. A comprehensive understanding of plants’ water use and drought mechanism is required for a successful mixed green space planning and maintenance. Drought, in plant biology, is the state of considerable, long-term water scarcity and dry weather that results in damage to endemic vegetation in a geographical region. Since drought is a common environmental phenomenon in nature, living organisms have evolved different mechanisms to survive it.



577

Plants have been more seriously prone to water stresses like water scarcity, salinity, and low or high temperatures. The drought resistance then could be defined as the integrated capabilities and mechanisms in plants to sense and adapt to harsh environmental conditions caused by drought scarcity. This capacity has been acquired in the course of evolution to allow plants to adapt to specific habitats for the benefit of their growth and development. Plants generally utilize complementary mechanisms with respect to the extent and severity of water deficit in their rhizosphere, like reducing water loss via rapid stomatal closure, leaf rolling [227], and increasing wax accumulation on the leaf surface in many plant species [18,205], which help reduce the water transpiration rate. Overall, the survival strategies vary among plant species, depending on water stress type and duration, and under different climatic conditions. However, plant drought resistance mechanisms could be generally discussed under four main categories: drought avoidance or shoot dehydration, drought tolerance, drought escape, and drought recovery [121,241]. The drought tolerance and drought avoidance mechanisms are known to be more commonly applied among plants [269]. Drought avoidance is the capability of a plant to maintain its fundamental and normal physiological processes under mild or moderate drought stress conditions by adjusting certain morphological structures or growth rates to avoid the negative effects of the drought stress [17]. The maintenance of high plant water potentials under water shortage conditions is the main characteristic of this mechanism [36]. Plants utilize these strategies through (1) keeping the water current from root to shoot and simultaneously uptaking nutrients from the soil; (2) enhancing the water uptake ability through a root system with increased rooting depth, rooting density, or root/shoot ratio and improving the water storage capacity in specialized parts of a plant such as fleshy water-storing tissues of cacti and succulents [164,227]; and (3) accelerating or decelerating the conversion from vegetative growth to reproductive growth to avoid complete abortion at the severe drought stress stage [150]. Moreover, some plants like cacti open their stomata only at night to capture carbon dioxide necessary for photosynthesis, when cooler temperatures mean a reduction in water loss. Many herbaceous plants use this strategy to avoid drought by completing their life cycles before the annual dry period. A variety of ornamental and turf grasses grow well in hot, dry areas once they have established their early stages of growth and development under stressed conditions. Therefore, the plant germination capacity to withstand different levels of environmental stresses, or “the factor of origin” [208], is an important parameter for selecting the proper landscape plant species for arid or semiarid urban areas. Drought tolerance is, however, a mechanism by which plants sustain a certain level of physiological activities under severe drought conditions through the regulation of a plethora of genes and metabolic pathways to alleviate or repair stress damage [245]. Since physiological interactions within plant cells to cope with water stresses seriously affect the biochemical balance in cell protoplasm, one of the main goals in response to harsh conditions is adjusting the activities of cell defense enzymes to reduce the accumulation of hazardous substances. Another strategy that is naturally practiced by some plants and could also be applied artificially under landscape management principles is referred to as drought escape. This solution is through the adjustment of the growth period, life cycle, or planting time of plants to prevent the growing season from encountering local seasonal or climatic drought [150]. Plants like yellow star thistle [185] and alyssum [12] that live in hot deserts or cold plateaus are examples of plants that use the drought escape mechanism to survive and reproduce in the usually very short period of moderately favorable conditions. Overall, these strategies serve one basic rule, which is to sustain the plant under water-stressed situations in the mid or long term. As mentioned earlier, highly adapted plants combine many of these mechanisms to survive and develop under water scarcity. The plant species’ selection, particularly the introduction of exotic flora into new environments for their visual attraction or stress tolerance capacities, has been the scope of many studies [44,49,84,119, 131,146,151,178,191,214,229,247,268]. Table 30.1 provides some examples of various drought-tolerant trees, shrubs, bushes, and turf or grass species that could be used in urban green space settings under semiarid or arid climates. Planning green space settings based on enough knowledge about plant

Ephedra intermedia Artemisia sieberi Pennisetum orientale Perovskia abrotanoides Karel Verbascum songaricum Satureja hortensis Vitex agnus castus Hesperaloe sp. Capparis spinosa Berberis gagnepainii Buddleia davidii Buxus hyrcana Caesalpinia gilliesii Chaenomeles japonica Cotoneaster horizontalis Elaeagnus pungens Euonymus japonicus

Bush

Shrub

Shrub

Shrub

Shrub

Shrub Shrub Shrub

Bush Bush Shrub

Bush

Bush

Bush

Bush

Bush Bush

Scientific Name

Type

Evergreen euonymus

Silver berry

Rock cotoneaster

Butterfly bush — Bird of paradise bush Japanese quince

Monk’s pepper tree — — Bergenia moench

—

—

—

Artemisia —

Ephedra

Common Name

Celastraceae

Elaeagnaceae

Rosaceae

Rosaceae

Loganiaceae Buxaceae Caesalpiniaceae

Asparagaceae Caryophyllales Berberidaceae

Verbenaceae

Leguminosae

Scrophulariaceae

Lamiaceae

Compositae Poaceae

Ephedraceae

Plant Family

Evergreen

Evergreen

Evergreen

Deciduous

Deciduous Evergreen Deciduous

Deciduous Deciduous Evergreen

Deciduous

Deciduous

Deciduous

Deciduous

Deciduous Deciduous

Deciduous

Foliage Type

High

High

Moderate

Low

Moderate Moderate High

High High Moderate

High

High

High

High

High High

High

Salinity Tolerance

High

High

High

High

High High High

High High High

High

High

High

High

High High

High

Drought Tolerance

TABLE 30.1 Examples of Suitable Plant Species for Arid and Semiarid Urban Green Landscapes

High

High

High

High

High High High

High High Moderate

High

High

High

High

High High

High

Air Pollution Tolerance

High

High

Moderate

Moderate


High High High

High

High

High

High

High High

High

Tolerance to Soil Alkalinity


Low

Low

Moderate

Low

Low Low Low

Moderate Low Low

Low

Low

Low

Low

Low Low

Low

Soil Fertility

−15

−20

−25

−25

−15 −15 −5

0 0 −20

0

0

0

0

0 0

0

Low Temp. (°C)

Fast

Fast

Fast

Fast

Fast Fast Fast

3–4.5

3–4.5

0.6–0.9

0.6–0.9

1.5–3 3–7 3

1–1.5 0.6–0.9 1.5

1.5–3

0.4–0.6

1–1.5

0.5–1

0.6–0.9 0.6–0.9

1–1.5

Max. Height (m)

(Continued)

Fast Moderate Fast

Fast

Fast

Fast

Fast

Fast Fast

Fast

Growth Speed

578 Handbook of Drought and Water Scarcity

Scientific Name

Shrub Photinia serrulata Shrub Pittosporum tobira Shrub Cercis chinensis Shrub Cercis occidentalis Shrub Cercis siliquastrum

Shrub Symphoricarpos albus Shrub Vitex agnus castus Shrub Laurus nobilis

Shrub Euonymus japonicus var. argenteovariegata Shrub Euonymus japonicus var. aureo-variegata Shrub Euonymus japonicus var. viridi-variegata Shrub Forsythia intermedia Shrub Lonicera fragrantissima

Type

Celastraceae

Celastraceae

Oleaceae

Evergreen euonymus

Evergreen euonymus

Showy Forsythia

Deciduous Deciduous

Caesalpiniaceae Caesalpiniaceae

Caesalpiniaceae

Judas tree redbud

Deciduous

Evergreen

Evergreen

Evergreen

Moderate

Moderate Moderate

High

Low

High

High

Deciduous

Pittosporaceae

Rosaceae

Lauraceae

High

Moderate

High

High

High

High

Deciduous

Semideciduous

Deciduous

Evergreen

Evergreen

Evergreen

Foliage Type

Japanese Pittosporum Chinese redbud Western redbud

Sweetbay grecian laurel Chinese Photinia

Winter Caprifoliaceae honeysuckle, bush honeysuckle Common Caprifoliaceae snowberry Chaste tree Verbenaceae

Celastraceae

Plant Family

Evergreen euonymus

Common Name

Salinity Tolerance

High

High High

High

High

High

High

High

High

High

High

High

High

Drought Tolerance

High

High High

High

High

High

High

High

High

High

High

High

High


TABLE 30.1 (Continued) Examples of Suitable Plant Species for Arid and Semiarid Urban Green Landscapes

High

High High

Low

High

High

High

High

Moderate

High

High

High

High



Low

Low Low

Moderate

Moderate

Moderate

High

Low

Moderate

Moderate

Low

Low

Low

Soil Fertility

−23

−23 −23

0

−17

−18

−18

−38

−15

−30

−15

−15

−15

Low Temp. (°C)

Moderate

Moderate Moderate

Slow

Slow

Slow

Fast

Fast

Fast

Fast

Fast

Fast

Fast

Growth Speed

(Continued)

4.5–10

1.5–6 5–7.5

2.4–3.6

4.5–7.5

3.6–4.5

3–5.5

0.6–1.8

1.8–3

2–3

3–4.5

3–4.5

3–4.5

Max. Height (m)

Water Scarcity and Sustainable Urban Green Landscape 579

Japanese maple Norway maple

Sycamore maple

Acer palmatum Acer platanoides

Acer pseudoplatanus

Tree

Aceraceae

Aceraceae Aceraceae

Aceraceae

Box elder

Tree Tree

Caesalpiniaceae

Fabaceae

Honey locust

Papilionaceae

Spanish broom

Amaranthaceae

Deciduous

Apocynaceae Rosaceae

Oleander Scarlet firethorn

Saxaul

Evergreen

Myrtaceae

Common myrtle

Deciduous

Deciduous Deciduous

Deciduous

Deciduous

Evergreen

Evergreen Evergreen

Evergreen

Evergreen

Lythraceae

Deciduous

Rosaceae Deciduous Evergreen

Evergreen

Rosaceae

Foliage Type

Hydrangeaceae Celastraceae

Willowleaf cotoneaster English hawthorn

Shrub Cotoneaster salicifolius Shrub Crataegus oxyacantha Shrub Deutzia scabra Shrub Euonymus europaeus Shrub Lagerstroemia indica Shrub Myrtus communis Shrub Nerium oleander Shrub Pyracantha coccinea Shrub Spartium junceum Shrub Halimodendron halodendron Shrub Haloxylon persicum Tree Gleditsia triacanthos Tree Acer negundo

Plant Family

Fuzzy deutzia European spindle tree Crape myrtle

Common Name

Scientific Name

Type

High

Moderate High

Moderate

High

High

High

High

High Moderate

High

Moderate

Moderate Moderate

High

Moderate

Salinity Tolerance

Moderate

Moderate Moderate

High

High

High

High

High

High High

High

High

High High

High

High

Drought Tolerance

Low Moderate– high High

Moderate

High

High

High

High

High High

High

High

High High

High

High



Moderate

Low Moderate

High

High

High

High

High

High High

High

Moderate

High High

High

Moderate



Moderate

Moderate– high Moderate Moderate

Low

Low

High

Low

Low High

Moderate

Low

High Low

Moderate

Moderate

Soil Fertility

−30

−28 −33

−40

−39

−5

−10

−10

−10 −19

−12

−5

−20 −40

−25

−36

Low Temp. (°C)

Fast

Fast Fast

Fast

Fast

Moderate

Moderate

Moderate

Moderate Moderate

Moderate

Moderate

Moderate Moderate

Moderate

Moderate

Growth Speed

(Continued)

15–30

8 12–15

15–18

15–22.5

4.5–5

1.5–2

1.8–3

2.5–5.5 1.8–4.5

1.5–1.8

3–9

2–3 2

5.5–7.5

3–4.5

Max. Height (m)


Ailanthus altissima Albizia julibrissin Caragana arborescens Cedrus deodara Celtis reticulata Celtis sinensis Cinnamomum camphora Elaeagnus angustifolia Eucalyptus gunnii Fraxinus excelsior Fraxinus ornus Fraxinus velutina Gleditsia triacanthos sunburst Maclura pomifera Melia azedarach Melia azedarach umbraculifera Morus alba Populus euphratica

Tree

Tree Tree

Tree Tree

Tree

Tree Tree Tree

Tree

Tree

Tree

Tree Tree Tree Tree

Tree

Tree

Scientific Name

Type

Persian lilac Texas umbrella Tree White mulberry Willows

Osage orange

Flowering ash Velvet ash Thornless honey locust

European ash

Cider gum

Russian olive

Deodar cedar Desert hackberry Chinese hackberry Camphor tree

Pea tree

Silk tree

Tree of heaven

Common Name

Moraceae Salicaceae

Meliaceae Meliaceae

Moraceae

Oleaceae Oleaceae Caesalpiniaceae

Oleaceae

Myrtaceae

Elaeagnaceae

Pinaceae Ulmaceae Ulmaceae Lauraceae

Papilionaceae

Mimosaceae

Simaroubaceae

Plant Family

Deciduous Deciduous

Deciduous Deciduous

Herbaceous

Deciduous Deciduous Deciduous

Deciduous

Evergreen

Deciduous

Evergreen Evergreen Evergreen Evergreen

Deciduous

Deciduous

Deciduous

Foliage Type

High High

High High

High


Moderate

High

High

Low Moderate Low High

High

Moderate

High

Salinity Tolerance

High High

High High

High

High High High

High

High

High

High High High High

High

Moderate

High

Drought Tolerance

High High

High High

High

High High High

High

High

High

High Moderate Moderate High

High

High

High



High Moderate

High High

High

High Moderate High

High

High

High

Moderate High Moderate Moderate

High

High

High



Low Low

Low Low

Low

High Low Low

High

Moderate

Moderate

Moderate Moderate Moderate Moderate

Moderate

Low

Low

Soil Fertility

−27 −18

−10 −10

−23

−18 −25 −40

−29

−15

−42

−17 −18 −18 10

−39

−5

−25

Low Temp. (°C)

Fast Fast

Fast Fast

Fast

Fast Fast Fast

Fast

Fast

Fast

Fast Fast Fast Fast

Fast

Fast

Fast

Growth Speed

(Continued)

6–18 15

9–15 9

18

15 9–15 15–24

18–24

10–25

4.5–6

12–18 8 12–18 12–15

6

4.5–7.5

18–25

Max. Height (m)


Tree

Tree

Tree Tree Tree Tree

Common juniper

Persian conifer

Common horse chestnut Pignut hickory Western catalpa Cedar of Lebanon Chinese juniper

Aesculus hippocastanum Carya glabra Catalpa speciosa Cedrus libani Juniperus chinensis Juniperus excelsa sp. Juniperus communis

Tree

Tree

Tree Tree

Tree

Tree

Tree Tree

Tree

Populus nigra Lombardy italica Populus Quaking aspen tremuloides Prunus lusitanica Portugal laurel Robinia Black locust pseudoacacia Robinia Umbraculifera pseudoacacia black willow umbraculifera Tamarix aphylla Tamarix aphylla athel Acacia salicina Willow acacia Eucalyptus River red camaldulensis eucalyptus Acer campestre Hedge maple

Tree

Common Name

Scientific Name

Type

Deciduous

Evergreen Evergreen

Evergreen

Deciduous

Evergreen Deciduous

Deciduous

Deciduous

Foliage Type

Cupressaceae

Cupressaceae

Juglandaceae Bignoniaceae Pinaceae Cupressaceae

Evergreen

Evergreen

Deciduous Deciduous Evergreen Evergreen

Hippocastanaceae Deciduous

Aceraceae

Fabaceae Myrtaceae

Tamaricaceae

Papilionaceae

Rosaceae Papilionaceae

Salicaceae

Salicaceae

Plant Family

High

High

Low Moderate Moderate High

Low– moderate High

High High

High

High

High High

High

Moderate

Salinity Tolerance

High

High

High High High High

High– moderate Moderate

High High

High

High

High High

High

High

Drought Tolerance

High

High

Low High High High

High

High

High High

High

High

High High

High

High



High

Moderate

High High High Moderate

High

High

Moderate Moderate

High

High

Moderate High

High

High



Low

Low

Moderate Moderate Moderate Low

High

Low

Moderate Moderate

Low

Low

Moderate Low

Moderate

Low

Soil Fertility

−40

−25

−33 −39 −35 −25

−15

−28

−5 −5

−15

−36

−15 −36

−57

−30

Low Temp. (°C)

Slow

Slow

Slow Slow Slow Slow

Slow

Slow

Fast Fast

Fast

Fast

Fast Fast

Fast

Fast

Growth Speed

(Continued)

0.5–6

15–20

15–25 15–24 12–15 15–20

25–75

6–9

6.5–12 12–15

4–6

4.5–6

3–6 12–21

6–18

12–30

Max. Height (m)


Punica granatum Taxus baccata Ziziphus jujuba Cedrus atlantica Celtis australis

Celtis occidentalis Crataegus pinnatifida Cupressus arizonica Cupressus sempervirens Cupressus sempervirens Juniperus virginiana

Tree Tree Tree Tree Tree

Tree

Tree

Tree

Tree

Tree

Tree

Tree Tree Tree

Virginian red cedar

Columnar cypress

Italian cypress

Arizona cypress

Fruiting olive Persian parrotia Austrian black pine Pomegranate English yew Jujube Atlas cedar European hackberry Common hackberry Hedge thorn

Goliath

Golden rain tree

Koelreuteria paniculata Magnolia grandiflora Olea europaea Parrotia persica Pinus nigra

Tree

Tree

Common Name

Scientific Name

Type

Cupressaceae

Cupressaceae

Cupressaceae

Cupressaceae

Rosaceae

Ulmaceae

Punicaceae Taxaceae Rhamnaceae Pinaceae Ulmaceae

Oleaceae Hamamelidaceae Pinaceae

Magnoliaceae

Sapindaceae

Plant Family

Evergreen

Evergreen

Evergreen

Evergreen

Deciduous

Evergreen

Deciduous Evergreen Deciduous Evergreen Evergreen

Evergreen Deciduous Evergreen

Evergreen

Deciduous

Foliage Type

High

High

High

Moderate

High

Moderate

Moderate Moderate High Moderate Moderate


Moderate

Moderate

Salinity Tolerance

High

High

High

High

High

High

High High High High High

High High High

High

High

Drought Tolerance

High

High

High

High

High

High


High High High

High

High



High

High

High

High

High

High

High Moderate High High High

High High High

Low

Moderate



Low

Low

Low

Low

Moderate

Moderate

Moderate Moderate Low High Moderate

Moderate High Low

Low

Low

Soil Fertility

−41

−15

−15

−25

−20

−39

−5 −36 −20 −20 −18

−14 −10 −30

−14

−27

Low Temp. (°C)

Moderate

Moderate

Moderate

Moderate

Moderate

Moderate

Slow Slow Slow Moderate Moderate

Slow Slow Slow

Slow

Slow

Growth Speed

(Continued)

12–15

12–20

12–20

9–12

6

15–27

4.5–6 6–12.5 4.5–10.5 18–40 12–21

7.5–10 6–12 12–18

18–30

9–12

Max. Height (m)


Morus nigra Pinus eldarica Pinus pinea Pinus ponderosa Pistacia atlantica

Pyrus calleryana Quercus ilex Quercus suber Sophora japonica

Tamarix gallica Taxodium distichum Ulmus carpinifolia

Tree Tree Tree Tree Tree

Tree Tree Tree Tree

Tree Tree

Tree

Tree

Smooth leaf elm

Japanese flowering crabapple Black mulberry Mondell pine Stone pine Ponderosa pine Mount Atlas pistache Bradford Holly oak Cork oak Japanese pagoda tree Tamarix Bald cypress

Eastern red cedar

Silver eastern red cedar

Juniperus virginiana glauca Juniperus virginiana var. virginiana Malus floribunda

Tree

Tree

Common Name

Scientific Name

Type

Ulmaceae

Tamaricaceae Taxodiaceae

Rosaceae Fagaceae Fagaceae Papilionaceae

Moraceae Pinaceae Pinaceae Pinaceae Anacardiaceae

Rosaceae

Cupressaceae

Cupressaceae

Plant Family

Deciduous

Evergreen Deciduous

Herbaceous Evergreen Evergreen Deciduous

Deciduous Evergreen Evergreen Evergreen Deciduous

Deciduous

Evergreen

Evergreen

Foliage Type

Moderate

High Moderate

Moderate High Moderate High

High High High High Moderate

Moderate

High

High

Salinity Tolerance

High

High High

High High High High


High

High

High

Drought Tolerance

High

High High

High High High High

High High High Low Moderate

High

High

High



High

High Low

High High High High

High High Low Low High

High

High

High



Moderate

Low Moderate

Moderate High High Low

Low Moderate Low Low Moderate

Moderate

Low

Low

Soil Fertility

−29

−15 −28

−33 −5 −10 −10

−15 −23 −19 −43 −20

−41

−41

−41

Low Temp. (°C)

21–27

3–5 18–24

7.5–15 25 21–30 1.5–6

10–15 9–24 10.5–18 15–18 20

6–10

12–15

12–15

Max. Height (m)

(Continued)

Moderate

Moderate Moderate

Moderate Moderate Moderate Moderate

Moderate Moderate Moderate Moderate Moderate

Moderate

Moderate

Moderate

Growth Speed


Turf

Turf

Turf

Turf Turf Turf

Fine fescues

Tall fescue

Elfin thyme

Blue festuca Sheep fescue Couch lawn

Zoysia grass Buffalo grass

Seashore saltgrass

Distichlis spicata (L.) Greene Zoysia matrella Bouteloua dactyloides Festuca glauca Festuca ovina Cynodon dactylon Thymus serpyllum Festuca arundinacea Festuca sp.

Grass

Turf Turf

Lacebark elm

Ulmus parvifolia

Tree

Common Name

Scientific Name

Type

Poaceae

Poaceae

Lamiaceae

Poaceae Poaceae Poaceae

Poaceae Poaceae

—

Ulmaceae

Plant Family

Evergreen

Evergreen

Evergreen

Evergreen Evergreen Evergreen

Evergreen Evergreen

Semideciduous Evergreen

Foliage Type

High

High

High

High High High

High High

High

Moderate

Salinity Tolerance

High

High

High

High High High

High High

High

High

Drought Tolerance

High

High

High

High High High

High High

High

High



High

High

High

High High High

High High

High

Moderate



Moderate

Moderate

Moderate

Moderate Moderate Moderate

Moderate Moderate

Low

Moderate

Soil Fertility

−12

−8

−15

−12 −15 −10

−7 −5

—

−36

Low Temp. (°C)

Fast

Fast

Fast

Moderate Fast Fast

Slow Slow

Slow

Moderate

Growth Speed

10–15

30–50

15–20

20–25 15–20 25–45

8–12 10–25

0.4–0.9

12–15

Max. Height (m)


586


capacities for drought resistance and about a detailed landscape site layout for topology, irrigations systems cover, and water availability to each section could facilitate matching drought-tolerant species to the appropriate sites in the landscape, which is a crucial task for developing successful sustainable landscapes [111].


30.9.2 Soil and Substrate Condition Soils form the foundation for many ecological processes and interactions, such as nutrient cycling, distribution of plants and animals, and ultimately the location of human habitation. Soils can function in urban landscapes by reducing the bioavailability of pollutants, storing carbon and mineral nutrients, serving as habitat for soil and plant biota, and moderating the hydrologic cycle through absorption, storage, and supply of water [126,180]. Urban land development practices, including clearing, topsoil removal, surface grading, compacting, and building construction, lead to degraded urban soils with low vegetative cover, high bulk densities, low infiltration rates, and disturbed carbon cycles [39,109,258]. Soil quality deterioration is increasingly recognized as an important cause of poor tree performance in urban parks and other urban areas [130,271]. Stressors include deficiency of plant-available nutrients and disrupted nutrient cycling, low organic matter content, elevated soil pH, soil compaction [104], and prolonged exposure to de-icing salt due to vehicle traffics [184]. Urban park location and soil age could also be associated with the urban soil quality deterioration [125,137,172]. Studies have explored the relationship between soil quality deterioration and park management practices [69]. The deficiency of essential plant nutrients due to urban abiotic factors such as land use, street width, tree pit characteristics, pH, and type of cover material and underlying surficial geologic material makes street trees susceptible to occasional harsh urban environments [107]. The most common nutrient deficiencies facing plants in urban zones include deficiency in nitrogen (N) that limits tree growth [15], phosphorous (P) that results in growth stunting, and to a lesser extent magnesium (Mg) that might result in chlorosis [73]. Most soils contain enough available potassium (K) and calcium (Ca) to support the urban plant species’ growth. However, in some cases, the apparent over-accumulation of calcium (Ca) in urban soils occurs due to the widespread use of concrete and gypsum as construction materials, which eventually degrade and get redistributed in the landscape [103]. Calcium (Ca) deficiency might make the soil so acidic that it becomes toxic for plants. On the other hand, the elevated concentrations of aluminum (Al) and manganese (Mn) due to high acidity in soil can become toxic to vegetation. Soil acidification can increase trace metal mobilization in soils and increase metal uptake by plants [19]. This also causes easy leaching away of some special trace minerals like magnesium (Mg). The soil chemical properties, including soil nutrient concentrations, also vary among urban land zones (e.g., commercial vs. residential) [130,201]. Heavier traffic in commercial zones in temperate regions may expose the soil nearby the streets to increased concentrations of de-icing salt [182]. Sodium from the de-icing salt can affect nutrient availability for street trees [9,184]. Furthermore, sodium (Na) from de-icing salt and carbonate released by the weathering of buildings and concrete structures can increase soil pH [184]. The soil biota, or the soil microorganisms, is another important component of the soil ecosystem, which actively contributes to the soil formation by altering its physicochemical properties [180]. The contamination of tree pit soil with trace metals can affect the availability of nutrients by changing the cation exchange capacity of the soil or altering the population of mycorrhizal species, which facilitate releasing the chelated nutrients for plants [108,184]. The decreased microbial activity, consequently, might further degrade the structure of soil aggregates. The mineralization of organic matter and fixation of atmospheric nitrogen (N) by some bacteria are two ways that nitrogen (N) may become available in soils [201]. Organic matter is often the main source of nutrients in landscape soils. Leaf litter, a potentially important source of organic matter, is often returned to soil only in small quantities. The resulting limited cycling of organic matter can further reduce soil fertility [184]. Soil fertilization with sewage water,



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pond sludge, or inorganic fertilizer may increase the levels of soil nutrients but, at the same time, might add toxic heavy metals to the soils [24,123]. The soil’s physical properties in areas designated for landscaping influence the success of the vegetation as well. Most of the soils in urban areas are prone to different land development practices like clearing, topsoil removal, surface grading, compacting, and building construction that severely disturb the soil’s natural physical structure [39,109,258]. Therefore, it is more appropriate to preallocate the urban green space locations on the general city development plan in order to minimize the negative impact of construction activities on the soil, if any urban expansion plan is foreseen. Moreover, for those land lots that are made available opportunistically for urban greenery, proper soil preparation and improvement will be required before any vegetation setup to return the soil to its natural physical and biochemical life-supporting standards. Soil modifications are not always implemented to introduce life-supporting standards into soils. Recently, many organic and environmentally safe materials have been developed that improve soil physical characteristics like the water-holding capacity of the soil near plant root zones [3,216], soil aeration index [45], and prevent soil compaction. The application of some compounds with water-b uffering capacity in poorly structured soils could considerably contribute to water saving and maintenance activities. Moreover, the contamination risk of soils in cities due to the reuse of understandard urban or industrial runoffs for vegetation irrigation is a critical risk for long-term soil health. Therefore, the application of superabsorbent polymers for improving the ecological chemistry of degraded or polluted lands has shown promising results [92]. Bullock and Gregory [28] presented a detailed discussion on various aspects of soils in the urban environment. Finally, the protocols for soil quality assessment in landscape monitoring programs should be considered as well. The index of soil quality is an integration of the physical, biological, and chemical aspects of soil. Such indices could act as early-warning systems for preventing unrecoverable damage to urban soils due to the over-accumulation of toxic chemicals and urbanization activities. The holistic assessment of soil quality in landscape and urban planning is ignored in most cases and is typically replaced only by some chemical analyses. Therefore, the development of protocols that provide a comprehensive assessment of the soil’s ability to perform critical environmental functions at a relatively modest cost and help define management and remediation approaches [203] is critical for keeping a healthy landscape soil.

30.9.3 Maintenance Strategies Landscape maintenance involves responding to plants’ demands of water, nutrition, and health to support their natural growth and development. Thus, this process considers all the parameters and aspects of a green landscape, including plants, systems control and maintenance, and water management. With respect to plants, it should be noted that the physiological responses of the landscape and agricultural plant species to climatic and nutrition stresses like soil salinity or water quality are similar. However, Costello [52] asserted that despite the existence of common characteristics between agricultural and landscape systems, there were still significant dissimilarities from a management and maintenance viewpoint. Urban landscapes usually comprise plant species from different environments with various water, nutrition, and climatic requirements. They are primarily grown for their attractive appearance, environmental benefits, and ecological functions, not for food or feed. A large portion of the urban greenery elements are long-lived perennials. Therefore, the occurrence of urban environmental complexities and plant variation within landscape sites can complicate the prediction and management of the actual response of the plants to stress conditions at different growth and development stages. The first step toward successful green landscape maintenance is proper designing. Without an adaptive and comprehensive design that encompasses almost all the known sustainable and waterwise parameters of landscaping, especially the uniformity of plant species, any maintenance effort for sustaining the vegetation under unfavorable conditions will not be efficient. One general strategy for


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water-saving maintenance is to discourage water-demanding new growth of plants during the periods of severe water supply shortage. Wade et al. [242] suggested some designs and low-maintenance solutions for water-saving urban landscapes referred to as xeriscape-type landscaping. This strategy includes applying less fertilizers and using slow-release fertilizers; proper mowing and avoiding heavy shearing or pruning to eliminate water-demanding new growth of plants; aerating turf grass to facilitate soil water movement down into soil; and controlling weeds and pests so they do not compete with plants for water and nutrients. Nonetheless, the best maintenance program could be applied based on proper and timely response to plant demands. If only plants could tell when they need water or a given mineral, then by connecting such visually detectable signals to maintenance systems like irrigation controllers, we could promptly and easily satisfy their need, and consequently the water and energy efficiency would be tremendously significant. This is already a hot topic for the scientific community: to develop plants that could change color when they need water (Bioconst Pty, Glenelg, South Australia, Australia). The maintenance strategies are considered the most important water and plant management tools in existing urban green spaces, because there are fewer opportunities for the cost-effective redesigning or changing of the plant species for such zones. The trend of landscape maintenance protocols and techniques is rapidly changing, and the advent of new technologies and advancement in various plant sciences play a significant role in this process. For example, our updated understanding about the light wavelength effects on the plant photosynthetic apparatus as well as the regulation of some genes’ expression has proved quite practical in at least two ways. First, today we are almost sure that from the full light spectrum, only a few wavelengths at particular intensities are harvested by the plants. Various studies on different annual plant species (e.g., vegetables and ornamental species and flowers), which were grown under some selected light wavelengths, showed generally faster and healthier growth conditions [171,190]. Many perennial plant species like eucalyptus, which is a tree of choice in arid and semiarid landscape settings, have also meaningfully responded to specific light treatments [154]. However, the advantages of light treatment are not limited to plant growth parameters. Plants grown under specific light wavelengths were more tolerant to environmental stresses such as drought and high temperatures [190]. Therefore, the application of light sources with specific wavelengths in green spaces and also during the early stages of seed germination and seedling establishment might improve the innate capacity of the plants for resistance to more intense environmental conditions like water shortage and heat. Second, the need of plants to a limited number of wavelengths could be an advantage for introducing more efficient and energy-saving light sources in urban landscapes. The light-emitting diode (LED) technology is a promising option to this end. LEDs could emit any specific light wavelength with the required intensity with minimum energy requirement and heat emission. Dayani et al. [59] recently reviewed the history, principles, technologies, and latest scientific discoveries and business trends on the effect of light and light technology, with emphasis on LEDs, on plants’ growth and resistance parameters.

30.10 Summary and Conclusions The public green spaces in urban zones provide a wide range of services and advantages for city dwellers, the economy, and the environment. The rapid pace of global urbanization and global climate change, which in many densely populated regions of the world are followed by water scarcity and drought, has urged the development of more efficient and water-wise communities. The green landscapes in cities are known to be among the most water-demanding sections, while maintaining a standard ratio of urban greenery for human well-being is critically important and cannot be compromised. This review attempted to provide a comprehensive although not very detailed prospect of the current knowledge about the public urban green spaces, including the advantages, modeling, urban water issues, and the concept of sustainability in urban greenery. It was shown that developing a waterwise sustainable urban green space is a multidimensional task, which requires considering a plethora of



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parameters from multiple disciplines. Despite the efforts to cover almost all the known parameters that could play a role in maintaining successful urban landscapes by many authors, there might be still topics that are neglected or less explored like the issue of private green spaces in cities and their effects on overall contribution to public greeneries. As the green space and human health cities is gaining in attention among different scientific, political, economic, and other stakeholders, new scientific modeling and multidisciplinary strategies need to be devised for evaluating and improving the existing green spaces and developing more efficient future green zones in cities. The water issue is a pivotal and nonnegotiable factor in any solution and planning. The future world inevitably would be more dependent on strategies and technologies that help sustain the drinking water reservoirs because the current sources in arid and semiarid regions are already seriously at risk. Despite the tremendous efforts by the private and public groups in various fields and levels, we still need more reliable solutions and technologies to keep up with this trend. In the end, based on the discussed topics and presented literatures, a couple of suggestions for future studies and considerations are noteworthy. The identification and introduction of endemic drought- or water-stress-tolerant plant species with acceptable aesthetic values into the urban landscaping should be put on the agenda of research and urban management. There are a number of modeling and estimation methodologies that could improve the planning and development of enduring green spaces; however, they still need to be fine-tuned based on different climatic zones and geographical conditions. Therefore, identifying crop coefficients for landscape plants in order to improve the reliability and efficiency of current models is a demanding task and should be conducted under scientific supervision and encouragement of urban authorities for any climatic zone and its plant species. Developing solutions and technologies that facilitate human–nature communication could be a breakthrough. The idea is already being acknowledged by the scientific community, and some prototype plants that project some predefined visual signals about their physiological status are developed, like plants that change color to say they are thirsty before they die or suffer unresolvable damage or lose their aesthetic values. Although such genetic modifications may require a considerable amount of time for approval and application, they are still among the promising options for wise and efficient human interactions with nature in the future. Finally, integrated utilization of new technologies could be suggested in the context of our everexpanding knowledge of nature as a practical strategy to tackle the existing and emerging challenges in human societies and nature.

Authors Soleyman Dayani is a senior research fellow in plant biotechnology with the Department of Agricultural Biotechnology of Payame Noor University (PNU). His research interest focuses on plants genetics and physiology under abiotic stresses. Dayani actively participates in research and development programs aimed at improving crop plants performance and yield under unfavorable environmental conditions, enhancing the efficiency of controlled-environment agriculture, plant tissue culture and gene cloning, and plant nutrition. He has coauthored a couple of peer-reviewed articles and book chapters in the field. Dayani received the academic achievement awards of the Faculty Top Student and the Distinguished Graduate Researcher in agriculture. He has also received a number of national honors including the National Elite Youth Award for his applied scientific and humanitarian contributions. Mohammad R. Sabzalian graduated from Isfahan University of Technology where he received his PhD in 2008. He is now an associate professor of genetics and plant breeding at Isfahan University of Technology, and he has taught statistics, experimental design, biometrical genetics, and plant breeding for stress environments for several years. He and his MSc and PhD students are working on breeding grasses using symbiotic fungal endophytes for better adaptation to drought stress conditions. Polymers and some advanced materials are also applied in some studies to mitigate environmental stress damage on crops and strategically tree species.

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Mahdi Hadipour is a graduate research fellow in molecular plant breeding from Sari Agricultural and Natural Resources University. He has been conducting research and lecturing at the Agricultural Biotechnology Research Institute of Iran (ABRII) and Payame Noor University for 6 years. He has worked on molecular markers and secondary metabolites in several medicinal plants including Papaver bracteatum. His main research interests are proteomics, plant tissue culture, and the effect of medicinal plants’ secondary metabolites in cancer treatment. Hadipour has extensive experience in honeybee’s proteomics and metabolite studies. Saeid Eslamian is a full professor of hydrology and water resources engineering in the Department of Water Engineering at Isfahan University of Technology, Iran, where he has been since 1995. He received his PhD from the University of New South Wales, Australia, under the supervision of Professor David Pilgrim. His research focuses mainly on water resources planning and management and statistical and environmental hydrology in a changing climate. Formerly, he was a visiting professor at Princeton University, New Jersey, and the University of ETH Zurich, Switzerland. On the research side, he has started a research partnership from 2014 with McGill University, Canada. He has contributed to more than 500 publications in journals and books or as technical reports. He is the founder and chief editor of both International Journal of Hydrology Science and Technology (Scopus, Inderscience) and Journal of Flood Engineering. Currently, he has been the author of more than 100 book chapters and books. Recently, Professor Eslamian has started the editorship of several handbooks published by Taylor & Francis Group (CRC Press). A three-volume Handbook of Engineering Hydrology (2014), Urban Water Reuse Handbook (2015), a three-volume Handbook of Drought and Water Scarcity (2017), and Underground Aqueducts Handbook (2017) are published ones.

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