Keywords

8.1 Introduction

Urbanization has changed the land cover of an area. From the natural environment, the area is being converted to a built environment. The changes to the urban built environment have altered the environment due to the significant change of the built area materials, substituted with artificial materials such as concrete and asphalt. Combining the artificial materials with sealed surfaces and less vegetation will influence the radiation balances of the urban environment (Butera et al., 2018).

Urban population is one of the main drivers of urbanization where more than half of the global population are currently residing in the urban areas, and it is projected that in 2050, this will increase up to 68% (United Nations, 2018). With the population growth, many more developments will be made available, and this is unavoidable to meet the needs of urban residents. This will further deteriorate the urban environment and, specifically, will alter the heat energy balances and consequently the urban climate (Mohajerani et al., 2017). The urban climate is typically characterized by higher air temperature and drier conditions compared to the surrounding rural area, and this is often called as urban heat island (UHI) effect (Zhao et al., 2014). The increase in temperature in cities exacerbated by urban heat and global warming can reduce the thermal comfort of the urban residents.

One of the strategies in mitigating the adverse effect of urban climate issues is the implementation of Blue-Green Infrastructure that promotes urban cooling through the nature-based solution (Sanusi & Bidin, 2020). Due to its ecosystem services, Blue-Green Infrastructure has been utilized in the urban environment as part of the planning strategy. However, with the detrimental effect of landscape changes that influences microclimate conditions and human thermal comfort, there is still a lack of information on how the utilization of the Blue-Green Infrastructure in the urban area can improve urban cooling and thus humans. From a systematic review by Lourdes et al. (2021), they found that recreation, mental and physical health (n = 54) and moderation of extreme events services (n = 54) were the most studied ecosystem services among the provisioning, regulating, supporting and cultural domains. This shows that the influence of Blue-Green Infrastructure on urban climate and human well-being is an important issue, and therefore, studies that relate Blue-Green Infrastructure to microclimate and human thermal comfort should be emphasized in future urban studies. This paper aimed to review the past literature covering the current status of research on Blue-Green Infrastructure on microclimate and human thermal comfort. It addresses three specific research questions as follows:

  1. 1.

    Does the Blue-Green Infrastructure on microclimate and human thermal comfort research focus on specific types of Blue and Green Infrastructure or the combination of Blue-Green Infrastructure? In what year and country they are being assessed?

  2. 2.

    What are the data collection processes of the Blue-Green Infrastructure research on microclimate and human thermal comfort and what types of Blue-Green structures are being assessed?

  3. 3.

    Does the research done on Blue-Green Infrastructure specifically measure human thermal comfort?

Further recommendations on the future research direction based on the findings will be given to conclude this review.

8.2 Materials and Method

In this review, literature from previous studies was searched using search string composed terms of the selected topic in the Scopus database. The search string composed terms were focused on the specific research concerning Blue-Green Infrastructure, microclimate and human thermal comfort. The search string applied to the database is as shown below:

  • (“blue-green infrastructure” OR “green space” OR “green infrastructure” AND “blue spaces” OR “blue infrastructure”) AND “microclimate” AND “human thermal comfort” OR “thermal stress” AND “urban” OR “city” OR “cities”.

From the literature search string on Blue-Green Infrastructure, microclimate and human thermal comfort, a total of 61 articles were returned including journal articles, reviews and book chapters (Fig. 8.1). For the document type, 17 reviews and a book chapter were excluded as this study includes only journal articles. Moreover, 12 papers were further excluded as there was no clear relation to the investigated topics. In these performed searches, all abstracts were read carefully to extract important information that can provide some insight into the research within the context of the selected topic. Specifically, this paper looked into six types of information which were (1) types of infrastructure, (2) year of studies, (3) country of origin, (4) data collection process, (5) Blue-Green structure and (6) human thermal comfort studies (Fig. 8.1).

Fig. 8.1
A hierarchical chart of a literature search on research related to blue-green infrastructure, microclimate, and humans are divided into 3, document types of review, articles that were excluded, and articles selected in this report. The six types of information extracted are types of, year of studies, country of origin, data collection, and blue-green.

Schematic representation from the search string on articles related to Blue-Green Infrastructure, microclimate and human thermal comfort

Full size image

8.3 Results and Discussion

8.3.1 General Patterns of Returned Articles

From the searches, the trend of research on Blue-Green Infrastructure to microclimate and human thermal comfort was acquired, where only 33 papers were related to this topic. This shows that future studies should be further expanded on the climate and human thermal comfort perspectives as in the context of urban areas, urban heat island poses a threat to heat-related human health issues (Aghamohammadi et al., 2021). On the other hand, the research trend or direction towards the impact of urban Blue-Green Infrastructure on urban climate and human well-being in Southeast Asia is also quite promising (Lourdes et al., 2021).

From a thorough analysis of the returned articles, not all the research focused on the specific combination of Blue-Green Infrastructure (Fig. 8.2). Only 33% of the returned articles researched the contribution of Blue-Green Infrastructure on urban cooling (n = 11). On the other hand, green infrastructure was the main focus in the research with 55% (n = 18), while the lowest studied was blue infrastructure with only 12% (n = 4). Although research that combines blue and green infrastructures is still progressing, it was unbalanced and needed further studies to see its contribution to urban climate and human thermal comfort especially the blue infrastructure. This is because Blue-Green infrastructure is seen to be a future key strategy in sustainably planning and managing the urban environment (Din Dar et al., 2021). For instance, applying both elements in parks could reduce the UHI due to its synergistic cooling capability (Gunawardena et al., 2017). However, although the application of Blue-Green Infrastructure has received great interest globally due to its ecosystem services (Dai et al., 2021; Nguyen et al., 2021), there is still a need to further determine the influence of applying the Blue-Green Infrastructure in urban environments (Ghofrani et al., 2017). Lack of understanding on how the Blue-Green Infrastructure can be beneficial and in what way it is best to be implemented in urban environments may also lead to future exploration of its benefits and application (Bedla & Halecki, 2021).

Fig. 8.2
3 bar graphs on types of infrastructure include data on blue, green, and blue green infrastructure based on the year of publication. The countries of origin from 2018 to 2021. The last bar graph is on various countries like Hong Kong, United States, Australia, China, Czech, Italy, Hungary, Japan, Malaysia, Turkey, Spain, Egypt, and Austria.

The general information extracted from all the returned articles of types of infrastructure, year of publication and countries of research origin

Full size image

Moreover, from these findings, it was apparent that the green infrastructure was predominantly studied. This might be due to green infrastructures being widely used in mitigation strategies in combating urban climate issues such as the UHI. The green infrastructure not only provides various environmental benefits including the urban cooling and mitigation of UHI but also consequently serves the urban residents with a thermally comfortable environment, thus improving their health and well-being (Wilis & Petrokofsky, 2017). Furthermore, the possibility of the green and blue spaces that could differently affect the cooling effect of the environment (Hu & Li, 2020) may also lead to more studies being done to look at the infrastructure individually.

In terms of year of publication, when relating the urban microclimate and human thermal comfort to Blue-Green Infrastructure, it was noticeable that relevant articles from the searches only started in the year 2018 (3%) and a gradual increase was apparent until 2021 (58%) (Fig. 8.2). Thus the spike of research from 2018 to 2021 signified the growth of interest in linking the Blue-Green Infrastructure contribution to urban cooling and human health and well-being.

In addition, despite the growth of interest in this research area, it was largely studied in China with 28%, followed by Hong Kong with 15%, Australia and multiple cities (study conducted in more than one city) with 9% and the Czech Republic and Turkey with 6% (Fig. 8.2). Moreover, countries such as Italy, Hungary, Japan, Malaysia, Sri Lanka, Spain, Egypt and Austria were with 3% each. According to de Macedo et al. (2021), cities in China also had a high contribution of studies on the concept of urban Blue-Green Infrastructure in the Global South, specifically related to local sustainable development. This indicates that the interests of big cities like those in China are to strategically address the urban issues through the application of Blue-Green Infrastructure. It is also suggested that most studies on the ecosystem services, at least in Southeast Asia, are predominantly carried out in more developed countries such as Singapore, Thailand, Indonesia and Malaysia, while less developed countries such as Myanmar, Cambodia and Laos are less studied (Lourdes et al., 2021).

8.3.2 Data Collection Process, Blue-Green Infrastructure Structure and Human Thermal Comfort Studies

For all studies, their data collection process varies according to the objectives they wanted to achieve. All the data collection involved the microclimate measurements; however, all the measurements differed depending on the data collection approach. From the returned articles, four types of data collection involved biophysical modelling, field study, mixed-method and survey (Table 8.1, Fig. 8.3). In terms of microclimate measurements for the biophysical modelling, the microclimate parameters were typically collected using on-site mobile or fixed meteorological stations and remote sensing data, where these data were calculated, simulated and modelled according to the desired urban landscapes and their conditions for each study (Table 8.1). Similarly, the mixed method also involved the mobile or fixed meteorological stations and remote sensing data but mixed with other data collections (Table 8.1). For mixed method, it involved the combination of different approaches as follows: (1) biophysical modelling and fieldwork, (2) biophysical modelling and survey and (3) fieldwork and unmanned aerial vehicle. While for field studies, the microclimate parameters were measured on-site at different urban settings such as parks, water bodies, tree coverages, peri-urban woodland, rooftop, pergola and pondside (Table 8.1). On the other hand, the survey data collection involved on-site microclimate measurements that were combined with the survey questionnaire at desired urban landscapes to see the interaction of microclimate with personal factors and perception (Table 8.1).

Table 8.1 The main findings of the returned articles related to Blue-Green Infrastructure, microclimate and human thermal comfort
Full size table
Fig. 8.3
3 bar graphs on blue green structure have data for biophysical modeling, field, mix method, and survey. The second graph is human thermal comfort studies on multiple, golf, Urban parks, Remnant, tree, Lawn, Vertical greening, artificial pond, and river. The last graph is on survey, Biophysical modeling, field, and biophysical modeling plus survey.

The specific information on the data collection process, Blue-Green Infrastructure (BGI Blue-Green Infrastructure, GI Green Infrastructure, BI Blue Infrastructure) and human thermal comfort studies of all the returned articles

Full size image

Further analysis showed that the biophysical modelling involving the process, mechanistic or land cover modelling and analysis was mainly used as a tool in accessing the contribution of Blue-Green Infrastructure to urban microclimate and human thermal comfort with a total of 58% of the returned articles. This is then followed by field study with 30%, mixed-method with 9% and survey with 3% (Fig. 8.3). The variety of approaches in assessing the Blue-Green Infrastructure indicates that the assessment can be conducted in many ways. Finding the best methodology may be hard, but applying the suitable methodology to the context of its study is more relevant when assessing the contribution of Blue-Green Infrastructure to urban microclimate and human thermal comfort. Despite this, it is also important to note that as biophysical modelling was mainly used in the assessment, it showed that the data and findings of these studies were largely relied on the process, mechanistic or land cover modelling and analysis level rather than the site level. The biophysical modelling could help spatially plan the city and can further identify vulnerable zones through a clear and compelling model aimed to be used in the decision-making and planning processes (Khorrami & Malekmohammadi, 2021). However, field data should also be seen as one of the options as it could access the impact of Blue-Green Infrastructure at the micro level, and when talking in the context of human thermal comfort, it will be most relevant as it is directly related to the urban residents.

This review further elaborates that when researching the Blue-Green Infrastructure, it involved both blue and green elements in their research (Fig. 8.3). Most of the research involved multiple Blue-Green spaces and multiple green spaces. For green infrastructures, the structure varies where the research was done in areas of trees, golf course, urban parks, remnants, lawn and vertical greening. This further elaborates the importance of green structures such as trees in the urban microclimate and human thermal comfort as they could enhance the urban environment through urban cooling and therefore reduce thermal stress (Sanusi et al., 2016, 2017; Sanusi & Bidin, 2020). On the other hand, for blue infrastructure, it was studied in river, artificial pond and multiple blue spaces.

It is also apparent that there was alack of studies that measured human thermal comfort from all the returned articles. So far, only 14 studies were estimating the impact of Blue-Green Infrastructure on human thermal comfort, where most of the studies were using biophysical modelling, followed by fieldwork (Fig. 8.3). This indicates that most studies only relatively discussed this in general without actually estimating the impact on urban residents and the research was primarily based on a biophysical modelling approach. It has been known that many studies look at the importance of vegetation especially trees to provide ecosystem services such as mitigation of urban climate and improving human health and well-being especially for the reduction of heat-related illnesses (Chianucci et al., 2015; de Abreu-Harbich et al., 2015; Sanusi & Livesley, 2020). Therefore, without physically estimating the impact at the micro level, less evidence can be provided in estimating the human thermal comfort benefits when applying Blue-Green Infrastructure in the urban environment. This would be an important knowledge gap that is needed to address further understanding on the role of Blue-Green Infrastructure in urban areas including on how far the Blue-Green Infrastructure can ameliorate the urban climate and therefore improve the urban thermal conditions. Further description of all the extracted information from the returned articles and related findings that are discussed in this current review is detailed out and summarized in Table 8.1.

8.4 Conclusions

From this review, the Blue-Green Infrastructure research on microclimate and human thermal comfort research were specifically focused on the Green Infrastructure followed by the combination of Blue-Green Infrastructure, while studies on Blue Infrastructure were limitedly being studied upon. Moreover, there was a gradual surge in the Blue-Green Infrastructure studies with an increasing trend of 55% studies from 2018 to 2021. The findings suggest that there are still many knowledge gaps on the contribution of Blue-Green Infrastructure, and with the growing interest, there is a need to further determine the influence of applying Blue-Green Infrastructure in the urban environment, especially in terms of microclimate and human thermal comfort. Furthermore, most research was largely studied in China, followed up by Hong Kong and Australia. This further highlights the need of expanding research on related topics to many other regions.

It was also notable that biophysical modelling was the most used approach and followed by fieldwork. Moreover, only 36% of the total study measured the human thermal comfort parameters in the field, thus indicating limited proof to link Blue-Green Infrastructure’s role to mitigate the urban microclimate and improve the thermal comfort of urban residents at the micro level.

The application of Blue-Green Infrastructure would synergistically improve the microclimate and the heat-related health problems for urban cooling. This review addresses the knowledge gap of Blue-Green Infrastructure research on microclimate and human thermal comfort. It is concluded that specific research should be further expanded on the discussed knowledge gaps to ensure more accurate decisions can be made for future urban planning efforts using Blue-Green Infrastructure as a nature-based solution.