A systematic review on urban blue-green infrastructure in the south Asian region: recent advancements, applications, and challenges

The escalating urbanization in developing countries poses significant challenges to urban sustainability. As cities expand, natural habitats are often fragmented or destroyed. Such occurrences have led to increased pollution, habitat degradation, compromised livability, and the loss of green spaces, further exacerbating environmental challenges (Aguilera et al. 2020; Liu & Wang 2020; Ulpiani 2021). The rapid influx of people into urban areas strains resources, infrastructure, and services. This surge in population often leads to increased energy consumption, waste generation, and heightened demands. Addressing the impact of rising urbanization on sustainability requires holistic and integrated approaches, particularly in developing countries where it lacks. Balancing urbanization with sustainable practices is crucial for ensuring the well-being of developing countries' current and future urban populations.

In response to rising urbanization, cities in developing countries are actively promoting the implementation of blue-green Infrastructure (BGI) in order to protect the ecosystem services (ES) and enhance the overall well-being of their residents (Plieninger et al. 2022). The concept of ‘green infrastructure’ (GI) was first introduced as a strategic method of land conservation that takes into account the adverse social and ecological impacts of urban sprawl in the United States (Mell 2017). This approach is deemed crucial in addressing intricate socio-ecological challenges through strategic planning, design, and the management of interconnected networks. International entities like the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES) and the Intergovernmental Panel on Climate Change (IPCC) have also acknowledged the conservation and reintroduction of green spaces in built environments as a nature-based solution (NBS) for cities (Pörtner et al. 2021).

BGI applications or services can be broadly divided into provisioning, regulating, and cultural services. Provisioning services encompass various essential aspects such as food security, poverty alleviation, access to clean water, and clean energy. Crucial regulatory functions include water regulation, thermal regulation, carbon storage, air quality improvement, and climate regulation. Important cultural services encompass a range of offerings, such as chances for recreation, appreciation of aesthetic qualities, and preservation of biodiversity (Zinia & McShane 2021). BGI play a vital role in urban planning and design, with the shared goal of fostering more sustainable, healthy, and aesthetically pleasing living environments. The threats and challenges created by climate change can be addressed through BGI strategies, although the specific scale, extent, problems to be dealt with, desired results, etc., may differ globally (Krishnan & Shanthi Priya 2022). Since BGI encompasses various components, some are labeled differently but describe comparable elements based on local characteristics. The different terms associated with BGI are elaborately discussed in the following section. Consequently, the objective of this article is to examine the scholarly literature on urban BGI in the lower-middle-income countries of the South Asian region, with the aim of analysing prominent patterns, components, and ongoing research endeavours in the field of urban BGI. Additionally, the objective was to ascertain areas with insufficient knowledge that might be explored in future research endeavours.

The selected papers for this study contain multiple mentions of GI. The municipal authorities in several cities are using GI methods to enhance ES. For instance, in Khyber Pakhtunkhwa (KP), Pakistan, these tactics are employed to regulate the local climate (Rayan et al. 2022a). Similarly, in Bengaluru, India, efforts are made to reduce surface runoff and achieve carbon sequestration benefits (Shreewatsav & Sheriff 2022), while in Dhaka, Bangladesh doing both (Safayet et al. 2017; Begum et al. 2021). There are a relatively limited number of papers that discuss blue space or blue infrastructure (BI). BI is responsible for a vast array of ES, many of which are beneficial to individuals, with the regulatory and cultural services standing out as particularly significant. Although they provide different kinds of ES, both wet and dry lakes are found to be useful to the people who live nearby. The degree to which individuals were exposed to urbanisation significantly impacted how they perceived various ES provided by lakes. However, the socio-demographic disparities in assessments were rather minimal (Plieninger et al. 2022).

Further, it was noted that the existing body of research on ES provided by urban BGI has predominantly focused on regions with low to medium population densities in economically prosperous areas of Europe, North America, and Asia. However, a limited number of studies are conducted in the ‘Global South’, which primarily includes middle to low-income countries of Asia, South America, and Africa (Fletcher et al. 2021). In order to promote urban sustainability, it is imperative to conduct comprehensive investigations into BGI that offer societal benefits while actively engaging with communities and stakeholders (Ranagalage et al. 2020). Nature-based solutions (NBS) like BGI are considered a significant potential option in developing countries, most notably in small cities. Therefore, this study aims to enhance comprehension of the specific challenges in the lower-middle-income countries within the South Asian region. It explores how authorities like urban local bodies can effectively utilise BGI techniques to mitigate rising risks of unsustainable urban growth, improve the health of urban ecosystems and enhance the urban living experience echoing the ethos of sustainable urban development.

Developing BGI is a new concept and a collective term involving integrating various natural and semi-natural green spaces, such as parks, green belts, green roofs, etc., with ‘blue spaces’ or water bodies like lakes and ponds through ecologically active linkages (García Sánchez & Govindarajulu 2023). In this section, we delve into various terms linked with BGI. Although some of these terms may depict comparable land features, they are not precisely synonymous. Table 1 summarizes the terms and their definitions.

BGI is a network of interconnected green and blue spaces, such as parks, wetlands, and green roofs, providing various ecosystem services, including carbon sequestration, water management, and biodiversity conservation (Hamel & Tan 2022). BGI is a departure from traditional ‘grey’ infrastructure, which relies on man-made structures like concrete and steel to manage urban challenges. BGI offers a more sustainable and cost-effective alternative to grey infrastructure, as it can provide multiple benefits while also enhancing the aesthetic and recreational value of urban spaces.

Moreover, BGI is a flexible and adaptable approach that can be tailored to local conditions and needs. It can be implemented at different scales, from small-scale interventions like green roofs to large-scale projects like urban forests (Puppim de Oliveira et al. 2022). BGI also offers opportunities for community engagement and participation, as it can involve citizens in the planning, designing, and managing green and blue spaces (Lamond & Everett 2019). Overall, the novelty of the BGI approach lies in its recognition of the value of natural systems and processes in addressing urban challenges and its potential to provide multiple benefits for both people and the environment.

BGI, such as urban forests, green spaces, and vegetated corridors, can sequester carbon dioxide from the atmosphere, helping to mitigate the impacts of urban emissions (Weber et al. 2006; Liu et al. 2022). Green roofs, parks, and urban forests can help reduce the urban heat island effect by providing shade, evaporative cooling, and reducing surface temperatures, thereby decreasing the energy demand for cooling (Lehmann 2014; Liu et al. 2022). BGI can support active transportation modes, such as walking and cycling, by providing green corridors and pedestrian friendly pathways, thereby reducing the reliance on motorized vehicles and lowering emissions from transportation (Magrinyà et al. 2023). Moreover, blue infrastructure elements, such as green roofs, rain gardens, and permeable pavements, can help manage stormwater, reducing the energy intensive processes associated with conventional drainage systems and wastewater treatment (Rujner et al. 2022; Chikhi et al. 2023). Constructed wetlands and green retention ponds can help manage stormwater runoff, reduce pollution, and improve water quality, thereby supporting aquatic biodiversity and ecosystem health (Ferreira et al. 2023). BGI components like green roofs and urban vegetation can improve building energy efficiency by providing insulation and reducing the need for heating and cooling (Bevilacqua 2021). Thus, by integrating BGI into urban planning and design, cities can effectively contribute to reducing greenhouse gas emissions while enhancing the livability and sustainability of urban environments.

This study adhered to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines to provide a structured framework for reporting systematic reviews and meta-analyses in the methodology phase to carry out a comprehensive literature review (Moher et al. 2009). The quantity and characteristics of publications indicate scientific interest and the state of BGI implementation across various countries. PRISMA guidelines used in the methodology for this SLR are detailed below.

This study followed PRISMA guidelines to carry out a comprehensive literature review across various academic disciplines, including urban studies, environmental sciences, environmental studies, ecology, geography, green sustainable science technology, water resources, remote sensing, biodiversity conservation, architecture, forestry, and agriculture. A comprehensive search was conducted in the Web of Science (WoS) and Scopus databases to locate pertinent papers. The inclusion criteria for conducting the Systematic Literature Review (SLR) in this study consist exclusively of peer-reviewed journal articles published within the past ten years, specifically from 2013 to 2022, and written in the English language. It was observed that the amount of research in this domain has significantly increased over the last decade. This could be due to the growing concern for the urban health of developing countries in the South Asian region. All these countries have incorporated the development of new smart cities in their policies. In this regard, BGI is crucial. Hence, the past decade has witnessed a rapid escalation in BGI research; therefore, this time frame was selected for the current study.

The search terms utilised in the WoS and Scopus databases to ascertain the components of urban BGI in the South Asian region were ‘urban green’ OR ‘urban tree’ OR ‘urban forest’ OR ‘urban agriculture’ OR ‘urban farming’ OR ‘urban garden’ OR ‘green space’ OR ‘green infrastructure’ OR ‘green belt’ OR ‘green roof’ OR ‘green wall’ OR ‘urban river’ OR ‘urban lake’ OR ‘urban pond’ OR ‘urban stream’ OR ‘urban wetland’ OR ‘urban drainage’ OR ‘blue space’ OR ‘blue infrastructure’ OR ‘urban waterbody’ OR ‘stormwater pond’ OR ‘detention pond’ OR ‘bioswale’ OR ‘permeable pavement’ OR ‘rain garden’ OR ‘nature-based solution’.

In order to identify relevant articles, the search was initially limited to specific components of BGI; due to the limited number of articles retrieved using broad terms such as ‘blue space’, ‘blue infrastructure’, ‘waterbody’, etc., more specific terms such as ‘river’, ‘lake’, ‘stream’, ‘pond’, ‘wetland’, etc., were included. The search strings were used by combining all of the specified components of BGI (applying suitable synonyms wherever applicable for each term) with the word ‘urban’ and the names of countries such as India, Bangladesh, Sri Lanka, Bhutan, Nepal, and Pakistan, a significant number of articles were found.

A search in the WoS and Scopus database returned a total of 2,581 research articles. The search results were then subjected to further analysis using the PRISMA method. The steps are discussed further.

• A review of the titles was carried out, and duplicate entries were eliminated. Additionally, conference proceedings and book chapters were excluded.

• Several papers were removed depending on the titles that excluded the core keywords, with no particular emphasis on urban BGI issues.

• Following the title and keywords, the abstracts were thoroughly examined and comprehended to ascertain whether or not the studies refer to urban BGI.

• Following that, a subsequent review was conducted on the full text of those articles from the initial keyword search.

• Many studies were excluded where authors were affiliated with South Asian countries, but the study areas were not from this region.

• Some articles did not mention BGI directly and seemed to belong to a dissimilar field (fisheries, agriculture). Still, after a full review, such papers were included if they highlighted the scope of BGI applications.

• Then, for the final round of screening, more records that matched the search criteria in the reference lists of the full-text papers were identified.

A meta-analysis was carried out to comprehend the current research trends on urban BGI in the selected countries of interest; results were presented as graphs, diagrams, tables, and illustrative examples through statistical analysis. All of the 77 relevant articles were reviewed in depth.

Throughout the study, it has been recognized that the approach employed for data collecting has a crucial role in shaping the outcomes. Despite utilizing inclusive search phrases, it remains plausible that alternative or additional search keywords could have impacted the number of articles included in the sample. Furthermore, this review exclusively examined English-language scientific papers sourced from the WoS and Scopus databases while excluding publications in other languages and grey literature from consideration.

The higher number of publications and citations for India can probably be attributed to the degraded urban health conditions and also due to the recent actions taken to improve urban air quality. Multiple metropolitan and megacities in India rank very high among global cities regarding poor air quality indices. Hence, India has initiated a number of missions to promote green and sustainable cities (National Smart Cities Mission, Green Cities campaign, Training on Urban Green Planning in Climate Smart Cities). Such initiatives have drawn attention from the research world, and thus, many recent publications have focused on BGIs.

It was also observed that the total BGI mentions of 150 for the selected six countries is significantly larger than the sample of 77 that were included in this literature review because some abstracts referred to multiple components of BGI (refer to Figures 9 and 10). The analysis showed a significant variation in the study of the elements of BGI, where the maximum variation was found in India, followed by Bangladesh (see Figure 12).

The analysis of the trends by country reveals that 50% of the articles on vertical greening, urban forestry, rooftop farming, gardens, parks, green walls, green belts, and street trees are written about either Bangladesh or India. When compared to the other countries, India is found to have the most publications mentioning the term BGI. In articles concerning Pakistan, GI appears the most frequently. In Sri Lankan cities, the studies mainly mentioned the terms green space, green infrastructure, and urban greening/greenery; none of the articles referred to the term blue space or blue infrastructure in Sri Lanka, Bhutan, Nepal, and Pakistan (see Figure 11). Further, no articles about Nepal mentioned green infrastructure or blue-green infrastructure.

Provided the wide range of studies on different BGI approaches, it was observed that researchers are emphasizing the benefits to community health, mental well-being, quality of life, and economic performance along with the ecosystem and environmental benefits. These studies span different cities in India, Bangladesh, Sri Lanka, Nepal, and Pakistan, providing a comprehensive overview of the challenges and opportunities in the region. The following table (refer Table 2) provides an overview of different BGI services, focus areas mentioning individual services, and a summary of some notable articles.

The presented results showcase a diverse range of studies related to sustainable urban development, the BGI approach, and the impact of NBS on various aspects of urban life. Our sample of assessments reported several indicators in BGI services aligning with the previous review papers (Maes et al. 2016; Prudencio & Null 2018; Almaaitah et al. 2021). Some notable findings from the review are listed below.

• i.

  There was emphasis on implementing BGI solutions to address water shortage and urban flooding. This approach involves collecting surface run-off and recharging groundwater, showcasing the potential for integrated solutions to mitigate conflicting urban challenges. Similarly, the effectiveness of rain gardens in managing stormwater contributes to a well-planned drainage system and maintenance free landscapes is highlighted.

• ii.

  The comparison of urban BGI levels in different cities with varying service levels demonstrates the importance of not only the extent of green infrastructure but also its efficiency. The cooling and pollution removal impacts vary significantly between cities, emphasizing the need for context specific and well executed blue-green initiatives.

• iii.

  The role of trees in urban environments is emphasized in studies where street trees are shown to lower humidity, temperature, and pollution levels. This aligns with the idea that increasing the percentage of green spaces, avoiding complex shaped greenery, and growing trees in strategic patches can enhance the cooling effect more efficiently.

• iv.

  The studies also delve into the social and cultural aspects of green spaces. The importance of well designed urban green spaces for socialization, healthy living, and aesthetic satisfaction is highlighted. The contribution of parks and rooftop gardens to cultural ecosystem services and recreation further underscores the multifaceted benefits of green infrastructure.

• v.

  Additionally, the studies shed light on economic aspects, such as the monetary value of carbon sequestration benefits, the willingness of residents to adopt green roofs, and the economic potential of rooftop farming in different regions. These findings not only emphasize the environmental benefits of green initiatives but also their potential financial contributions and the need for supportive policies.

• vi.

  Challenges in the adoption of green practices are also addressed. Issues such as the dislike of certain tree species, the fear of large loads in rooftop gardening, and the lack of awareness and policies for modern farming technology in developing countries underscore the barriers that need to be overcome for widespread implementation.

• vii.

  However, a limited number of scholarly works discussed urban lakes, ponds, streams, wetlands, and bioswales, specifically in India and Bangladesh. Subsequent investigations may direct their attention toward blue components, such as urban rivers and stormwater ponds, to ascertain their relevance within the context of BGI studies.

Based on the analysis of this review, it is evident that the articles encompass a diverse array of urban BGI components or typologies and dimensions, particularly emphasizing socio-economic disparity (Arshad & Routray 2018; Sathyakumar et al. 2019), ecological considerations, and socio-cultural concerns. For instance, the articles looked into topics such as the sustainable management of stormwater and urban lakes (Kaur & Gupta 2022; Santhanam & Majumdar 2022), the cooling effects generated by urban blue and green spaces (Ali & Patnaik 2018; Rahul et al. 2020; Shah et al. 2021), and investigations into the availability, accessibility, and perception of urban green areas. Moreover, the articles also explored the issue of addressing the growing food demand through rooftop farming and examined the willingness to pay for green adaptation strategies, particularly in the context of Dhaka, Bangladesh (Ali & Patnaik 2018).

Urban BGI studies have been found to strongly emphasize analytical tools, field research, conceptual frameworks, and modeling; however, there are variations in how BGI services are distributed between studies. For instance, there is a large number of research on the availability, accessibility, and perception of urban green spaces (Gupta et al. 2016; Mukherjee et al. 2017; Singh 2018; Pussella & Li 2019; Turaga et al. 2020; Basu & Nagendra 2021; Sen & Guchhait 2021; Kamble et al. 2022). Similarly, numerous studies emphasize the temperature regulating benefits of BGI (Vailshery et al. 2013; Chandramathy & Kitchley 2018; Rahul et al. 2020), which may be attributable to the availability of free thermal imageries (such as Landsat data). As observed, many research investigations have examined the relationship between land use/land cover and the formation of urban heat islands or land surface temperature (LST) (Ranagalage et al. 2017; Ranagalage et al. 2018a, 2018b; Maheng et al. 2019; Arshad et al. 2022; Karunaratne et al. 2022). Consequently, the future proportional distribution of green space allocation within cities should be guided by a scientific approach considering the association between LST and vegetative cover. This should be followed by strategically incorporating new parks into the city's masterplan (Anguluri & Narayanan 2017).

Few studies convey innovative and novel approaches, mainly from India and Bangladesh. The practices carried out in these countries, as observed from the literature review, are listed in Table 3.

In recent years, scholars have concentrated on providing solutions for mitigation and adaptations to thermal regulation, air pollution, and disaster risk reduction, primarily in India, Sri Lanka, and Nepal. To ensure urban sustainability, it is also crucial to reduce the risk of disaster, and therefore, NBS has been identified as a potentially effective approach. The cities of the lower-middle-income countries in the South Asian region must do an in-depth assessment to identify the regulatory challenges to integrate different levels of governance to capture these multidimensional benefits of BGI.

Though South Asian countries still lack in utilizing the full potential of BGIs, many developed countries have already applied BGI to enhance the quality of urban life. Construction of eco corridors, rain gardens, permeable pavements, water retention basins, green sunken plazas, and green walls has drastically improved the urban environment in developed cities (Asleson et al. 2009; Jennings 2016; Conger et al. 2019; Ronchi et al. 2020). Cities like New York in USA and Melbourne in Australia have implemented policies mandating or incentivizing the installation of green roofs on new buildings or as part of retrofitting efforts. Cities like Amsterdam in Netherlands and San Francisco in USA have adopted policies and initiatives to promote circular economy principles, including waste reduction, recycling, and sustainable resource management, aiming to minimize emissions associated with resource extraction, production, and disposal. Tokyo (Japan) and Singapore have implemented policies relating to urban forest expansion. Cities such as Copenhagen in Denmark and Bogotá in Colombia have implemented policies prioritizing cycling infrastructure, pedestrian-friendly urban design, and efficient public transportation systems to reduce reliance on fossil fuel-powered vehicles and lower transportation-related emissions.

All such methods cannot be duplicated in the exact manner in South Asian countries since most of these methods are vastly resource and technology dependent. However, a few of them can be adapted in South Asian cities. Among these, blue green solar roofs and green walls should be prioritized. Since these methods do not require new land acquisition and there are already high-rise buildings in large cities, landscaping policies should identify suitable locations for green facades. Tree canopies over bus stops can be easily planned. Promoting cheaper low carbon transport can also help in meeting sustainable goals.

The majority of the articles emphasized the methodology, analytical tools, conceptual framework, field investigation, and modeling in urban BGI research. Since 2017, there has been a noticeable surge in scientific interest in urban BGI solutions across the South Asian region. However, articles discussing a holistic methodology to implement BGI as a tool for promoting urban sustainability are still lacking. The review exhibited that many BGI focused projects incorporate multi-criteria decision making, like incorporating policy builders, GIS specialists, architects and designers, botanists, and environmental engineers together. Approaching BGI with one or two dimensions has led to insufficient utilization of its potential. Also, blue spaces were clearly less investigated compared to green spaces. Both green and blue spaces should be integrated effectively for better environmental applications. The literature on the governance of BGI, focusing on its effective administration and strategies for enhancement, is limited; hence, further research is required to explore this domain better.

Multiple studies have emphasized the necessity of stakeholder and public participation in BGI development for urban areas for their ecosystem services in the selected countries. However, there is a lack of comprehensive studies conducted to thoroughly investigate the significance of stakeholder engagement in identifying specific goals relevant to BGI development in the cities of the selected countries. There are still unresolved issues in finding a suitable approach and framework for identifying, establishing, overseeing, and governing BGI and its integration with pre-existing grey infrastructure.

The other notable finding was the lack of implementing BGI solutions in these South Asian countries. It can be concluded from the review that reconstruction and new recreational landscaping in densely built cities were major hindrances. Many adaptation methods require vast technical resources. The developing countries must come up with location-based solutions that are specific to each city. Emphasis can be given to the suburbs and satellite cities that are still experiencing ongoing urbanization. These places can be built with sufficient eco corridors.

It was evident that BGI research exhibits geographical variations, wherein an observation can be made regarding the relatively fewer studies conducted on themes related to provisioning services compared to regulatory and cultural services. Food security should play a vital role in highly populated developing country cities. Thus, the focus should be on utilizing BGI with the goal of enhancing provisional services. Simultaneously, a limited number of studies were found to have examined numerous benefits concurrently, such as temperature management, water regulation, carbon capture, and others, inside single research. The research papers mainly focused on analysing individual BGI services, such as the impact of rooftop gardens on temperature control. Therefore, it is recommended that future research endeavors further investigate the various advantages associated with the BGI component within a single comprehensive study.

However, it is crucial to understand that the attributes of our data collection impact our findings. Even though inclusive search keywords were utilized, different alternative search terms may have influenced the publications in our sample since the terminologies vary from region to region. Additionally, we only looked for English-language scientific publications from the Web of Science and Scopus database. Hence, our findings may not universally apply to publications in languages other than the one studied and to grey literature.

Finally, the findings of this study are expected to contribute to a deeper understanding of the utilisation of urban BGI in lower-middle-income countries of the South Asian region, including its conceptual framework and practical implementations. Each country has made positive strides in certain aspects of blue-green infrastructure practices. Encouraging knowledge exchange and learning from successful strategies can contribute to more effective BGI implementation. Furthermore, it is necessary for decision-makers to persistently explore strategies that overcome the difficulties of implementing BGI, which means recognizing regulatory challenges and promoting intergovernmental cooperation to leverage the various benefits of BGI effectively. This study can assist municipal authorities in effectively addressing challenges and framing issues in context, thereby preventing the mere duplication of strategies employed by high-income countries.

Writing – editing original draft preparation, Aman Gupta; proofreading and supervision, Bhaskar De. All authors have read and agreed to the published version of the manuscript.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.