Nature-Based Solutions for Water Management in Peri-Urban Areas: Barriers and Lessons Learned from Implementation Experiences

Nature-based solutions (NBS) are defined by the European Commission as “actions that are inspired by, supported by, or copied from nature . . . ” and that solve societal challenges and multiple benefits. As a result, NBS are often promoted as alternative responses that solve complex societal challenges such as watershed management, while delivering a systemic approach of multiple benefits for well-being, human health, and sustainable use of resources. Despite rising interest in NBS, further identification of experiences implementing NBS could advance our understanding of the operationalization of this comprehensive concept. For this purpose, we analyzed 35 peer-reviewed articles on implementation experiences of NBS for water management in peri-urban areas, on aspects related to (i) NBS problem–solution: water challenges, ecosystem services, scales, and types; (ii) NBS governance and management. From the insights of the analysis, this paper asks what lessons are learned, and which barriers are identified, from implementing NBS for water management in peri-urban areas? As a result, this study presents a detailed analysis of each aspect. We conclude by highlighting accountancy, monitoring, and communication as potential success factors for integration and development while diminishing the overall barrier of complexity, which leads to technical, institutional, economic, and social uncertainty.


Introduction
Societal challenges such as carbon sequestration, coastal resilience, ecosystem restoration, and watershed management underpin the need for systemic ways to address them. In this sense, nature-based solutions (NBS) enable natural processes into the technical response to address these challenges, with the aim of creating multiple benefits for society [1], well-being, human health, and the sustainable use of resources. NBS are delivering the benefits through open green spaces (e.g., urban parks), green/blue infrastructures (e.g., wetlands, river parks, rain gardens), and at a building level with elements such as green roofs or green walls. For example, addressing water challenges through NBS, i.e., flood risks, droughts, water pollution, freshwater withdrawals, or difficulties related to stormwater and urban water management, promotes the development of multifunctional landscapes, e.g., river parks that could benefit human well-being and physical and mental health. Dealing with the complex and dynamic impacts of urbanization processes and climate changes through NBS could be particularly relevant in areas that combine rural and urban dynamics, identified as peri-urban areas, peripheries, sprawls, and suburbs, among others [2].
NBS has been defined as solutions inspired and supported by nature to face societal challenges while delivering benefits that are ecological, social, and economic [3]. There are plenty of arguments areas expose the community consensus, or the lack of it, for the support and up-take of NBS as an innovation development.
Purposely, NBS and peri-urban concepts have been used as keywords in this literature review, and the references that exposed case studies were selected as implementation experiences of 'NBS' for water management in 'peri-urban areas'. The analysis followed an outline structure (i.e., social, environmental, economic, and governance) to gather the descriptors that could be supporting the comprehensive approach of NBS. Specifically, we examined NBS as a problem-solution relating to the analysis of physical and spatial aspects, and NBS governance and management to identify the actors involved and the policy instruments supporting the implementation. What lessons learned and barriers are identified in implementing NBS for water management in peri-urban areas? This article identifies the characteristics of implementing NBS in peri-urban areas as actionable knowledge in lessons learned, and the barriers as observed limitations or negative aspects. Section 2 outlines the methodology implemented, qualitative-focused multi-methods. Section 3 describes the results as NBS: problem-solution and NBS governance and management. Section 4 presents a further discussion on the analysis and significance of the barriers and lessons learned focusing on the collected evidence that could promote an advancement on the operationalization of NBS.

Materials and Methods
Our multi-methods research is qualitative-focused and comprises a literature review in combination with, content analysis, and descriptive research. The SLR helps to collect, examine, and integrate the different scientific contributions under the keyword combination "nature-based solutions' + water + peri-urban'. An initial search of peer-reviewed articles was conducted in December 2019. Two databases have been used, Scopus and Web of Science for the search of the keywords combination. As the exclusion criteria, we used the date to limit it to sources from 2015, type to gather only articles in peer-review journals and because of language limitations, we excluded references when not written in English. As the inclusion criteria, we selected articles that explicitly expose case studies of NBS and its implementation experiences. The Systematic Literature Review (SLR) was developed with the purpose of review and analysis of 'NBS implementation', dealing with water management in 'peri-urban areas', recognizing that these terms are mainly used in the European context, while other terms are used globally for similar purposes.
A literature search using digital databases to find experiences in implementing NBS, using selected keywords presented 160 references, of which 3 references were excluded because of our lack of access or they were not written in English. Of the 157 peer-reviewed publications, we first read the title and abstract to search for the terms, "water" and "case study"; 66 references mentioned case studies and were included. The excluded 91 references were related to literature reviews, conceptual, modeling, and assessment publications. We then read the 66 articles to determine their proper fit as a case study, to conduct a more intensive review and selected 35 references (Figure 1). The examination followed an outlined structure of aspects including location, environment, economics, social dimensions, and descriptors about governance, instruments, actors involved, and its financing. To support the systematic documentation of the lessons learned and barriers highlighted in the experiences, the analysis and synthesis strategy was assisted by NVivo (Qualitative Analysis Software). In terms of the review criteria, the references excluded were out of the scope of NBS implementation; because of not using cases dealing with water or to the concept of NBS, e.g., not mentioning water in the process as input, output, or benefit, or not mentioning NBS or other green terms, such as ES, GI, EbA.
Data were gathered from 35 references published in 2016 (1), 2017 (6), 2018 (6), 2019 (15) to 2020 (7), detailed information and codes for the references are presented in Appendix A, Table A1. The references present studies in different locations, but mainly from Europe where the 'NBS concept' is promoted and funded, followed by Asia, America, Africa, and Oceania. Yet, it is acknowledged that due to the concepts used, the review is predominantly focused on European experiences. Besides, even if our specific search including 'peri-urban' as a criterion; some case studies are linked to urban and rural areas, reinforcing the idea of interlink among built environments, beyond the administrative borders.  (15) to 2020 (7), detailed information and codes for the references are presented in Appendix A, Table A1. The references present studies in different locations, but mainly from Europe where the 'NBS concept' is promoted and funded, followed by Asia, America, Africa, and Oceania. Yet, it is acknowledged that due to the concepts used, the review is predominantly focused on European experiences. Besides, even if our specific search including 'peri-urban' as a criterion; some case studies are linked to urban and rural areas, reinforcing the idea of interlink among built environments, beyond the administrative borders.
The analysis of NBS has been structured as (i) NBS problem-solution: Water challenges, ES, types, and scales, for the technical and spatial response; and (ii) NBS governance and management, for identifying the specific factors that support NBS implementation. As our search strategy required an iterative process to determine if the case studies were included or excluded in each criterion, the case studies mentioned in each sub-section might vary. To facilitate the analysis, each aspect is included along the results using codes and references, while detailed information on each implementation experience is presented in Appendix A. The barriers and lessons learned emerged from these insights, identifying the positive descriptions in the collected evidence of the implementation as 'lessons learned' or actionable knowledge; and the negative aspects as 'barriers' or observed limitations for operationalizing NBS as a comprehensive concept. The analysis of NBS has been structured as (i) NBS problem-solution: Water challenges, ES, types, and scales, for the technical and spatial response; and (ii) NBS governance and management, for identifying the specific factors that support NBS implementation. As our search strategy required an iterative process to determine if the case studies were included or excluded in each criterion, the case studies mentioned in each sub-section might vary. To facilitate the analysis, each aspect is included along the results using codes and references, while detailed information on each implementation experience is presented in Appendix A. The barriers and lessons learned emerged from these insights, identifying the positive descriptions in the collected evidence of the implementation as 'lessons learned' or actionable knowledge; and the negative aspects as 'barriers' or observed limitations for operationalizing NBS as a comprehensive concept.

Results
The dominant discourse of NBS as a comprehensive approach is to achieve systemic interventions, delivering multiple benefits to multiple stakeholders in a resource-efficient manner [3]. As an integral feature of the concept, NBS link the problem addressed to the solution, within the aim of sustainable development-in other words, facing social, environmental, economic, and institutional barriers [9]. Thus, when referring to complex challenges, the aim for systemic interventions is to deliver results at different environmental-technical, and social levels. In this sense, we analyzed NBS implementation for water management to identify the specificities of the systemic response. The next sub-sections present NBS examined from two standing points. Our first search criteria aimed to examine the NBS problem-solving feature from the technical and spatial aspects (Section 3.1), responding to the ecological dimension of the concept. Second, the NBS governance and management (Section 3.2) to identify the socio-economic aspects that support NBS implementation.

NBS Problem-Solution: Challenges, ES, Scales, and Types
This section will explore NBS by focusing on the technical and spatial factors of NBS implementation for water challenges (Section 3.1.1), the ES delivered (Section 3.1.2), the scales of the solutions (Section 3.1.3), and types implemented (Section 3.1.4). To close this section a representation of the links among NBS types, challenges, and ecosystem services is presented. This section is complemented by Appendix A, with detailed information on each implementation experience.

Challenges
In terms of challenges, the experiences describe a variety of issues related directly and indirectly to water, reporting pressures on water resources, and the system management ( Table 1). The direct challenges are flood risks, urban water systems management, freshwater withdrawals, climate regulation, freshwater supply, stormwater management, climate change mitigation and/or adaptation, water pollution purification/filtration, and drought/water scarcity. Indirect challenges are related to the effective-incorporation of socio-cultural services when mentioning concerns as recreation, human well-being, and social cohesion. See also Appendix A (Table A2).
The literature on NBS usually explains water challenges as results of the pressures from the climate influence and/or urbanization effects, and as causal mechanisms of interdependence among other challenges. This is observed, for example, on the hydrological impacts of urbanization processes, such as a reduction in perviousness, infiltration, and surface retention, which could be the causal mechanism for increasing storm rainfall going to a runoff, leading to floods [16]. Thus, flood risks are linked to stormwater management, not only by runoff and peak flows but also by conveying pollutants to nearby surface waters [17]. In this order of ideas, shifting natural landscape in peri-urban areas, towards an urbanized one, diminishes the natural land pervious cover, its infiltration, and its retention capacities. This shift could increase environmental risks, not only locally but also in other areas, which will require higher investments in infrastructure, services, and management over the long-term.
In fact, the literature frequently mentioned NBS for risk management, including floods, droughts, heatwaves, sea-level rise, and earthquakes. Figure 2 shows the relations between water challenges and risk management addressed in the literature. The thickness of the edges (nexus) are proportional to the number of articles that relate to both vertices (nodes). The strongest relations show more publications that relate risks to water challenges are 'flood', which relates to flood risk, urban water systems and pollutants purification/filtration, and "climate change" (climate change mitigation/adaptation, climate regulation and flood risk). Consequently, integrating changes in land covers and flows controls, while maintaining a certain water quality and flow, is a multi-level challenge for water management in peri-urban areas. In this sense, it is relevant to notice that water systems are also influenced by the dynamics of urban and rural systems, and decisions in other sectors as risk management.  [8,17,[45][46][47][48] The literature on NBS usually explains water challenges as results of the pressures from the climate influence and/or urbanization effects, and as causal mechanisms of interdependence among other challenges. This is observed, for example, on the hydrological impacts of urbanization processes, such as a reduction in perviousness, infiltration, and surface retention, which could be the causal mechanism for increasing storm rainfall going to a runoff, leading to floods [16]. Thus, flood risks are linked to stormwater management, not only by runoff and peak flows but also by conveying pollutants to nearby surface waters [17]. In this order of ideas, shifting natural landscape in periurban areas, towards an urbanized one, diminishes the natural land pervious cover, its infiltration, and its retention capacities. This shift could increase environmental risks, not only locally but also in other areas, which will require higher investments in infrastructure, services, and management over the long-term.
In fact, the literature frequently mentioned NBS for risk management, including floods, droughts, heatwaves, sea-level rise, and earthquakes. Figure 2 shows the relations between water challenges and risk management addressed in the literature. The thickness of the edges (nexus) are proportional to the number of articles that relate to both vertices (nodes). The strongest relations show more publications that relate risks to water challenges are 'flood', which relates to flood risk, urban water systems and pollutants purification/filtration, and "climate change" (climate change mitigation/adaptation, climate regulation and flood risk). Consequently, integrating changes in land covers and flows controls, while maintaining a certain water quality and flow, is a multi-level challenge for water management in peri-urban areas. In this sense, it is relevant to notice that water systems are also influenced by the dynamics of urban and rural systems, and decisions in other sectors as risk management.

NBS and ES
NBS implementation as a solution addressing, primarily, the issues related to water challenges; has the potential of delivering multiple benefits in a resource-efficient manner and adaptable manner. In this review, these benefits are identified through ES and its categorization of provisioning, regulating, cultural, and supporting services ( Table 2). The regulating services that are frequently

NBS and ES
NBS implementation as a solution addressing, primarily, the issues related to water challenges; has the potential of delivering multiple benefits in a resource-efficient manner and adaptable manner.
In this review, these benefits are identified through ES and its categorization of provisioning, regulating, cultural, and supporting services ( Table 2). The regulating services that are frequently mentioned are the moderation of extreme events, waste-water treatment, among others. Cultural services are mainly related to recreation, mental, and physical health. Provisioning services are mainly represented through freshwater. Supporting services expose the habitat for species. See also Appendix A, Tables A3 and A4.  [27,41,42,44,48] Although, the multiple benefits and services provided through nature are recognized; the debate in the literature is more oriented to the proper assessment of these services, and its integration into different sectors, to recognize the added value of NBS. In the policy sector, advancement by the European Commission established NBS priority areas as: the regeneration and well-being in urban areas, carbon sequestration, coastal resilience, watershed management, and ecosystem restoration, to enhance the insurance value of ecosystems and to foster sustainable use of matter and energy [3].

NBS Scales
NBS implementation for water management in peri-urban areas respond to different spatial scales from site to national level (Table 3). Cases are mentioned as sites (6), neighborhood (6), municipality (20), metropolitan area (7), regional-basin level (10), and national level (1). In some cases, no explicit reference was found, thus, the category 'other' was applied (3). Implementation experiences were mainly on a municipality scale, which is associated with the level of urban planning competencies.
Despite, the spatial scale of intervention cannot address or control the overall impact of the water challenges; it is a recognition of the limits of the NBS and the need for articulated responses at different spatial scales. This shows the role of the governance level of decision-making, that the type of NBS could determine the sectors involved, and that there must be instruments allowing the required integration of governance and management for supporting NBS implementation.

NBS Types
As technical responses, different types of NBS are presented in the implementation experiences (Table 4). NBS are wetland-related approaches, such as natural wetlands, constructed wetlands, and purpose-built wetlands (15); sustainable urban drainage systems (SUDS) (11); green-roofs/walls (11); river parks (9); agroforestry (9), parks (9); permeable pavement (4); Phytorid sewage treatment (3), which is a wastewater treatment using a specific variety of plants in constructed wetlands [13]; rain gardens (3); bioswales (2); and others (24). In this review, the solutions are differentiated elements, since there is no clear boundary between what is conceptualized as SUDS in the literature. Specifically, solutions referring to SUDS for their drainage or filtration functions could be permeable pavement, rain gardens, bioswales, green roofs, detention and retention basins, wetlands.  [5,6,13,18,22,23,25,[27][28][29][30][31][32][33][36][37][38]40,41,43,45,[47][48][49] NBS are not implemented independently but are combined within hybrid approaches of green, blue, and/or gray infrastructures. NBS as place-based interventions shift the approach to landscape management, compared to traditional infrastructural projects. This is done by emphasizing the link among the green (vegetation), and blue (floodable areas, water) areas; and the influence of changes in land covers, and land uses. To illustrate this aspect using flood risk management, some publications claim that the hybrid approach is the most widely used, followed by a green approach and then by a blue approach [50]. In contrast, others argue that the green approach, represented as a green storage, is modified and influenced by land cover and land-use change; thus, it is more vulnerable than the blue approach [23]. Despite its vulnerability, NBS propose a shift from the design and use of gray flood control infrastructure or NBS planning for water management [7].
In addition, other types of NBS could be regarded as linked to natural landscapes or to build-up landscapes ( Table 5). The first solutions refer to integrating soil, vegetation, floodable areas, and water, while the latter is related to vegetation, floodable areas, and water. To close this sub-section, NBS is represented ( Figure 3) as a simplified scheme flowing from problems or challenges (left) towards a response delivering benefits (right). In the literature, the analysis of the ES supports how NBS might deliver reinforced benefits through several and simultaneous ES. Thus, this scheme exposes the problem-solving feature in the spatial and technical aspects of the NBS, as the response addressing the interconnected water challenges and delivering a multiplicity of services; rather than establishing a causal link for a fixed categorization. See also Appendix A, Tables A5 and A6. To close this sub-section, NBS is represented ( Figure 3) as a simplified scheme flowing from problems or challenges (left) towards a response delivering benefits (right). In the literature, the analysis of the ES supports how NBS might deliver reinforced benefits through several and simultaneous ES. Thus, this scheme exposes the problem-solving feature in the spatial and technical aspects of the NBS, as the response addressing the interconnected water challenges and delivering a multiplicity of services; rather than establishing a causal link for a fixed categorization. See also Appendix A, Tables A5 and A6.

NBS Governance and Management
This section continues the analysis of NBS by identifying the governance and management factors that are supporting NBS implementation in peri-urban areas as the policy instruments (Section 3.2.1), the involvement of stakeholders (Section 3.2.2), and financing (Section 3.2.3). This section is complemented by Appendix A, Table A7 with detailed information on each implementation experience.

Policy Instruments
Most of the implementation experiences mentioned some kind of policy instrument (27) supporting its implementation, such as projects, programs, and plans (Table 6). When analyzing the governance level of these instruments, the regulations could link supra-national regulations to

NBS Governance and Management
This section continues the analysis of NBS by identifying the governance and management factors that are supporting NBS implementation in peri-urban areas as the policy instruments (Section 3.2.1), the involvement of stakeholders (Section 3.2.2), and financing (Section 3.2.3). This section is complemented by Appendix A, Table A7 with detailed information on each implementation experience.

Policy Instruments
Most of the implementation experiences mentioned some kind of policy instrument (27) supporting its implementation, such as projects, programs, and plans (Table 6). When analyzing the governance level of these instruments, the regulations could link supra-national regulations to national, regional, or municipal initiatives. Most instruments correspond to local regulations and initiatives at the municipal level, which complements the findings of the spatial scale of the projects (See Section 3.2.2). In the European Union context, the multi-level link is often developed under the EU Water Framework directive, which exposes the vertical coordination or agreement on NBS implementation. In addition, these regulatory frameworks cover long-term and cross-sectoral agendas, such as Sustainable Development Goals (SDGs); and sectoral documents, such as water planning, water management, risk management, and urban planning, displaying the interrelation among sectors for NBS while promoting specific supporting tools.

Involvement of Stakeholders
In NBS development, complex societal challenges reveal the efforts of different actors (Table 7). However, a key aspect of its implementation is the cooperation between stakeholders to address sectoral barriers, fragmentation at different governance levels, and multidisciplinary consensus. In terms of stakeholders, the categories implemented correspond to representatives of public authorities, academics and researchers, the business and private representatives, the citizens and community, including NGOs, and other water-related actors. In our review, academic and public authorities were the most mentioned actors in the case studies reviewed in this study (22), followed by the civil society (16), water-related actors (10), and business and private representatives (5).
The public sector is leading the NBS implementation effort to address the challenges described, and it takes on technical and economic leading roles. Scientific interest is reflected in the effort when research opportunities are available. The involvement of private actors and industry depends on the development of business models. The role of citizens is related to users and recipients of the benefits. However, a limitation of this study is that only peer-reviewed publications in scientific databases were considered, which may explain also the importance of the academic actors. Accordingly, further research could include the analysis of other literature sources, to complement the actors and roles. The articles exposed different social and/or cultural values when implementing NBS. Whilst some case studies (16) involved the civil society, only one-third of them (10) explicitly mentioned its role: in the participatory process [32]; participatory modeling [17]; and personal interviews and collective meetings [41]. Also, cultural values were mentioned with terms such as recreation [13,18,24,41,42,[44][45][46]48]; aesthetics [13,36,48]; social cohesion [42]; educational and therapeutic activities [47]; and cultural and historical heritage [18]. Local knowledge was slightly mentioned in educational activities [24]; [22], in activities to increase awareness [44]; and in bottom-up initiatives [31].

Financing NBS
For financing, almost half of the studies in the reviewed literature mentioned sources (Table 8). NBS is mostly funded by the public sector (14), the private sector (9), and few mentioning public-private partnerships (6). Public funding comes from local governments (municipalities), regional governments, ministries or departments of national governments or supranational institutions, such as the European Union. Research funding at the supranational level is a key aspect of this European context. Private funding comes mainly from sources such as foundations, non-profit organizations, and private corporations; or by conducting before-after simulations [43,48].
NBS has been developed as an amenity supported by public interest or through a non-profit aim, rather than through specific business models. The lack of identification of innovative business models behind NBS, reveals an open arena for identifying who is involved in the development of NBS for water management, and which roles they play. Although some references mentioned aspects related to the economic feasibility of NBS, specifically through aspects as life-cycle costs, cost-benefit analysis, or operating and maintenance costs, there were not included as part of this study; but, it should be covered in research by others. In fact, the economic feasibility of NBS is a barrier in its implementation, especially for avoiding uncertainty in its operationalization.   [13,24,29,30,35,44] In summary, the results in this section indicate that implementation experiences of NBS in peri-urban areas are addressing different challenges, at different spatial scales, but mainly at the municipal level, and are executed through several types as hybrid approaches. The governance and management aspects of the cases suggest that NBS is linked to municipal, metropolitan and regional basin scales, i.e., by interventions across administrative borders, and delivered through agreements and consensus supported by policy instruments. The recognition of the actors involved indicates the leading role of public authorities, although, in some cases, other actors as academia and industry are involved. Funding schemes executed by privates are rarely mentioned. Finally, the implementation of NBS could be considered a process of a participatory nature. NBS as a socio-technical innovation needs to advance in the economic aspects. As a further barrier, none of the case studies examined referred to the gender perspective, which could be seen as a knowledge gap in sustainable development and the NBS-well-being relationship. The next section, therefore, moves from these insights on to discuss the barriers and lessons identified.

Discussion
This paper analyzes NBS for water management in peri-urban areas, using peer-reviewed literature on the implementation experiences, with a detailed view on specific spatial and technical aspects; and more general information on the governance, and management aspects. It supports in particular a previously cited barrier of NBS: there is still a need for NBS to be operationalized to be able to collect evidence on its effectiveness [16]. The added value of NBS in terms of measuring the technical performance is presented in the literature through the recognition of ES, although, the debate is more oriented on the assessments, classifications, and scenario planning. In this review, the added value of NBS in the management and governance focused on the policy instruments and involvement of stakeholders, and economic aspects are only examined by the funding sources; but is an aspect to be researched by others, since it is equally relevant for informed cost-benefits, assessments on life-cycle costs, or operating and maintenance costs.
What lessons were learned, and which barriers were identified, by implementing NBS for water management in the peri-urban?

Lessons Learned
Water challenges expose pressures due to climate, risks, and urbanization. These challenges are interdependent, dynamic, and linked to the quality and quantity of the resource, revealing the complexity of water management in peri-urban areas. A common aspect is addressing water challenges through a mix of green/blue, green/gray, and green/blue/gray infrastructure approaches. NBS in the peri-urban area ranges from macroscales, such as river basins and agroforestry, to buildings as a microscale. Although there is not a fixed spatial scale, it is understood that peri-urban areas could tend to municipal levels for their planning competencies or bigger scales for implementations linking rural and urban systems. The multiple benefits delivered could be regarded as interrelated services, influencing different fields as landscape management; risks and climate regulation; recreation, physical and mental health, and well-being [35]. This approach is open to contextualization as various GI and ES are mentioned when referring to implementation experiences, e.g., in the use of NBS or other green terms such as GI, ES, Eb; also, its open to adaptation, since NBS could be known under other terms such as LID, WSUD, SUDS, IUWM.
NBS deliver structural physical changes, exposed as spatial elements enhancing water management in different material manners while being resource-efficient. These changes result from integrating different fields, instruments, and mechanisms to promote shifts in the practices of cross-sectoral expertise, e.g., infrastructure design and water management. These new practices shift different domains, for example in landscape management [11], to support built-up landscapes or natural landscapes; as well as improvements in the spatial resources, risk management, and social well-being. Specific examples are the identification of flood-prone areas and influencing factors for flood occurrence, such as distance, slope, and land cover [22]; design green infrastructure for increasing awareness of previously unnoticed natural features, such as sudden incidental nature experiences [45]; and improve multi-functionality of the urban green space [41].
NBS as an alternative practice, to enhance the dominant culture of gray infrastructure with interventions and experiments of blue and green approaches, could increase the potential of a peri-urban area to support human well-being. NBS implementation can deliver multiple benefits at multiple aspects; depending on the involvement and support of different levels of responsibility, territorial scales, and sectors. The operationalization of NBS could be enhanced by developing frameworks for capturing its comprehensive approach, considering it as a process, and including the multiple dimensions of its solution (e.g., spatial, technical, ecological, social, economic, etc.).
Finally, accountancy on the multi-functionality and the benefits delivered could be a critical success factor to involve cross-disciplinary approaches into NBS for water management. In this sense, monitoring NBS as a process is crucial for integrating the different scales of NBS: the spatial scale of the implementation, the scale of the challenge, and the scale of the impacts. Besides, communication about NBS could support the involvement of different actors, and the integration of sector, at different levels of decision-making to improve infrastructure planning and assessments of multiple benefits.

Barriers
NBS cannot control the overall impact of water challenges [32], nor can it meet all needs (e.g., high runoff volumes, high contaminant loads, etc.), that could be related to high technical uncertainty [43]. To avoid uncertainty, systemic implications of NBS require taking into account the benefits, services, and the potential risks or unintended consequences of their up-take [14]. Limitations to this purpose could be financial but could also extend to the lack of technological capacity or deficiency in infrastructure, as shown by the region-wide gap in groundwater monitoring systems and data [38]; or that affect institutional capacity [29,31]. These also affect the NBS market uptake and the creation of alternative business models and practices that support it, and thus, limiting partnerships and involvement. For instance, market uptake of NBS benefits as a field still requires legal regulations [8].
Even if NBS implementation involves different stakeholders, the promoting role is mainly done, and funded, by the public sector. The lack of interactions among the different actors involved compromises the perception of NBS, which could be negative for aspects such as costs, benefits in the short-and long-term, and impact of the solution [52]. This could lead to difficulties due to inhabitant resistance to changes [29,35], passive involvement, and insignificant increase of social cohesion [8], fear of the unknown, and uncertainty [16,17]. Some NBS functions could lead to disservices, which are perceived negatively or affect safety perceptions, i.e., fire risks on green spaces or drowning risks in SUDS [31]. In this sense, the cases revealed how social dynamics in terms of behavior and practices are shaped through socio-cultural values, traditions, and perceptions [16,31]; and how they influence the uptake and use of NBS [26]. Study cases revealed that NBS increases individual and public awareness for lifestyle shifts [8]. Major aspects to consider for precaution in implementing NBS are displacement, gentrification commodification, social justice regarding access to nature and human well-being, among others [1,11,16].
A common aspect of the lessons learned and barriers is the knowledge demand, associated with the NBS uptake or decision-making for long-term and co-benefits [16], and on its effectiveness for comparison to conventional approaches. Therefore, NBS as a process should also be open to monitoring, i.e., for its maintenance and operation, and for examining ES and disservices. A crucial factor is to account for NBS perceptions throughout its implementation, e.g., costs, benefits; while acknowledging that both positive and negative perceptions need to be managed [52].
Limitations of this review are related to the use of European promoted terms; the lack of economic aspects related to cost-benefits, maintenance, and operation, and that none of the case studies examined referred to the gender perspective, which could be seen as a knowledge gap in the NBS-well-being relationship. To provide greater insight into the novelty and comprehensive approach of NBS, further research could work on these gaps.

Conclusions
NBS address different water challenges, produced by urbanization processes, changes in climate, and risks while allowing the delivery of other services. Through a systematic review, in combination with content analysis, and descriptive research, this study examined 35 articles of experiences with NBS implementation in peri-urban areas. The review presented NBS from two standing points, first from the spatial scale and technical aspects of its problem-solving feature, to respond to the ecological dimension of the concept. Second, from the governance and management, to identify the socio-economic factors that support its implementation.
Based on the insights of this analysis, we identified lessons learned and barriers. Mainly, accountancy, monitoring, and communication could be a potential success factor in NBS for water management. In this sense, accountancy to involve cross-disciplinary approaches on the multi-functionality and the benefits delivered. Monitoring on NBS as a process for integrating the different scales of NBS: the spatial scale of the implementation, the scale of the challenge, and the scale of the impacts. Communication about NBS could help to implicate different actors at different levels of decision-making. In a second order, this work identified that NBS could deliver multiple benefits, regardless of the type, scale, and location. However, in its implementation as a systemic response, its benefits are usually acknowledged as ES, which are integrated at multiple spatial scales and social aspects beyond the green infrastructure (GI). Since NBS cannot meet all needs, neither control the overall impact of water challenges, its implementation should be supported on different levels of responsibility, territorial scales, and sectors. This means that physical changes in water management should be supported on social consensus established among different stakeholders, sectors, and organizations.
Yet, a major barrier for NBS implementation is the complexity of a comprehensive approach, which leads to technical, institutional, economic, and social uncertainty. Limitations could be the lack of technological capacity or deficiency in infrastructure, as shown by the region-wide gap in groundwater monitoring systems and data [38]. Even if public authorities are playing a vital role in the promotion of NBS, by funding it, promoting research and policies; limitations could be related to the institutional capacity [29,31]; and extend to economic aspects, e.g., to promote legal regulations required for the market uptake of NBS benefits [8]. This weakness in the creation of alternative business models and practices could restrain partnerships and the support of NBS by private actors. Further NBS uptake is depending on the enhancement of technical, institutional, and financial capacities, but also on the involvement of the different actors, including lay citizens. In fact, the interaction between the different actors involved could promote greater advances to actionable knowledge, perspectives, and discourses on this solution.
Nevertheless, the effort in identifying the systemic implications of NBS, in terms of benefits, services, potential risks, and unintended consequences aids to manage the negative perceptions around NBS implementation, which could be an advancement for overcoming alternative practices as NBS. In this sense, water management requires addressing its related challenges and social aspects in an integrated way. The cases reviewed have implemented NBS as an environmental and socio-technical system, which provides a feasible approach to managing water challenges and their associated pressures. As our research focused on NBS for water management in peri-urban areas, it is limited in identifying meaningful influential factors among NBS types, ES, or built-up context. Water management in peri-urban areas could be further explored to understand the influence of urban boundaries on NBS types.
As a systematic review, this paper has different limitations. First, 'NBS' and 'peri-urban' terms used in inclusion criteria are mostly used in Europe. Although, other relevant experiences may be found under similar concepts for NBS such as LID, WSUD, SUDS, IUWM, or to peri-urban areas as suburbs, fringe, peripheries, suburbs, sprawls, etc. Second, the literature selected only covered peer-reviewed articles, and there may be significant evidence of experiences, lessons, and barriers, in other bodies of literature as NBS is in the intersection of science-policy-innovation. Third, our analysis has a wide-spread view of the implementation of NBS reported in the scientific literature, which usually focuses deeply on a specific discipline, therefore further analysis could advance in the operationalization of NBS in terms of frameworks for capturing its comprehensive approach. Further analysis could contrast this 'NBS in peri-urban areas' to other bodies of literature, including other terms, and explore this alternative approach from complementary concerns as the economic aspects (e.g., cost-benefits, life-cycle costs, operating and maintenance costs).
Despite these limitations, this review offers a widespread overview of the comprehensive approach of NBS regarding implementation experiences of NBS for water management in peri-urban areas. The contribution of this paper is the analysis of NBS in its different aspects throughout the implementation, identifying the lessons learned and barriers behind them. The results are describing each of the aspects analyzed in the selected references, to provide an overview of what is considered the problem-solving feature of NBS. Furthermore, complementary information of the cases (location, challenges, ES, types, scales) is used to report the specificities of the implementation experiences. There are still many opportunities and knowledge gaps to facilitate NBS operationalization, such as the different narratives around NBS and the local search for collaborations as processes that not only reveal the technical effectiveness of the challenges addressed but also the advances to the solution as a fixed vision or as a cross-boundary scheme that requires cross-sectoral dialogues.  . This version is integrating valuable insights from the fruitful discussions within the NATWIP meetings. We would like to thank Gerard Martinez Görbig for the support during the selection of references for its formal analysis.

Conflicts of Interest:
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses or interpretation of data; in the writing of the manuscript; or in the decision to publish results.