1.1 How is urban water managed?

Urban water management usually considers water as a resource. For instance, water management generally focuses on obtaining clean water, managing stormwater and wastewater, and performing flood control, based on technological solutions that do not consider the hydrological dynamics of the watershed as a system. This understanding of water as a resource, rather than a fundamental component of a system, promotes the occurrence of socioenvironmental impact. This impact is expressed in environmental degradation caused by changes in land use that do not consider watershed dynamics. These changes are driven by the need to obtain water from outside the urban territory, through artificial transfers of water from external sources, as well as the need to transport poor quality water out of the city [24]. For example, Mexico City Metropolitan Area imports clean water from the upper part of the Balsas River watershed and exports poor quality water to the Panuco River watershed [25–27]. This kind of urban water management fosters problems of inequality, marginalization, and poverty. These processes of water transportation and cleaning, based on the use of fossil energy, have an additional operating cost. Therefore, the decision where water is distributed and how much water is disinfected or treated, as well as how these processes are performed, becomes a sociopolitical issue related to water governance issues [28]. Simultaneously, the use of technology to deal with water management, by supplanting or complementing natural processes lost or reduced by urbanization, is not always effective and efficient. Furthermore, in some cases, constructed infrastructure is abandoned with the consequent environmental impact and waste of space, time, financial resources, energy, and materials that will become obsolete [13].

Therefore, the urban water management decision-makers currently face a combination of problems: an increasing water demand related to population growth and industrialization, the deterioration or obsolescence of hydraulic infrastructure, the interests of different societal sectors and the competition for water use, as well as alterations and socioenvironmental challenges intensified by climate change [29]. According to projections of urban growth, these effects will continue to increase over time [22]. To face this scenario, it is necessary to understand how to improve water management in cities, and an adequate vision from a practical standpoint should be based on the functional hydrological units: the watersheds.

1.2 Urban areas inside the watershed dynamic

To incorporate natural dynamics and processes in urban water management an integral view of the territory, beyond the urban limits, is necessary. In this sense, the territory delimited and covered by a watershed is considered the optimal area to establish such management [30–32]. This is because within this territory, depending on the biophysical characteristics of the watershed, hydrological processes, such as infiltration, aquifer recharge, spring formation, connectivity between water bodies and aquatic ecosystems, will occur. A watershed encompasses the entire geographic space in which water drains towards the same point and has clear limits established by natural elements, making possible to isolate and to understand the inflow and outflow of water from the system [33, 34]. In the space delimited by the watersheds, many of the processes that control the dynamics of the ecosystem occur, including the dynamics and water processes related to the cities immersed in the watersheds. Therefore, watersheds’ functionality and integrity make them strategic ecosystem management units [12, 30, 33, 35].

Additionally, groundwater dynamics must be considered since cities are generally supplied, at least partially, by groundwater. Groundwater dynamics occur in a system with a complex structure where aquifers coexist at different depths and are characterized by belonging to water flows at different spatial and temporal scales, as well as local to regional flows that keep superficially separated systems interconnected [36]. These dynamic properties of groundwater are determined by the geological characteristics of the area. When managing the land and water sources, it is necessary to understand that surface and groundwater dynamics are different but closely related [37, 38].

Water management with a watershed approach proposes managing a geographic space, considering the watershed as a complex system. In this sense, it is based on understanding water as a central and integrative element of natural systems proposing its management through its consideration as a common good [39] instead of a commodity. Considering the watershed as a resource system and water as the resource unit requires organizing the actors involved for its governance so that everyone can benefit from the system of resources without any actors evading their responsibilities or acting opportunistically [39]. Therefore, a comprehension of the interactions that occur inside the watershed system is needed, including: those between the biophysical components determined by environmental conditions; the types of territories’ appropriation regarding social organization, economy, technology; and the institutions related to water management at diverse scales and levels [31, 40, 41].

To consider the dynamics and the interactions inside the watershed could contribute to the solution of various urban water-related problems. However, although the benefits of a watershed approach are well known, the application of this approach has been difficult. The challenges for implementing this approach are related, for instance, with the requirements for respecting the natural boundaries and dynamics, which do not generally coincide with administrative limits and institutional coordination capacities at the basin level. This approach also presents a time scale that is often not compatible with the need for an urgent response to critical short-term situations and with the brevity of the political-electoral cycles [42]. Furthermore, the implementation of this approach might require an economic investment for generating urban water management changes which could be incompatible with the economic resources and investment priorities of those responsible for water management. These difficulties for the implementation of a watershed approach in space management increase when entering urban areas. The management of the territory in urban areas seems to be conceptualized as isolated from natural processes, so generally the management of urban territory is thought and executed through artificial processes mediated by technology and energy expenditure.

In this context, many cities have experienced water crises in the last decades, and some are on the way to reaching a critical situation regarding water called “Day Zero”. This concept was established in 2017 by officials in Cape Town, South Africa, and marks the moment when an open water supply ends and access to water begins to be rationed, so the population must collect their portion of water every day [29]. There are many cities that will possibly reach a similar situation if the authorities do not modify their urban water management. For example, the cities of Sao Paulo, Brazil and Monterrey, Mexico experienced a dry season in 2015 and 2022, respectively. This caused water shortages, with some parts of the city only receiving water two days a week [20, 43]. In Mexico City, the capital of Mexico, part of its population experiences water scarcity, either with no water or only at certain times of the day or week [27]. In addition, Sao Paulo in Brazil, Mexico City and Buenos Aires in Argentina, experience flooding problems related to deficiencies in water management [10, 13]. In contrast, some cities in different regions, such as Melbourne, Australia, Portland and New York in the United States, and Chinese cities such as Shanghai, Wuhan and Shenzhen, have introduced changes in urban water management to include the consideration of natural dynamics and processes related to water. These changes were achieved through watershed-based programs, as well as laws, policy modifications and specific urban interventions, to generate changes promoting long-term urban sustainability [44–46]. At a country level, New Zealand incorporated since 1868 a watershed approach in national water regulations and implemented a territorial organization around river basins [47].

Even if there are published case studies and literature reviews on the importance of a watershed approach in land and water management, they generally study cases in rural or natural areas, are single case-studies of cities or countries, or compare conceptual methods or models related with the watershed approach instead of comparing cases with an actual implementation of this approach [48–51]. Thus, in order to learn from the experience of cities worldwide which implemented changes in urban water management to include natural dynamics and a watershed perspective, a broader understanding is needed. Specifically, it is necessary to assess which are the drivers for cities to implement those actions, the kind of actions implemented from this approach in urban areas in diverse regions, as well as the main challenges and results obtained. Through a scoping review, this research aims to analyze how paths towards urban sustainability could be generated by incorporating the consideration of the dynamics and natural processes related to water, in urban water management, through a watershed approach. This research was performed through a scoping literature review [52] and guided by the following research question: What is known from the literature about the incorporation of a watershed approach in urban water management? This study aims to identify the existing research related to the incorporation of natural dynamics and process into urban water management, as well as to recognize any existing gaps in knowledge.

Step 1: Research questions

The initial research question for the investigation was: What is known from the literature about integrating the watershed approach into urban water management? This question was essential for guiding the scoping review and defining the limits of the research. Additionally, four specific questions were formulated: 1) What are the main drivers reported in the literature for incorporating a watershed approach into urban water management? 2) What are the main actions identified in the literature to integrate a watershed approach to water management in cities? 3) What are the main challenges identified in the literature for cities to integrate a watershed approach in their water management actions? 4) What are the main results reported in the literature related to the implementation of the identified actions?

Step 2: Search strategy

In order to identify suitable case studies for analysis, a search was conducted to identify cities that had incorporated, to some degree, a watershed approach in their water management. Such a search was performed in three databases: Web of Science, Google Scholar and SciELO (Scientific Electronic Library Online) and included scientific works that partially or generally included a watershed approach in urban water management. The search for information used the following combination of key words: “watershed” and "urban water management"; “watershed” and "cities water management"; "river basin" and "urban water management"; "watershed" and "policy" and "urban" and "water management"; "watershed management" and "urban" and "case" and "water management"; "watershed approach" and "urban" and "case" and "water management"; "water sensitive" and "watershed" and "cities" and "water management"; "water resources management" and "urban" and "watershed" and "case"; “socioecosystem” and "urban water management" and "case"; and "socio-ecological system" and "urban" and "water management" and "case."

Step 3: Selection criteria and eligibility of documents

The initial phase of the document review process involved in the examination of the first 20 search results in each database for each keyword combination. All the documents identified in the databases that were not part of the first 20 results were excluded. Within those first 20 results, inclusion and exclusion criteria were applied to evaluate and screen the documents [54]. Related to the applied criteria, the first filter was that the documents had to be written in Spanish or English. All identified documents that were in a language other than Spanish or English were excluded. Afterwards, the documents’ title and summary were analyzed in order to identify if the document content was related to the research question. Only documents that could be accessed based on the institutional access of the Universidad Nacional Autónoma de México (UNAM in Spanish), that presented a title and a summary that allowed knowing the scope and objectives of the investigation and that were related with the research question were selected [27]. The inclusion criteria focused on documents that provided information about urban water management with a watershed approach or that included natural dynamics of water in water management in a specific city or a case comparison between two or more cities. The exclusion criteria, conversely, removed any documents that focused on general water management, water management in rural areas, or water management at a national, regional, or global level.

Step 4: Data collection

All the documents found were analyzed to determine if they contained the information sought through the establishment of mandatory and complementary categories of information. These categories were divided in two compulsory categories and two complementary categories. The compulsory categories were as follows: policies or actions that include a watershed approach (Actions) and problems solved through these policies or actions (Drivers). In addition, the two complementary categories were the following: positive consequences and negative consequences (Results), as well as the challenges or barriers to implementing these policies or actions (Challenges).

Based on this analysis, the last filtering of documents was performed, leaving only the documents containing information of at least one of the mandatory categories, and the data were recorded. In the cities where information from the mandatory categories was identified; the complementary categories were registered. Documents that analyzed cases with theoretical models, recommendations, or scenarios for watershed implementation, which failed to include specific actions or policies implemented, were excluded. Only documents with specific actions or policies implemented for solving urban water problems within specific cities were included in the following step of analysis (Table 1).

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Table 1. Cities selected as case studies and references from the scoping review.

https://doi.org/10.1371/journal.pone.0309239.t001

Two reviewers were in charge of applying the selection criteria and collecting the data from each document. First, the reviewers worked independently in the literature review process and then comparing and double-checking the work collected by their peer during this process to confirm the data included was accurate.

Step 5: Data analysis

The collected data were analyzed to understand what changes were undertaken in each city to incorporate natural dynamics in water management from a watershed approach. For each document, the cities analyzed were identified and each document was classified according to the city or cities addressed in each study (Table 1). Subsequently, all pertinent information corresponding to the mandatory categories (drivers and actions) and the supplementary categories (challenges and outcomes) were identified for each city, based on an exhaustive review of all pertinent documents. That information was classified for each category (actions, drivers, results and challenges) and analyzed in the content and the Tables 2–5, details in the Results section. Lastly, the data analysis allowed identifying the implications of these incorporations in the transition towards more sustainable urban conditions and the challenges of implementing these water management initiatives for cities, details in the Discussion section.

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Table 2. Drivers for including a watershed vision in cities water management based on the selected case studies.

https://doi.org/10.1371/journal.pone.0309239.t002

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Table 3. Actions to include a watershed vision in cities’ water management based on the selected case studies.

https://doi.org/10.1371/journal.pone.0309239.t003

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Table 4. Challenges for including a watershed vision in cities water management based on the selected case studies.

https://doi.org/10.1371/journal.pone.0309239.t004

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Table 5. Results of actions aiming to include a watershed vision in cities’ water management based on the selected case studies.

https://doi.org/10.1371/journal.pone.0309239.t005

Step 6: Complementary data collection and analysis

Based on the data analyzed (Step 5), some information gaps were identified in the cities analyzed regarding the drivers, actions, challenges, or results for incorporating watershed dynamics in water management. For instance, the analysis of the drivers, actions, challenges, and results revealed, in some cities, a potential for bias due to the limited number of documents identified. This was addressed through the implementation of an additional step in the methodological process. For this step, a search was undertaken specifically for all the cities identified (Step 4) and analyzed (Step 5). Such a search was performed in the same three databases (Step 2). The search for this complementary information used the following combination of key words: “city name” +” urban water management”. Afterwards, the same selection criteria were applied (Step 3) and the list of documents regarding each city was updated with the new results (Table 1). Lastly, the complementary information was analyzed to have a more robust understanding of the main actions, challenges, drivers, and results for each city (case study) incorporating a watershed approach in water management (Step 5), details in the Results and the Discussion section.

3.1 Drivers of change

The main drivers that motivated changes in urban water management were associated with urban water problems, such as water scarcity and floods. Water availability for satisfying urban demand was a motivation in 88% of the cities analyzed (Table 2). This motivation was mainly related to the presence of prolonged droughts, low quality of water sources or the intensive exploitation of water sources in those cities, which limits water availability and promotes water scarcity. Besides, 88% of the cities analyzed were forced to change their water management policies because of flood problems (Table 2). In order to address flood problems in Melbourne, Portland and Berlin, those cities incorporated measures such as the rehabilitation of surface water sources and associated ecosystems generating corridors and preventing surface runoff [44, 45, 56, 63, 79].

Besides, it was observed that environmental and social drivers, related to urban surface water quality, motivated changes in urban water management. For instance, 76% of the cities analyzed included water management changes to face water quality alteration problems in their surface water bodies, as well as aquifers (Table 2). Concern for water quality is mainly based on water availability, issues of public health, the recreational use of natural spaces and the habitability of the city. In this sense, for Chinese cities, even in cities where water resources are abundant, poor water quality is a threat for water security [60, 67]. In order to respond to this problem, through the "Sponge City" program, China promoted the recycling of rainwater and wastewater treatment to optimize the urban water drainage system for water distribution and wastewater purification [56]. While in Philadelphia, in the United States, actions were performed to reduce runoff, to decrease contaminants input, and to prevent the overflow of the combined sewage system, as well as to address health issues such as yellow fever outbreaks [45, 91].

Regarding political or institutional motivations for changing urban water management, some cities have made changes in their water management because of regional and national policies, regulations and budgets that promoted a watershed approach incorporation (76% of the cities) (Table 2). The majority of these cities are located in Asian countries, such as China and Singapore, where a robust institutional framework guides the implementation of necessary changes. This framework is characterized by a top-down approach, which enables the effective and efficient implementation of required changes [56, 57, 70–74]. For instance, in Chinese cases, the cities were used as pilots to test new nationally developed water management strategies and frameworks, known as the “Sponge City” concept [56, 57], which is a framework developed to address and overcome water problems related to surface runoff, water quality, water storage, and climate change through green infrastructure applications, for example, green roofs, rain gardens or bioretention [57]. China released the national strategy for the Sponge City construction through programs, policies, incentives, and the selection of some cities to act as pilots in the application of this water management framework [56, 59]. In other cities, such as New York, Melbourne, Seoul and Berlin, the existence of consolidated institutions with national frameworks, regulations and budgets promoting water management changes were identified as key drivers for implementing actions [76, 79, 81, 92].

Habitability was reported in 71% of the cities as a driver for introducing changes in urban water management (Table 2). This was measured by a range of life quality factors, including access to fresh water and the possibilities of performing recreational activities in the city. For instance, in Melbourne and Vienna, some modifications to urban water management were prompted by the impact of current management practices on recreational activities due to alterations in the water quality of some recreation spaces [44, 86].

Another environmental driver identified was related to the mitigation of the impact of urbanization on the environment through the introduction of changes in urban water management; found in 47% of the cities analyzed (Table 2). These changes were not directly related to the different uses of water but are based on avoiding the degradation of terrestrial and aquatic ecosystems [44]. It is crucial to underscore that the bibliographical review reveals a close correlation between the occurrence of floods and the deterioration of surface water quality, which are both influenced by surface runoff processes intensified by urbanization.

Economic motivations were also found to be a reason for promoting changes in urban water management (Table 2). In 29% of the cities analyzed, the current or potentially future increase in the costs of water management was mentioned as a direct motivation for changing water management policies or actions [44]. For example, in New York City, the city’s motivation to implement changes in water management was driven by the potential for significant financial costs associated with the installation of filtration systems, as required by the U.S. Environmental Protection Agency (USEPA) under the United States Safe Drinking Water Act (SDWA), if the city’s drinking water quality continued to decline [92]. Likewise, even if it was not reported directly, several of the motivations found in the other cities were closely related to economic issues. For example, the increase in the cost of flood control, the decrease of clean water availability, the interruption of tourism because changes in water quality, the increased cost of cleaning water, among others, were mentioned in the cases even if they were not explicitly stated as drivers of management changes. Even in cities where economic drivers were not the main motivation to start changes, in some cities market incentives facilitated the implementation of some actions. For instance, in Melbourne the construction of third-pipe systems, that is nowadays being installed in many developments around Melbourne’s fringes, was promoted by developers since it was considered cost-neutral or actually saving money in comparison with other discharge options [79].

Another identified driver was striving to become more resilient in confronting climate change. However, in the context of the current uncertainty surrounding climate change it is notable that only 24% of the cities analyzed explicitly indicated drivers related to becoming more resilient to face the effects of climate change (Table 2). For example, in Rotterdam, the Netherlands, the primary driver for modifying water management strategies was the necessity to enhance resilience to climate change. Given that Rotterdam is a low-lying port city, particularly vulnerable to rising sea levels as a consequence of climate change, it was imperative to implement measures to mitigate this risk [44, 63].

Lastly, in 24% of the cities, social factors were identified as central motivations for incorporating changes in urban water management (Table 2). For instance, public awareness in water issues and citizens’ demand for environmentally friendly policies were drivers for implementing and maintaining a watershed approach in cities such as Berlin, Portland, Melbourne, Singapore, and Vienna [63, 77, 81, 87]. For instance, in Portland, a bottom-up approach was central for the promotion of a watershed management approach. In this case, citizen advocacy, led by informed and active social stakeholders, filed a lawsuit against the city of Portland, alleging a violation of the Clean Water Act, which triggered Oregon’s Department of Environmental Quality (DEQ) to order the reduction of water contamination in the city of Portland [63]. Also, transdisciplinary networks with academics, activists, and corporations, facilitated and promoted a science-based, continuous and incremental implementation of water management changes across different political administrations [72, 79]. Lastly, political factors based on national and local history and geopolitics were key drivers in Berlin and Singapore cases for implementing a watershed approach related to increasing water security and reducing water dependency on imported sources [70–74, 81].

3.2 Actions implemented

The identified drivers promoted the incorporation of changes in water management through specific actions in each city (Table 3). The vast majority of cities (76%) implemented actions that represented a shift from gray to green and blue infrastructure. These actions included conversion of impermeable gray spaces in cities to increase infiltration for water availability and flood control using urban design modifications. Actions also included implementation of new infrastructure in cities that considered the water cycle as a priority from project design stages, requiring a strong collaboration between land use planners and water resource managers. Some of the actions implemented were described in the case studies as Green Infrastructure (GI), nature-based solutions (NBS) and Low Impact Development strategies (LID) [69, 71, 74, 92]. For instance, in Berlin, Germany, a series of initiatives were implemented with the objective of sustaining the natural water cycle within the city in order to ensure its water supply remained self-sufficient. Among these initiatives, the filtration of riverbanks, the artificial recharge of groundwater, and the ecological improvement of surface water, as well as the infiltration of rainwater, stand out as particularly noteworthy [45]. Also, in cities of the United States of America, such as Philadelphia and Portland, several actions were taken to promote this shift from gray to green infrastructure. In Philadelphia, storm water regulations and protocols were implemented to increase the use of green infrastructure for storm management by urban developers [91]. Based on those actions, Philadelphia became one of the cities with the highest number of installed green roofs nationally [91]. Similarly, in Portland, based on its “Grey to Green” initiative, over 900 green streets, 400 eco-roofs and 32,000 street trees were implemented to alleviate loadings on the piped infrastructure system, to prevent floods and reduce surface water contamination [63, 90]. Likewise, in Chinese sponge cities the transformation of gray to green infrastructure was central among the implemented actions [62, 66]. For instance, in Wuhan 389 projects were developed including urban gardens, parks and green space for water infiltration [62].

In addition, among the actions identified, the conservation, restoration, and rehabilitation of surface water bodies and their associated ecosystems, such as riparian ecosystems, stand out as particularly noteworthy (Table 3). These types of actions were present in 71% of the cities analyzed and appeared across diverse regions and contexts from Asia to Europe, America, and Oceania. The rehabilitation of bodies of water aims to manage rainwater so that it contributes to flood control. This rehabilitation was also used for runoff control and for natural water treatment to improve water quality through urban wetlands [46]. It is crucial to underscore that these initiatives were implemented through the restoration and rehabilitation of existing bodies of water or through the construction of new bodies of water in close proximity to urban areas. For example, in Melbourne, Australia, the Melbourne Water Living Rivers Program promoted stormwater management devices such as rain gardens and wetlands [79]. Besides, in Singapore, a program began in 2006 to transform its drains, reservoirs and canals into preserved waterways and natural lakes [44]. In cities such as Philadelphia, United States of America, several land acquisitions were implemented (9,200 acres), resulting in the creation of the Fairmount Park, to protect Philadelphia’s water sources and, consequently, protecting drinking water quality [91].

Additionally, several actions were implemented to increase water availability from alternative water sources (41% of the cities analyzed) (Table 3). Singapore is an exemplary case in this matter, since it faces severe water availability constraints and implemented a large capital investment to increase water supply. Some of the actions realized by Singapore were increasing the storage and protection of rainwater, generating urban infrastructure for treatment and reuse of wastewater and expanding its desalination capacities [71, 72]. In other cities such as Melbourne, alternative water sources were collected and reused in greenfield areas, such as recycled wastewater for non-drinking uses, rainwater and stormwater harvesting from roofs and after touching the ground [79].

Finally, regarding pollution control policy actions, this type of action was reported in 41% of the cities analyzed (Table 3). For example, in Philadelphia, actions were taken to control combined sewer overflows, which sought to eliminate between 80% and 90% of annual pollutants in combined sewer areas through the use of green infrastructure to control stormwater runoff, even on site [45]. Similarly, pilot projects were designed and implemented in Melbourne based on networks of scientific institutions, policy-makers, land developers, municipalities, and water practitioners. The objective was to increase the use of stormwater quality treatment technologies, such as gross pollutant traps, constructed wetlands, and biofilters [76].

3.3 Challenges for the implementation

Despite the drivers of several cities around the world to include a watershed approach in their policies and actions, achieving changes in urban water management towards a watershed approach faces numerous challenges (Table 4).

In several of the urban areas studied, economic investment and market constraints have been identified as significant obstacles to the implementation of watershed-based water management strategies (47% of the cities) (Table 4). The costs of infrastructure building, operation, and maintenance was mentioned as a key challenge to overcome, not only to implement urban water management changes, but also for maintaining, adjusting, and expanding them over time [59, 69]. For example, in Singapore it was found that a barrier to these changes was the high installation and maintenance costs required for new urban water management systems [45]. It is noteworthy that despite the absence of economic investment as a challenge in the Chinese cities included in the case studies, China as a country posits that while it may be economically viable to implement changes in water management in specific cities, economic investment represents a limitation for the implementation of these projects at a basin-wide level [56]. Besides, market barriers were identified, such as local unavailability and high costs of products and materials required for changes in water management, such as Green Infrastructure. The absence of materials locally, such as geo-membranes used in green roofs and rain gardens, increased construction costs and limited a broader implementation of changes in cities such as Seoul [69].

The review also exposed a lack of experience, capacities, and incentives, along with technical and logistical barriers for implementing urban water management changes in governmental and private stakeholders. In 41% of the cities studied, these variables were reported as an obstacle for effectively implementing the watershed approach in their policies and actions (Table 4). For instance, in Melbourne, insufficient knowledge and operational capacities of water retail companies limited their actions for delivering recycled water systems [76]. Additionally, the lack of clarity regarding the determination of necessary treatments within the watershed constituted a barrier in this case [79]. Similarly, the lack of knowledge and experience among city planners in Seoul presented challenges for the effective implementation of LID and GI actions [69].

Additionally, social factors hindered including a watershed approach in urban water management. In 35% of the cities analyzed, the lack of social support was an obstacle for including the watershed approach in water management (Table 4). This was illustrated by the lack of environmental education of the population, the social perception toward new projects and the need and difficulty of forming interdisciplinary groups to address the issue. In Ningbo, China, even if in the early stages of the planning and construction process for a new water management system, public participation was not a mandatory requirement. The lack of education and communication with stakeholders and the general population impeded future participation in infrastructure maintenance. This was because the beneficiaries should appreciate the social and environmental benefits of the new water management mode, so that the potentially high building and maintenance costs of the model are offset. These cases highlighted the need for appropriation, empowerment, societal support, and economic resources to perform the projects. Therefore, it is suggested that the implementation of new water management models could use the population’s participation, information, and consultation, as well as the need for collaboration with the academic sector in order to sustain urban water changes, particularly over reuse of water, on scientific data [64]. A similar situation occurred in Melbourne, where a challenge to implement changes that would facilitate water reuse was the negative opinion and lack of confidence of the population and the political sector regarding the treatment of wastewater to make it available for consumption [79]. Even if the government of Melbourne was convinced of the merits of promoting stormwater harvesting as an alternative water source approach, it was necessary to await the emergence of scientific advancements and new evidence related to biofiltration technologies before proof could be provided that treated water could achieve sufficient quality levels. Until that moment, the context enabled to implement harvested stormwater and recycled wastewater initiatives [76]. Collaborative research and learning process were required to develop evidence, guidelines, and regulations for safely delivering reused water services and to progressively instill trust in political and social actors. It is necessary to emphasize that the potential uses of the treated water depend not only on scientific evidence, but also on community perceptions in each country and locality, for instance, about drinking treated water [76, 79].

Institutional design problems and coordination barriers were also identified as obstacles for implementing the new urban water management in 29% of the cities analyzed (Table 4). The main institutional and political barriers identified were: institutional fragmentation between different areas of the city government; coordination problems between local and national authorities; unclear requirements, roles and responsibilities for implementing and giving maintenance to the new infrastructure generated; and bureaucratic obstacles which problematized the implementation of structural changes [76, 82, 89]. For example, in Melbourne, the difficulty in establishing alliances to employ a new kind of water management was an identified challenge [44]. Additionally, in this city, there was a lack of clear financial evaluation criteria for new water management model projects due to the absence of a systematic approach to quantify the social and environmental benefits of such initiatives, which would justify subsidies and cost-sharing arrangements [79]. In contrast, cities such as Shenzhen where all water-related government functions were combined in a single governmental agency, an integrated management of water permitted to improve water management with a more holistic approach and to avoid conflicts due to overlapping functions and ambiguous functions [65].

The gray infrastructure built in cities and the consequent transformation, fragmentation and degradation of urban green spaces increased the difficulty for introducing changes with a watershed approach in urban water management. For example, 24% of the cities analyzed reported a historical transformation of green spaces into gray spaces as a challenge for including a watershed approach in water management (Table 4). The authors argued that it was easier to include changes in new urban developments than to try to modify the structure of existing urban areas. For example, in Ningbo, in China, converting the old blue and green infrastructure of the city into gray infrastructure was one of the main barriers identified for implementing the Chinese "Sponge City" program [64]. For this reason, the modernization of the old part of the city was considered harder than the installation of green or blue infrastructure in newly developed areas.

National and local regulations were identified as a barrier for implementing a watershed vision in water management in 18% of the cities analyzed (Table 4). In some cases, the current legislation in their territory discouraged implementing innovative changes in urban water management from a watershed approach. For example, in the case of Berlin, there was a limitation for applying new solutions from research and a pilot project because of the characteristics of the existing legislation [45]. In Melbourne, in highly valued environmental areas where additional stormwater management should occur, it was impossible to impose additional requirements on developers because of the existing regulatory framework [79]. Therefore, existing regulations might be difficult to overcome when applying innovative solutions, both in research and pilot projects [45]. Conversely, in some cities regulation at the national level enabled or even promoted implementing watershed-focused policies. For instance, such as in Chinese cities, changes were guided by national guidelines and specific implementation rules of a national watershed approach [56, 57]. These regulations, national policies, and guidelines were aligned with the change towards watershed-focused actions in water management, facilitating the implementation of these actions at the urban level.

The geographical, geological, and topographic characteristics of the cities were also identified in some cases as an environmental challenge for including a hydrographic watershed approach in urban water management (Table 4). For instance, in two of the cities analyzed, its geographical characteristics were considered a challenge to include water management changes. This was the case of the Chinese city of Chongqing, which is located in a mountainous environment that inhibits engineering solutions for managing high-speed surface runoff from mountains [57]. Also, in the case of Shenzhen, the city’s geographical and natural conditions, such as short river channels with limited self-purification capacity, were identified as factors that limits water pollution reduction [65].

3.4. Results of the actions

Based on the conducted literature review, it was difficult to extract the impact from the implementation of actions and policies of water management that included a watershed approach (Table 5). The reviewed literature described the actions undertook, their drivers, and challenges, but it rarely evaluated or illustrated the effects of the laws, the policies, and the actions performed by cities to tackle water challenges to move towards sustainability approaches. For some cases, such as the Chinese pilot cities implementing a Sponge City approach, the authors described the cases as having an excellent performance, but they did not specify impact indicators in terms of sustainability and water issues [57]. Even in Shenzhen and Wuhan cases where the implementation of Sponge City’s actions was described as outstanding, the monitoring data was considered insufficient by the authors and basin level ecosystem impacts have not been evaluated [62, 66].

Additionally, some other cases were found in the literature review with a brief description of results from the implementation of water management changes and actions related to a watershed approach incorporation. The literature regarding cities such as Melbourne, Stockholm, and Singapore illustrated a positive impact, which resulted from implementing green infrastructure for rainwater harvesting, drainage networks’ interventions, recycled wastewater use, runoff, and floods control. For instance, the Hornsgatan project in Stockholm, which is a rainwater harvesting project based on green infrastructure, was perceived as successful by the community. Some of the emphasized positive results from this project were direct, such as adequate wastewater treatment and water collection from runoff and rooftops; other indirect social benefits included improved air quality, better safety perceived and improved open areas from planted trees that were irrigated mainly through rainwater infiltration [89]. In Melbourne, the incorporation of “integrated water management” provisions in their planning instruments related to rainwater harvesting from roofs and stormwater harvesting (from the ground) was also described as having positive results. For example, the use of stormwater harvesting schemes became an extended practice in Melbourne’s urban developments and parks, where citizens reduced the need of purchasing drinking water [79]. In Singapore, rainwater collection and the drainage network in place with grass swales and filter strips, reduced waterlogging and flooding throughout the islands even during heavy storms [57]. Collected rainwater was also a new unconventional water supply source there, accounting for 20% of the total water supply sources [45]. In Chongqing’s case, an improvement of environmental conditions was identified and water safety was evaluated as having an upward trend [60].

Lastly, it was remarkable that Vienna explicitly presented specific indicators to describe the impact from their actions and water management policies, which included a watershed approach. In this case, the city government reportedly managed to maintain the desired water quality of a surface water body related to tourist use [86]. Moreover, surrounding this water body, a substitution and reforestation of the riparian ecosystem with native species were achieved through public and private cooperation [86].

4.1. Drivers or challenges for urban water management transformation

Whereas some characteristics were identified as drivers for change in some cities, in other cities the same factors were considered as challenges which limited the possibilities of implementing changes in water management. Three factors were identified that shared this duality: a) legislation and policies; b) economic factors; and c) social and political factors.

4.2. Actions to integrate urban watersheds from a socio-ecological perspective

Two general categories of actions were identified among the cities studied. On the one hand, a group of actions focused on reversing the impacts of urbanization processes, through the restoration of rivers or urban green spaces. These actions represent most of the actions identified in the case studies and seek to recover the benefits of ecosystems by preserving or restoring natural processes. On the other hand, there are actions that seek to incorporate the understanding of variables, dynamics and natural processes through innovations such as rainwater harvesting or reuse of treated water.

A crucial element identified in the first group of actions, is the effect of land use change promoted by urbanization. The degree of fragmentation, reduction, and disappearance of natural spaces (urban green spaces and surface water bodies), as well as their degree of conservation must be considered in order to incorporate a watershed approach to urban water management. These characteristics of urban land use will hinder or promote the incorporation of changes in water management from a watershed approach and will determine the level of complexity and socio-economic costs that the proposed changes in urban water management would imply. For instance, 71% of the cities analyzed addressed actions for the conservation, restoration or rehabilitation of surface water bodies and their associated ecosystems, which implies a high degree of economic investment and other challenges to overcome in order to recover natural processes or dynamics that benefit the city. This investment makes it difficult and very costly to implement these types of actions, and it is particularly in cities of the Global South. In this sense, cities that maintain a higher level of natural spaces conservation could achieve a more solid incorporation of a watershed approach and face fewer challenges with lower implementation costs. This highlights the role of natural spaces in cities and the importance of territorial and ecological planning that promotes the conservation of natural water processes and dynamics in the urban areas.

A second group of actions identified through the case studies are the innovations implemented, such as rainwater harvesting and the reuse of treated water. These types of innovations aim to integrate water dynamics into urban planning and to adapt the technology of use to the hydrological dynamics. Regarding these innovations, something that was rarely present in the literature review, but which is undoubtedly essential to consider when thinking about this type of innovation, is the relationship of the cities with climate change projections and the uncertainty that this represents [108, 109]. Climate change scenarios add complementary difficulties and uncertainties for water management that must be considered with a view of more sustainable and resilient cities in a short term. This is evidenced by the fact that changes in urban water regimes pose a growing risk and have already led to the collapse of water supply, stormwater and wastewater disposal systems in several cities around the world. Even in cities with a strong tradition of a watershed approach, such as Melbourne, technical analyses for the implementation of water management measures do not take into account future climate change scenarios, for example in the case of rainwater harvesting systems, which are climate-dependent [78].

In both types of actions, either ecosystem restoration actions or the promoted innovations, the incorporation of a watershed approach in urban water management is facilitated if it is based on understanding and respecting the physical and biophysical components of the system of the city and its watershed [110, 111]. This promotes the existence of resilient ecosystems (biophysical components) in which cities and their inhabitants can live and sustain ecological services as well as their common goods. Thus, a socio-ecosystem approach is useful to achieve because it understands water systems as composed of physical components in which the biophysical as well as human, social and institutional components are nested [111]. This type of alignment promotes long-term benefits of blue and green spaces, such as those identified in the case studies, which outweigh short-term economic savings and reduce future investments to restore lost blue and green spaces.