A Systemic Approach for Assessing the Design of Circular Urban Water Systems: Merging Hydrosocial Concepts with the Water–Energy–Food–Ecosystem Nexus

Abstract

Urban Water Systems (UWS) are complex infrastructures that interact with energy, food, ecosystems and socio-political systems, and are under growing pressure from climate change and resource depletion. Planning circular interventions in this context requires system-level analysis to avoid fragmented, siloed decisions. This paper develops the Hydrosocial Resource Urban Nexus (HRUN) framework that integrates hydrosocial thinking with the Water–Energy–Food–Ecosystems (WEFE) nexus to guide UWS design. We conduct a structured literature review and analyse different configurations of circular interventions, mapping their synergies and trade-offs across socioeconomic and environmental functions of hydrosocial systems. The framework is operationalised through a typology of circular interventions based on their circularity purpose (water reuse, resource recovery and reuse, or water-cycle restoration) and management scale (from on-site to centralised), while greening degree (from grey to green infrastructure) and digitalisation (integration of sensors and control systems) are treated as transversal strategies that shape their operational profile. Building on this typology, we construct cause–effect matrices for each intervention type, linking recurring operational patterns to hydrosocial functionalities and revealing associated synergies and trade-offs. Overall, the study advances understanding of how circular interventions with different configurations can strengthen or weaken system resilience and sustainability outcomes. The framework provides a basis for integrated planning and for quantitative and participatory tools that can assess trade-offs and governance effects of different circular design choices, thereby supporting the transition to more resilient and just water systems.

1. Introduction

The growing scarcity of resources and competing objectives have intensified the need for system-level analysis in urban water management [1,2,3]. Applying systems thinking when designing Urban Water Systems (UWS) helps clarify the structure of complex problems and reveal cross-sector interdependencies within the Water–Energy–Food–Ecosystems (WEFE) nexus [4,5]. This approach reduces unintended conflicts from siloed solutions and enables the identification of leverage points that trigger cascading positive effects [6,7,8,9,10,11,12,13,14].

Moreover, UWS design has implications not only for other resources within the WEFE nexus but also for socio-political systems. Researchers are increasingly studying the integration of hydrological and sociological aspects within urban environments, leading to a proliferation of concepts and frameworks in the water literature [1,3,15,16,17,18,19,20]. Studies on the hydrosocial cycle often stem from either natural science or sociological studies. While the former focuses on hydrology and overlooks critical social influences [15,21], the latter centres on institutional frameworks, governance structures, and community behaviours, but often overlooks the technical constraints that shape water management practices [16,17,22]. Most of these approaches do not clearly explain how elements and subsystems within UWSs interact to generate emergent system functions that support resilience. The structural and functional logic linking system components to hydrosocial outcomes remains underexplored. To improve UWS design and assessment, an integrated conceptual model is needed—one that bridges natural and social science perspectives and explicitly connects infrastructure design strategies to system-wide performance and resilience [16].

In response, the WEFE nexus and hydrosocial studies perspectives have gained traction as conceptual tools to understand how UWSs co-evolve with natural and societal processes. The WEFE lens enables integrated resource planning, while the hydrosocial approach foregrounds the political, spatial, and institutional dimensions of water governance [1,3,16,18,19,23]. Merging both offers a richer basis to capture the complexity of the UWS and to analyse urban water resilience.

Yet, although considerable research has focused on improving UWS design, most studies analyse structural design strategies—scale, circularity, greening, and digitalisation—either in isolation or in partial combination (Figure 1). When interactions between strategies are considered, they are typically explored within a limited scope, often confined to one subsystem of the UWS (e.g., water supply, wastewater, or drainage). For instance, several studies examine the implications of scale for circularity [24,25,26,27,28,29,30] or explore synergies between greening and circular solutions [11,31,32], sometimes extending to greening configurations across scales [7,12,13,33]. Others assess the role of digitalisation in enabling more efficient and adaptive UWS operation [6,34,35,36,37]. A few exceptions begin to bridge these domains [6,36,38], but none, as far as our knowledge goes, provide a fully integrated analysis across all four strategies and all three UWS subsystems.

In contrast, this study adopts a configuration-based approach in which each UWS intervention is understood as a systemic expression of all four design strategies—defined through its operational water management scale (decentralisation or centralisation gradient), degree of circularity, greening infrastructure profile, and digital integration (see Figure 1 and Box S2, in the accompanying Supplementary Material). This framing enables a more holistic analysis of how design decisions produce synergies and trade-offs across UWS compartments, WEFE Nexus, and the social and economic dimensions of hydrosocial systems. In doing so, we address not only the infrastructural and functional dynamics within UWS, but also the broader resource interdependencies and hydrosocial processes that shape urban resilience.

Grounded in General Systems Theory (GST) and systems thinking [19,39,40], our framework integrates the WEFE nexus and hydrosocial perspectives to assess how interventions generate synergies and trade-offs in hydrosocial functionalities. This approach offers practical guidance for systemic infrastructure planning and institutional co-design in the transition toward resilient hydrosocial urban systems. It does so by focusing on different configurations of circular interventions that are shaped by interactions between their circularity purpose and three design strategies: scale, greening and digitalisation. Based on a systematic literature review, the paper addresses the following research questions:

1.

How can the interactions between environmental, social, and economic elements of the hydrosocial system—and their links to the WEFE Nexus—be captured in a unified framework?

2.

How do synergies and trade-offs arising from circular interventions influence hydrosocial resilience?

To answer these questions, the article is structured as follows: Section 2 describes the methodological approach, including the literature review and formulation of intervention typologies; Section 3 presents the findings, detailing hydrosocial system functionalities and the qualitative assessment of synergies and trade-offs across different configurations of circular interventions. Section 4 presents practical implications and policy relevance and Section 5 includes final conclusions.

2. Materials and Methods

2.1. Systematic Review

The literature on hydrosocial systems, UWS design strategies, resilience and WEFE nexus was retrieved from the Web of Science (WoS) database, focusing on English-language articles worldwide. We first built a comprehensive topic search that combined several blocks of terms to capture: (i) urban settings (e.g., city, municipality, circular cities, urban greening); (ii) water and wastewater domains (e.g., water supply, stormwater, wastewater, reclaimed water); (iii) management and governance approaches (e.g., urban water management, socio-hydrology, circular economy, resilience, climate adaptation); (iv) infrastructures and design strategies (e.g., centralised and decentralised systems, green–blue–grey infrastructure, nature-based solutions, reuse technologies and control systems); (v) performance, impacts and indicators (e.g., benefits, risks, ecosystem services, costs, MCDA, spatial footprint); and (vi) assessment methods and planning tools (e.g., reviews, decision-support tools, lifecycle assessment, cost–benefit analysis, scenarios and pathways). These strings generated a large initial dataset of 6225 publications (Figure 2). To complement this broad search, we ran a funding-text query that targeted major international projects and initiatives on nature-based solutions, circular cities, water-sensitive design and digital water (e.g., NATURANCE, Nature4Cities, HYDROUSA). This helped to identify additional studies linked to key research programmes that might not be fully captured by the topic search. We also developed three sets of thematic queries to fill specific gaps: (i) Green infrastructure and co-benefits; (ii) WEFE, energy use, scale and resource-recovery strategies in urban water systems; and (iii) resilience, hydrosocial perspectives and social dimensions of water governance. The full query strings are reported in Box S1 (Support Material). To identify complementary seminal and landmark papers, the Connected Papers web tool was used to explore citation networks and relationships among key references.

The diagram summarises the identification, screening and eligibility stages leading to 161 sources read in full text. The screening process followed a two-level approach: (i) title and abstract screening and (ii) full-text review. The primary selection criterion was citation count, with different thresholds set based on the publication period. To overcome the challenge of a large initial dataset, citation thresholds varying with publication year were employed. Older publications required a higher citation count to be included, while more recent articles with fewer citations were still considered if they presented novel insights or emerging perspectives. This approach helped avoid the exclusion of seminal and foundational articles, while ensuring the inclusion of recent contributions that may not yet have accumulated significant citations but are potentially influential. In addition to citation thresholds, we applied explicit inclusion and exclusion criteria. We included studies that: (i) analysed urban water systems or closely related socio-hydrological systems; (ii) engaged with at least one of our core themes (hydrosocial systems, design strategies, resilience or WEFE nexus); and (iii) provided conceptual, methodological or planning-oriented insights at system level. After applying the eligibility criteria, a total of 143 scientific articles were reviewed in full. In addition to peer-reviewed papers, 18 complementary sources (e.g., white papers, reports, manuals, laws, and briefings) were included, bringing the total number of reviewed sources to 161.

2.2. Building the New Conceptual Framework: Hydrosocial–Resource Urban Nexus

The framework integrates the WEFE nexus and hydrosocial studies. A systems thinking approach is used to interpret and explore interrelations between system elements. These frameworks provide the foundation for analysing how design interventions in UWS influence hydrosocial system resilience. Rather than describing system compartments and elements alone, we focus on the emergent functions arising from their interactions. The definition of these functionalities is derived from established frameworks that specify urban challenges and water resilience goals [38,41,42,43], as well as from literature about the social nature of water [44,45,46]. We then categorise these functionalities into different environmental and socioeconomic domains.

2.3. Qualitative Assessment of Circular Interventions

To enable a systematic assessment of synergies and trade-offs triggered by changes in the design of UWS, we first developed an intervention typology matrix to classify circular interventions. We classify circular interventions into typologies defined by their circularity purpose (water reuse, resource recovery and reuse, or water-cycle restoration) and by how other design strategies (scale, greening and digitalisation) shape their configuration and modes of operation; see

Table 1. Matrix with interventions typology based on the possible values adopted for each design strategy.

Matrix		Centralized								Decentralized
Water	DPR	Centralized DPR								Decentralized DPR
			Greening options:								Greening options:
			x	Grey	x	Green-grey		Green			x	Grey	x	Green-grey		Green
			Digitalization options								Digitalization options
			x	Integrated Control	x	Localized control	x	Non-Digital			x	Integrated Control	x	Localized Control		Non-Digital
	IPR		Centralized IPR								Not applicable
			Greening options:
				Grey	x	Green-grey		Green
			Digitalization options
			x	Integrated Control	x	Localized control	x	Non-Digital
	UPR		Centralized UPR								Not applicable
			Greening options:
				Grey	x	Green-grey		Green
			Digitalization options
			x	Integrated Control	x	Localized control	x	Non-Digital
	NPR		Centralized NPR								Decentralized NPR
			Greening options:								Greening options:
			x	Grey	x	Green-grey		Green			x	Grey	x	Green-grey	x	Green
			Digitalization options								Digitalization options
			x	Integrated Control	x	Localized control	x	Non-Digital			x	Integrated Control	x	Localized Control		Non-Digital
Resources	RRR		Centralized RRR								Decentralized RRR
			Greening options:								Greening options:
			x	Grey	x	Green-grey		Green			x	Grey	x	Green-grey	x	Green
			Digitalization options								Digitalization options
			x	Integrated Control	x	Localized control	x	Non-Digital			x	Integrated Control	x	Localized Control		Non-Digital
Water Cycle	WCR		Centralized WCR								Decentralized WCR
			Greening options:								Greening options:
				Grey	x	Green-grey	x	Green			x	Grey	x	Green-grey	x	Green
			Digitalization options								Digitalization options
			x	Integrated Control	x	Localized control	x	Non-Digital			x	Integrated Control	x	Localized Control	x	Non-Digital

Notes: DPR—direct potable reuse; IPR—indirect potable reuse; UPR—unplanned potable reuse; NPR—non-potable reuse; RRR—resource recovery and reuse; WCR—water-cycle restoration. “x” indicates the typology combinations considered/applicable for each scheme (greening and digitalization options); blank cells indicate combinations not considered/not applicable.

.

The feasibility of each combination was informed by the literature. For instance, potable reuse scenarios necessarily rely on grey or green-grey infrastructures due to potable water quality requirements, while indirect potable reuse (IPR) is typically linked to large-scale applications due to the use of environmental buffers next to centralised treatment plants [7,47,48]. By contrast, NPR configurations allow greater flexibility in greening, especially at smaller scales, when source separation facilitates the use of green infrastructure for treatment [7,11]. WCR interventions were only considered when green infrastructure (functional, engineered, or natural green) is present. For more details about the classification of green infrastructure, see Box S2 in Supplementary Material.

Based on the feasibility screening, we selected ten circular intervention typologies (e.g., centralised DPR WMUs, decentralised DPR WMUs, decentralised NPR WMUs, centralised UPR WMUs, centralised RRR WMUs, centralised and decentralised WCR WMUs, etc.). For each typology, we compiled a cause–effect matrix in which key design aspects—relating to technological mechanisms, operational organisation, and contextual conditions—were listed and their positive and negative impacts on hydrosocial system functions were qualitatively assessed (Tables S3–S12). Within each typology, greening and digitalisation options were treated as design features that modulate specific effects (amplifying or mitigating them).

Trade-offs were considered when an intervention simultaneously advances some functionalities while causing regress in others, reflecting conflicting outcomes that require balancing. In contrast, synergies were identified when the combined effect of integrating multiple design strategies into one circular typology produces outcomes that exceed the sum of the individual strategies’ progress [23]. At the level of whole intervention typologies, we interpreted patterns of accumulated co-benefits as evidence of synergies between design strategies—for example, combinations of reuse, greening and digitalisation that jointly provide stormwater management, irrigation and ecosystem services. Such cases were associated with multifunctional design, whereas configurations with many competing impacts across functions were characterised by stronger trade-off profiles. This structured approach supported a qualitative assessment of how circular interventions influence hydrosocial system functionalities and overall system resilience. To visualise these influences, we used radar charts to depict impacts on hydrosocial functionalities and an alluvial diagram to represent impacts on the WEFE nexus and society. For the alluvial diagram, we first compiled an interlinkages matrix, identifying whether each hydrosocial functionality affects each component of the WEFE nexus and society directly or indirectly. Direct interlinkages were assigned a weight of 1 and indirect interlinkages a weight of 0.5; these weights were used to scale the accumulated synergy and trade-off scores and derive the flows from interventions to WEFE resources and society (see Table S2).

3. Findings

Drawing on the systematic literature review, we first develop the HRUN conceptual framework, describing the system structure, key environmental and socioeconomic functionalities, and their interactions with WEFE-related resources and society. We then use this framework to qualitatively assess how circular interventions affect hydrosocial functionalities and resilience.

3.1. Hydrosocial–Resource Urban Nexus

Urban water management is shaped by both technical constraints and socio-ecological dynamics. Systems thinking emphasises how elements of a system interact, exchange resources and information, and generate feedback loops and dynamic behaviour over time [19,39,49,50]. In the resource domain, this perspective is operationalised through the WEFE nexus, which highlights interdependencies between water, energy, food and ecosystems and the need to identify synergies and trade-offs in integrated planning [23]. In parallel, Social–Ecological Systems (SES) frameworks and hydrosocial studies stress that infrastructures, resource systems, actors and governance systems form coupled SES whose outcomes emerge from their interactions [16,17,44,51,52,53].

Building on this literature, we conceptualise the hydrosocial urban system as a complex adaptive system composed of multiple elements, grouped into environmental and socioeconomic subsystems tightly interlinked (Figure 3). This follows the SES view of systems composed of several subsystems and internal variables at multiple levels [51,52]. The ‘environmental subsystem’ includes natural components (water, energy, nutrients, ecosystems) and the built environment, comprising UWSs and the urban fabric shaping water demand, runoff and exposure. The ‘socioeconomic subsystem’ encompasses actors and governance systems [51,52]. This means residents, utilities, institutions, political actors and organised groups, as well as the cultural norms, behaviours and power relations that govern how water services are accessed, accepted and contested. It also encompasses the flows of funds and value associated with water—tariffs, investments, avoided damages and new business opportunities—which both enable and constrain adaptation and mitigation measures [19,40,44,53].

These subsystems interact through flows of water, energy, materials, information and funds [16,19,40] that connect UWSs with upstream and downstream water bodies and with wider WEFE and socio-political systems [52,54]. Natural conditions influence water availability and risks; social behaviour affects consumption, reuse acceptance and demand for protection; and economic choices shape which infrastructures and technologies are deployed. The tight coupling between ecological, social and technical elements fosters self-organisation and leads to emergent properties at system level, including resilience or vulnerability [3,44,55].

Within this hydrosocial system, the UWS forms the technical backbone and engineering system [56,57,58] for water supply, wastewater and stormwater management. We represent it as a set of Water Management Units (WMUs)—individual solutions physically connected by pipes and nature-based waterways that treat, infiltrate, filter, retain, convey or reuse water and wastewater resources [59]. The physical form and operational logic of the UWS are shaped by four main design strategies, which act as structural levers of the system (Figure 4):

• Circularity: the extent to which water and resource loops are closed within WMUs and the whole UWS. It ranges from linear flows to fully closed loops through different circular pathways of recovery, reuse and regeneration [24,25,28,60,61,62,63].
• Scale: refers to the degree of centralisation at unit and system level, from on-site and cluster-scale WMUs to medium- and large-scale centralised configurations [59].
• Greening: captures how far natural, engineered and functional green infrastructure is integrated into WMUs and the whole system, spanning a continuum from fully green to fully grey solutions [64,65].
• Digitalisation: expresses the extent to which data, automation and real-time control within WMUs and across the UWS support monitoring, optimisation and forecasting [6,34,36,53,66,67].

Each intervention in UWSs can be positioned along these four design axes, which represent a continuum of implementation levels offering multiple alternatives. Together, they determine how WMUs are distributed in space, how they interact with each other and with the environment, and how they make use of external resources and information. In the Hydrosocial Resource Urban Nexus (HRUN) framework, they are treated as exogenous design choices that reconfigure the hydrosocial urban system and, ultimately, its resilience. For more detailed definitions of categories within each axis (e.g., DPR, IPR, types of greening or levels of digital integration), we refer to Box S2.

To understand how these design strategies affect hydrosocial resilience, we organise the hydrosocial urban system into a multi-layered, nested structure (Figure 5) as detailed below:

System components—the hydrosocial urban system described in systems-thinking terms as elements, flows, rules, interactions and structure. This layer includes infrastructure, ecosystems, actors, institutions and the resource units within WEFE-related flows [3,19,39,44,51,55].

Hydrosocial functionalities—the term is not always defined consistently in the literature, but here it is understood as the set of services, tasks and processes that the system must sustain to remain effective under stress [68,69,70]. They are closely related to the notion of system services or “service standards” defined by the Safe & SuRe framework [56,57]. From a systems perspective, these functionalities are emergent properties of the coordinated operation of interdependent components [3,40]. In hydrosocial systems, they correspond to environmental and socioeconomic functions that the system is expected to perform. They represent the operational interface between technical design (e.g., changes in UWS configuration) and socio-institutional dynamics, such as behaviours, policies and organisational capacity [3,45,55,71].

Resilience attributes—generic, cross-cutting properties that describe how infrastructures and institutions are configured to support hydrosocial functionalities under stress [3,45,55,71]. Building on Butler’s notion of “system standards” [56,57], they capture design and operationally desirable qualities such as redundancy, modularity, diversity, connectivity or resourcefulness [55,70,72].

Resilience capacities—absorptive, restorative, adaptive and transformative capacities that describe how the system responds over time to shocks and long-term pressures, including a possible shift into a ‘new normal’ [44,71,73]. These capacities ultimately define system resilience.

Resilience performance—how the level of service associated with a given functionality changes over time, particularly during and after disturbances. Performance-based approaches represent this as a curve in which service levels drop after a shock (see graph on Figure 5). The depth, duration, and recovery trajectory of the drop are used to interpret how resilience attributes support system functionality [3,71].

The HRUN framework makes these functionalities explicit and uses them as the bridge between UWS design and resilience assessment. UWS design strategies modify system components; this reconfigures functionalities; and from the behaviour of these functionalities under stress we infer resilience attributes and capacities. Resilience is therefore conceptualised as a multi-layer, emergent property arising from the coordinated behaviour of the hydrosocial urban system.

To make this structure operational, we identify a set of hydrosocial functionalities from previous frameworks in the social and natural sciences, as described in Section 2.2. These functionalities are grouped into environmental and socioeconomic domains (Figure 6). Taken together, they provide a structured way to trace how interventions affect both environmental and socioeconomic resilience dimensions. For each functionality,

Table 2. Hydrosocial system environmental functionalities and WEFE interlinkages.

Hydrosocial Functionalities	Description				Interlinkages
Water Supply	Water supply from climate-dependent (fresh water) and non-dependent sources (wastewater). Functionality can be improved through the diversification of water portfolio through water reclamation and fit-for-purpose supply, increased landscape storage capacity, or by reducing losses (e.g., leakage) [41,42,74,75,76].				Water	Energy	Food	Ecosystem	Society
Net Energy Surplus	Ability of using low carbon-based energy matrix, to reduce energy intensity of process and augment energy recovery from wastewater [8,42,77].				Water	Energy	Food	Ecosystem	Society
Nutrients and Food/Biomass Production	Ability of recovering and/or reusing nutrients and other materials harvested from water/wastewater. It also refers to the ability to reuse recovered resources to produce food and biomass [11,13,32,41,42].				Water	Energy	Food	Ecosystem	Society
Restore Hydrological Cycle and Flood Mitigation	Ability to restore the landscape’s natural capacity to regulate water exchanges between the atmosphere, biosphere, water bodies and soil, while attenuating runoff and reducing flood risks. This is achieved through green or hybrid green-grey infrastructure [41,42,43,78].				Water	Energy	Food	Ecosystem	Society
Urban Ecosystem Regeneration	Ability to regenerate urban landscape, biodiversity, natural socio-ecological systems. It involves the regeneration of biotic resources and abiotic resources (air quality amelioration, cooling, soil recovery, natural water bodies recovery, cooling) [11,32,41].				Water	Energy	Food	Ecosystem	Society
Safe Resource Reuse, Disposal, and Discharge	Ability to properly treat water and recover embedded wastewater resources for various purposes, ensuring adequate quality for reuse, disposal, or discharge, thereby minimising risks to human health and ecosystems associated with their use and release [42,48,79].				Water	Energy	Food	Ecosystem	Society
Legend:		Direct interlinkages		Indirect interlinkages

Note: Cell background pink refers to direct interlinkage and green to indirect interlinkage.

and

Table 3. Hydrosocial system socioeconomic functionalities.

Hydrosocial Functionalities	Description				Interlinkages
Operation and Monitoring	Ability of utilities and other responsible actors to operate, maintain and monitor WMUs and the connecting infrastructure so that they perform as intended, given the complexity of the system and the degree of digitalisation adopted by the UWS [6,34,35,36,48,61,66,80]				Water	Energy	Food	Ecosystem	Society
Participatory and Cross-sectoral Planning	The capacity of institutional parties and individuals to be organised for making decisions over societal needs, based on participative process. The society capability to work coordinated to mitigate, adapt and recover from disruptions reflects the strength and quality of relationships and interactions between agents (social cohesion). Cross-sectoral decisions are made through the coordinated work between agents of different sectors, cities, or countries to achieve synergetic goals [11,13,27,42,60,81,82,83,84,85,86].				Water	Energy	Food	Ecosystem	Society
Learning and Behavioural Change	The ability to learn from past events and build knowledge among individuals and institutions to prevent and adapt to disruptions. This ability is closely related to governance elements of the system (perceptions, instruments and actions) that shape—and are shaped by—day-to-day practices, institutions and shared values. Learning also influences human behaviour, by reshaping values and perceptions, and strengthening knowledge and awareness. Behaviour, in turn, affects trends in water consumption, acceptance of water reuse and other aspects that determine how social actors interact with water (eco)systems. UWS design therefore influences cognitive resilience and human behaviour [19,41,43,44,46,59,60,81,87].				Water	Energy	Food	Ecosystem	Society
Environmental Justice and Wellbeing	Ability to realise rights under political conditions, promote water justice (prevent and minimise social inequalities and injustices related and sustained by water interactions) and other environment outcomes from ecosystem services provided by the water infrastructure. It is related to quality-of-life improvements in terms of mental and physical health due to changes in the landscape, increased biodiversity, and the reduction in climate change impacts [32,36,41,88,89].				Water	Energy	Food	Ecosystem	Society
Actors’ Agency	Agency is the ability of individuals or groups to act independently, mobilise resources and make their own free choices. It is associated with a resourceful system (natural and economical assets) in an inclusive society [44,90].				Water	Energy	Food	Ecosystem	Society
Investment and Financial Efficiency	Capacity to invest in structural and non-structural measures aimed at improving UWS services for all actors. It relates to the ability to avoid economic losses, ensure cost recovery and create value through the economic gains generated by water reuse, resource recovery and ecosystem regeneration processes [27,91].				Water	Energy	Food	Ecosystem	Society
Economic Development and Decent Employment	Capacity to generate business opportunities in the water sector (e.g., resource recovery and green-space services) and to diversify other economic activities (e.g., tourism, fishing and transport) through adequate governance and improved management of UWS infrastructure and natural assets [41,92].				Water	Energy	Food	Ecosystem	Society
Legend:		Direct interlinkages		Indirect interlinkages

Note: Cell background pink refers to direct interlinkage and green to indirect interlinkage.

describe its meaning and direct or indirect links to the WEFE and socio-political systems, emphasising that changes in UWS design propagate beyond the water domain. Additional details on these interlinkages are provided in Tables S1 and S2.

In line with resilience thinking in water governance [44], the HRUN framework understands resilience in three complementary ways. First, resilience as a property refers to attributes and capacities that a resilient system is expected to exhibit under stress. Second, resilience as a process captures the ongoing performance of hydrosocial functionalities—how infrastructures supply, reuse and restore water, recover resources and regenerate ecosystems, and how institutions plan, learn, coordinate and redistribute benefits and risks. Third, resilience as an outcome refers to the temporary states that emerge from these cross-scale interactions, aligned with the definition of “outcomes” in SES studies [51,52]. These outcomes reflect whose values and interests shape the rules of the system, which trade-offs are accepted, who is protected, and which cultural vision of a “resilient city” becomes the new status quo. They feed back into the system by reshaping rules, influencing future trajectories. Cognitive-ecological resilience [46] fits within this view, as people’s ability to adapt, regulate emotions and sustain meaningful relations with their environment depends on the properties and processes of the hydrosocial system in which they are embedded. In our terms, cognitive resilience is one expression of resilience-as-outcome, reflecting how the hydrosocial system is felt and lived by people. By focusing on hydrosocial functionalities and on how design choices in scale, circularity, greening and digitalisation reconfigure them, the HRUN framework helps to analyse how structural and non-structural interventions can change these cognitive outcomes.

In this study, the HRUN framework is applied qualitatively to assess how UWS design strategies reconfigure hydrosocial functionalities and, through them, resilience attributes and capacities. For each circular intervention typology, we identify potential synergies (e.g., when a decentralised reuse–green solution simultaneously reduces potable demand, mitigates floods and enhances urban cooling) and trade-offs (for example, when decentralised facilities are introduced without adequate participatory planning and operational support, limiting local agency and leading to difficulties in safe operation, and new inequalities in service provision).

Conceptually, HRUN advances the state of the art by specialising SES and hydrosocial thinking for urban water design decisions and making explicit the direct and indirect interlinkages with other resource and socio-political systems for each functionality. It clarifies the relationships between system components, hydrosocial functions, resilience attributes, capacities and performance in a coherent, system-consistent language. The framework also integrates environmental and socioeconomic effects of circular strategies within a single structure that can be used both conceptually and in decision-support tools. From a planning and policy perspective, HRUN offers a common language for utilities, planners and other stakeholders to discuss how UWS interventions contribute to broader resilience goals.

3.2. Assessment of Synergies and Trade-Offs on Hydrosocial System Resilience

This section presents the qualitative assessment of synergies and trade-offs resulting from different circular intervention typologies, derived from combinations of UWS design strategies (see Figure 4). The analysis is organised by circularity purpose, distinguishing three circular pathways for WMUs: water reuse, resource recovery and reuse (RRR), and water-cycle restoration (WCR). For each pathway, we examine how synergies and trade-offs affect hydrosocial functionalities and shape outcomes within the WEFE nexus and society.

3.2.1. Circular Interventions for Water Reuse

Water reuse strategies are central to circular urban water systems, enabling the diversification of water sources and reducing pressure on freshwater abstraction. This category includes four key configurations: Direct Potable Reuse (DPR), Indirect Potable Reuse (IPR), Unplanned Potable Reuse (UPR), and Non-Potable Reuse (NPR). These interventions operate across different degrees of (de)centralisation, greening and digitalisation.

DPR and IPR are planned potable reuse strategies, typically centralised, using advanced treatment technologies. DPR can be membrane-based (e.g., RO followed by AOP) or non-membrane-based (e.g., ozonation, Biologically Activated Filtration), with the former offering greater contaminant removal but higher energy use (0.23–2.5 kWh/m³ vs. 0.16–0.5 kWh/m³) [47,48,93]. IPR uses environmental buffers for natural attenuation, improving health protection, but adding about 0.48 kWh/m³ due to extra conveyance. DPR without redundant drinking water treatment and non-membrane based IPR represent the lowest energy options for planned potable reuse [93]. UPR emerges from unplanned reuse, when downstream potable water systems abstract upstream-treated wastewater. UPR benefits from gravity-driven flows and low energy intensity (~1.22 kWh/m³) [84] but poses public health risks due to the absence of engineered barriers or real-time monitoring [48,62,79,94]. IPR and UPR support basin-scale regulation, coordinated water allocation, and can support maintenance of ecological flows in scarce basins [62]. However, UPR lacks traceability and formal risk management.

NPR systems serve non-potable applications like irrigation and industrial use. Centralised NPR units are highly suitable to supply high-volume consumers near treatment plants, maintaining economies of scale through high volumes and reducing dual-piping requirements due to shorter distances [24]. Decentralised NPR enables stream segregation and local reuse, allowing short conveyance distances and gravity-driven supply from elevated areas to users at lower elevations [95]. Energy demands on centralised NPR interventions typically range from 0.1 to 0.84 kWh/m3 [96,97,98,99]. MBR-based decentralised NPR is energy-intensive (up to 7.2 kWh/m³) [27,96,98], but nature-based systems, such as high-rate algae ponds followed by wetlands, can operate at 0.10 kWh/m³, while offering co-benefits (e.g., nutrient recycling and biomass production) [100].

Greening degree varies across reuse types. NPR offers the highest greening potential, especially in decentralised applications that integrate wetlands or vegetated buffers. IPR uses natural buffers (e.g., aquifers, reservoirs) that support ecosystem functions. DPR is typically grey, requiring advanced polishing stages to meet potable standards, but can integrate green pre-treatment units [7,48,93].

Digitalisation is essential for planned potable reuse. DPR and IPR require real-time monitoring, predictive control, and automated barrier systems [6,63]. In decentralised NPR, digitalisation supports adaptive system optimisation, flow management, and multi-source integration. Combining centralised DPR and decentralised NPR increases operational complexity and requires robust digital infrastructure, adding capital investment and cybersecurity risks [9,12,36,101,102,103].

Tables S3–S8 summarise the impacts of reuse interventions across hydrosocial functionalities. All configurations contribute to diversifying water supply portfolio, reducing pressure on freshwater sources. NPR shows strong co-benefits in ecosystem regeneration and nutrient recovery, especially when associated with green infrastructure. For instance, irrigation with non-potable water, when associated with urban agriculture, can support food/biomass production and contribute to social equity by democratising access and strengthening community cohesion. DPR and IPR contribute to public health resilience and reliability, especially in water-scarce regions, but may involve trade-offs due to high energy demand, infrastructure costs, and public acceptance barriers. Moving from unplanned to formal potable reuse schemes with multibarrier treatment and real-time digital monitoring improves safety and accountability but typically increases system complexity and energy demand. At the same time, while reuse generally demands more energy than conventional discharge, it remains less energy-intensive than desalination and can lower total energy consumption by decreasing the need for water transfers from distant sources [47,93]. Therefore, viability must be assessed relative to the current solution in place. This analysis should consider the impact on the system’s net energy surplus and associated greenhouse gas emissions, which depend on infrastructure configuration, freshwater sources characteristics, and the energy matrix.

At the basin level, reuse configurations must also consider systemic trade-offs. For example, centralised reuse may reduce pollutant discharge but simultaneously limit downstream water availability where WWTP effluent contributes to baseflows [62]. Decentralised reuse, while enhancing equity and modularity [33,95], suffers from scale inefficiencies, and increases operational and regulatory burdens due to the dispersion of monitoring points.

In summary, reuse interventions can produce substantial co-benefits when suitable to local context, infrastructure readiness, and hydrosocial priorities. DPR and IPR are most appropriate in centralised systems operated by institutions with high technical capacity, particularly in areas under acute water scarcity. In contrast, NPR can be implemented in centralised settings—especially when large-scale users are located near treatment facilities—or in decentralised, modular configurations that prioritise local empowerment, spatial adaptability, equitable access to underserved areas, and co-benefits from integrating green infrastructure.

3.2.2. Circular Interventions for Resource Recovery and Reuse (RRR)

Circular WMUs for Resource Recovery and Reuse (RRR) transform wastewater into valuable secondary resources—such as nutrients, biosolids, biogas, and thermal energy—thus closing resource loops, enhancing environmental sustainability, and reducing dependency on exogenous energy sources and synthetic fertilisers, improving self-sufficiency across the WEFE nexus [11,12].

RRR strategies span centralised and decentralised configurations. In centralised systems, economies of scale and stable inflows support the use of mature technologies such as anaerobic digestion, struvite crystallisation, and biosolids valorisation for nutrient and energy recovery [26,82]. However, these systems face challenges related to resource dilution and logistics for redistribution [24].

Decentralised RRR systems can achieve higher recovery efficiencies by segregating streams and separating sources. Urine-diverting systems recover over 50% of domestic bioavailable nutrients and reduce losses in sewers [13,104]. They are especially effective for localised nutrient reuse, supporting food production and soil regeneration in community gardens and peri-urban farms. Lifecycle assessments show that urine-diverting and composting systems yield lower environmental and economic impacts than centralised treatment [24]. However, their effectiveness depends on community acceptance and engagement, regulatory clarity, and biosafety controls.

Energy recovery is also feasible across scales. At large scales, anaerobic digestion is the most widespread and mature technology, enabling biogas generation and, in some cases, energy self-sufficiency. In some WWTPs, increasing scale has more than doubled the energy output-to-input ratio [24,82]. Thermal energy recovery, using heat exchangers or pumps, is gaining traction due to the high heat content of municipal wastewater—up to 2.5 times its chemical energy content [8]. Technologies such as Aquifer Thermal Energy Storage (ATES), though promising, demand strict water quality monitoring and high institutional capacity [105,106].

At smaller scales, options such as wastewater heat exchangers and micro-hydropower systems (e.g., pumps as turbines) can be integrated with smart meters for leak detection and energy recovery [24,80]. Compact digesters processing blackwater can produce biogas but present operational and biosafety challenges at the household or building scale [13,26,82]. In such decentralised settings, co-digestion with food waste can significantly increase biogas yields and, in some cases, make these alternatives energetically viable [107].

Green infrastructure plays a dual role in decentralised RRR, acting both as a treatment mechanism and as the point of use for recovered resources. Nature-based solutions like high-rate algae ponds, wetlands, and composting units allow nutrient-rich water to be reused in short cycles [13,100]. In this way, green infrastructure for RRR contributes to multifunctional design, being especially suited for fertigation and edible landscapes [11,12].

Digitalisation further enhances RRR efficiency and safety. In centralised WMUs, real-time control, biosensors, and process automation optimise energy and nutrient recovery. In decentralised WMUs, smart irrigation, soil monitoring, and leak detection improve fertigation practices and reduce operational risks, such as leaching, salinity build-up, and microbial risks [37,80,108]. Micro-hydropower systems used to control network pressure offer dual benefits of energy recovery and network monitoring, with positive outcomes on leakage reduction [80]. These technologies require investment and skilled operation but hold significant potential to improve system responsiveness and learning in community-managed systems [7,12,109,110].

Tables S9 and S10 summarise the RRR intervention impacts across hydrosocial system functionalities. Centralised RRR enhances energy recovery, safe resource reuse, and nutrient recovery, particularly when recovering biogas and biosolids. Decentralised RRR adds more value when coupled with water reuse in green spaces, especially if employed in community-scale urban farms, potentially strengthening environmental justice, actors’ agency, and building capacity [12,104].

However, important trade-offs must be considered. For instance, optimising biogas yield can reduce the nutrient recovery efficiency of the digestate [13]. Decentralised RRR of energy and nutrients reduces organic and nutrient loads in sewers, decreasing risks of hydrogen sulphide formation and pipe corrosion [9,111], on one hand improving circularity and network operation safety, but on the other hand increasing operational complexity, and biosafety concerns. Fertigation reduces fertiliser needs but lacks the controlled release of commercial fertilisers and carries leaching risks if not supported by precision irrigation [11,112]. Likewise, thermal energy recovery at small scale can be hindered by downstream fouling, variability in flows, and maintenance barriers. Additionally, all RRR pathways must address potential contamination from emerging micropollutants such as pharmaceuticals, microplastics, and industrial compounds—particularly when targeting reuse in food production [113].

Centralised RRR is suited to cities with institutional capacity and viable markets for recovered products [114]. Decentralised systems perform well where land, proximity, and modular reuse are possible, especially in new developments or peri-urban areas [33]. User acceptance, policy clarity, and coordinated governance are key to operational success. Ultimately, RRR interventions enhance hydrosocial system resilience when designed to balance efficiency, equity, and circularity. Their multifunctional benefits justify greater integration into urban water planning, particularly when targeted toward local food security, climate mitigation, energy self-sufficiency goals, and to strengthen community cohesion.

3.2.3. Circular Interventions for Water-Cycle Restoration (WCR)

Circular WMUs for WCR aim to re-establish disrupted hydrological processes in urban areas through nature-based and hybrid systems that promote infiltration, evapotranspiration, flow regulation, and ecosystems regeneration. These interventions are fundamentally rooted in greening strategies—natural, engineered, and functional—which represent foundational components for managing runoff, restoring ecosystem connectivity, and mitigating Combined Sewer Overflows (CSOs) [11,32,42].

Centralised WCR interventions, such as floodplain reconnection, constructed wetlands, and large detention basins, are typically located near rivers or flood-prone zones, but can also consist of large green patches with sizes larger than 10 ha, commonly called urban forests [115,116,117]. These systems provide multifunctional stormwater storage, reduce peak flows, and buffer pollutant loads before discharging to natural water bodies [118,119]. Engineered floodplains and CSO wetlands often combine green and grey elements—vegetated zones with gates, storage tanks, or sedimentation units—to achieve greater control and treatment reliability [120]. While these systems offer substantial hydrological and ecological benefits, they are land-intensive and best suited to peri-urban settings, where their extensive footprint can also be leveraged for nutrient and resource recovery practices [117]. In contrast, decentralised WCR systems—such as rain gardens, green roofs, permeable pavements, and bioswales—are embedded in the built environment and operate at building or neighbourhood scale. They emphasise local retention and infiltration, reducing source runoff, protecting headwaters, and enhancing microclimatic regulation [38,121]. Their integration within urban morphology improves connectivity, mobility, and environmental justice outcomes by equitably distributing green space benefits across neighbourhoods [122,123]. However, their limited capacity and fragmentation can constrain hydrological performance unless green connectivity (e.g., green corridors and green based conveyance systems) is achieved across the city [124,125].

Empirical evidence on green space size and configuration refines this comparison between centralised and decentralised WCR. Trade-offs include increased irrigation demand with higher green space coverage, and elevated erosion risk or marginal service returns at larger patch sizes. For example, the capacity to store more carbon per area and support more pollinators increases for patches up to 10 ha, while patches larger than 10 ha exhibited no additional areal benefit [115,116,117]. At the same time, 10 ha appears to be the minimum size required to effectively support bird communities [115]. These findings suggest that large and well-connected networks of smaller green spaces play complementary roles in balancing synergetic effects across functionalities.

The adoption of green infrastructure remains the central design axis of WCR interventions. Natural green infrastructure (e.g., floodplain forests) maximises long-term ecosystem services, but they typically require large land areas and extended maturation periods. Engineered and functional green infrastructure (e.g., wetlands, green walls, green roofs) provide flexibility in compact settings, offering precise infiltration, cooling, and pollutant removal while adapting to specific urban typologies [11]. Hybridising with grey infrastructure—e.g., filtration units, storage tanks, or flow regulators—enhances system performance, especially under high-flow conditions [101,126], bridging ecological ambition and operational feasibility under escalating climate pressure [120].

Digitalisation further extends the adaptive capacity of WCR systems by enabling real-time control, performance monitoring, and climate forecasting. Interconnected smart sensors facilitate the dynamic regulation of stormwater flows, predictive irrigation, and autonomous gate/pump actuation, particularly in hybrid green-grey systems [12,66,127]. Examples such as smart rain barrels, infiltration well sensors, and monitors on constructed wetland offer scalable, decentralised control with enhanced responsiveness during storms or dry seasons [128,129,130]. Although challenges such as sensor fouling and data variability persist, digital tools reduce operational burdens, support resilience, and strengthen agency by enabling centralised supervision of distributed units in the network [6,12].

Synergies and trade-offs of centralised and decentralised WMUs designed for WCR are shown across the hydrosocial functionalities in Tables S11 and S12. From a functional perspective, WCR WMUs restore key components of the hydrosocial system, including water retention, infiltration, and ecosystem regeneration. They alleviate pressure on drainage infrastructure, reduce CSOs, and support aquifer recharge, while also contributing to biogeochemical cycling and habitat connectivity [11,42,119]. In decentralised configurations, they can enhance urban resilience through localised cooling and groundwater replenishment, and promote higher spatial distribution, facilitating equitable access [42,131]. WCR units maximise synergies across the WEFE nexus when combined with nutrient reuse strategies. For example, using green spaces for stormwater management and food production can potentially increase energy savings, and produce positive outcomes on human well-being [11,12,43].

Trade-offs must still be acknowledged. Larger WCR units may displace benefits spatially, contributing to green gentrification unless equity planning is embedded. Small-scale systems may underperform when fragmented or disconnected from ecological networks. Climatic variability may intensify evapotranspiration, raising irrigation demand during droughts [132]. Furthermore, large green spaces (over 10 ha) may carry increased ozone exposure or require soil amendments to mitigate erosion risks [117,133].

Nonetheless, when strategically implemented, WCR WMUs deliver co-benefits across multiple domains. They mitigate flood risk, restore hydrological cycles, regenerate ecosystems, and enhance public health through improved access to nature. They also serve as instruments for climate adaptation, system learning, and participatory planning—shifting water management from purely technical solutions to socio-ecological resilience strategies [89,134,135,136,137,138].

4. Practical Implications and Policy Relevance

The HRUN framework is applied to synthesise how the four design strategies jointly shape hydrosocial functionalities and, through them, socio-ecological resilience. Building on the preceding analysis, Figure 7 summarises, via radar charts, the total synergies and trade-offs obtained from the cause–effect matrices for each circular intervention typology. At the same time, these effects are redistributed across resources within the WEFE nexus and socio-political systems using alluvial diagrams (Figure 8). The flows are obtained by weighting direct and indirect interlinkages between hydrosocial functionalities and WEFE–Society components (see Table S2). These matrices are based on literature-informed design aspects and their positive and negative impacts on hydrosocial functionalities (see Tables S3–S12) as previously explained in Section 2.3.

Scale conditions how resources are mobilised, treated and redistributed. Centralised WMUs reduce unit costs and energy intensity at the treatment stage and simplify operational control, but they require long conveyance distances, leading to higher pumping needs and to complex water-quality matrices. Dilution of pollutants and nutrients can hinder efficient resource recovery and complicate quality control, particularly when inflows from multiple upstream sources are mixed [24,25,139]. Yet, large-scale recovery plants can more easily guarantee product quality and long-term process stability [13,26,82]. In addition, some recovery techniques are only feasible above minimum flow thresholds, which favours the adoption of larger-scale systems [13,25,140]. Decentralised configurations, in contrast, bring services closer to users and resource sources. Modular layouts allow flexible reconfiguration and make it easier to match local supply and demand, especially when topography is used to take advantage of gravity for reclaimed water distribution [33,141]. However, achieving equivalent total recovery volumes requires a greater number of units, which increases monitoring and maintenance needs, complicates risk assessment and may lengthen permitting procedures when many small WMUs must be licenced [33,48,74,79]. Digital devices often become essential to manage this distributed complexity [6].

Circular interventions should adopt different management scales according to the flow type (greywater, blackwater, rainwater, urine, sludge), leading to hybrid UWS that combine centralised and decentralised WMUs [74]. Source-separation schemes that treat greywater, rainwater, urine and sludge at different scales reduce mixing losses, enable higher recovery efficiencies and deliver multiple fit-for-purpose water qualities, while centralised assets can remain focused on potable supply or bulk treatment, taking advantage of investments already made [6,33,36,101,102].

The optimal management scale depends on the water stream, the circularity purpose (DPR, IPR, UPR, NPR, RRR or WCR), the quality requirements and the local hydrosocial context (infrastructure legacy, hydrogeology, demand profiles, community acceptance and governance capacity). Building on the optimisation framework proposed by [98] for identifying the decentralisation scale that minimises key performance indicators (e.g., energy use, GHG emissions and costs) for non-potable reuse; Figure 9 generalises this concept to multiple circular pathways. We use conceptual curves to show at which scale these indicators are minimised, and how this optimum scale shifts across DPR, IPR, UPR, NPR and WCR (panel a) and across different RRR options (panel b). Potable reuse (DPR, IPR) tends to favour larger, centralised WMUs, due to stringent potable standards that increase per-unit treatment costs [25,28,47,97]. The efficient scale range tends to move downwards for UPR, NPR and WCR. In UPR, lower water-transfer costs (downstream reuse without pumping) make higher degrees of centralisation more attractive. NPR interventions favour more decentralised implementations: lower quality targets reduce treatment energy intensity, while source separation and dual piping increase transport costs. Unlike conventional reuse pathways, WCR typically achieves lower operational costs at smaller scales; it decreases runoff entering drainage systems, reducing pumping needs and CSOs during rainfall events. For RRR, suitable scales depend strongly on the type of resource to be recovered and on whether the benefits of source separation (e.g., reduced losses, avoided dilution) outweigh the advantages of higher total recovery volumes, while still meeting safety requirements at higher degrees of decentralisation.

The greening design strategy relates both to the process of renaturing cities (through greenspace creation) and to the distributed implementation of multifunctional treatment facilities operating at different degrees of decentralisation. Adopting green infrastructure for water treatment typically entails lower treatment intensity, making decentralised scales technically viable and shifting the optimal scale point to the left in Figure 8. At the same time, these interventions require a larger spatial footprint per unit of service, which can constrain their deployment in dense urban fabrics and intensify competition for land [142]. Stormwater management through green infrastructure is particularly suitable for neighbourhoods with high levels of sealed surfaces [143,144], as it helps restore the hydrogeological functioning of urban landscapes [11,32,42,145]. Nevertheless, managing large stormwater volumes often requires interventions at larger scales. River and stream restoration with floodplains—including, where appropriate, hybrid green-grey alternatives that combine restored floodplains with grey infrastructure—can be critical to reducing flood risks. This multiscale, hybrid perspective is echoed in the Water Europe Vision, which identifies as one of its five innovation concepts the creation of a “resilient and reliable hybrid grey and green water system, designed to withstand severe external and internal shocks—such as climate-change induced floods and droughts—without compromising essential functions” [101].

Greening also has profound social implications. Larger green spaces offer richer nature experiences, deeper immersion, and broader recreational opportunities. These improved amenities encourage people to walk longer distances and spend more time outdoors [146]. Everyday nature exposure and active mobility require a complementary network of smaller, well-distributed green spaces that are closely integrated with daily destinations such as shops and schools [134]. When green spaces are accessible within walking distance, they can effectively reduce car dependency and support healthier, more sustainable urban lifestyles [134,146]. Small-scale green infrastructure, though limited in multifunctionality due to spatial constraints, are vital in reducing urban fragmentation and enhancing connectivity for both people and ecosystems. In densely built areas, supplying a mix of large and small green spaces, pedestrian zones, and slow traffic areas mitigates the potential compensatory behaviours observed in compact cities, where individuals may seek nature contact through long-distance travel on weekends and holidays—a phenomenon that can neutralise the environmental benefits of urban compactness [123].

As seen, renaturing cities produces positive effects on climate-change mitigation and adaptation, as well as improvements in citizens’ quality of life [32,65,89,134,147,148]. Trade-offs are mostly associated with competition for water supply, especially on water-scarce zones [132]. Other trade-offs are associated with increased operational complexity and environmental justice; for example, depending on how they are implemented, interventions can either improve equity or act as drivers of green gentrification [122].

In this context, non-structural measures and multifunctionality play a key role. For example, fertigation units that employ green infrastructure as part of the treatment can recover and reuse water and nutrients from wastewater, and potentially produce food, if under adequate risk assessment [5,12,107,110]. Multifunctional designs that integrate green infrastructure also present significant influence over social systems, by deepening the relationships between people and place [149].

The human affinity for natural environments suggests that design strategies promoting routine contact with soils, water, and vegetation can alleviate stress, enhance cognitive functioning, and improve mental and physical well-being [89,134,135]. When circular UWS interventions embed accessible green spaces into the urban fabric, they provide more than hydrological or ecological services—they create conditions for regular, embodied experiences with nature. These repeated interactions foster environmental familiarity, promote place-based identity, and reinforce pro-environmental attitudes, which are foundational to improve positive behavioural change, participatory engagement, and well-being.

To further enable these outcomes, policymakers should support diverse governance models—especially common-property arrangements—that empower communities to co-manage shared green infrastructure [136]. Urban green commons not only function as socially neutral meeting grounds, but also anchor community routines (e.g., gardening, stewardship, education) that build local knowledge, reinforce intergenerational learning, and foster a sense of collective responsibility. By expanding access to shared natural resources and social networks, these spaces provide tangible means and collective support for individuals to pursue personal and shared goals, fostering actors’ agency.

This aligns with the notion of cognitive-ecological resilience, which recognises that people’s resilience capacity is shaped by the broader social-ecological context—including UWS design itself [46]. In HRUN terms, community-accessible green spaces serve as a socio-technical interface that simultaneously enhances actors’ agency, behavioural change, and cross-sectoral coordination—particularly when supported by inclusive policies and digital tools that facilitate shared monitoring and decision-making [12,81,122]. At the same time, bottom-up participatory design processes reduce failure risks, by fostering awareness, preventing cultural mismatch or loss of place identity, and enabling the integration of traditional and local knowledge into design choices. When these processes reflect local values and lived experiences, they also strengthen social cohesion, foster emotional attachment, and increase satisfaction with outcomes [81,150]. These dynamics are especially critical when circular interventions involve the creation or retrofit of green infrastructure, where socioeconomic functionalities play a pivotal role in determining their long-term success. Moreover, when communities are involved in the design, implementation, and maintenance of these infrastructures, such interventions can catalyse new forms of economic development and decent employment, reinforcing both resilience and equity [41,92].

Digitalisation underpins operation and monitoring functionality. Sensors, remote control and data platforms enable real-time observation of distributed circular WMUs, support demand-side management, enhance leak detection and allow the implementation of digital twins for optimised control [6,36]. These tools are especially important when adopting modular treatment trains, multi-barrier risk management and complex hybrid configurations [6,61]. Demand-side management, coupled with district metered areas and micro-hydropower generation from pressure control, can potentially decrease energy inputs without compromising performance [34,37]. At higher digitalisation degrees, forecasting demand and the fraction of wastewater generation diverted to reclamation factories can reduce problems with flow variability.

Digitalisation can reinforce several resilience attributes—such as responsiveness, redundancy in monitoring and adaptive capacity—but it also introduces new vulnerabilities and ethical challenges. Data-driven solutions often rely on stable connectivity, skilled staff and robust data governance; where these conditions are absent, performance may be uneven and false alarms or blind spots may emerge. Moreover, questions of data ownership, algorithmic transparency, workforce displacement and the potential for disproportionate surveillance or pressure on vulnerable users raise concerns about digital justice [22,34,120,151,152]. If not carefully governed, digital tools intended to improve efficiency and safety may restrict actors’ agency, reduce trust and deepen existing inequalities—for instance, if used to support policies that control water consumption more strictly to low-income households.

The design of UWS interventions influences how social systems perceive, engage with, and adapt to engineered and ecological environments. The more cognitive resilience is supported through inclusive, embedded, and nature-integrated design, the more adaptive and participatory the system becomes. When circular interventions are spatially embedded within communities and integrate green infrastructure, they not only promote systems’ flexibility and redundancy, but also expand opportunities for stewardship, social learning, and attachment to both natural and physical assets of UWS [81,150]. Digital technologies, when designed for transparency and user engagement, can complement these interfaces by supporting shared oversight, feedback loops, and interconnectivity between places, people, and resources. Circular UWS configurations also increase opportunities for entrepreneurship, expanding new markets linked to the water, nutrients, food, and energy [10,103].

HRUN can be used as an analytical framework to guide the design of circular interventions, organised into six main steps: (i) Clarifying circularity purposes and WEFE demands, by identifying which resources and services are most critical in the given context (e.g., water supply, nutrients, energy, cooling, flood protection, community capacities) and how they relate to local challenges and vulnerabilities; (ii) Selecting priority circular pathways by determining, on the basis of this diagnosis, which combinations of water reuse, RRR and WCR should be prioritised at city and neighbourhood scales; (iii) Locating and specifying WMUs by identifying suitable and priority sites using spatial analysis techniques that include indicators from environmental and socioeconomic domains, and then selecting technologies consistent with the circularity purpose; (iv) Defining the optimal water-management scale and configuration by choosing, for each pathway, the decentralisation scale and hybrid arrangement that minimise energy use and cost while supporting targeted hydrosocial functionalities and respecting equity and governance constraints. Figure 8 illustrates typical scale–performance trade-offs for different pathways. Examples of suitability variables used to define the water management scale can be found in [59]; (v) Assessing trade-offs by evaluating how benefits and burdens are distributed across social groups and territories. Particular attention should be given to whether vulnerable communities gain or lose in terms of service reliability, environmental quality, agency and health. These activities should preferably be conducted through a participatory process so that different perspectives are considered. To deepen understanding of global synergies and trade-offs, this step may be supported by qualitative analysis and visualisation tools, such as cause–effect matrices, causal loop diagrams, network interlinkages and radar charts [153,154], and by models focused on studying potential implications for different social groups (e.g., agent-based modelling, system archetypes) [155,156]. Qualitative insights should preferably be combined with quantitative tools, such as those proposed by the Multi-Sectoral Water Circularity Assessment Framework [54] (e.g., multi-sectoral material-flow, lifecycle studies and hydro-biogeochemical models); (vi) Maximising gains by combining structural and non-structural strategies. These strategies modulate outcomes through greening and digitalisation, increase interconnectivity between space, processes and people, and reduce social trade-offs through participatory planning and policies that improve cross-sectoral outcomes. For example, by adding substantial amounts of new green space in low-income, underserved communities while protecting nearby affordable housing to avoid gentrification issues [122].

A strong normative agenda has emerged to guide the circular transition in urban water systems. Globally, the 2030 Agenda for Sustainable Development—particularly SDG 6—calls for integrated water resources management, improved water quality and strengthened governance. The European Union has translated these ambitions into policy packages such as the Green Deal, the Circular Economy Action Plan, the Biodiversity Strategy and the forthcoming Blue Deal. In the water domain, instruments including the Water Framework Directive, the European Water Resilience Strategy and the revision of the Urban Wastewater Treatment Directive [157,158] promote circularity, ecological restoration, digitalisation and stakeholder collaboration. They introduce binding targets, including the elaboration of wastewater management plans, prioritising stormwater source control, expanding water reuse and nutrient recovery, and moving treatment towards energy-neutral, digitally optimised systems. For example, Spain, like other countries exposed to droughts and intense rainfall, has further mandated integrated wastewater management plans [159] (‘PIGSS—Planes Integrales de Gestión del Sistema de Saneamiento’) that adopt decentralised, climate-resilient solutions and hydraulic performance indicators to reduce CSOs and water-quality impacts.

Building on this agenda, our framework also operationalises key circular economy (CE) principles in the urban water domain [160]: (i) Regenerate Natural Capital by linking CE ambitions to water-cycle restoration and urban ecosystem regeneration; (ii) Keep Resources in Use by making explicit the analysis of water, energy and nutrient recovery across scales; and (iii) Design Out Waste Externalities by identifying cross-sector trade-offs and distributional effects. On this basis, the framework helps to operationalise the emerging policy agenda at city scale. By combining a hydrosocial–WEFE conceptual framework with a cause–effect assessment of circular intervention typologies, our analytical framework makes explicit how different circularity pathways, degrees of decentralisation and mixes of green and grey infrastructure, together with varying levels of digitalisation, redistribute benefits and risks across water, energy, food, ecosystems and society. In doing so, it helps avoid burden shifting between sectors—for example, reducing water scarcity at the expense of higher energy demand or nutrient losses—and instead guides the participatory co-design of coherent portfolios of circular interventions that advance water resilience, decarbonisation and just urban transitions.

Within this architecture, the socioeconomic dimension foregrounds environmental justice—in its distributive, procedural, recognitional, restorative and intergenerational forms [161]—and links structural design changes in UWSs to their effects on actors’ cognitive-ecological resilience. As interventions reshape UWSs, they simultaneously reconfigure the hydrosocial system—understood as the SES in which actors are embedded—by altering established relations between people, institutions and places, as well as the associated governance processes. In doing so, the framework shows that socioeconomic functionalities such as agency, participation, capacity building, employment and inclusive economic development are not ancillary to environmental performance but a precondition for UWSs to remain resilient and capable of adapting under increasing pressure and, when needed, transforming into a ‘new normal’.

5. Conclusions

The impacts of UWS interventions on hydrosocial system functionalities are highly contingent on the systemic alignment across design strategies—scale, circularity, greening and digitalisation. Hydrosocial functionalities such as nutrient recovery, urban ecosystem regeneration, actor agency and safe resource reuse do not emerge from isolated interventions, but from the coordinated interplay of design choices. The HRUN framework makes these interdependencies explicit by linking the configuration of circular interventions to cross-sectorial synergies and trade-offs in hydrosocial resilience. Decentralised configurations with integrated greening and digital monitoring can foster multifunctionality, participatory planning and distributive environmental justice, yet often introduce operational complexity and financial inefficiencies. Conversely, centralised systems can enhance energy recovery, investment efficiency and operational control, but may dilute nutrients, limit actor engagement and weaken local ecosystem connectivity. Ultimately, hydrosocial resilience is not the sum of technical optimisations, but an emergent capacity that navigates these trade-offs, balancing environmental, social and economic goals across scales and sectors.

Building on this perspective, the HRUN framework offers a structured basis for rethinking how we plan and govern urban water systems. By integrating hydrosocial thinking with the WEFE nexus, it answers the need for a unified lens that connects environmental, social and economic elements of UWSs to their systemic outcomes. The framework shows how different circularity purposes (water reuse, resource recovery and reuse, water-cycle restoration), degrees of decentralisation and mixes of green, grey and digital infrastructure redistribute benefits and risks across water, energy, food, ecosystems and society. In doing so, it helps to make trade-offs visible, avoid burden shifting between sectors and support the co-design of coherent portfolios of circular interventions. Within this architecture, socioeconomic functionalities—such as participation, learning, agency and inclusive economic development—are recast as preconditions, rather than by-products, of resilient hydrosocial systems.

Future work should translate this qualitative, heuristic framework into quantitative and participatory tools that can be applied in diverse urban contexts. Priorities include applying metrics to quantify WEFE synergies and trade-offs, testing how different decentralisation and governance models affect equity and resilience, and exploring how digitalisation can support adaptive management without undermining digital justice. Comparative applications in cities with contrasting hydrosocial conditions would help refine the typology of interventions and assess the transferability of HRUN. By embedding systemic trade-off analysis into UWS planning, the framework can support more integrative approaches to water governance—where circularity becomes a guiding strategy, and where links to energy, food, ecosystems and society are explicitly considered. Planning with these interdependencies in mind is essential to advance just transitions towards circular and resilient urban water systems that benefit both present and future generations.