Balancing security, resilience, and sustainability of urban water supply systems in a desirable operating space

Urban water system trajectories evolve through several phases, starting from the provision of basic services ('water supply city') for protecting human health, to a 'water sensitive city' for protecting environmental health [1]. Arden and Jawitz [2] propose phases following a hierarchy of human needs beginning with human health issues with the need for water supply distribution and treatment, sewage collection and treatment, and the need for stormwater management to protect environment and economy, followed by the need for pollution prevention for the protection of local and global environment. The authors connect each stage in the hierarchy with increasing system complexity and associated costs. We adopt the notion of increasing complexity in the evolution of urban water systems, and integrate this with evolving spatial and temporal boundaries, as UWSS move from security (provision of services), to resiliently responding to shocks, and finally achieving local and global sustainability.

The security of UWSS focuses on local, present conditions and has clearly defined spatial, temporal and sectoral system boundaries that contain the natural, engineered and human elements required for system functioning (figure 2, top layer). These elements (or 'capitals', see below) are accounted for in the assessment of UWSS security [20].

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Persistence during and recovery from shocks requires buffering capacity, e.g. water drawn from diverse sources during a drought, often from beyond the local system boundaries, and which are allowed to replenish in the absence of shocks [20, 21]. This stretches the spatial and temporal scales of resilience and requires the consideration of additional inter-sector dependencies. Cross-sectoral linkages account for the risk of drinking water contamination resulting from the lack of sanitary infrastructure and inadequately maintained water distribution pipes, and supply gaps due to electricity failures (figure 2, middle layer).

Legacy effects of decisions taken in the past can constrain system sustainability and governance strategies implemented today will influence long-term sustainability [22]. Externalities associated with the impairment of global sustainability through governance strategies that are indirectly connected to UWSS can occur: a typical strategy for overcoming local resource scarcity is to import such resources from elsewhere [23–25], or to export the production of goods and resource exploitation processes that cause environmental degradation [26, 27]. The ability to overcome local constraints to development by import and export strategies has allowed populations to grow beyond local resource and environmental constraints by increasing their ecological and water footprints, oftentimes beyond global carrying capacity [28, 29]. This has extended the spatial scales across which water is imported into cities embedded in food and other consumption goods. Thus, system boundaries of UWSS sustainability are expanded across time, from local to global scales, and incorporate additional cross-sector interactions [3, 30, 31]. Ecological and water footprints account for land and water (quantity and quality) requirements embedded in consumption goods [28, 32, 33] (figure 2, bottom layer).

2.2.1. Security

2.2.2. Resilience

2.2.3. Sustainability

UWSS operation is constrained by contributions of security (CP), resilience (1-CT), and sustainability (GP). We determined CP, (1-CT) and GP values based on empirical results, model simulations of the earlier work mentioned above, and results of the sustainability assessment implemented here. Balance was assessed using relative contributions of SRS. We converted absolute values (∣x_i∣), where i = [CP, (1-CT), GP] for each case study (x) into relative values x_j, where j = [CP_r, (1-CT)_r, GP_r], such that:

$$\begin{eqnarray}&&{x}_{j}=\displaystyle \frac{\left|{x}_{i}\right|}{{{\rm{\Sigma }}}_{i}^{n}\left|{x}_{i}\right|}.\end{eqnarray} \tag{ 1 }$$

As a result, the sum of relative values is equal to one [CP_r + (1-CT)_r + GP_r = 1]. For externalized footprints (GP_global < 0), we maintained the negative sign when plotting relative data (see results). We empirically determined lower and upper boundaries of the desirable operating space, which were drawn around the limits of the case study data for resilience and security, and we drew the sustainability boundary where footprints exceeded global carrying capacity (GP_global = 0). Thus, the DOS represents the space in which UWSS provide services without the externalization of costs, i.e. where global sustainability remains ≥ 0, and cities have shown a likelihood of recovery from shocks >50% [21]. The triangular shaded area in the upper area of the SRS space represents the DOS, where (CP ≈ (1-CT) ≈ GP). A shaded gradient towards a more balanced Pareto optimum indicates maximized sustainability, where trade-offs in the achievement of SRS are minimized. The space below the sustainability boundary shows the possibility of exceeding global per capita carrying capacity for water and ecosystems. Externalized footprints of individual cities can be balanced to some degree, as long as global footprints remain below carrying capacity. We therefore mirror the DOS by a viable operating space (VOS), where increasing unsustainability decreases viability. Figure 3 summarizes the methods described above and used to determine the positions of the analyzed UWSS in the SRS space.

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We analyze the sustainability of UWSS for the seven case study cities described in Krueger et al [20, 21]. These were selected from a broad range of hydro-climatic and socio-economic regions on four continents, and were found to represent three types of UWSS along a gradient from low to high security and resilience. Singapore, Melbourne (Australia) and Berlin (Germany) were categorized as water secure and resilient. Chennai (India) and Ulaanbaatar (Mongolia) were identified as water-insecure and non-resilient, with insufficient UWSS and recurring shocks, which results in high probability of service collapse. Amman (Jordan) and Mexico City have intermediate levels of water security and resilience, and relatively high levels of risk. The confluence of different sets of constraints in natural, physical and socio-economic capitals triggers diverse adaptation strategies, which are employed to improve UWSS. Detailed descriptions of these case studies can be found in [20].

We find that cities with high levels of security and resilience (Melbourne, Berlin, Singapore) also have relatively high levels of local sustainability, but negative global sustainability, while in cities with lower levels of security and resilience (Amman, Mexico City, Chennai, Ulaanbaatar), local and global sustainability are more variable across cities, as shown in figure 4. These four cities have relatively low GP_local resulting from insufficient efforts towards wastewater treatment and infrastructure integration of water supply, wastewater, energy, and nutrient recovery; financial dependence of the water sector; lack of coordination and information exchange across sectors, and lack of demand management and community awareness (see table S.5 in SI).

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In contrast to local SRS, which for all capitals are ≥0, global sustainability can be less than zero, representing (externalized) water and ecological footprints in excess of global per capita carrying capacity. A general pattern indicates that cities increasingly invest into local SRS as they attain the ability to externalize footprints, which decreases global sustainability. Ulaanbaatar is an exception diverging from this general pattern.

The relationship observed between UWSS security and resilience and sustainability is shown in figure 5. Resilience and local sustainability show positive relationships with security (R² = 0.62, p < 0.035 and 0.84, p < 0.0035, respectively), while global sustainability shows a negative trend with security (R² = 0.61, p < 0.038).

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Observed values for absolute sustainability were in the ranges [0.3 ≤ GP_local ≤ 0.8] and [−2.2 ≤ GP_global ≤ 0.3]. Empirical evidence of absolute values of UWSS security showed [20] that lower limits were CP $\lesssim$ 0.3, as only a fraction of urban residents received services and cities relied on community adaptation for securing UWSS services. More reliable services were provided for CP $\gtrsim$ 0.4. Stability analyses and numerical simulations [21] showed that values (1-CT) $\lesssim$ 0.5 characterized systems that quickly collapsed in response to shocks. A higher likelihood of 'survival' was achieved in cities with (1-CT) $\gtrsim$ 0.7. Maximum values for security and resilience were CP $\gtrsim$ 1.24 and (1-CT) = 1, respectively.

Converting combinations of absolute to relative numbers resulted in UWSS data distributed within the SRS space as shown in figure 6 (compare table S.5). UWSS in Amman, Mexico City, Chennai, and Ulaanbaatar operate on a combination of public supply and community efforts to reduce public service deficits (total services = public services + A). Total services are represented by color-filled shapes as defined in the column labeled 'total services' of figure 6. Open shapes represent public services. Total services are within the viable operating space for four of the seven cities (Amman, Mexico City, Chennai and Berlin). The remaining three cities (Melbourne, Singapore, Ulaanbaatar) are outside the VOS in the unsustainable area extending beyond the lower tip of the VOS triangle. When accounting for global sustainability, only Chennai is located within the DOS (see discussion section).

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To be in the DOS, the three UWSS dimensions need to be balanced. Outside the DOS, cities may provide viable services, albeit in an unbalanced state, where securing services and making systems resilient comes at the cost of sustainability, or citizens may be at risk of losing services in response to shocks. Externalization of costs to the global scale moves systems across the global sustainability boundary, where GP ≤ 0.

For several systems located outside the DOS/VOS, Monte-Carlo simulations presented in [21] showed that recurring shocks eventually led to system collapse. We marked those systems with red shaded areas and discuss them below in the context of poverty traps. These include public services for Chennai and Mexico City, as well as public and total services in Ulaanbaatar. Community adaptation is critical for moving these UWSS into the viable space. Comparison of Ulaanbaatar (local) and Ulaanbaatar (global) indicates the tension arising between local and global governance goals: from a global perspective, the city should reduce its global environmental footprint, however it simultaneously needs to invest heavily into higher security at the local scale. Extremely low levels of services, and strong reliance on international imports puts this city into a red area ('poverty trap') while being in the globally unsustainable space ('rigidity trap', see discussion section).

Melbourne, Singapore and Berlin operate only on public services, and community adaptation is not required (A = 0; right column in figure 6). From a local sustainability perspective, the three cities are located in the DOS (shapes defined in the left column of figure 6), but large global environmental footprints move them across the sustainability boundary (right column of figure 6). These systems operate within highly connected international virtual water trade systems, which can be highly sensitive to impacts of global change [44], but also inflexible, as we can observe from slow uptake of transformative action to global change pressures [22, 45]. The large global footprints for Melbourne and Singapore move their urban water supply systems outside the VOS. We discuss systems in the unsustainable area outside the VOS in the context of rigidity traps below.

In rigidity traps, focusing resources and efforts to adapt to specific external forces and internal demands leads to highly connected, self-reinforcing systems [48]. Sunk-cost and legacy effects of centralized, inflexible infrastructure impede adaptive management [22]. The development of excess water infrastructure, which requires maintenance at high financial cost, contributes to the rigidity trap. Gradual loss of robustness can be triggered through globally unsustainable management, which exacerbates climate and other global change processes, bearing the risk of losing resilience and shifting to an alternate state of low services [21]. Cumming et al [49] propose that cities have been successful at resolving the trap of unsustainable local consumption through global upscaling. We suggest that due to the lack of direct feedback loops, with current population and consumption levels, global upscaling is a maladaptive strategy that can only be a temporary solution and bears a global systemic risk [11, 12, 22, 44]. When global resource limits are reached, collapse could percolate through a globally connected urban planet with catastrophic consequences [13, 29].

In our analysis, traps are outside the boundaries of the VOS/DOS. Within the VOS, individual cities can have per capita consumption footprints above per capita carrying capacity, if they remain in balance (i.e. below carrying capacity) globally. In our analysis, Melbourne and Singapore have exceeded the limits of the VOS. Their governance strategy has been to meet high consumption demand by investing into highly (globally) connected systems which could prove vulnerable to failure. In this case, improved GP_local comes with externalized environmental costs resulting in the overexploitation of global resources beyond global hydro- and biocapacities. With high financial capital availability and a strong governance system, these cities may be able to respond resiliently to the consequences of crossing a global tipping point. However, Ulaanbaatar is also located outside the VOS, and while struggling to provide services to its citizens, it also has global water and ecological footprints that are highly unsustainable. This makes its situation particularly precarious and vulnerable not only to local shocks, but also to global systemic risk.

The diffusive character of global (un-)sustainability resulting from the externalization of environmental costs makes the rigidity trap somewhat elusive [49]. It is revealed only by determining consumption relative to global carrying capacity [13, 29]. Increasing impacts of global change, such as sea level rise, will put increasing pressure on current management paradigms [50]. Besides persistence, adaptation and short-term system resilience, long-term perspectives of transformation [3, 6, 51] will be critical for moving cities into the DOS, where urban services are not just secure and resilient, but also locally and globally sustainable.

The poverty trap is 'a situation in which connectedness and resilience are low, and the potential for change is not realized' [48]. A daily pre-occupation with accessing basic services prevents urbanites from improving their economic situation [52], and lack of investment into the avoidance of water-related risk can lead systems into a poverty trap [53]. The lack of and inability to marshal critical capitals results in UWSS well below tolerable levels, and shocks quickly lead these systems into collapse [21].

Underserviced areas in Chennai, Mexico City, and Ulaanbaatar experience such a poverty trap, as these households spend large fractions of their time and income to access alternative services, such as water bought from tanker trucks or privately drilled wells, and rely on water that is unsafe for drinking [54–56]. Local sustainability in these cities is low, another indicator of a poverty trap [49]. Resilience simulations [21] showed that Chennai's UWSS converged towards collapse even in the absence of shocks due to lack of resilience [(1-CT)=0]. Adaptation to chronic service deficits is common, however adding multiple severe disruptions causes the degradation of adaptive capacity over time. Community adaptation relieves some pressure from urban managers, however the urban community bears the costs of coping with low service levels and locally unsustainable conditions. While Chennai's citizens showed strong coping capacity during the 2003/2004 drought [57], their ability to adapt was more constrained as another high magnitude event occurred in 2019 with the suspension of piped water supply for the second time in less than two decades [58]. In the face of global change, it is crucial that urban managers transform their systems to meet demands through public services in a balance of SRS—security and resilience must align with sustainability goals [3].

Avoiding traps and moving back into the DOS requires UWSS governance approaches that are desirable from a sustainability perspective, i.e. that optimize resource use across sectors in a circular economy [59, 60], including the use of renewable energy in the water sector [37, 61], the reuse of water, waste, and nutrients [35, 36]. Design of modular, coordinated, flexible and participatory systems are needed, in which information is shared and stakeholders are linked across hierarchies and sectors, from decision-makers, managers and operators to the served community [62–65]. Different urban infrastructure systems need to be integrated [66, 67] and dependence on external resources, including external funding, should be minimized. All of these measures combined require transformational urban agendas [3, 6, 7] that must be developed in an open process and with broad stakeholder participation.

Eakin et al [68] discuss individual-level and system-level capacities for securing human livelihoods as a basis for creating socio-economic well-being ('generic'), and for risk management and response to shocks ('specific'). These four categories are comparable to our understanding of public services (system-level), community adaptation (individual-level), security ('generic'), and resilience ('specific'). The authors suggest that measures to improve one dimension (i.e. specific or generic) can undermine the other, which is in line with the idea of SRS trade-offs described here. However, while the authors suggest that a simultaneous existence of specific and generic capacities is an indicator of 'sustainable adaptation', we propose here that systems that are secure and resilient are not necessarily sustainable. (Global) sustainability adds a dimension that requires additional consideration.

The goal of this study was to present a framework as a new way of approaching UWSS security, resilience and sustainability issues, and sustainable development goals more generally. The methods for quantifying SRS are data intensive. Quantification of the chosen metrics is based on sparse data and expert judgment. Therefore, while our results match with reported information and narrative descriptions, the location of data points in the SRS space should be considered approximations. Future research should focus on identifying robust metrics that are more easily accessible at the global scale. Application to the global scale (e.g. all major cities worldwide) would allow investigations towards how a global sustainability balance across cities can be achieved. Such global studies would furthermore allow a more reliable delimitation of the VOS boundaries, which are here based on the selected case studies and earlier modeling efforts [30].