The changing nature of the water – energy nexus in urban water supply systems: a critical review of changes and responses

This paper provides a review of the changing nature of the water – energy nexus in urban water supply systems (UWSSs) due to the primary long-term drivers of climate change, population growth and technological development from the ‘ energy for water ’ perspective. We identify both the physical changes in UWSSs, as well as the changes in the attributes of the system, both of which contribute to the changing nature of the water – energy nexus. We provide an overview of responses to this change in the water – energy nexus through the lens of four application areas, namely long-term planning, system design, system operation and system rehabilitation, based on the review of 52 papers. Ten responses in three categories are found to be commonly considered in each of the four application areas. The three categories are energy or greenhouse gas reduction, integrated modelling and planning, and improving social bene ﬁ ts. The main drivers of these responses may vary with the application area. Based on the review outcomes, we outline the gaps in the responses in relation to the changing nature of the water – energy nexus in UWSSs, providing directions for future research on improving UWSS ef ﬁ ciency considering the long-term drivers.


GRAPHICAL ABSTRACT INTRODUCTION AND BACKGROUND
The relationship between water and energy is well recognised (Nair et al. ; Lee et al. ), as water is needed for energy production and energy is needed for water production and supply (Rothausen & Conway ; Sharif et al. ). It is also well understood that population growth is likely to lead to significant increases in water and energy demand in the future, making the availability of water one of the limiting factors enabling future energy demands to be met, and making the availability of energy one of the limiting factors enabling future water demands to be met (Carrillo & Frei ; Wakeel & Chen ), thus further strengthening this nexus. When considering the water-energy nexus, this can be analysed either by considering 'energy for water' or 'water for energy' (Nair et al. ; Vakilifard et al. a). The majority of previous studies in this field have focussed on the latter, with the energy requirements of water systems receiving comparatively less attention (Lee et al. ). Consequently, this paper is concerned with the 'energy for water' side of the nexus, with a particular focus on urban water supply systems (UWSSs).
As a result, wastewater systems will only be considered when they are linked to water supply through recycling.
UWSSs are an important part of the urban infrastructure system. Although the energy use for UWSSs varies significantly from city to city (Kenway et al. ), the total life cycle energy consumption of urban water systems can account for up to 7% of total energy use in a city (Wakeel & Chen ). UWSSs generally consist of a number of components, including those used for abstraction, storage, conveyance, treatment, distribution and enduse (Rothausen & Conway ; Lee et al. ). As can be seen in Figure  papers that considered aspects of the water-energy nexus in the optimisation of water supply systems.
The above reviews of the water-energy nexus have generally focussed on traditional sources of water (e.g., reservoirs and groundwater) and energy (e.g., coal and gas) production. However, these sources are changing in response to a number of long-term drivers, such as climate change, population growth and technological development (Rothausen & Conway ; Hamiche et al. ), resulting in the use of a range of non-traditional sources of both water (e.g., rainwater, stormwater, desalinated seawater and wastewater) and energy (e.g., wind, solar and pumped hydro) (e.g., Paton et al. b; Guidici et al. ). This is having an impact on the nature of the water-energy nexus in UWSSs. For example, the introduction of solar and wind energy is changing energy supply patterns, which affects energy pricing and in turn impacts on the operation of UWSSs. Similarly, the use of non-traditional sources of water is a catalyst for the construction of third-pipe systems for the distribution of non-potable water supplies, which is, in turn, having an impact on both the embodied and operational energy requirements of UWSSs. While Nair et al. () considered the energy intensity of decentralised water systems and corresponding end-uses, as well as the potential impact of the feedback loop between water scarcity and climate change, there has not been a comprehensive review of how the nature of the water-energy nexus in UWSSs is changing in response to the use of non-traditional sources of water and energy, and to what degree this has been addressed in the research literature.
In order to address this shortcoming, the purpose of this paper is to provide a framework that articulates the changing nature of the water-energy nexus in UWSSs and to provide a critical review of the papers that have described responses to this change. The remainder of this paper is organised as follows. The framework outlining the relationship between long-term drivers of change, including climate change, population growth and technological development, the corresponding changes in the provision of energy and water, as well as the resultant changes in the nature of the water-energy nexus of UWSSs, from the 'energy for water' perspective, is presented and discussed in the third section. This is followed in the fourth section by a critical review of papers that have focussed on responses to this changing nature of the nexus through the lens of the planning, design, operation and rehabilitation of UWSSs. Conclusions and future directions are provided at the end.

REVIEW PROCESS
To identify papers that address the changing nature of the water-energy nexus for UWSSs from the 'energy for water' perspective, keyword searches were conducted in the ISI Web of Science database, focussing on system planning, system design, system operation and system rehabilitation.
It should be noted that this paper does not represent a systematic review of all relevant literature. Instead, the purpose is to understand how the changing nature of the water-energy nexus has been considered in UWSS-related studies, answering the following questions: • Which long-term drivers and associated changes in the water-energy nexus in UWSSs have been considered?
• What responses have been proposed in order to address these changes and the underlying long-term drivers?
Consequently, the papers included in this review have been selected to provide a diversity of perspectives on how the changing nature of the water-energy nexus for UWSSs has been considered in the research literature. This has resulted in a critical review of 52 papers, including 9 papers on system planning, 15 papers on system design, 12 papers on system operation and 16 papers on system rehabilitation.
To enable the literature to be reviewed in a structured manner, a framework outlining the changing nature of the water-energy nexus for UWSSs from the 'energy for water' perspective is presented, as well as which aspects of this framework have been considered in the papers included in the review. The responses to the changing nature of the nexus provided in the selected papers are then critically reviewed through the lens of the framework, considering system planning, design, operation and rehabilitation, in order to identify areas with practical solutions and gaps that should be addressed in future research.

THE CHANGING NATURE OF THE WATER-ENERGY NEXUS IN UWSSS
The proposed framework outlining the changing nature of the water-energy nexus for UWSSs from the 'energy for water' perspective is shown in Figure 2. As can be seen, the framework consists of four primary components, including (i) long-term drivers of change (orange boxes), the resulting impacts on both water supply (blue boxes and arrows) and energy (purple boxes and arrows) systems and the corresponding changes in the nature of the waterenergy nexus (red arrows). Each of these components is discussed in the following sub-sections.

Long-term drivers
The primary long-term drivers impacting on the nature of the water-energy nexus in UWSSs are climate change, population growth and technological development. These drivers have an impact on both water and energy supply, as indicated by arrows 1 and 9 in Figure 2. Climate change affects the water side of the nexus by increasing the variability in both water availability and demand, as well as their absolute values, with increases expected in some areas and decreases in others. In terms of the energy side of the nexus, climate change is a driver for reducing GHG emissions from energy production, but is also expected to increase energy demand (due to increased air conditioning requirements, for example). Population growth increases both water and energy demand, whereas technological development is likely to increase the feasibility and reduce the cost of both renewable energy and alternative water sources, making their use more prevalent.

Changes in water supply
As mentioned in Section 2, the combination of climate change, population growth and technological advancements has resulted in the need to consider alternative, non-traditional sources of water, such as rainwater, stormwater, recycled wastewater and desalinated seawater, to supplement/replace existing sources, as indicated by arrow 2 in Figure 2. This use of alternative water sources has led to changes in the required storage, treatment and distribution infrastructure, as indicated by arrow 3 in Figure 2.
In terms of storage, the increased prevalence of alternative sources of water has resulted in the usage of household rainwater tanks to capture roof runoff, aquifers to store harvested stormwater (e.g., managed aquifer recharge) and wetlands for storing (and treating) stormwater. The use of alternative sources of water has also resulted in the need for alternative treatment technologies, such as for the desalination of seawater and the purification of stormwater, greywater and blackwater, as well as alternative infrastructure systems, such as third-pipe systems for the distribution of non-potable water.
These changes to the infrastructure components of UWSSs that have an increased penetration of alternative sources of water also have an impact on their planning, design and rehabilitation, as shown by arrow 5 in Figure 2, especially due to increased diversity in a system scale (e.g., an increase in the number of systems at cluster or household scales). However, they also have an impact on system operation, as indicated by arrows 4 and 6 in Figure 2, primarily because of an increase in system complexity resulting from (i) the need to consider the availability of different quantities of water from different sources, (ii) the need to match water quality with the end-use type, as different source waters are likely to be of different quality (e.g., potable versus non-potable) and (iii) the need to consider the temporal variability in the availability of different sources. Due to the increase in complexity in operation arising from the inclusion of alternative sources of water, there is increased benefit in including operational considerations during the planning, design and rehabilitation of infrastructure, as shown by arrow 7 in Figure 2.

Changes in energy supply
As mentioned in Section 2, population growth and climate and technological changes are drivers for the increased consideration of alternative sources of energy, such as solar, wind, hydropower and biofuels (e.g., methane), to either replace or supplement existing supplies, as highlighted by arrow 10 in Figure 2. These changes modify some of the attributes of the energy used by UWSSs, such as (i) the GHG emissions associated with their use, as different alternative The intermittency and changes in the levels of GHG emissions of alternative sources of energy impact UWSSs via changes in their embodied energy, which is used in the manufacture and construction of water system infrastructure (arrow 16, Figure 2), as well as their operational energy, which is used on an ongoing basis to operate water systems (arrow 18, Figure 2). There is also a feedback loop between the reduction in GHG emissions resulting from the use of alternative sources of energy and the need to introduce these sources in response to climate change, as indicated by arrow 20 in Figure 2.

Changes in water-energy nexus
As discussed previously, the introduction of alternative sources of water has resulted in a greater diversity of the water supply system, with a larger number of processes and infrastructure types with different energy demands, costs, water quality and availability. From an energy perspective, the introduction of alternative sources has changed a constant supply with fixed pricing (e.g., tariffs) and emission factors to a supply for which availability, pricing and emissions factors vary over time. The introduction of alternative sources has also resulted in increased complexity and diversity of scales, for both water and energy systems.
The above changes to water and energy systems caused by the introduction of alternative sources have a number of implications for the water-energy nexus. The fact that the different types of infrastructure associated with alternative water sources operate at different scales, not just at centralised scales, as is the case with traditional water supply systems, can create a range of trade-offs that need to be considered (arrows 16 and 18, Figure 2). For example, some decentralised systems, such as household rainwater tanks, require less energy for the collection and re-distribution of water, resulting in a reduction in embodied energy (Green However, the pumps and treatment systems used at these smaller scales are generally less efficient than those used at larger scales, resulting in an increase in operational energy (Vieira et al. ). Which scale provides the best trade-off is unknown and case study dependent. The changing nature of the energy supply associated with water supply systems that use a greater proportion of alternative sources of supply can either have a positive or negative impact on climate change (e.g., if the energy requirements of a particular alternative source is higher (e.g., desalination), then this has a negative impact on climate change and vice versa), as shown by arrow 8 in Figure 2.
The intermittency in energy supplies to UWSSs resulting from the use of alternative sources of generation results in the variation in the total emissions associated with the construction and operation of water supply systems (arrows 16 and 18, Figure 2). As the temporal variation in energy from alternative sources generally also results in a temporal variation in electricity prices, the costs associated with the operation of water supply systems also vary over time in  Figure 2). In addition, the desire to reduce net energy usage and GHG emissions of water systems has resulted in the use of water systems for energy production, such as the use of anaerobic digestion of sewage sludge for biomethane production (Esposito et al. ), which can be used to provide the energy needs of the treatment process itself (arrow 21, Figure 2).
A summary of the number of papers included in this review that have addressed the different aspects of this changing nature of the water-energy nexus in UWSSs from the 'energy for water' perspective is given in Table 1. As can be seen, the table distinguishes between papers that have considered different long-term drivers (climate change, population growth and technological development), different physical changes to energy and water systems in response to these drivers (consideration of alternative sources of water and energy, and consideration of different system scales and complexity) and changes in the attributes of the energy and water supplied from alternative sources (variability/changes in energy and water availability, pricing, GHG emissions, water quality and user behaviour/demand).
In addition, there is a distinction in terms of whether the planning, design, operation or rehabilitation of water supply systems was considered. Table 1 shows that climate change and population growth are common long-term drivers considered together with technological development in studies on system planning, design and rehabilitation. However, technological development is the main driver for changes in system operation. Although most physical changes are considered in all application areas of planning, design, operation and rehabilitation, the different system scales (mainly resulting from decentralised systems) have not been a focus for system operation studies. The most commonly considered system attribute related change is varying availability of energy/ water supply, which is considered in all application areas; followed by varying GHG emissions, which is considered in system planning, design and operation. Varying energy/ water prices and water quality are both considered in two of the application areas. In contrast, user behaviour was only considered in system planning studies.

RESPONSES TO THE CHANGING WATER-ENERGY NEXUS IN UWSSS
System planning UWSS planning involves defining the goals to be achieved by the system and identifying the best actions to achieve these goals. These actions are usually compiled into a water supply plan for a city or region. For example, the goals might be to supply potable water to a city at minimum cost and an acceptable level of reliability while satisfying specific environmental and social criteria. The plan would identify what water sources will be used, where the raw water will be stored, where and how it will be treated, as well as the broad layout of the distribution system for the treated water. The reliability of the system, as well as the water quality, environmental and social criteria to be met are usually also identified as part of the plan (see Appendix Urban water supply planning usually takes place for large centralised systems and over a long time horizon  These smaller systems still need to be integrated into longterm planning.
A total of nine papers were reviewed that considered the water-energy nexus in UWSSs in the context of system planning. Three main responses were identified in these papers, including: (1) Considering energy/GHG reduction as an objective (in a multiobjective optimisation framework); (2) Developing integrated models for water and energy planning; and (3) Developing decentralised solutions for water and/or energy production. all responses to each of the drivers, there was at least one paper that discussed each driver-response combination.
This could be due to the nature of long-term system planning, which tends to consider all possible drivers over the planning horizon. The three principal responses are discussed in more detail below.
Considering energy/GHG reduction as an objective (e.g., via a single-or multiobjective optimisation framework) One direct response to climate change and increased energy consumption due to increased demand resulting from population growth is to include energy or GHG reduction directly as a planning objective. The most straightforward way to achieve this is to include them in existing objectives, e.g. operating cost reduction ( Including energy/GHG reduction as an objective can also be achieved using a multiobjective optimisation approach, where the trade-offs between energy/GHG reduction and other potentially conflicting objectives, such as cost reduction, can be explored. This is primarily enabled by advances in computing hardware and optimisation algorithms, as well as in response to the increased system complexity resulting from the consideration of alternative water sources. Such an approach has been applied in a

Developing integrated models for water and energy planning
Another common response to the long-term drivers is to develop integrated models for the long-term planning of both water and energy systems. This response draws on alternative energy and water sources and is necessitated by System design UWSS design involves defining the layout of, and interactions between, the various subsystems needed to achieve current and future planning goals. These subsystems fall into three main categories: water source, water treatment and water distribution systems. The design of these systems will depend on the long-term planning goals. For example, if the goal is to supply fit-for-purpose water at least cost and energy input, then all water sources available at varied spatial and temporal scales need to be considered, spanning from lot to precinct scales, as well as sub-daily to seasonal scales.
A total of 15 papers were reviewed that considered the responses to the changing nature of the water-energy nexus in UWSSs through the lens of system design. Three main responses were identified in these papers, including:

Considering energy/GHGs reduction as an objective
One of the most significant impacts of climate change, population growth and technological development are increases in water and energy demand, leading to increased GHG emissions that further reinforce climate change. Therefore, the most direct response in system design is to include energy/GHG reduction as a UWSS design objective. sources. This is the first time such a dynamic and multiobjective approach has been used to solve this type of problem.
The authors tested their approach for Ustica Island in Italy, and the results showed that the designs using renewable energy sources outperformed the traditional design with respect to different sustainability indicators, reduced investment costs and environmental impacts.
Considering the design and operation of water supply systems together In traditional UWSS design, the focus is often on sizing the system to achieve certain objectives (e.g., cost minimisation) while satisfying design constraints, without fully considering the impact from long-term operation (Wu et al. b). However, the ongoing operation of these systems can have a significant impact on their performance over their lifetime, e.g. the total cost or energy consumption and associated With a case study in China, the authors showed that the integrated design of UWSSs, together with the design of urban areas, can improve overall system efficiency and lead to multiple benefits, such as reduced water and energy demand, reduction in the urban heat island effect and enhanced liveability outcomes. In addition, better use of waste generated from urban areas, including wastewater (and treatment byproducts) and municipal solid waste, will not only provide renewable energy, but also lead to reduced waste generation and disposal, enhanced water quality outcomes for receiving water bodies, reduced landfill areas needed for waste disposal and reduction in GHG emissions (Wang et al. ).

System operation
A UWSS incorporates a number of controllable components (1) integrated management of energy and water supply systems; (2) more efficient and flexible control of water supply systems and (3) recovering energy from existing systems.

Integrated management of energy and water supply systems
Desalination and other energy-intensive water production technologies are being used increasingly to respond to reduced water availability and growing water demand, tightening the interdependency between water and energy in UWSSs. In this context, an increasingly explored solution is to connect UWSSs to renewable energy sources and jointly operate the coupled water-energy system in either grid-connected or off-grid mode, reducing fossil fuel depen- There are also studies that deal with grid-connected systems

System rehabilitation
UWSS rehabilitation refers to the use of repair, renewal and replacement technologies to return functionality to the system (EPA ). Since most UWSSs were constructed many years ago, rehabilitation is also used as an opportunity to use new technologies to improve or expand the functionality of these systems to meet new challenges, including increased demand due to population growth and switching to renewable energy sources to reduce GHG emissions.
There is also a myriad of connections between water and energy in UWSS rehabilitation, as discussed previously.
System rehabilitation is also a straightforward way of making changes to existing UWSSs in response to longterm drivers, the resulting changes and their impact. As mentioned previously, 16 papers considering the waterenergy nexus within the application area of system rehabilitation were reviewed. These included five main responses in relation to changes in the water-energy nexus due to the long-term drivers considered in these studies, including • considering energy/GHG reduction as an objective; • considering renewable energy sources; • recovering energy from existing systems; • using water storage as energy-balancing storage and • bringing benefits to disadvantaged communities.
The relationships between these five responses and the long-term drivers, as well as the number of papers considering each of the responses, are summarised in Figure 6.

Considering energy/GHG reduction as an objective
Despite the myriad connections between energy and water supply system rehabilitation, energy consumption is The first and most direct response to climate change drivers in system rehabilitation is to include energy or GHG emission-related objectives in the rehabilitation process.
The inclusion of energy considerations in UWSS rehabilitation indicates an increased awareness of the connection between water and energy in urban water systems. This is reflected in a few recent studies, where energy reduction was directly considered in urban water system rehabilitation

Considering renewable energy sources
The use of renewable energy sources to replace conventional electricity generated from burning fossil fuels has become A pump-as-turbine-based system was included in a small Alpine UWSS in Austria (supplying water to 2,000 residents) to generate energy from excess flows during low water demand seasons. The study concluded that a significant amount of profits can be achieved for both single and interacting twin systems. A similar concept was used in a study in Brazil when rehabilitating a trunk main of a UWSS, when a pump-as-turbine was added in the system to recover energy from normal system operation with a by-pass system for pressure increase during emergencies (Meirelles et al. ). In addition, pump-as-turbine was also used in high-rise buildings to replace pressure-reducing valves for leakage reduction and excess energy recovery (Du et al.  () discussed above, energy can be recovered from water treatment facilities to finance the upgrade of the facility itself. It is therefore possible that in the future, various upgrades to UWSSs can be carried out with achieving economic profit as an objective, which brings other environmental or social benefits (e.g., reduced overall energy consumption and emissions and creation of jobs) at the same time.

Summary of responses
In response to long-term drivers and the resulting changes to UWSSs, various responses have been proposed in system planning, design, operation and rehabilitation, as discussed above. A summary of these responses is provided in which is a direct response to technological development.
The reduction in the use of conventional energy or resulting GHG emissions can also be achieved through the use of renewable energy sources in place of conventional energy sources, which has been considered in all four application areas. Incorporation of renewable energy sources has become a main aim for system rehabilitation in many studies, and typically solar energy is incorporated so that future system operation will rely less on conventional energy sources and generate fewer GHG emissions. In addition, energy recovery from existing systems is an alternative way to reduce total energy consumption and the resulting emissions. This response has mainly been driven by population growth and technological development, which made it technically feasible. Energy recovery is typically considered in both system operation and rehabilitation studies.
Another commonly proposed response to the three drivers is the use of an integrated approach for urban water system management, typically through integrated modelling.
This can be conducted on a small (or cluster) scale, where the design and operation of a single water supply system are considered jointly during the system design phase, so that the life cycle cost and energy (or emissions) can be taken into consideration at the initial stage of system design. Integrated modelling can also be applied to an extended scale, where water and energy system planning is integrated into the planning of urban areas, leading to multiple benefits, such as better management of waste, energy recovery, reduced landfill and reduced GHG emissions.
The integration of water and energy systems has been considered in all four application areas as a direct response to all three drivers. It has been widely recognised that overall system efficiency can be improved through integrated planning of water and energy systems, in response to the tighter coupling between water and energy as the changes induced by the long-term drivers take effect. The consideration of renewable energy sources, such as wind and solar, increases the variability of power supply for water systems, which need to be balanced with large energy reserves. The potential of using hydropower storage in water storages (e.g., reservoirs and water tanks in high-rise buildings) as a balancing energy storage has been explored, primarily for existing systems through system rehabilitation.
Finally, advances in technologies make new technologies not only more feasible, but also more accessible (economically and physically) to communities who could not afford them previously. A few studies on system rehabilitation have explored this opportunity to improve water supply to disadvantaged communities, including the development of small-scale solar-powered devices for mitigating arsenic contamination problems in water sources for offgrid communities in Latin American countries and the use of energy recovered from water system upgrade to fund the upgrades themselves. However, this response is yet to be included as a main focus in other application areas.

CONCLUSIONS AND FUTURE DIRECTIONS
We provide a review of how the water-energy nexus within UWSSs has changed due to the primary long-term drivers of climate change, population growth and technological development, as well as the responses to this change in the nexus that have been presented in the literature on the planning, design, operation and rehabilitation of UWSSs. The traditional water-energy nexus in UWSSs is represented in Figure 1, as discussed previously; and the changed nexus due to the long-term drivers is represented in Figure 7. By comparing the two figures, it can be seen that the longterm drivers have led to changes in both the physical UWSS and the attributes of the system, and in turn the inherent water-energy nexus within the system. The drivers have led to the consideration of alternative water sources and renewable energy sources in urban water supply. This has increased the physical complexity of the system due to additional system components and the different scales of the systems involved, e.g. centralised desalination plant and decentralised rainwater-harvesting systems. The development of decentralised systems is also assisted by technological development, which has made many of the technologies more feasible on a smaller scale and more accessible to individual users (e.g., rooftop rainwater collection systems and solar panels). The consideration of alternative water and energy sources has also changed attri- proportion of the total energy consumed in UWSSs and be accounted for using LCA-based approaches.
Integrated modelling is a common response considered in all application areas; however, integrated modelling of combined centralised and decentralised systems and the optimisation of their integration has only been considered in a limited number of studies (Wu et al. ). For example, water-sensitive urban design, such as an integrated approach considering both centralised (e.g., stormwater harvesting) and decentralised alternative water supply sources (e.g., rooftop rainwater), has mainly been considered in the context of city planning (Brodnik & Brown ) and has received less attention from UWSS designers and managers (Sharma et al. ). In addition, the integrated consideration of UWSSs and energy systems in urban area planning presents promising opportunities for improving overall system efficiency and should be investigated further in the future. One tool that can be used for this purpose is urban metabolism modelling, which applies an integrated and interdisciplinary approach to account for all material (including water) and energy flows in urban systems (Dijst et al. ). However, research on this topic is still largely limited to the urban planning field (Perrotti ). Consequently, the general applicability of a totally integrated modelling approach of all material flows and their overall performance over different UWSSs needs to be further investigated.
Although technological development is the primary driver of many responses and has been considered in all four application areas of UWSSs, the scope of technological development considered in the current literature is quite limited. Almost all of the responses linked to technological development are due to the advancements in renewable energy or new network components (e.g., variable speed pumps, pump-as-turbine and photovoltaic pumps) and the resulting increased accessibility (e.g., reduced prices and more portable devices) due to these technologies. In contrast, consideration of recent developments in smart meters and smart sensors has been limited in scope (e.g., using machine learning techniques to extract information from increased data collected using these sensors) (Cominola et al. ). However, the full potential of these smart technologies is yet to be explored. The insights into water usage provided by smart meters can assist end-users to be more water and energy efficient, and assist service providers to better tailor their services to users' needs, improving the overall efficiency of the system (Sønderlund In addition, how developments in policy and governance can be used to respond to the changing water-energy nexus in UWSSs is yet to be investigated. Government policies will have an impact on the development of water and energy markets, which can have a significant impact on how UWSSs should be designed and operated. Market mechanisms allow the prices of water and energy to vary over time, which will increase the complexity of decisionmaking for end-users, as well as system operators and water authorities. These varying prices will also change users' behaviour in terms of when to use water and energy and by how much, or result in the use of alternative supplies (e.g., roof top solar panels and rainwater tanks), increasing the complexity of system operation. System operation can be complicated further by allowing the water in the system and energy generated from UWSSs to be traded in a market. Integrated modelling considering not only the physical connections between the water and energy systems, but the dynamics introduced by human decisions are key in fully tapping into the benefits of the enhanced water-energy nexus to improve system efficiency in the future. This paper focuses on the water supply side of urban water systems, with wastewater systems being considered only when they are linked to water supply (e.g., considering recycled wastewater). It is well known that wastewater treatment is one of the major energy consumers in urban water systems, and the water-energy nexus in wastewater treatment systems has been well studied (Xu et