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Review

A Review of the Water–Carbon Nexus in Urban Systems

by
Xiao Han
,
Wei-Yu Shi
* and
Yu-Xia Yao
Chongqing Jinfo Mountain Karst Ecosystem National Observation and Research Station, School of Geographical Sciences, Southwest University, Chongqing 400715, China
*
Author to whom correspondence should be addressed.
Water 2023, 15(6), 1005; https://doi.org/10.3390/w15061005
Submission received: 2 February 2023 / Revised: 22 February 2023 / Accepted: 1 March 2023 / Published: 7 March 2023
(This article belongs to the Section Water-Energy Nexus)
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Abstract

:
The rapid development of cities has brought a lot of carbon emissions and water consumption, leading to disasters, such as the greenhouse effect and drought. However, there is a lack of systematic review on the key nodes for the water–carbon nexus and the key points of water-saving and emission reduction improvement in the urban systems. This study reviewed the definition of the water–carbon nexus, analyzed its changing mechanism in different urban systems, and concluded the major methodologies applied in the nexus studies. The main findings are as follows: (1) the water/wastewater treatment in water systems and the structure transformation in energy systems are the key nodes for the water–carbon nexus. (2) From the perspective of methodologies, the research focus has gradually changed from single system and process analysis to multi-system and whole process analysis. (3) There is a tendency to sacrifice large water consumption in exchange for carbon reduction in the urban systems, calling for explorations in the water–carbon balance. (4) More comprehensive methods, systematic data support, and accurate definition of system boundaries are required to quantify the nexus. This study systematically reviewed the existing achievements on the water–carbon nexus, providing support for urban metabolism and related policy makings.

1. Introduction

The urban systems are regarded as spaces where human beings closely relate to the natural world and living and nonliving elements coexist and interact with each other [1]. In urban systems, water is an essential resource for human beings, as it provides the necessary guarantee for social production and human survival [2]. Additionally, human activities, such as fossil energy combustion, water supply, and wastewater treatment will cause large CO2 emissions and climate deterioration, which in turn lead to water shortages [3]. However, from the perspective of urban metabolism, urban systems can be unsustainable because of dependence on material and energy importation to sustain themselves and to export waste to the ecosystems [1]. The unsustainability, together with the unavoidability of both water consumption and carbon emissions in urbanization, make the water–carbon coordination significant for the sustainable development of cities.
However, under the rapid urbanization, it is expected that 66% of the total population will be urbanized by 2050, making CO2 emissions and water resources come under increasing pressure and threat [4]. For example, during the last century, the world’s population has increased by only four times, while municipal water consumption has increased by surprisingly 10 times [5]. Additionally, the threat to water resources will be exacerbated without control, leading to more people suffering from water shortages [6]. Meanwhile, cities play an important role in CO2 emissions, accounting for about 71% of the global carbon emissions [7]. Additionally, increasing emission is causing catastrophic effects, such as global warming, floods, and extreme droughts [8]. However, there is a complex interaction between carbon emissions and water consumption, and they are closely related to each other through the product supply chains in the urban systems, which hinder water saving and emission reductions in cities [9].
Therefore, in order to explore the mechanism of carbon and water interaction in urbanization and the model of urban sustainable development, a large number of studies have focused on the water–carbon nexus in urban systems. They tried to figure out the mechanism in the water–carbon nexus and gave targeted suggestions for policy making, some of which have achieved results. However, there is a lack of systematic reviews of existing achievements. What are the key nodes for the water–carbon nexus? Where should improvements be focused on in water saving and emission reduction? The answers differed a lot on multiple scales. Therefore, in order to answer the questions above, this paper focused on the academic literature on the water–carbon nexus in urban systems published in recent years and tried to elaborate on the key directions for the nexus improvement in urban systems and to provide scientific suggestions for future research and policy formulation. We first illustrated the definition of the water–carbon nexus. Then, we systematically reviewed the achievements of water–carbon nexus research in different directions in urban systems and the methodologies applied in these studies. Finally, we discussed the existing limitations and the possible research directions in the water–carbon nexus in urban systems in the future.

2. Definition of the Water–Carbon Nexus

Wolman [10] firstly present the city as an ecosystem, and the concept of metabolism is used to describe the interactions between subsystems within an urban region. Some studies have also defined urban metabolism as an exchange process by which materials, energy, and water are converted into built environment, human biomass, and waste [11]. As the main resource and pollutant output in the cities, both water and carbon are important parts of the urban metabolism. Additionally, the complex relationship between them is defined as the water–carbon nexus.
The concept of nexus is defined as the interactions among different subsystems within the systematic boundary. It can be regarded as a dynamic relationship reflected in the process of product production and consumption, as well [12]. Additionally, the complex relationship between energy, carbon, and water is called the energy–carbon–water (ECW) nexus [13]. This study constructed an ECW framework in urban system based on the theory of urban metabolism. Figure 1 showed that the ECW in the urban system mainly consists of three parts: the energy system, the water system, and the consumption system. During the urbanization process, energy systems consume resources such as fuels to support water supply and treatment in water systems and production in consumption systems. Meanwhile, it consumes water resources for cooling and emits great CO2. Then, the water systems require large energy consumption for the transport, distribution, and treatment of water, while discharging wastewater and CO2 into the environment. At last, the consumption systems, including household, industry, and tourism, consume great energy, water, and cause CO2 emissions to produce products in support of social operation. In this case, the water–carbon nexus is defined to investigate the relationship between water and carbon related to energy [14,15], non-energy, or less-energy processes [3]. It happens both inside the urban system and between the urban system and the environment. Mahlknecht et al. [16] further prompted that the interaction between carbon and water includes inter-linkages, synergies, and trade-offs. In a word, the water–carbon nexus in urban systems is a process of interdependence, mutual exclusion, and co-development between carbon and water throughout the urban metabolism.
Wang et al. [17] suggested that the water–carbon nexus study aimed to explore the development path under the dual constraint of water and carbon. Most of the previous studies paid attention to the water–carbon interaction in energy-related systems [18,19] or the carbon emissions in water-related systems [20]. In addition, the water–carbon nexus in households [21], construction [22], and agricultural sectors [23] has attracted public attention. The research scales and methodologies of recent water–carbon nexus studies were summarized in Table 1 and will be systematically illustrated in Section 3 and Section 4.

3. Research Focuses

3.1. The Water–Carbon Nexus in Water Systems

The urban water system (WUS) is composed of water intake, treatment, distribution, use, wastewater treatment, and disposal [44]. Studies have shown that the urban water system accounted for 3–10% of the global warming potential (GWP) of greenhouse gas (GHG) emissions in many European countries [24]. This is because the water distribution and wastewater treatment in water systems have a high demand for energy, in which carbon emissions will be generated [45,46]. For example, in the United States, water and wastewater treatment consumed energy equivalent to about 45 million tons of GHG emissions per year, which raised concerns for the sustainable development of urban water systems [47]. Additionally, the increasingly strict standards of municipal wastewater treatment also resulted in huge CO2 emissions [48]. Therefore, a large number of studies have been carried out to analyze the water–carbon nexus in urban water systems.
Some studies promoted that water and carbon emission savings varied a lot under different water and energy-based interventions [13]. The water company has become the focus of water–carbon nexus since they were required to treat water and wastewater during operation [49]. It was found that water companies can reduce GHG emissions by 7.5% for each supply of the same volume of drinking water [50]. Additionally, more researches analyzed the carbon contributions from water supply and wastewater treatment separately. Wastewater treatment is widely considered to be the most important contributor to carbon emissions, but its contribution may be underestimated due to the lack of attention to wastewater. In Antalya, a city of Turkey, water supply and sanitation accounted for about 26% and 74% of the total GHG emissions, respectively [51]. However, in Mexico, about 90% of CO2 emissions were caused by water supply, while the wastewater industry contributed less to carbon emissions as over 80% of wastewater was discharged without treatment [3]. There are studies that paid attention to the carbon emission mechanism in water systems as well. Some of them depicted that water sources, water quality, and terrain characteristics were the main determinants of CO2 emissions in water supply [27]. Additionally, CO2 emissions in wastewater treatment greatly depended on electricity fuel mix and wastewater treatment technologies [52].
Targeted suggestions have been proposed to improve the urban water systems. Although the indirect carbon emissions caused by the use of chemical products were limited, water quality can still be improved by reducing the use of chemicals, such as pesticides, thereby reducing carbon emissions during water treatment [27,53]. Additionally, previous studies have shown that the improvement in water treatment technologies will greatly reduce carbon emissions [48,54,55]. Moreover, other researchers promoted that traditional centralized wastewater treatment systems increased indirect CO2 emissions during the transportation of wastewater [56]. Thus, decentralized wastewater treatment systems may be superior in sustainable development due to less energy input and more efficient resource recovery [57,58].

3.2. The Water–Carbon Nexus in Energy Systems

Energy generation requires large water resources and generates large amounts of CO2 emissions [59]. On the one hand, low-carbon technologies in energy systems can place an additional burden on water resources despite the reduction in CO2 emissions [31]. On the other hand, a lack of water resources will in turn limit energy production and affect carbon emissions [60]. Therefore, the energy system is considered as an important node of water–carbon nexus, and the sustainable development of urban energy system puts forward urgent requirements for further exploration in its water–carbon interactions [61].
The energy sector is considered as a major CO2 emitter, as it contributes more than 40% of the total CO2 emissions [62]. Additionally, it is also the world’s second-largest water user [63]. However, the low-carbon strategies may exacerbate the conflict between water and carbon in the power system. Previous studies suggested that, during the process of reaching the peak of carbon emissions, thermal power may bring 34.85 Gt of water consumption growth in China [30]. During the energy structure transformation, the application of alternative energy sources may also lead to an increase in carbon emissions and water use. Qin et al. [64] promoted that the source of natural gas has a significant impact on the water–carbon relationship. Due to its dependence on natural gas, California is expected to produce more CO2 emissions from 2015 to 2025 [65]. Meanwhile, the shift in fuel from coal to natural gas could lead to a 32% decrease in water consumption in the state of Illinois [66]. Besides, the accumulated water utilization per accumulated GHG reduction in hydropower will decrease as the power station operates, which makes hydropower a low-carbon-friendly alternative energy source [67]. Therefore, many studies have been carried out to analyze the mechanism of carbon emission and water consumption in hydropower stations [68,69]. It is confirmed that there is a significant positive correlation between water loss and carbon emissions [33]. However, hydropower is the largest water consumer of all energy sources, putting enormous pressure on water resources, especially for areas suffering water shortages [70]. In addition, the rationality of water use in clean energy sources, such as nuclear power, has been analyzed [71]. The results showed that nuclear energy will also consume a lot of water resources once generating electricity [70].
Facing the increasingly serious water–carbon conflict in the energy system under low-carbon policies, Wang et al. [17] suggested that the improvement pressure of water-saving technology is greater than carbon emission reduction technology. Cooper et al. [72] promoted that substituting low-water-density energy sources, such as wind or solar photovoltaic for thermal power, may relieve water pressure. Additionally, replacing coal investment with 1% share of solar and hydropower investment every year may reduce carbon and water footprint while increasing land and cost footprint [73]. In addition, Shaikh et al. [70] compared the water–carbon consumption of energy development in different scenarios, and the Renewable Energy Focused Development Plan was found to use the least water and emit the least carbon.

3.3. The Water–Carbon Nexus in Urban Economic Systems

A number of researches on water–carbon nexus in independents system in cities have been carried out. However, due to the increasingly frequent resource flows, trade, and freight transport, the water and CO2 exchange is widespread among different sectors and regions. Thus, these studies in independents system in cities are limited by the narrow boundary definition as [74,75], and the analysis of water–carbon nexus in a complete urban system can provide a more comprehensive understanding of how complex water–carbon relationships affect human activities. These studies mainly focused on multiple sectors in consumption systems in the urban metabolism at the same time, such as manufacture, construction, etc. Some studies tended to explore the water–carbon linkages across multiple sectors [76], while others focused on water–carbon flows between regions [35,77].
Many studies in the water–carbon nexus have been carried out in a single sector. However, the water–carbon nexus wildly exists among different sectors, and this dynamic relationship varies greatly among sectors [76]. The key nexus sectors have been identified based on direct or embodied water–carbon nexus [78]. For example, the service and construction sectors are considered as major water consumers and CO2 emitters [36,79]. Additionally, Wang et al. [80] identified the manufacturing and agriculture sectors as the key nodes in the water–carbon nexus. However, Hu et al. [79] further put forward that, when considering the consumption-based and production-based carbon emissions, respectively, the key sectors may differ. In conclusion, the key sectors in the water–carbon nexus may vary in different regions from multiple perspectives.
On the national scale, Wang et al. [77] found that the EU27 countries reduced CO2 emissions by 1.4 Gt and water use by 64.5 Gm while maintaining economic output comparable to 2014, where Germany, France, and Italy were the main beneficiaries. On the provincial scale, Fang et al. [37] analyzed the water–carbon flow between 34 provinces in China, finding that Beijing was a major importer of water resources and carbon emissions in China, so it could transform the environmental pressure to other regions, while Hebei was the main consumer of local water resources and under great environmental pressure. Moreover, more studies have been carried out at an inter-regional scale [81]. For example, the water use and CO2 emissions per unit output in Shanghai were greater than those in Beijing, indicating more significant environmental pressure on development in Shanghai [39]. Besides, carbon and water also flow across regions through water-intensive and carbon-intensive products with inter-regional trade activities [37,82]. Therefore, virtual water and CO2 emissions have been analyzed at national [83] and sectoral [84] scales.
Many suggestions have been put forward to reduce carbon emissions and water use in urban systems. Technically, improving resource utilization efficiency may reduce water consumption and CO2 emissions [39]. Following an efficiency-oriented development model is expected to reduce both CO2 emissions and water consumption, but increase the system cost [85]. From a political perspective, strengthening trade cooperation could improve the water–carbon conflict [86]. Additionally, Sperling et al. [87] further suggested that key departments, such as local governments and households, should be allocated CO2 emissions and water ‘budgets’ to keep water use and carbon emissions in control.

3.4. Other Research Focuses

Other studies did not focus on energy and water systems, nor on the complete consumption systems, but they focused on individual sectors in the consumption system, such as agriculture, trade, household, etc.
Agriculture is the largest consumer of freshwater resources, consuming 85% of the world’s freshwater resources [88]. Additionally, large CO2 emissions generated by heating and fossil fuel burning can make agriculture to be significantly affected by the water–carbon nexus [89]. For example, Bieber et al. [90] illustrated how policies such as energy mix and agricultural investment affected water consumption, carbon reduction, and the cost of food production. A recent study showed that taking potatoes as a staple in China may reduce the total carbon-land-water impacts of staple crops [91]. Additionally, Yang et al. [92] also explored how changes in crop species affected the carbon–soil–water relationship in a desert ecosystem. The application of clean power, intelligent systems, and other water-saving technologies may reduce the cost of water and carbon in agriculture, and some of them have been applied and achieved economic benefits [93,94].
On a smaller scale, the water–carbon nexus in the household sector has attracted wide attention. The water use of the household sector mainly includes washing, drinking water, and water consumption during energy use. Additionally, the CO2 emissions mainly originated from space heating, cooling, lighting, and so on. Li et al. [95] found that income improvement plays a leading role in the increase in household ECW, while household water use is more sensitive to socioeconomic development than carbon emissions. Additionally, according to Song et al. [42], food waste in the household sector could increase the carbon and water footprints.
Moreover, Tobarra et al. [14] analyzed the water–carbon nexus in the trade of out-of-season products and found that not all domestic production contributed to its footprint reduction. It is suggested that the total tourism expenditure was a main driver for water consumption increase in China tourism [40]. Cheng detected the water–carbon nexus in high-speed railway construction, finding that the construction of bridges contributed the most CO2 emissions and water consumption [41]. Besides, Al-Kez et al. [96] also analyzed the nexus in digitization and suggested that storing dark data without proper treatment may cause large amounts of global carbon and water footprint.

4. Methodologies

4.1. Input-Output Model

Input–output analysis (IOA) is an economic mathematical model used to analyze the quantitative dependence between input and output resources in the socioeconomic system, which has been widely applied in water–carbon nexus research [97,98]. It can be used to investigate the role of key sectors in the water–carbon nexus and trace environmental pressures [99] and to identify the main drivers of the environmental influence [100].
However, IOA is limited in evaluating interactions between multiple regions and sectors under different policies [101]. Therefore, extended IOA methods, such as the multi-regional input–output (MRIO) and the multivariate statistical input–output analysis were developed to analyze the nexus across regions and sectors [36]. For example, the MRIO method was used to identify the similarities and differences between water and carbon footprints in China during 1990–2010 [102]. Based on the MRIO model, Ali [103] compared the carbon and water footprints in different world regions, and Sun et al. [104] suggested that considering eating out will obviously increase the food-related carbon and water footprints in rural areas.
However, it should be noticed that both the IOA model and the MRIO model are highly dependent on input–output tables that the sector aggregation in input–output tables may cause difficulties in quantifying the nexus [105]. Additionally, these massive systematic data may be time-lagged and bring trouble for the actual operation [106].

4.2. Life-Cycle Analysis

Life-cycle analysis (LCA) is a bottom-top approach based on the individuals, and it is capable of quantifying the flow of materials and energy in the whole production process [107]. For example, Meldrum et al. [28] compared the life cycle water use in different power sources and promoted that the total life cycle water consumption of photovoltaic and wind power generation was the lowest.
The LCA method is often used to analyze the nexus combined with other models. For instance, as a top-bottom approach, the input–output life-cycle assessment (IO-LCA) approach is developed by combining the LCA method with the IOA model. Thus, the IO-LCA method is allowed to focus on the environmental influence of different products and services within a more complete system boundary [108]. Feng et al. [61] and Li et al. [109] analyzed the savings in life cycle CO2 emissions and water use when using different low-carbon power generation technologies based on the IO-LCA method. Additionally, Malik et al. [110] quantified changes in economic output and employment when introducing a new sugarcane-based bio-fuel industry in Australia. Besides, other extended LAC methods were applied in the water–carbon nexus studies as well. The process-based LCA (PLCA) was used to detect the carbon footprint of stone fruit production [43]. Additionally, Laurenzi et al. [111] investigated the life cycle of greenhouse gas emissions and water use in Bakken tight oil. A multiregional hybrid life-cycle assessment (MRHLCA) model was built by Liu et al. [34] to evaluate the carbon emissions and water use in the non-electric energy sector, and the spatially explicit life cycle assessment (SELCA) was used to evaluate the carbon emission intensity in the water supply process in California [26].
However, the LCA method still has some limits. For example, the dynamic interactions of the water–carbon nexus cannot be captured by the traditional LCA approach, and there are difficulties in detecting the regional interactions [12]. There may exist aggregation errors in the IO-LCA method because sectors with similar functions or positions may be classified as one in IO tables [112]. Additionally, the PLCA method may lead to significant truncation errors in the calculations [61]. In this case, the hybrid life-cycle analysis, which is combined with IO-LCA and PLCA methods, may be superior in nexus studies.

4.3. Material Flow Analysis

Material flow analysis (MFA) provides researchers with a path to detect the flows and stocks of materials during the whole life cycle in urban systems [113]. It can be used to describe the relationship between urban systems and the environment by calculating the carbon and water flows to and from an urban area [114]. Liang et al. [38] quantified the food–energy–water nexus in the Detroit Metropolitan Area by using the material and energy flow analysis. Elshkaki et al. [29] analyzed the water flows and CO2 emissions in electricity generation systems under different scenarios. Sun et al. [32] discussed the synergy between materials and energy in the iron and steel industry.
Previous studies tended to use this method to analyze material flows in a complete urban system, involving many departments or systems, or focused on the material flow in different situations in one department. However, there is a lack of studies about how the materials, water, and carbon are re-utilized and flow between cooperative sectors, which is important to the sustainable development of cities.

4.4. Other Methods

Studies carried out in recent years showed that various methodologies have been applied in water–carbon nexus studies. Linkage analysis, originating from input–output analysis, has been used by Fang et al. [37]. This method is able to describe the carbon and water flows in the urban systems by detecting the backward linkages and forward linkages [115]. Additionally, principal components analysis (PCA) was used to discuss the energy-water–carbon nexus pressure transmitted in sectors in Shanghai, China [116]. Additionally, the ecological footprinting was regarded as an alternative approach to MFA, which was used to calculate the land that needs to provide resources based on equivalences [117]. Besides, the system dynamic model [13], data envelopment analysis [25] and the relative aggregate footprint method [31] have been used in the nexus studies as well.

5. Limits and Directions of the Future Research

The low-carbon strategy has exacerbated the carbon-water conflict, which is required for the balance in the diversified energy structure. The extensive energy structure transformation promoted the transition from electric energy to non-electric energy, in which the latter one often has higher requirements for water resources. In other words, there is a tendency of large water consumption in exchange for carbon reduction. However, there is still a lack in the quantification of the internal two-way response mechanism between carbon and water, that is, the proportion of carbon–water conversion in different sectors is not clear, which hinders the improvement in the water–carbon balance. For example, when the hydropower station has been put into operation, it had the tendency to sacrifice a small water consumption in exchange for a large amount of carbon emission reduction, and this benefit grew as the hydropower plant’s operating time increased [67]. However, in other sectors and scales, it is still unclear whether there is such a stable relationship between carbon and water. Since excessive water consumption is detrimental to urban development, how to keep the water–carbon balance in urban systems will be the focus of future research.
Second, the systematic boundary of the nexus needs to be improved. On the one hand, the urban system is not independent from other regions or systems. However, most studies were limited to a single sector or country, ignoring the water–carbon interactions across different sectors or regions. Additionally, for the studies on the whole urban system, although sectors such as manufacturing and agriculture are widely considered to be the key nodes of the carbon–water nexus, the unclear definition of academic boundary leads to obvious differences in the results and lack of universal conclusion. On the other hand, the water and carbon indicators can be further completed since some energy-independent processes were not considered. For example, some nexus studies in agriculture may only take the carbon emissions from energy consumption into consideration, but others ignore the indirect emissions from the decomposition of pesticides because of the limits in data collection. Therefore, the precise definition of the system boundary, refined data management, and more complete models need to be considered. Additionally, nexus studies in interregional and international trade are necessary.
Besides, the studies in water–carbon nexus are limited by the accuracy of data management, especially for studies based on IO tables. Take the construction industry as an example—different types of construction are not identified, respectively, in the construction sector, and the statistical accuracy of different data may be different [118]. Therefore, a more accurate and systematic data management model may provide the possibility for a more detailed analysis of intra-and inter-sectoral water–carbon nexus in the future.

6. Conclusions

Large amounts of CO2 emissions and water consumption during the urbanization process have brought global environmental deterioration and climate anomalies in recent decades, which have promoted the transformation of energy structure, the technology development, and ultimately affected the path of urbanization. There are studies analyzing the water–carbon nexus in urban systems, revealing the water–carbon interaction mechanism in the urban systems. Additionally, this study aimed to figure out the key nodes in the water–carbon nexus in the urban systems and to explore feasible ways for improvement.
This study first reviewed the definition of the water–carbon nexus. Additionally, then, we concluded the main research focuses on the urban systems and summarized methods applied in the nexus research. At last, we discussed the limits existing in previous studies and gave suggestions for future improvement. The main conclusions are as follows:
(1) From the perspective of urban metabolism, the water/wastewater treatment in water systems and the structure transformation in energy systems are the key nodes for the water–carbon nexus, in which the technical improvements are required. Additionally, the key nodes in consumption systems differed a lot due to the influence of the boundary definition, data sources, and research methods.
(2) From the perspective of methodologies, many methods, such as the IOA, LCA, and MFA methods, have been widely used to analyze the nexus from different scales. The research focus has gradually changed from single system and single process analysis to multi-system and whole process analysis.
(3) The tendency to sacrifice large water consumption in exchange for carbon reduction widely existed in the urban systems due to the low-carbon policies and energy structure transformation, which makes the water–carbon balance the focus of future researches. However, the mechanism of water–carbon transformation is still unclear, which requires more rigorous methodologies to quantify the carbon–water nexus.
(4) Besides, the inaccurate definition of the systematic boundary and the lack of accurate data are the main obstacles to current theoretical researches on water–carbon nexus. Thus, more comprehensive methods, systematic data support, and accurate definition of system boundaries are required to quantify the nexus.
Overall, this study comprehensively reviewed the water–carbon nexus in different sectors and scales of urban systems. This work will largely improve our understanding of urban metabolic processes and provide strategic recommendations for sustainable development of cities.

Author Contributions

Writing—Original draft preparation, X.H. and W.-Y.S.; writing—review and editing, W.-Y.S. and Y.-X.Y. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (No. 41975114), the Chongqing Outstanding Youth Science Foundation (No. cstc2021jcyj-jqX0025), JSPS BRIDGE Fellowship (No. BR221301), and the Chongqing elite-innovation and entrepreneurship demonstration team (to Wei-Yu Shi).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

No new data were created or analyzed in this study. Data sharing is not applicable to this article.

Conflicts of Interest

The authors declare no conflict of interest.

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Water 15 01005 g001 550
Figure 1. Energy–carbon–water nexus under the urban metabolism framework.
Figure 1. Energy–carbon–water nexus under the urban metabolism framework.
Water 15 01005 g001
Table
Table 1. Main research scales and methodologies of water–carbon nexus studies.
Table 1. Main research scales and methodologies of water–carbon nexus studies.
DirectionScaleMethodologyReference
Water systemsWastewater treatment plantRemote sensingSamuelsson et al., 2018 [24]
Water supplyData envelopment analysisMa et al., 2022 [25]
Spatially explicit life-cycle assessmentFang et al., 2015 [26]
City water systemSystem dynamic analysisChhipi-Shrestha et al., 2017 [13]
Life-cycle analysisVenkatesh et al., 2014 [27]
Energy systemsElectricity generation industryLife-cycle analysisMeldrum et al., 2013 [28]
Material flow analysisElshkaki, A., 2019 [29]
National energy technology power modelTang et al., 2020 [30]
Electricity generation technologyRelative aggregate footprintRistic et al., 2019 [31]
Iron and steel industryMaterial flow analysisSun et al., 2020 [32]
Hydropower industryLife-cycle analysisWang et al., 2019 [33]
Non-electric energy sectors Multiregional hybrid life-cycle analysisLiu et al., 2018 [34]
Urban economic systemsSectors in urban agglomerationsEnvironment extended input-output analysisLuo et al., 2022 [35]
Multivariate statistical input-output modelWang et al., 2022 [36]
Provincial economic sectorsMultiregional input-output modelFang et al., 2018 [37]
Sectors in citiesMaterial flow analysisLiang et al., 2019 [38]
Principal components analysisYang et al., 2018 [39]
OthersTourismEnvironmentally extended input-output modelLee et al., 2021 [40]
International tradeMultiregional input-output modelTobarra et al., 2018 [14]
ConstructionHybrid
input-output life-cycle analysis		Cheng et al., 2020 [41]
HouseholdLife-cycle analysisSong et al., 2015 [42]
AgricultureProcess-based life-cycle analysisNunez-Cardenas et al., 2022 [43]
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Han, X.; Shi, W.-Y.; Yao, Y.-X. A Review of the Water–Carbon Nexus in Urban Systems. Water 2023, 15, 1005. https://doi.org/10.3390/w15061005

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