The energy–water nexus: are there tradeoffs between residential energy and water consumption in arid cities?

Water scarcity, energy consumption, and air temperature regulation are three critical resource and environmental challenges linked to urban population growth. While appliance efficiency continues to increase, today’s homes are larger and residents are using more energy-consuming devices. Recent research has often described the energy–water nexus as a “tradeoff” between energy and water due to reduced temperatures resulting from irrigated vegetation. Accordingly, some arid cities have implemented landscape-conversion programs that encourage homeowners to convert their yards from grass (mesic) to drought-tolerant (xeric) landscapes to help conserve water resources. We investigated these relationships in Phoenix, Arizona by examining energy and water data for the summer months of June–September 2005 while temperature variability was analyzed from a local heat wave. Results show parallel consumption patterns with energy and water use strongly correlated and newer homes using more of both. The counterintuitive findings show that “drought-resistant” models may not be beneficial for community health, environment, or economics and that this issue is further complicated by socio-economic variables.     

African Urbanization: Assimilating Urban Metabolism into Sustainability Discourse and Practice

Shaping sustainable, equitable African cities requires strengthened investigations into the cities’ current resource flows, infrastructure systems, and future resource requirements. The field of urban metabolism (UM) offers multiple forms of analysis with which to map, analyse, and visualize urban resource profiles. Challenges in assessing UM in African cities include data scarcity at the city level, difficulty in tracking informal flows, lack of standardized methods, and the open nature of cities. However, such analyses are needed at the local level, given that city practitioners cannot rely purely on urban planning traditions of the global North or the typically broad studies about urban Africa, for supporting strategies toward sustainable urban development. This article aims to draw together the concepts of sustainable development and UM and explore their application in the African context. Further, the article estimated resource profiles for 120 African cities, including consumption of biomass, fossil fuels, electricity, construction materials, and water, as well as emissions of carbon dioxide. These resource profiles serve as a baseline from which to begin assessing the current and future resource intensity of these cities. It also provides insights into the cities’ relative resource impact, future consumption trends, and potential options for sustainability interventions.     

How much heat can we grow in our cities? Modelling UK urban biofuel production potential

Biofuel provides a globally significant opportunity to reduce fossil fuel dependence; however, its sustainability can only be meaningfully explored for individual cases. It depends on multiple considerations including: life cycle greenhouse gas emissions, air quality impacts, food versus fuel trade‐offs, biodiversity impacts of land use change and socio‐economic impacts of energy transitions. One solution that may address many of these issues is local production of biofuel on non‐agricultural land. Urban areas drive global change, for example, they are responsible for 70% of global energy use, but are largely ignored in their resource production potential; however, underused urban greenspaces could be utilized for biofuel production near the point of consumption. This could avoid food versus fuel land conflicts in agricultural land and long‐distance transport costs, provide ecosystem service benefits to urban dwellers and increase the sustainability and resilience of cities and towns. Here, we use a Geographic Information System to identify urban greenspaces suitable for biofuel production, using exclusion criteria, in 10 UK cities. We then model production potential of three different biofuels: Miscanthus grass, short rotation coppice (SRC) willow and SRC poplar, within the greenspaces identified and extrapolate up to a UK‐scale. We demonstrate that approximately 10% of urban greenspace (3% of built‐up land) is potentially suitable for biofuel production. We estimate the potential of this to meet energy demand through heat generation, electricity and combined heat and power (CHP) operations. Our findings show that, if fully utilized, urban biofuel production could meet nearly a fifth of demand for biomass in CHP systems in the United Kingdom's climate compatible energy scenarios by 2030, with potentially similar implications for other comparable countries and regions.     

Assessing influence in regression analysis with censored data.

In this paper we show how to evaluate the effect that perturbations to the model, data, or case weights have on maximum likelihood estimates from censored survival data. The ideas and methods also apply to other nonlinear estimation problems. We review the ideas behind using log-likelihood displacement and local influence methods. We describe new interpretations for some local influence statistics and show how these statistics extend and complement traditional case deletion influence statistics for linear least squares. These statistics identify individual and combinations of cases that have important influence on estimates of parameters and functions of these parameters. We illustrate the methods by reanalyzing the Stanford Heart Transplant data with a parametric regression model.     

The metabolism of U.S. cities 2.0

In the fifty years since Abel Wolman first published an estimate of U.S. urban metabolism, the field of urban metabolism has begun to thrive, with cities outside the United States being much of the focus. As cities attempt to meet local and international sustainability goals, it is time to revisit the metabolism of cities within the United States. Using existing empirical databases for material flows (the Freight Analysis Framework) and a published database on urban water flux, we provide a revised estimate of urban metabolism for the typical U.S. city. We estimate median values of metabolism for a city of one million people, considering water resources, food, fuel, and construction materials. Food consumption and waste production increased substantially to 3,800 metric tons per day and 4,900 metric tons per day, respectively. To facilitate a second generation of urban metabolism, we extend traditional analyses to include the embedded energy required to facilitate material consumption with important implications in determining sustainable urban metabolism. We estimate that a city of one million people requires nearly 4,000 gigajoules of primary energy per day to facilitate its metabolism. Our results show high heterogeneity of urban metabolism across the United States. As a result of the study, we conclude that there is a distinct need to promote policies at the regional or city scale that collect data for urban metabolism studies. Urban metabolism is an important educational and decision‐making tool that, with an increase in data availability, can provide important information for cities and their sustainability goals.     

A Database to Facilitate a Process‐Oriented Approach to Urban Metabolism

In view of urbanization trends coupled with climate‐change challenges, it is increasingly important to establish less‐harmful means of urban living. To date, urban metabolism (UM) studies have quantified the aggregate material and energy flows into and out of cities and, further, have identified how consumer activity causes these flows. However, little attention has been paid to the networks of conversion processes that link consumer end‐use demands to aggregate metabolic flows. Here, we conduct a systematic literature search to assemble a database of 202 urban energy, water, and waste management processes. We show how the database can help planners and policy makers choose the preferred process to meet a specific resource management need; identify synergies between energy, water, and waste management processes; and compute optimal networks of processes to meet an area's consumer demand at minimum environmental cost. We make our database publicly available under an open‐source license and discuss the possibilities for how it might be used alongside other industrial ecology data sets to enhance research opportunities. This will encourage more holistic UM analyses, which appreciate how both consumer activity and the engineered urban system work together to influence aggregate metabolic flows and thus support efforts to make cities more sustainable.     

Exploring two methods for statistical downscaling of Central European phenological time series

In this study we set out to investigate the possibility of linking phenological phases throughout the vegetation cycle, as a local-scale biological phenomenon, directly with large-scale atmospheric variables via two different empirical downscaling techniques. In recent years a number of methods have been developed to transfer atmospheric information at coarse General Circulation Model's grid resolutions to local scales and individual points. Here multiple linear regression (MLR) and canonical correlation analysis (CCA) have been selected as downscaling methods. Different validation experiments (e.g. temporal cross-validation, split-sample tests) are used to test the performance of both approaches and compare them for time series of 17 phenological phases and air temperatures from Central Europe as microscale variables. A number of atmospheric variables over the North Atlantic and Europe are utilized as macroscale predictors. The period considered is 1951–1998. Temporal cross-validation reveals that the CCA model generally performs better than MLR, which explains 20%–50% of the phenological variances, whereas the CCA model shows a range from 40% to over 60% throughout most of the vegetation cycle. To show the validity of employing phenological observations for downscaling purposes both methods (MLR and CCA) are also applied to gridded local air temperature time series over Central Europe. In this case there is no obvious superiority of the CCA model over the MLR model. Both models show explained variances from 40% to over 70% in the temporal cross-validation experiment. The results of this study indicate that time series of phenological occurrence dates are very compatible with the needs of empirical downscaling originally developed of local-scale atmospheric variables.     

Ecological network analysis of urban–industrial ecosystems

Sustainability of urban areas is paramount in the coming years as cities continue to grow in population and resource consumption. A number of methods to model cities have been developed, including material flow analysis and urban metabolism, but these accounting methods do not fully analyze the complex network dynamics present within cities. Ecological network analysis (ENA) provides a new perspective into these urban areas by using metrics designed for analysis of natural ecosystems. This study analyzes 29 urban–industrial ecosystems using ENA, comparing the networks to each other as well as comparing them to previously analyzed eco‐industrial parks and natural food webs. It is found that these systems perform similar to other human‐designed systems, which consistently lack in ecological performance when compared with the natural ecosystems. Additionally, the impact of specific actor types within these networks is shown indicating the importance of industry, agriculture, and the natural environment. Finally, the types of networks are determined to affect the ecological metrics, with the more linear‐based energy networks having the worst performance. This new dataset of ecologically analyzed networks provides a deeper understanding of urban networks and their infrastructure, while providing useful information on how to potentially improve their sustainability.     

基于投入产出模型的黄河“几字弯”城市群用水特征与节水关键区域

黄河“几字弯”区域是黄河流域的关键节点,是实现中西部崛起的关键区域,也是黄河流域生态环境保护和高质量发展的战略支撑。然而,缺水已经成为制约“几字弯”地区经济社会发展的最大瓶颈。利用投入产出模型,从生产侧的实体水和消费测的虚拟水出发,分析“几字弯”地区18个城市的用水量、结构变化和流入流出特征,结合生产与消费两个方面提出了“几字弯”地区城市群节水的重点区域及产业供水调整的建议。（1）“几字弯”区域虚拟水用量由2007年648.5亿m3增加至2017年778.4 亿m3,增加了20.0%,但实体水用量仅变化了-0.04%,各城市虚拟水增长类型差异较大。（2）18个城市实体水占虚拟水的比例由2007年41.7%下降至2017年34.8%,水资源利用效率逐步提高;实体水的用水大户是农业,虚拟水的用水大户是制造业和农业,服务业实体水和虚拟水用量剧增。（3）区域18个城市虚拟水流入量增长明显,但整体仍呈现出虚拟水净流出的特征。制造业虚拟水流出量均在188.9亿m3以上,流入量均不足108.9亿m3,制造业在区域经济中具有一定的比较优势。（4）巴彦淖尔、中卫、吴忠、石嘴山和白银市是“几字弯”18个城市中重点节水的对象。在这些城市中,农业用水比重较大,产出较低,农业是今后节水的重点。鄂尔多斯、包头、石嘴山、乌海制造业具有比较优势,巴彦淖尔的农业用水系数远远高于18个城市的平均值,需要控制其农业用水。对于鄂尔多斯、包头、石嘴山、乌海、巴彦淖尔5市,需要向当地的优势产业制造业和服务业进行供水倾斜、提高比较优势。     

The Changing Metabolism of Cities

Data from urban metabolism studies from eight metropolitan regions across five continents, conducted in various years since 1965, are assembled in consistent units and compared. Together with studies of water, materials, energy, and nutrient flows from additional cities, the comparison provides insights into the changing metabolism of cities. Most cities studied exhibit increasing per capita metabolism with respect to water, wastewater, energy, and materials, although one city showed increasing efficiency for energy and water over the 1990s. Changes in solid waste streams and air pollutant emissions are mixed.
The review also identifies metabolic processes that threaten the sustainability of cities. These include altered ground water levels, exhaustion of local materials, accumulation of toxic materials, summer heat islands, and irregular accumulation of nutrients. Beyond concerns over the sheer magnitudes of resource flows into cities, an understanding of these accumulation or storage processes in the urban metabolism is critical. Growth , which is inherently part of metabolism, causes changes in water stored in urban aquifers, materials in the building stock, heat stored in the urban canopy layer, and potentially useful nutrients in urban waste dumps.
Practical reasons exist for understanding urban metabolism. The vitality of cities depends on spatial relationships with surrounding hinterlands and global resource webs. Increasing metabolism implies greater loss of farmland, forests, and species diversity; plus more traffic and more pollution. Urban policy makers should consider to what extent their nearest resources are close to exhaustion and, if necessary, appropriate strategies to slow exploitation. It is apparent from this review that metabolism data have been established for only a few cities worldwide, and interpretation issues exist due to lack of common conventions. Further urban metabolism studies are required.     

Energy efficient job scheduling with workload prediction on cloud data center

Data centers are the backbone of cloud infrastructure platform to support large-scale data processing and storage. More and more business-to-consumer and enterprise applications are based on cloud data center. However, the amount of data center energy consumption is inevitably lead to high operation costs. The aim of this paper is to comprehensive reduce energy consumption of cloud data center servers, network, and cooling systems. We first build an energy efficient cloud data center system including its architecture, job and power consumption model. Then, we combine the linear regression and wavelet neural network techniques into a prediction method, which we call MLWNN, to forecast the cloud data center short-term workload. Third, we propose a heuristic energy efficient job scheduling with workload prediction solution, which is divided into resource management strategy and online energy efficient job scheduling algorithm. Our extensive simulation performance evaluation results clearly demonstrate that our proposed solution has good performance and is very suitable for low workload cloud data center.     

Robust optimization for energy-efficient virtual machine consolidation in modern datacenters

Energy efficient virtual machine (VM) consolidation in modern data centers is typically optimized using methods such as Mixed Integer Programming, which typically require precise input to the model. Unfortunately, many parameters are uncertain or very difficult to predict precisely in the real world. As a consequence, a once calculated solution may be highly infeasible in practice. In this paper, we use methods from robust optimization theory in order to quantify the impact of uncertainty in modern data centers. We study the impact of different parameter uncertainties on the energy efficiency and overbooking ratios such as e.g. VM resource demands, migration related overhead or the power consumption model of the servers used. We also show that setting aside additional resource to cope with uncertainty of workload influences the overbooking ration of the servers and the energy consumption. We show that, by using our model, Cloud operators can calculate a more robust migration schedule leading to higher total energy consumption. A more risky operator may well choose a more opportunistic schedule leading to lower energy consumption but also higher risk of SLA violation.     

基于多角度基尼系数的江西省资源环境公平性研究

资源环境基尼系数是衡量和评价资源消耗、污染物排放在各个城市或地区间的公平性与合理性的有效途径。对基于GDP(gross domestic product)、人口、国土面积等角度推算的资源环境基尼系数方法的优劣性进行了评价,提出了"生态负荷系数"的概念,界定了"生态容量"的概念与内容,并从GDP、人口和生态容量的角度出发,以江西省11地市作为评价对象,选取2009年能源消耗、水资源消耗、COD(chemical oxygen demand)排放及SO2排放等4个评价因子,构建了江西省资源环境基尼系数、生态负荷系数的计算与评价方法,并对计算结果进行了对比分析。结果表明:(1)不同角度计算的基尼系数不尽相同,从生态负荷系数来看,在江西11个城市中,南昌、鹰潭、萍乡、景德镇、新余等经济发达城市是引起能源消耗、SO2排放主要的不公平因子,是需要转变经济发展模式的城市,更须注重经济与环境的协调发展;(2)从生态容量的角度推算的资源环境基尼系数普遍高于从GDP和人口角度推算的资源环境基尼系数,且都高于"警戒线";(3)从理论上看,基于生态容量角度的资源环境基尼系数比基于GDP和人口角度的资源环境基尼系数更为科学与合理。文章最后讨论了不同角度的资源环境基尼系数及生态负荷系数的科学性与合理性的问题。     

“双碳”目标下东北三省水-能源纽带关系及网络特征分析

厘清水-能源联动关系,优化水-能源消费结构是实现“碳达峰、碳中和”的首要途径。构建区域资源-经济投入产出模型,以2017年为基准年对东北三省水资源和能源消耗进行测算,并通过展布虚拟水-能源足迹的空间转移变化,分析部门资源流通状况,在此基础上耦合多源数据构建水-能源系统生态网络分析模型,探究水-能源网络耦合特征。结果显示：(1)2017年东北三省直接水资源消费总量为561.97亿m~3,能源隐含水总量为30.48亿m~3,水资源隐含能消费量为699.25万tce,直接能源消费量为40579.95万tce,虚拟水足迹为474.13亿m~3,虚拟能源足迹为54668.6万tce;(2)虚拟水足迹部门转移变化为：辽宁省最大输入路径为食品制造及烟草加工业-农业,吉林省和黑龙江省输入路径则相反;虚拟能源部门转移变化中最大流通部门分别为：建筑业-第三产业/金属冶炼及制品业、采选业-化学工业。(3)水-能源网络循环率均低于40‰,竞争关系是水-能源网络效用矩阵关系中主流关系;在三省中,吉林省水-能源网络的协作性最优,生态网络处于更为稳定的状态之中;水-能源网络耦合后,各省虚拟资源网络共生指数呈现不同程...     

Regional normalisation figures for Australia 2005/2006—inventory and characterisation data from a production perspective

Background, aim, and scope  Under ISO 14040, normalisation is an optional step in life-cycle impact assessment designed to provide environmental context by indicating the relative contribution that the product system under investigation makes in the various impact categories, in comparison to a suitable reference scenario. The challenge for many studies, however, is to provide the appropriate context by adopting a normalisation reference scenario that is well matched to the product system’s parent environment. Australia has a highly urbanised population, mainly contained in just eight capital cities. In the context of normalising environmental impacts against the profile of an ‘average’ Australian, this poses a unique problem, compared to other industrialised regions of the world. This study aims to use publicly available data on environmentally relevant emissions and non-renewable resource consumption in 2005/2006 to develop regional normalisation data for Australia, at both inventory and characterisation levels. Methods  The regionalised inventory of emissions and resources production is constructed using a framework of 60 regional Statistical Divisions from the Australian Standard Geographical Classification system. Data from the National Pollutant Inventory, Australian Greenhouse Emissions Information System and the Australian Bureau of Agricultural and Resource Economics (energy and mineral statistics) are used as the basis for the inventories. These data could subsequently be used by any LCA practitioner to construct characterisation or normalisation data by impact category, according to any preferred life-cycle impact assessment methodology, for any of 60 regions in the country. In this study, the regionalised inventory data were assessed using the CML 2001 baseline and IMPACT 2002+ life-cycle impact assessment methods in SimaPro v.7.1.5. Results  Characterisation results from the two LCIA methods for Australia’s eight state and territory capital cities are presented, together with an overall national profile. These data are also shown on a per capita basis to highlight the relative environmental profiles of citizens in the different cities. Interestingly, many significant impacts occur outside of the capital cities but are linked to facilities providing the majority of their services and products to these urban centres (e.g. power stations, minerals processing). Comparison of the average Australian data with the Netherlands, Western Europe and the World shows the results to be broadly similar. Discussion  Analysis of the CML 2001 baseline characterisation results, on a per capita basis, shows substantial differences between the major cities of the country. In each impact category, these differences can be successfully traced to specific emissions in the raw data sources, the influence of prevailing climate conditions, or factors such as the mix of non-renewable energy resources in each state. Some weaknesses are also evident in the collection and estimation techniques of the raw data sources and in the application of European-based impact assessment models. Australia is a net exporter of many products, particularly natural resources. Therefore, a significant part of the characterisation data presented here for Australia represents products that will be consumed in other parts of the world. Similarly, at a regional level, there will be many inventory items produced in one area yet consumed in another. In this way, the impacts associated with consumption (particularly in densely populated but largely industry-free cities) are dissipated into other production centres. Conclusions  This study provides the first set of comprehensive inventory and characterisation data for Australia from a production perspective, disaggregated at a regional level. Despite Australia’s unique spatial demography, it is now possible to properly characterise the relative significance of environmental impacts occurring in any of 60 specific regions across the country. Recommendations and perspectives  Australia’s unique concentration of urban populations demonstrates the importance of regionally specific environmental assessments. Whilst the data presented in this study will be of most use to Australian LCA practitioners, it is also demonstrative of the broader global distribution of environmental impacts between urban and non-urban areas. The disconnection of environmental impacts between the place of production and the place of consumption is highlighted by this study and should be considered in any studies using these normalisation data for environmental profiling. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Ecosystem Evapotranspiration as a Response to Climate and Vegetation Coverage Changes in Northwest Yunnan,China

Climate and human-driven changes play an important role in regional droughts. Northwest Yunnan Province is a key region for biodiversity conservation in China, and it has experienced severe droughts since the beginning of this century; however, the extent of the contributions from climate and human-driven changes remains unclear. We calculated the ecosystem evapotranspiration (ET) and water yield (WY) of northwest Yunnan Province, China from 2001 to 2013 using meteorological and remote sensing observation data and a Surface Energy Balance System (SEBS) model. Multivariate regression analyses were used to differentiate the contribution of climate and vegetation coverage to ET. The results showed that the annual average vegetation coverage significantly increased over time with a mean of 0.69 in spite of the precipitation fluctuation. Afforestation/reforestation and other management efforts attributed to vegetation coverage increase in NW Yunnan. Both ET and WY considerably fluctuated with the climate factors, which ranged from 623.29 mm to 893.8 mm and –51.88 mm to 384.40 mm over the time period. Spatially, ET in the southeast of NW Yunnan (mainly in Lijiang) increased significantly, which was in line with the spatial trend of vegetation coverage. Multivariate linear regression analysis indicated that climatic factors accounted for 85.18% of the ET variation, while vegetation coverage explained 14.82%. On the other hand, precipitation accounted for 67.5% of the WY. We conclude that the continuous droughts in northwest Yunnan were primarily climatically driven; however, man-made land cover and vegetation changes also increased the vulnerability of local populations to drought. Because of the high proportion of the water yield consumed for subsistence and poor infrastructure for water management, local populations have been highly vulnerable to climate drought conditions. We suggest that conservation of native vegetation and development of water-conserving agricultural practices should be implemented as adaptive strategies to mitigate climate change.     

Scaling in urban building energy use and its influencing factors

Several studies have reported scaling relationships for energy consumption with respect to city size and other indicators. However, such scaling relationships have rarely been reported at the suburban level. This study explored the scaling relationships between energy use (EU) and building size (gross floor area [GFA]) at the building level in 16 urban regions in the United States from 2011 to 2019. We found that the scaling exponents of most of the examined regions changed from either super-linear or sub-linear to linear (β = 1) over the years. The scaling exponents of some cities (e.g., New York City) fluctuated around 1. These scaling exponents are negatively correlated with regional climate. This study reports that the scaling relationships between energy consumption and GFA at the building level in heterogeneous cities are evolving toward linear scaling. This study also found that different building types and building energy structures significantly impact building energy consumption. Hotels in New York City had the highest scaling exponent (β = 1.02), and strong correlations were observed between the scaling exponents and the share of electricity in building EU. These insights reveal the common nature of the relationships between building EU and GFA and the intersections between scaling exponents and building attributes. Our study highlights the importance of energy efficiency management in hotels and electricity-dominated buildings.     

基于数据包络分析方法的城市水资源利用效率研究

为了对我国城市水资源利用效率问题进行分析和评价,本文基于数据包络分析方法,从农业、工业、生活、社会等几个方面共选取了5个输入指标及6个输出指标,利用AIC信息准则(Akaike information criterion)进行了变量选择,构建了较为科学合理的用水效率评价指标体系.在此基础上,采用香农熵指数提升了传统CCR(由Charnes A,Cooper W W,Rhodes E提出)模型的识别能力,选取我国31个省会城市为研究对象,给出了省会城市水资源效率的完整排名。结果表明:①绝大部分城市综合效率得分(CES,Comprehensive efficiency score)普遍不高,投入产出比仍有较大的进步空间;②拉萨、北京、天津、银川、海口、上海等城市CES得分相对较高,这说明一个城市水资源利用效率的高低与经济发展水平可能没有必然的联系,其他城市应结合自身情况向CES得分靠前的城市进行学习;③重庆、南宁、南昌、长沙等水资源较丰富的城市CES得分反而较低,表明这些城市可能存在大量水资源被浪费,应建立起节水机制,同时优化产业结构。     

The Water Withdrawal Footprint of Energy Supply to Cities

Water footprints traditionally estimate water lost as a result of evapotranspiration (or otherwise unavailable for downstream uses) associated with producing a certain good, and the same embodied in trade across regions is used to estimate regional and national water footprints. These footprints, however, do not address risk posed to city energy supplies characterized by insufficient streamflow to support energy production, such as cooling water intake (e.g., withdrawals) at thermoelectric power plants. Water withdrawal intensity factors for producing goods and services are being developed at the national scale, but lack sufficient spatial resolution to address these types of water‐energy challenges facing cities. To address this need, this article presents a water withdrawal footprint for energy supply (WWFES) to cities and places it in the context of other water footprints defined in the literature. Analysis of electricity use versus electricity generation in 43 U.S. cities highlights the need for developing WWFES to estimate risks to transboundary city energy supplies resulting from water constraints. The magnitude of the WWFES is computed for Denver, Colorado, and compared to the city's direct use of water to offer perspective. The baseline WWFES for Denver is found to be 66% as large as all direct water uses in the city combined (mean estimate). Minimum, mean, and maximum estimates are computed to demonstrate sensitivity of the WWFES to selection of water withdrawal intensity factors. Finally, scenario analysis explores the effect of energy technology and energy policy choices in shaping the future water footprint of cities.