Water Footprint of Cities: A Review and Suggestions for Future Research

Assessment of vulnerability to water shortage in semi-arid river basins: The value of demand reduction and storage capacity

Interest in securing reliable water supplies has increased due to climate change and rapid population growth. This challenge is significant in growing areas with limited water supplies. To meet water demands, water managers are considering new storage infrastructure to increase the reliability of water supplies while also identifying opportunities to reduce water use per person. Although these strategies change water consumption patterns, their success at reducing shortages across space and time for different climate change scenarios remains unclear. In this paper, population- and climate-dependent future water supply and demand models are developed and integrated into a water allocation model calibrated for the South Platte River Basin of Colorado. Eight future climate scenarios are simulated using four statistically downscaled models from the Coupled Model Inter-Comparison Project Phase 5 (CMIP5) with two Representative Concentration Pathways (RCP). Lastly, findings from the water allocation model simulations are generalized beyond the study area using a novel approach by introducing dimensionless indices to characterize water shortage and basin conditions. Results reveal a threshold ratio of total storage capacity to mean water supply with a value of 0.64 above which additional storage has no effect on total water shortages. This threshold communicates the limitation of building storage infrastructure as a strategy to adapt to decreasing average water supplies for basins considering increasing storage capacity. However, basins with low current capacity are likely to fall below the threshold and could invest in reservoirs to mitigate future shortages.

Simulation of land use/land cover changes and urban expansion in Estonia by a hybrid ANN-CA-MCA model and utilizing spectral-textural indices

Over the recent two decades, land use/land cover (LULC) drastically changed in Estonia. Even though the population decreased by 11%, noticeable agricultural and forest land areas were turned into urban land. In this work, we analyzed those LULC changes by mapping the spatial characteristics of LULC and urban expansion in the years 2000-2019 in Estonia. Moreover, using the revealed spatiotemporal transitions of LULC, we simulated LULC and urban expansion for 2030. Landsat 5 and 8 data were used to estimate 147 spectral-textural indices in the Google Earth Engine cloud computing platform. After that, 19 selected indices were used to model LULC changes by applying the hybrid artificial neural network, cellular automata, and Markov chain analysis (ANN-CA-MCA). While determining spectral-textural indices is quite common for LULC classifications, utilization of these continues indices in LULC change detection and examining these indices at the landscape scale is still in infancy. This country-wide modeling approach provided the first comprehensive projection of future LULC utilizing spectral-textural indices. In this work, we utilized the hybrid ANN-CA-MCA model for predicting LULC in Estonia for 2030; we revealed that the predicted changes in LULC from 2019 to 2030 were similar to the observed changes from 2011 to 2019. The predicted change in the area of artificial surfaces was an increased rate of 1.33% to reach 787.04 km² in total by 2030. Between 2019 and 2030, the other significant changes were the decrease of 34.57 km² of forest lands and the increase of agricultural lands by 14.90 km² and wetlands by 9.31 km². These findings can develop a proper course of action for long-term spatial planning in Estonia. Therefore, a key policy priority should be to plan for the stable care of forest lands to maintain biodiversity.

City footprints and SDGs provide untapped potential for assessing city sustainability

Cities are recognised as central to determining the sustainability of human development. However, assessment concepts that are able to ascertain whether or not a city is sustainable are only just emerging. Here we review literature since the Sustainable Development Goals (SDGs) were agreed in 2015 and identify three strands of scientific inquiry and practice in assessing city sustainability. We find that further integration is needed. SDG monitoring and assessment of cities should take advantage of both consumption-based (footprint) accounting and benchmarking against planetary boundaries and social thresholds in order to achieve greater relevance for designing sustainable cities and urban lifestyles.

A Review of Water Stress and Water Footprint Accounting

Production and consumption activities deplete freshwater, generate water pollution and may further lead to water stress. The accurate measurement of water stress is a precondition for sustainable water management. This paper reviews the literature on physical water stress induced by blue and green water use and by water pollution. Specifically, we clarify several key concepts (i.e., water stress, scarcity, availability, withdrawal, consumption and the water footprint) for water stress evaluation, and review physical water stress indicators in terms of quantity and quality. Furthermore, we identify research gaps in physical water stress assessment, related to environmental flow requirements, return flows, outsourcing of water pollution and standardization of terminology and approaches. These research gaps can serve as venues for further research dealing with the evaluation and reduction of water stress.

Decomposition of Water Footprint of Food Consumption in Typical East Chinese Cities

Water scarcity has put pressure on city development in China. With a particular focus on urban and rural effects, logarithmic mean Divisia index decomposition (LMDI) was used to analyze the water footprint per capita (WFP) of food consumption in five East China cities (Beijing, Tianjin, Shanghai, Qingdao, and Xiamen) from 2008 to 2018. Results show that the WFP of food consumption exhibited an upward tendency among all cities during the research period. Food consumption structure contributed the most to the WFP growth, mainly due to urban and rural residents’ diet shift toward a livestock-rich style. Except in Beijing, the food consumption level mainly inhibited the WFP growth due to the decrease in food consumption level per capita in urban areas. Urbanization had less influence on WFP growth for two megacities (Beijing and Shanghai) due to the strictly controlled urban population inflow policy and more positive effects for other cities. The water footprint intensity effect among cities was mainly due to uneven water-saving efficiency. Meanwhile, Beijing and Tianjin have achieved advancement in water utilization efficiency.

Exploring consumption-based planetary boundary indicators: An absolute water footprinting assessment of Chinese provinces and cities

The water planetary boundary (PB) has attracted wide academic attention, but empirical water footprint research that accommodates local biophysical boundaries remains scarce. Here we develop two novel quantitative footprint indicators, the water exceedance footprint and the surplus water footprint. The first measures the amount of excessive water withdrawal (exceeded amount of water withdrawn against local water PBs) and the latter evaluates the potential of surplus water that can be sustainably utilised (amount of surplus water available within local water PBs). We quantify the extent to which demand for goods and services in Chinese provinces and cities are driving excessive withdrawal of local and global water resources. We investigate both territorial and consumption-based water withdrawal deficit and surplus against local water withdrawal PBs. We also trace how PB-exceeded water and surplus water are appropriated for producing certain commodities. In 2015, China's domestic water exceedance reaches 101 km³ while the total water exceedance footprint is 92 km³. We find that 47% of domestic excessive water withdrawal is associated with interprovincial trade. Exceeded water transfers were dominated by agricultural trade from the drier North to the wetter South. A revised virtual water trade network informed by exceedance and surplus water footprint metrics could help address sustainability concerns that arise from the trade of water-intensive commodities. Our findings highlight that policy targets need to accommodate PB exceedance of both direct and virtual water use.

Use of the Life Cycle Methodology to Calculate Energy Consumption of Urban Water Cycle: A Case Study of Ordos City

Water resources and energy constitute two broad categories of resources required for social and economic development. The water-energy nexus has become a focus of research in recent years. Although water resources are closely related to energy systems, the processes involved remain incompletely understood due to the diversity and complexity of energy types, processes, and consumption sectors. This study aimed to accurately calculate the energy demand of water resources and to identify an effective method of improving the energy utilization efficiency of water. The life cycle of water resources was divided into five stages based on the life cycle methodology: (1) extraction, (2) purification, (3) transportation, (4) utilization, (5) sewage treatment. The quantity and characteristics of energy consumed in each stage were studied, and an equation to calculate energy consumption for each stage was proposed. Using the city of Ordos in Northwest China as an example, energy consumption of water resources from 2013 to 2017 was analyzed. The results showed that from 2013 to 2017, energy consumption and per unit energy consumption of water resources in Ordos decreased by 33.3% and 30.6% from 1.62 × 108 kWh to 1.08 × 108 kWh and from 10.11 kWh·m−3 to 7.02 kWh·m−3, respectively. The majority of energy consumption over the entire life cycle of water resources occurred during the utilization stage at a proportion of total energy consumed of ~95%. Use of water for thermal power generation was identified as the single largest consumer of energy and, therefore, has the greatest potential for energy saving.

Integrating embedded resources and network analysis to understand food-energy-water nexus in the US

To find a sustainable way of supplying food, energy, and water (FEW) while simultaneously protecting the ecosystem services, it is imperative to build greater understanding on interconnections, feedback, and dependencies in FEW systems. The FEW nexus has developed as a field of study to provide frameworks for such pursuits. Building upon previous work in this paper, we analyze FEW resources through the development of a virtual water trade network using the US network of food and energy flows and their associated virtual water contents. Our main objective is to provide a quantitative estimation of the virtual water embodied in the internal US food and energy transfers and analyze the associated interdependencies of these connections. Three methodological advancements demonstrate the novelty of this work. First, unlike existing FEW virtual water modeling studies, our work separates corn into both food and energy resources accounting for the significant use of corn for ethanol in the United States. Second, we apply recently published water consumption values for energy commodities confirming the variation between previous water footprint studies and these more accurate accounting procedures. Third, we examine network properties of the trade flows furthering FEW nexus literature and showcasing avenues for future research. Our results indicate that accounting for the transfer of corn from the food commodity network to the energy commodity network leads to a virtual water footprint decline of 11% for the cereal grain virtual water network. Additionally, the food trade network shows highly dense and connected properties compared to the energy trade network. Finally, our results indicate that transfers of water footprints between water scarce and water abundant states differ substantially between food and energy virtual water networks. A quantifiable understanding of the water footprint network embodied in the food and energy trade can help in developing policies for promoting conservation and efficiency in the context of the FEW nexus.

Bridging the gap between city and water: A review of urban-river regeneration projects in France and Spain

The urban regeneration of European river spaces since the 1990s has allowed their transformation into new axes of centrality and socialization and has seen the concentration of green areas along the banks of Europe's cities. This process has been promoted by the implementation of various projects, conducted at different scales and with varying budgets, in a range of urban-river habitats; yet, all of them seek a common goal: the exploitation of river spaces as new areas for social uses. In Western Europe, both France and Spain have implemented major projects that serve as good examples of urban regeneration. This paper identifies, classifies and analyses the main French and Spanish urban-river regeneration projects described in studies published since 1990 through a literature review and a statistical treatment of data. We identified 47 publications, referring to 12 projects in France and 14 projects in Spain, that allow us to conduct an analysis of the characteristics of this urban regeneration process. Overall, the projects are highly varied, above all in terms of their scale, the way in which they use processes of public participation to reach citizen consensus, and their implementation of environmental measures as a complement to urban regeneration. The projects with the largest budgets, as well as those targeting the largest areas for regeneration, have been made possible thanks to EU funding. In contrast, smaller budgets and interventions of shorter length and area tend to characterize the projects without EU funds. Additionally, processes of public participation have also been a key element, above all in France, and most projects with environmental rehabilitation have been carried out in Spain.

A Bibliometrics Review of Water Footprint Research in China: 2003–2018

As water security becomes an increasingly important issue, the analysis of the conflict between water supply and demand has gained significance in China. This paper details a bibliometric review of papers published between 2003 and 2018 on the water footprint in China, one of the global hotspots of water resource research. The tendencies and key points of water footprint research were systematically analyzed based on 1564 articles, comprising 1170 original publications in Chinese from the China National Knowledge Infrastructure database and 394 publications in English from the Web of Science database. The results show that the literature associated with water footprint research has expanded significantly. The number of papers published increased from 104 in 2003–2006 to 735 in 2015–2018. Water footprint research has been applied to agricultural, industrial, and regional water resource management to quantify the impact of human activities on water resources and the environment. Water footprint metrics were extracted for regional comparisons. There are obvious regional characteristics of the water footprint in China, but the uncertainty of results makes further investigation necessary. Further water footprint modeling and field experimental research is needed to explore the water–ecological environment under complex systems.

Risk Assessment and Pressure Response Analysis of the Water Footprint of Agriculture and Livestock: A Case Study of the Beijing–Tianjin–Hebei Region in China

Excessive water consumption, associated with regional agriculture and livestock development and rapid urbanization, has caused significant stress to the ecological health and sustainable use of water resources. We used the water footprint theory to quantify the spatiotemporal characteristics and variation in the water footprint of agriculture and livestock (WF-AL) in the Beijing–Tianjin–Hebei region of China (2000–2016). We predicted the spatial distribution and sustainability of regional water resources at different levels of annual precipitation. Results showed that the average county WF-AL rose from 8.03 × 108 m3 in 2000 to 10.89 × 108 m3 in 2016. There was spatial heterogeneity compared to the average city WF-AL. The WF-AL varied between the mountains and the plains. The scale of the WF-AL was one of the main reasons for differences in the consumption and distribution of water resources. The development of regional water resources deteriorated from a stable state to an unstable state from 2000 to 2016. Only 5.8% of the areas maintained a stable state of water resources. Even in the predicted wet years, no improvements were found in the instability of water resources in four areas centered on the counties of Xinji, Daming, Luannan, and Weichang. To achieve a medium and long-term balance between WF-AL development and water resource recovery, the WF-AL should be limited and combined with reservoir and cross-regional water transfer.

Rural-Urban Migration and the Growth of Informal Settlements: A Socio-Ecological System Conceptualization with Insights Through a “Water Lens”

Informal settlements i.e., slums emerge from the interplay of multidimensional factors related to urbanization and sustainability. While the contribution of urban factors is well understood, the role of external drivers, such as uncontrolled migration to urban areas, is rarely addressed in research or policy-making. This study develops a novel conceptualization of slums by reviewing the pushing and pulling factors of migration and their contribution to informal settlements through 1) a socio-ecological system approach and 2) the concept of adaptive capacity. Further, it advances the discussion around synergistic and coherent policy-making in the urban context by reviewing three urban agendas and further using water as a case with the concept of cross-cutting domains. We show that the emergence of urban challenges can, and should be, linked to the root causes of flows into urban areas. Understanding these linkages through a socio-ecological system framework opens a window for knowledge-based policy development and addressing the question of how to avoid unsustainable urban development. Urbanization is one of the phenomena where the excessive complexity and dimensions of problems should not hamper action but instead, actions should be encouraged and enabled with synergistic and integrative pathways for sustainable urban development.

Cities as hotspots of indirect water consumption: The case study of Hong Kong

During the last years, the city of Hong Kong has made large investments to make its urban water supply system more water efficient and sustainable. As such, its municipal water abstraction - often defined as direct water use - has decreased from 355 litre per capita per day (l/cap/d) in 2005 to 326 l/cap/d in 2013. Due to its political history, Hong Kong is unique in the world in data availability on urban food consumption. It is therefore the ideal case study to show typical urban food consumption behaviour and its related indirect water use. The objective of this paper is to show the large water quantities associated with indirect water use and that the citizens of Hong Kong can additionally save much more water by looking at this indirect water use. The current average diet in Hong Kong is very different to the average Chinese diet. It is characterised by a high intake of water intensive products like animal products and sugar, leading to a food related indirect water use or water footprint (WF_cons) of 4727 l/cap/d. According to recommendations from the Chinese Nutrition Society for a healthy diet, the intake of some product groups should be increased (vegetables and fruit) and of other product groups reduced (sugar, crop oils, meat and animal fats). This would result in a reduction of the WF_cons of 40% to 2852 l/cap/d. Especially the reduced intake of meat (including offals) from currently 126 kg per capita per year (kg/cap/yr) to the recommended value 27 kg/cap/yrwould results in a substantial WF_cons reduction. Meat consumption in Hong Kong is extremely high. A pesco-vegetarian diet would result in a reduction of 49% (to 2398 l/cap/d) and a vegetarian diet in a 53% (to 2224 l/cap/d) reduction. Hong Kong citizens can thus save a lot of water through a change in their diet. Many of the products consumed, contribute to different levels of blue water scarcity in the regions of origin Hong Kong imports from. This poses a water-related risk to food security in Hong Kong. As all diet scenarios also result in a lower blue WF_cons, they decrease this risk. In order to become sustainable, (mega)cities should reduce their dependency on distant resources and ecosystems.

Large cities get more for less: Water footprint efficiency across the US

Many urban indicators and functional citywide properties have been shown to scale with population due to agglomeration effects. We hypothesize that scaling relations may also exist for water-related urban indicators such as the water footprint. The water footprint is an indicator of water use that measures humans' appropriation of freshwater resources. We analyze the scaling of the water footprint for 65 mid- to large-sized US cities using both empirical estimates and a social interaction network model of city functioning. The network model is used to explain the presence of any scaling exponent in the empirical estimates of the urban water footprint by linking to previous theories of urban scaling. We find that the urban water footprint tends to approximately show sublinear scaling behavior with both population and gross domestic product. Thus, large cities tend to be more water footprint efficient and productive than mid-sized cities, where efficiency and productivity are quantified, in a broad sense, as deviations from a linear scaling exponent. We find the sublinear scaling may be linked to changes in urban economic structure with city size, which lead to large cities shifting water intensive economic activities to less populated regions. In addition, we find that green water contributes to the scaling both positively by transferring the dependence of food consumption on population into the water footprint and negatively by increasing heterogeneity. Overall, the proposed scaling relations allow for the comparison of water footprint efficiency and productivity of cities. Comparing these properties and identifying deviations from the expected behavior has implications for water resources and urban sustainability.

Water consumption related to different diets in Mediterranean cities

Providing the sustainable development goals (SDGs) water, food and energy security to cities relies strongly on resource use outside city borders. Many modern cities have recently invested in a sustainable urban water system, and score high in international city rankings regarding water management and direct urban water use. However, these rankings generally neglect external resource use for cities. Here we quantify the water resources related to food consumption in thirteen cities located in Mediterranean countries, by means of the water footprint (WF) concept. These WFs amount from 3277l per capita per day (l/cap/d) to 5789l/cap/d. These amounts are about thirty times higher than their direct urban water use. We additionally analyse the WF of three diet scenarios, based upon a Mediterranean dietary pattern. Many authors identify the Mediterranean diet as cultural heritage, being beneficial for human health and a model for a sustainable food system. The first diet scenario, a healthy Mediterranean diet including meat, leads to WF reductions of -19% to -43%. The second diet scenario (pesco-vegetarian), leads to WF reductions of -28% to -52%. The third diet scenario (vegetarian), leads to WF reductions of -30% to -53%. In other words, if urban citizens want to save water, they need to look at their diets.

Estimating watershed degradation over the last century and its impact on water-treatment costs for the world's large cities

Urban water systems are impacted by land use within their source watersheds, as it affects raw water quality and thus the costs of water treatment. However, global estimates of the effect of land cover change on urban water-treatment costs have been hampered by a lack of global information on urban source watersheds. Here, we use a unique map of the urban source watersheds for 309 large cities (population > 750,000), combined with long-term data on anthropogenic land-use change in their source watersheds and data on water-treatment costs. We show that anthropogenic activity is highly correlated with sediment and nutrient pollution levels, which is in turn highly correlated with treatment costs. Over our study period (1900-2005), median population density has increased by a factor of 5.4 in urban source watersheds, whereas ranching and cropland use have increased by a factor of 3.4 and 2.0, respectively. Nearly all (90%) of urban source watersheds have had some level of watershed degradation, with the average pollutant yield of urban source watersheds increasing by 40% for sediment, 47% for phosphorus, and 119% for nitrogen. We estimate the degradation of watersheds over our study period has impacted treatment costs for 29% of cities globally, with operation and maintenance costs for impacted cities increasing on average by 53 ± 5% and replacement capital costs increasing by 44 ± 14%. We discuss why this widespread degradation might be occurring, and strategies cities have used to slow natural land cover loss.