Editing IPCC:AR6/WGII/Chapter-6 (section)

==== 6.2.2.3 Urban Water Scarcity and Security ====

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Urban water scarcity occurs when gaps exist between supply and demand of available freshwater resources (Zhang et al., 2019). Urban water security requires a sustainable quantity and quality of water to meet community and ecosystem needs in a changing climate ( [[#Romero-Lankao--2019|Romero-Lankao and Gnatz, 2019]] ; Allan, Kenway and Head, 2018; Huang, Xu and Yin, 2015; [[#Chen--2016|Chen and Shi, 2016]] ). Risks arising from urban water scarcity worldwide are ''very likely'' increasing due to climate drivers (e.g., warmer temperatures and droughts) and urbanisation processes (e.g., land use changes, migration to cities and changing patterns of water use including over extraction of surface and groundwater resources) affecting supply and demand ( ''high confidence'' ) (Allan, Kenway and Head, 2018; Crausbay et al., 2020; Haddeland et al., 2014; Pickard et al., 2017; De Stefano et al., 2015; Sun et al., 2019; Van Loon et al., 2016; Zhang et al., 2019; [[IPCC:Wg2:Chapter:Chapter-4#4.2.4|Section 4.2.4.4]] ; See Box 8.6 for case study on 2018 Cape Town drought). Flörke et al. (2018) estimates that nearly a third of all major cities worldwide may exhaust their current water resources by 2050. Globally, projections suggest that 350 million (± 158.8 million) more people living in urban areas will be exposed to water scarcity from severe droughts at 1.5°C warming and 410.7 million (± 213.5 million) at 2°C warming (Liu et al., 2018).

Decreased regional precipitation and associated changes in runoff and storage from droughts is exacerbating urban scarcity by impairing the quality of water available for its resource management in cities ( ''high confidence'' ). For example, less runoff to freshwater rivers can increase salinity and concentrate pathogens and pollutants that increases risks of urban water scarcity (Hellwig, Stahl and Lange, 2017; [[#Jones--2018|Jones and van Vliet, 2018]] ; [[#Leddin--2020|Leddin and Macrae, 2020]] ; [[#Lorenzo--2020|Lorenzo and Kinzig, 2020]] ; Ma et al., 2020; [[#Mosley--2015|Mosley, 2015]] ; Zhang et al., 2019; van Vliet, Flörke and Wada, 2017; see also Box 6.2). Drought also changes the dynamics of groundwater pollution, leading to increased environmental health risks when those sources are used for urban water supplies (Kubicz et al., 2021; Moreira et al., 2020; Pincetl et al., 2019). Changes in the nature of droughts, for example, hotter droughts ( [[#Herrera--2017|Herrera and Ault, 2017]] ), snow droughts (Cooper, Nolin and Safeeq, 2016; Mote et al., 2016) or ‘flash’ droughts (Otkin et al., 2016; Otkin et al., 2018; Pendergrass et al., 2020) can exacerbate urban water scarcity, exposing the limitations of engineered water infrastructure designed to accommodate historical patterns of supply and demand (Gober et al., 2016; [[#Ulibarri--2019|Ulibarri and Scott, 2019]] ; Zhao et al., 2018a).

Risks of urban water scarcity and security are compounded by vulnerabilities such as service availability and quality of infrastructure to supply water for increased urban demand from in-migration to cities ( ''medium confidence'' ) (Ahmadalipour et al., 2019; Dong et al., 2020; Reynolds et al., 2019; Thomas et al., 2017; [[#Mullin--2020|Mullin, 2020]] ). Risks to local water security in cities are also exacerbated by drivers such as dependence on imported water resources from distant locales that may be exposed to additional drought risks ( ''high confidence'' ) (Ahams et al., 2017; Li et al., 2019b; Marston et al., 2015; Zhao et al., 2020; Zhang et al., 2020); from considerable projected urban expansion in drought-stressed areas, for example, across drylands of Western Asia and North Africa (Güneralp et al., (2015); and by export of virtual water (i.e., export of water embedded in food and energy) from local sources to distant trading partners (Djehdian et al., 2019; D’Odorico et al., 2018; [[#Fulton--2015|Fulton and Cooley, 2015]] ; [[#Rushforth--2016|Rushforth and Ruddell, 2016]] ; Verdon-Kidd et al., 2017; Vora et al., 2017).

Droughts interact and manifest in complex ways in interconnected urban areas that ''likely'' increase risks of urban water scarcity (Tapia et al., 2017; [[#Rushforth--2015|Rushforth and Ruddell, 2015]] ). Urban interdependencies mean droughts in one region can limit water resources availability in another (e.g., Macao and Zhuhai, Hong Kong, Shenzhen in China, Singapore and Johor, in cities in Pakistan and India, and in the west and southwest USA) (Chuah, Ho and [[#Chow--2018|Chow, 2018]] ; Gober et al., 2016; Srinivasan, Konar and Sivapalan, 2017; Zhang et al., 2019; Zhao et al., 2020). Likewise, physical and social teleconnections mean decisions made about water resources in one region or location may impact another in unexpected ways ( [[#Moser--2015|Moser and Hart, 2015]] ; Liu et al., 2015).

Urban water security risks are confounded by inequities in economic opportunity, risk exposure and human well-being ( ''medium evidence'' ) (Sena et al., 2017; Stanke et al., 2013; [[IPCC:Wg2:Chapter:Chapter-4#4.2.4|Section 4.2.4.5]] ). Water scarcity is felt more acutely among low-income compared with high-income populations (Nerkar et al., 2016), and scarcity on top of inequities and political instability can lead to security issues, for example, conflict between different water users (Cosic et al., 2019; von Uexkull et al., 2016; Ahmadalipour et al., 2019; [[#Döring--2020|]] [[#Döring--2020|Döring, 2020]] ; Ide et al., 2021), particularly when road infrastructures and access to water are limited ( [[#Detges--2016|Detges, 2016]] ; Sena et al., 2017). Scarcity risks may also be exacerbated by human and ecosystem needs in water-short years (Srinivasan, Konar and Sivapalan, 2017). Finally, growing populations along with migration into water scarce regions can exacerbate water security issues ( [[#Akhtar--2020|Akhtar and Shah, 2020]] ; [[#Singh--2019|Singh and Sharma, 2019]] ).

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