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lead to reduced water scarcity, including increased efficiency, other allocation strategies and reducing agricultural water demand via diet change and food waste reduction. Here, we discuss different measures largely linked to the individual targets, addressing increasing access to water, sanitation and hygiene (WASH), decreasing water demand, increasing water supply and reducing water pollution.


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Investing in access to water, sanitation and hygiene Achieving the targets on drinking water and sanitation will require increased investment in infrastructure, especially sanitation (United Nations Conference on Trade and Development 2014; Hutton and Varughese 2016). Due to population growth, an additional 3.4 billion people will require access to sanitation by 2030, or 620,000 per day, 2.5 times the number of people served during the 2001-2015 period (Mara and Evans 2018). The current levels of investment are likely to cover the capital costs of basic service provision for access to WASH by 2030, but not enough for safely managed service provision. To achieve universal access to safely managed WASH services, investment levels will need to increase threefold (Hutton and Varughese 2016). Achieving universal access to safe water and adequate sanitation is as much about changing behaviour as it is about changing infrastructure. This requires better marketing, communication and community- led sanitation (Water and Sanitation Program 2004; Kar and Chambers 2008; Devine and Kullmann 2011).


Increasing water-use efficiency Water scarcity including groundwater often needs to be managed at the watershed or aquifer level (Scott et al. 2014). These can be within one country, but often there are multiple countries involved. In those cases, an international framework is needed to evaluate strategies to reduce water stress and maximize mitigation (Wada, Gleeson and Esnault 2014). Wada, Gleeson and Esnault (2014) conclude that four demand-side measures are required: increasing agricultural water productivity (more crop per drop), improving irrigation efficiency (reducing water losses), more efficient water use in domestic and industrial sectors including reducing water leakage and improving recycling, and limiting the rate of population growth. To maintain or even reduce the global population under water scarcity by 2050 and beyond, water- use efficiency for these demand-side measures needs to improve by more than 20-50 per cent globally (0.5-1.2 per cent improvement per year). Moreover, strategies for water management at the level of watersheds are necessary to deal with competing demands for agricultural production, industrial activities, household water use and ecological services. The precise mix depends on economic, social, legal and political issues such as international or subnational water treaties, rights or disputes (Wada, Gleeson and Esnault 2014). Various scenarios have shown that increased water efficiency in agriculture, households and industry can have a significant impact on reducing water scarcity (e.g. Bijl et al. 2017).


Increasing water supply Increasing water supply can be done using more conventional measures such as building more water storage or dams, by investing more in desalination capacity in coastal regions (Wada, Gleeson and Esnault 2014) or by wastewater reuse. Furthermore, groundwater resources could serve as a buffer during droughts or severe water scarcity because of their ubiquitous presence across the globe.


528 Outlooks and Pathways to a Healthy Planet with Healthy People


annually (Food and Agriculture Organization of the United Nations 2018). Although this amount is important for coastal regions, the global total currently accounts for much less than 1 per cent of water withdrawals worldwide (4,000 km3


Increasingly, countries are implementing desalination strategies – for example, in the Middle East, North Africa and the United States of America (e.g. California) (World Water Assessment Programme 2003; Hanasaki et al. 2016). The global amount of desalinated water use has been rapidly increasing since the 1990s and it is currently estimated to exceed 10 km3


). Hanasaki et al. (2016) projects that under different SSP scenarios (1-3), the use of seawater desalination will increase 1.4- to 2.1-fold in 2011-2040 compared with the present, and 6.7- to 17.3-fold in 2041-2070. The associated costs are in the order of US$2 billion to US$200 billion. The large spreads in these projections are primarily attributable to substantial socioeconomic variations in the SSP scenarios. To scale up desalination of seawater in coastal water-stressed basins, a 10- to 50-fold increase is projected to be required; however, this would imply significant capital and energy costs, and it would generate wastewater that would need to be disposed of safely (Wada, Gleeson and Esnault 2014; Hanasaki et al. 2016).


Wastewater reuse enables upgrading of unsuitable water quality originating from households and industry to sufficient quality for different purposes. The amount of wastewater reuse or recycling has been increasing worldwide especially for agriculture, as small-scale farmers in urban and peri-urban areas of developing countries depend largely on wastewater or wastewater-polluted water sources to irrigate high-value crops for market (Qadir et al. 2010). However, higher-quality water is needed for drinking purpose and the establishment of water reuse guidelines is critical (Bixio et al. 2006; Bixio et al. 2008). Ongoing technological innovations, such as the use of membranes, and dedicated economic instruments are expected to further increase the use of wastewater as a resource in various regions with limited surface- and groundwater resources. In order to reduce water limitations in urban areas or megacities, a similar magnitude of future scaling up is required for wastewater reuse combined with the desalination of seawater (Wada, Gleeson and Esnault 2014).


It should be noted, however, that these two supply-side measures require a large amount of economic investment and modernizing of existing infrastructure, which might not be feasible for many developing countries (Neverre, Dumas and Nassopoulos 2016). Alternatively, nature-based solutions may have high potential to increase and/or regulate water supply by reducing degradation of water quality, while limiting economic investments (Vörösmarty et al. 2010). Multiple ecosystem services or sustainable infrastructure can mitigate water pollution and increase water supply for humans and ecosystems (Reddy et al. 2015; Liquete et al. 2016). These examples highlight an important role for development and deployment of water conservation technologies and practices to achieve water-related SDG targets (Hejazi et al. 2014).


Reducing water pollution Experience in developed countries has shown that it is possible to reduce water pollution. Unfortunately, there is very little scenario literature addressing water pollution problems and ways to achieve future sustainability targets. However, there is some literature discussing reduced nutrient pollution,


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