Editing IPCC:AR6/SROCC/Chapter-2 (section)

===== 2.3.1.3.3 Drinking water supply =====

Only a few studies provide detailed empirical assessments of the effects of cryosphere change on the amounts of drinking water supply. Decreases in drinking water supplies due to reduced glacier and snowmelt water have been reported for rural areas in the Nepal Himalaya (McDowell et al., 2013 <sup>[[#fn:r400|400]]</sup> ; Dangi et al., 2018 <sup>[[#fn:r401|401]]</sup> ), but the tropical Andes have received the most attention, including both urban conglomerates and some rural areas, where water resources are especially vulnerable to climate change due to water scarcity and increased demands (Chevallier et al., 2011 <sup>[[#fn:r402|402]]</sup> ; Somers et al., 2018 <sup>[[#fn:r403|403]]</sup> ), amidst rapidly retreating glaciers (Burns and Nolin, 2014 <sup>[[#fn:r404|404]]</sup> ).

The contribution of glacier water to the water supply of La Paz, Bolivia, between 1963–2006 was assessed at 15% annually and 27% during the dry season (Soruco et al., 2015 <sup>[[#fn:r405|405]]</sup> ), though rising as high as 86% during extreme drought months (Buytaert and De Bièvre, 2012 <sup>[[#fn:r406|406]]</sup> ). Despite a 50% area loss, the glacier retreat has not contributed to reduced water supplies for the city, because increased melt rates have compensated for reductions in glacier volume. However, for a complete disappearance of the glaciers, assuming no change in precipitation, a reduction in annual runoff by 12% and 24% in the dry season was projected (Soruco et al., 2015 <sup>[[#fn:r407|407]]</sup> ) similar to reductions projected by 2050 under a RCP8.5 scenario for a basin in southern Peru (Drenkhan et al., 2019 <sup>[[#fn:r408|408]]</sup> ). Huaraz and Huancayo in Peru are other cities with high average contribution of melt water to surface water resources (up to ~20%; Buytaert et al., 2017 <sup>[[#fn:r409|409]]</sup> ) and rapid glacier retreat in their headwaters (Rabatel et al., 2013 <sup>[[#fn:r410|410]]</sup> ).

Overall, risks to water security and related vulnerabilities are highly heterogeneous varying even at small spatial scales with populations closer to the glaciers being more vulnerable, especially during dry months and droughts (Buytaert et al., 2017 <sup>[[#fn:r411|411]]</sup> ; Mark et al., 2017 <sup>[[#fn:r412|412]]</sup> ). A regional-scale modelling study including all of Bolivia, Ecuador and Peru (Buytaert et al., 2017 <sup>[[#fn:r413|413]]</sup> ) estimated that roughly 390,000 domestic water users, mostly in Peru, rely on a high (&gt;25%) long-term average contribution from glacier melt, with this number rising to almost 4 million in the driest month of a drought year. Despite ''high confidence'' in declining longer-term melt water contributions from glaciers in the tropical Andes (Figure CB6.1), major uncertainties remain how these will affect future human water use. Regional-scale water balance simulations forced by multi-model climate projections (Buytaert and De Bièvre, 2012 <sup>[[#fn:r414|414]]</sup> ), suggest a relatively limited effect of glacier retreat on water supply in four major cities (Bogota, La Paz, Lima, Quito) due to the dominance of human factors influencing water supply (Carey et al., 2014 <sup>[[#fn:r415|415]]</sup> ; Mark et al., 2017 <sup>[[#fn:r416|416]]</sup> ; Vuille et al., 2018 <sup>[[#fn:r417|417]]</sup> ), though uncertainties are large. Population growth and limited funding for infrastructure maintenance exacerbate water scarcity, though water managers have established programs in Quito and in Huancayo and the Santa and Vilcanota basins (Peru) to improve water management through innovations in grey infrastructure and ecosystem-based adaptations (Buytaert and De Bièvre, 2012 <sup>[[#fn:r418|418]]</sup> ; Buytaert et al., 2017 <sup>[[#fn:r419|419]]</sup> ; Somers et al., 2018 <sup>[[#fn:r420|420]]</sup> ).

In summary, there is ''limited evidence'' ( ''medium agreement'' ) that glacier decline places increased risks to drinking water supply. In the Andes, future increases in water demand due to population growth and other socioeconomic stressors are expected to outpace the impact of climate change induced changes on water availability regardless the emission scenario.

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