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

===== 2.5.3.6.1 Risks to the quantity and quality of drinking water =====

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Forests and other vegetated ecosystems assist the production of drinkable water by facilitating the infiltration of rainfall and snowfall into the ground, where water either moves through the saturated soil zone to supply streams and other surface waters or infiltrates further to recharge groundwater aquifers ( [[#Ellison--2012|Ellison et al., 2012]] ; [[#Bonnesoeur--2019|Bonnesoeur et al., 2019]] ). Globally, 4 billion people depend on forested watersheds for drinking water ( [[#Mekonnen--2016|Mekonnen and Hoekstra, 2016]] ). [[IPCC:Wg2:Chapter:Chapter-4|Chapter 4]] assesses the physical science of water supply, including precipitation, runoff and hydrology as well as the social aspects of human water use. This section assesses the ecological aspects of risks to freshwater supplies for people.

Diminished vegetation cover following wildfires ( [[#2.5.3.2|Section 2.5.3.2]] ) and tree mortality ( [[#2.5.3.3|Section 2.5.3.3]] ) can reduce long-term water infiltration, increase soil erosion and flash floods and release sediment that degrades drinking water quality. Widlfires increase impacts of extreme precipitation events due to climate change, which contribute to increased surface runoff and hence increased risks of land erosion, landslides and flooding ( [[#Ebel--2012|Ebel et al., 2012]] ; [[#Robinne--2020|Robinne et al., 2020]] ). Under current conditions, nearly half the global land area is at a moderate-to-high risk of water scarcity due to wildfires ( [[#Robinne--2018|Robinne et al., 2018]] ; [[#Robinne--2020|Robinne et al., 2020]] ). From 1984 to 2014, wildfires in the western USA affected 6–11% of stream and river length ( [[#Ball--2021|Ball et al., 2021]] ). Under a high-emissions scenario of a 3.5°C temperature increase, post-fire erosion across the western USA could double sedimentation and degrade drinking water quality in one-third of watersheds by 2050 ( [[#Sankey--2017|Sankey et al., 2017]] ). In Brazil, post-fire vegetation loss tends to increase runoff, reduce infiltration and reduce groundwater recharge and flow of springs ( [[#Rodrigues--2019|Rodrigues et al., 2019]] ). Runoff from wildfires can contain DOC precursors for the formation of carcinogenic trihalomethanes during chlorination of water for drinking ( [[#Uzun--2020|Uzun et al., 2020]] ) as well as chromium, mercury, selenium and other toxic trace metals ( [[#Burton--2016|Burton et al., 2016]] ; [[#Burton--2019|Burton et al., 2019]] ).

Net effects of deforestation and afforestation on runoff and water supply depend on local factors, leading to conflicting evidence of effects of land cover change ( [[#Ellison--2012|Ellison et al., 2012]] ; [[#Chen--2021b|Chen et al., 2021b]] ), but combinations of climate change and deforestation are projected to reduce water flows ( [[#Olivares--2019|Olivares et al., 2019]] ). In southern Thailand, the combination of the conversion of forest to rubber plantations and a one-third increase in rainfall could increase erosion and sediment load by 15% ( [[#Trisurat--2016|Trisurat et al., 2016]] ). In the watershed that supplies São Paulo, Brazil, afforestation could increase water quantity and quality ( [[#Ferreira--2019|Ferreira et al., 2019]] ). In most regions with dry or Mediterranean subtropical climates, projected climate change can reduce surface water and groundwater resources ( [[#Doell--2015|Doell et al., 2015]] ). In northeast Spain, reduced precipitation and vegetation cover under the high-emissions scenario of a 3.5°C temperature increase could reduce drinking water supplies by half by 2100 ( [[#Bangash--2013|Bangash et al., 2013]] ).

Changes in algal biomass development and the spread of cyanobacteria blooms, related to global warming, resemble those triggered by eutrophication with the well-known negative effects on the services lakes provide, particularly for drinking water provision and recreation ( ''robust evidence'' , ''high agreement'' , ''high confidence'' ) ( [[#Carvalho--2013|Carvalho et al., 2013]] ; [[#Adrian--2016|Adrian et al., 2016]] ; [[#Gozlan--2019|Gozlan et al., 2019]] ).

Based on a 10% increase in precipitation, ( [[#de%20Wit--2016|de Wit et al., 2016]] ) estimated an increased mobilisation of organic carbon from soils to freshwaters of at least 30%, demonstrating the importance of climate wetting for the carbon cycle. Browning negatively affects the taste of drinking water and this may be difficult to address ( [[#Kothawala--2015|Kothawala et al., 2015]] ; [[#Kritzberg--2020|Kritzberg et al., 2020]] ). It also often reduces attractiveness for recreational purposes, especially swimming ( [[#Arthington--2003|Arthington and Hadwen, 2003]] ; [[#Keeler--2015|Keeler et al., 2015]] ). Based on a worst-case climate scenario until 2030, ( [[#Weyhenmeyer--2016|Weyhenmeyer et al., 2016]] ) projected an increase in the browning of lakes and rivers in boreal Sweden by a factor of 1.3. The chemical character of DOM, as modified by climate change ( [[#Kellerman--2014|Kellerman et al., 2014]] ), determines its amenability to removal by water treatment ( [[#Ritson--2014|Ritson et al., 2014]] ). Therefore, in order to provide safe and acceptable drinking water, more advanced, more expensive and more energy/resource-intensive technical solutions may be required ( [[#Matilainen--2010|Matilainen et al., 2010]] ).

In summary, climate change increases risks to the integrity of watersheds and the provision of safe, acceptable freshwater to people ( ''medium evidence'' , ''medium agreement'' ).

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