Editing IPCC:AR6/WGI/Chapter-8 (section)

===== 8.4.1.7.4 Groundwater =====

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Groundwater projections were not assessed in AR5. Groundwater processes are not explicitly included in most current CMIP6 models and so must be calculated separately with hydrologic models (e.g., R.G. Taylor et al. , 2013; Cuthbert et al. , 2019a) . A range of factors are important in assessing groundwater projections, including the mean difference between precipitation and evaporation, the intensity of precipitation (R.G. [[#Taylor--2013|Taylor et al., 2013]] a), and in changes in snow ( [[#Tague--2009|Tague and Grant, 2009]] ), glaciers ( [[#Gremaud--2009|Gremaud et al., 2009]] ), and permafrost ( [[#Okkonen--2011|Okkonen and Kløve, 2011]] ). Climate impacts on groundwater are occurring in the context of severe and growing human-caused groundwater depletion (WGII; [[#Konikow--2005|Konikow and Kendy, 2005]] ; [[#Rodell--2018|Rodell et al., 2018]] ; [[#Bierkens--2019|Bierkens and Wada, 2019]] ), and water scarcity issues ( [[#Mekonnen--2016|Mekonnen and Hoekstra, 2016]] ). Climate-related changes to the water cycle can influence water demand (for example, precipitation decreases in an irrigated area), and anthropogenic groundwater depletion can influence the water cycle through interactions with surface energy fluxes, surface water, and vegetation ( [[#Cuthbert--2019a|Cuthbert et al., 2019a]] ), although uncertainties in estimates of future groundwater depletion are large ( [[#Smerdon--2017|Smerdon, 2017]] ; [[#Bierkens--2019|Bierkens and Wada, 2019]] ) . Some aspects of groundwater change will be irreversible, including the increase of saltwater intrusion into coastal aquifers with sea level rise ( [[#Werner--2009|Werner and Simmons, 2009]] ), and depletion of fossil aquifers and aquifers with very long recharge times ( [[#Bierkens--2019|Bierkens and Wada, 2019]] ).

Globally, two modelling studies have shown substantial decreases in groundwater in regions including the Mediterranean, north-eastern Brazil and south-western Africa, with less clarity for other regions ( [[#Döll--2009|Döll, 2009]] ; Portmann et al. , 2013) . Recent regional-scale analyses of the impact of water cycle changes on groundwater recharge (e.g., Meixner et al. , 2016; Tillman et al. , 2017; Shrestha et al. , 2018 ) suggest changes in both seasonality and spatial distribution, which are amplified under a higher greenhouse-gas emissions scenario (i.e., RCP 8.5 compared to RCP4.5). Seasonality changes are linked to increases during wet winter periods and declines during dry summer periods. Changes in spatial distribution are linked with increases in more humid regions and declines in more arid locations. Uncertainty in projections of groundwater were found to be substantially influenced by the conceptual and numerical models employed to estimate groundwater recharge ( [[#Meixner--2016|Meixner et al., 2016]] ; [[#Hartmann--2017|Hartmann et al., 2017]] ). Accordingly, current research on estimating water cycles change on groundwater includes a focus on improving the numerical representation of groundwater systems ( [[#Bierkens--2015|Bierkens et al., 2015]] ; [[#Döll--2016|Döll et al., 2016]] ).

In summary, based on known limitations in current modelling, no confident assessment of groundwater projections is made here, although important climate-related changes in groundwater recharge are expected. In many environments, such climate-related impacts are expected to occur in the context of substantial human groundwater withdrawals depleting groundwater storage.

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