Editing IPCC:AR6/WGII/Chapter-11 (section)

==== 11.3.3.2 Projected Impacts ====

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Projections indicate that future runoff in southeast and southwest Australia are ''likely'' to decline (median estimates of 20% and 50% respectively under 2.2°C global average warming) (Figure 11.3) ( [[#Chiew--2017|Chiew et al., 2017]] ; [[#Zheng--2019|Zheng et al., 2019]] ). These projections are broadly similar to those reported previously and in AR5 ( [[#Teng--2012|Teng et al., 2012]] ; [[#Reisinger--2014|Reisinger et al., 2014]] ). The range of estimates arises mainly from the uncertainty in projected future precipitation (Table 11.2a).

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'''Figure 11.3 |''' '''Projected changes in mean annual runoff for 2046–2075 relative to 1976–2005 for RCP8''' '''.''' '''5 from hydrological modelling with future climate projections informed by 42 CMIP5 GCMs.''' Projections for RCP4.5 are about three quarters of the aforementioned projections. Plots show median projection and the 10th and 90th percentile range of estimates. The boundaries are based on hydroclimate regions and major drainage basins. Source: ( [[#Zheng--2019|Zheng et al., 2019]] ).

The runoff decline in southern Australia is projected to be further accentuated by higher temperature and potential evapotranspiration ( [[#Potter--2011|Potter and Chiew, 2011]] ; [[#Chiew--2014|Chiew et al., 2014]] ), transpiration from tree regrowth following more frequent and severe wildfires ( [[#Brookhouse--2013|Brookhouse et al., 2013]] ) (Box 11.1), interceptions from farm dams ( [[#Fowler--2015|Fowler et al., 2015]] ) and reduced surface–groundwater connectivity (limiting groundwater discharge to rivers) in long dry spells ( ''high confidence'' ) ( [[#Petrone--2010|Petrone et al., 2010]] ; [[#Hughes--2012|Hughes et al., 2012]] ; [[#Chiew--2014|Chiew et al., 2014]] ). In the longer term, runoff will also be affected by changes in vegetation and surface–atmosphere feedback in a warmer and higher CO 2 environment, but the impact is uncertain because of the complex interactions, including changes in climate inputs, fire patterns (Box 11.1) and nutrient availability ( [[#Raupach--2013|Raupach et al., 2013]] ; [[#Ukkola--2016|Ukkola et al., 2016]] ; [[#Cheng--2017|Cheng et al., 2017]] ).

Climate change is projected to affect groundwater recharge and the relationship between surface waters and aquifers and through rising sea levels where groundwater has a tidal signature ( [[#PCE--2015|PCE, 2015]] ; [[#MfE--2017a|MfE, 2017a]] ). Groundwater recharge across southern Australia has decreased in recent decades ( [[#Fu--2019|Fu et al., 2019]] ), and this trend is expected to continue ( ''high confidence'' ) ( [[#Barron--2011|Barron et al., 2011]] ; [[#Crosbie--2013|Crosbie et al., 2013]] ). Climate change is also projected to impact water quality in rivers and water bodies, particularly through higher temperature and low flows ( [[#Jöhnk--2008|Jöhnk et al., 2008]] ) (Box 11.5) and increased sediment and nutrient load following wildfires ( ''high confidence'' ) ( [[#Biswas--2021|Biswas et al., 2021]] ) (Box 11.1) and floods (Box 11.4).

The projected changes in river flows in New Zealand are consistent with the precipitation projections (Table 11.2), with increases in the west and south of the South Island and decreases in the east and north of the North Island (Figure 11.4). In the South Island, the runoff increase occurs mainly in winter due to increasing moisture-bearing westerly airflow, with more precipitation falling as rain and snow melting earlier. In the North Island, the runoff decrease occurs in spring and summer ( [[#Caruso--2017|Caruso et al., 2017]] ; [[#Collins--2018a|Collins et al., 2018a]] ; [[#Jobst--2018|Jobst et al., 2018]] ; D. [[#Collins--2020|Collins, 2020]] ).

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'''Figure 11.4 |''' '''Projected percentage change in mean annual runoff for 2086–2099 relative to 1986–2005 from hydrological modelling informed by six CMIP5 GCMs for four RCPs.''' Maps show median projection from the six modelling runs. White indicates that the change is not statistically significant. Source: (D. [[#Collins--2020|Collins, 2020]] ).

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