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

=== 11.3.3 Freshwater Resources ===

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Climate change impacts on freshwater resources cascade across people, agriculture, industries and ecosystems (Boxes 11.3 and 11.5). The challenge of satisfying multiple demands with a finite resource is exacerbated by high interannual and inter-decadal variability of river flows, particularly in Australia ( [[#Chiew--2002|Chiew and McMahon, 2002]] ; [[#Peel--2004|Peel et al., 2004]] ; [[#McKerchar--2010|McKerchar et al., 2010]] ).

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==== 11.3.3.1 Observed Impacts ====

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Streamflow has generally increased in northern Australia and decreased in southern Australia since the mid-1970s ( ''high confidence'' ) ( [[#Zhang--2016|Zhang et al., 2016]] ). Declining river flows since the mid-1970s in southwest Australia have led to changed water management ( [[#WA%20Government--2012|WA Government, 2012]] ; [[#WA%20Government--2016|WA Government, 2016]] ). The large decline in river flows during the so-called 1997–2009 Millennium drought in southeast Australia resulted in low irrigation water allocations, severe water restrictions and major environmental impacts ( [[#Potter--2010|Potter et al., 2010]] ; [[#Chiew--2011|Chiew and Prosser, 2011]] ; [[#Leblanc--2012|Leblanc et al., 2012]] ; [[#van%20Dijk--2013|van Dijk et al., 2013]] ). The drying in southern Australia highlighted the need for hydrological models that adequately account for climate change ( [[#Vaze--2010|Vaze et al., 2010]] ; [[#Chiew--2014|Chiew et al., 2014]] ; [[#Saft--2016|Saft et al., 2016]] ; [[#Fowler--2018|Fowler et al., 2018]] ). The decline in streamflow was largely due to the decline in cool-season rainfall (which has been partly attributed to climate change) (Figure 11.2) ( [[#Timbal--2011|Timbal and Hendon, 2011]] ; [[#Post--2014|Post et al., 2014]] ; [[#Hope--2017|Hope et al., 2017]] ; [[#DELWP--2020|DELWP, 2020]] ), when most of the runoff in southern Australia occurs.

In New Zealand, precipitation has generally decreased in the north and increased in the southwest (Figure 11.2) (Harrington et al., 2014), but it is difficult to ascertain trends in the relatively short streamflow records. Glaciers in New Zealand’s southern alps have lost one third of their mass since 1977 ( [[#Mackintosh--2017|Mackintosh et al., 2017]] ; [[#Salinger--2019b|Salinger et al., 2019b]] ), and glacier mass loss in 2018 was at least 10 times more likely to occur with anthropogenic forcing than without ( [[#Vargo--2020|Vargo et al., 2020]] ).

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==== 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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[[File:9fcb9f5c1b607607af31216271fec297 IPCC_AR6_WGII_Figure_11_003.png]]

'''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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[[File:6ec3c5bd8d6ea2a7c37efbe154160579 IPCC_AR6_WGII_Figure_11_004.png]]

'''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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==== 11.3.3.3 Adaptation ====

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In Australia, prolonged droughts and projections of a drier future have accelerated policy and management change in urban and rural water systems. Adaptation initiatives and mechanisms, like significant government investment to enhance the Bureau of Meteorology online water information ( [[#Vertessy--2013|Vertessy, 2013]] ; [[#BoM--2016|BoM, 2016]] ), funding to improve agricultural water use and irrigation efficiency ( [[#Koech--2018|Koech and Langat, 2018]] ), enhanced supply through inter-basin transfers and upgrading water infrastructure and an active water trading market ( [[#Wheeler--2013|Wheeler et al., 2013]] ; [[#Kirby--2014|Kirby et al., 2014]] ; [[#Grafton--2016|Grafton et al., 2016]] ) are helping to buffer regional systems against droughts and facilitating some adaptation to climate change ( ''medium confidence'' ). However, these measures could also be maladaptive because they may perpetuate unsustainable water and land uses under ongoing climate change (Boxes 11.3 and 11.5).

The widespread 2017–2019 drought across eastern Australia (BoM, 2021b) has led to the Australian government establishing a Future Drought Fund ( [[#Australian%20Government--2019|Australian Government, 2019]] ) to enhance drought resilience and a National Water Grid Authority to develop regional water infrastructure to support agriculture. Nevertheless, the ability to adapt to climate change is compounded by uncertainties in future water projections, complex interactions between science, policy, community values and political voice, and competition between different sectors dependent on water (Boxes 11.3 and 11.5). The impact of declining water resources on agricultural, ecosystems and communities in southeastern Australia would escalate with ongoing climate change ( ''medium confidence'' ) ( [[#Hart--2016|Hart, 2016]] ; [[#Moyle--2017|Moyle et al., 2017]] ), highlighting the importance of more ambitious, anticipatory, participatory and integrated adaptation responses ( [[#Bettini--2015|Bettini et al., 2015]] ; [[#Abel--2016|Abel et al., 2016]] ; [[#Marshall--2021|Marshall and Lobry de Bruyn, 2021]] ).

Altered water regimes resulting from the combined effects of climatic conditions and water policies carry uneven and far-reaching implications for communities ( ''high confidence'' ). Acting on Indigenous Peoples’ claims to cultural flows (to maintain their connections with their country) is increasingly recognised as an important water management and social justice issue ( [[#Taylor--2017|Taylor et al., 2017]] ; [[#Hartwig--2018|Hartwig et al., 2018]] ; [[#Jackson--2018|Jackson, 2018]] ; [[#Jackson--2019|Jackson and Moggridge, 2019]] ; [[#Moggridge--2019|Moggridge et al., 2019]] ). Compounding stressors, such as coal and coal seam gas developments, can also severely impact local communities, water catchments and water-dependent ecosystems and assets, exacerbating their vulnerability to climate change ( [[#Navi--2015|Navi et al., 2015]] ; [[#Tan--2015|Tan et al., 2015]] ; [[#Chiew--2018|Chiew et al., 2018]] ).

In Australian capital cities and regional centres, water planning has focused on securing new supplies that are resilient to climate change. This includes increasing use of stormwater and sewage recycling and managed aquifer recharge ( [[#Bekele--2018|Bekele et al., 2018]] ; [[#Page--2018|Page et al., 2018]] ; [[#Gonzalez--2020|Gonzalez et al., 2020]] ). All major coastal Australian cities have desalination plants. Household scale adaptation, like rainwater harvesting, water-smart gardens, dual flush toilets, water-efficient showerheads and voluntary residential use targets, can help reduce water demand by up to 40% ( [[#Shearer--2011|Shearer, 2011]] ; [[#Rhodes--2012|Rhodes et al., 2012]] ; [[#Moglia--2018|Moglia et al., 2018]] ). Water utilities across Australia have established climate change adaptation guidelines ( [[#WSAA--2016|WSAA, 2016]] ). Coordinated efforts to reduce demand, design and retrofit infrastructure to reduce flood risk and harvest water and to practice water-sensitive urban design are evident ( [[#WSAA--2016|WSAA, 2016]] ; [[#Kunapo--2018|Kunapo et al., 2018]] ; [[#Rogers--2020b|Rogers et al., 2020b]] ). Transitioning centralised water systems to a more sustainable basis represents adaptation progress but is complex and faces many barriers and limits ( ''medium confidence'' ) ( [[#Morgan--2020|Morgan et al., 2020]] ). Developing multiple redundant or decentralised systems can enhance community resilience and promote autonomous adaptations that may be more sustainable and cost effective in the longer term ( [[#Mankad--2011|Mankad and Tapsuwan, 2011]] ; [[#WSAA--2016|WSAA, 2016]] ; [[#Iwanaga--2020|Iwanaga et al., 2020]] ).

In New Zealand, many water supplies are at risk from drought, extreme rainfall events and sea level rise (SLR), exacerbated by underinvestment in existing water infrastructure (in part due to funding constraints) and urban densification ( ''high confidence'' ) ( [[#CCATWG--2017|CCATWG, 2017]] ; [[#MfE%20and%20Stats%20NZ--2021|MfE and]] [[#Stats%20NZ--2021|Stats NZ, 2021]] ). Lessons can be learned from global experience (e.g., Cape Town, South Africa; [[IPCC:Wg2:Chapter:Chapter-4#4.3.4|Section 4.3.4]] ). Water quality has diminished, with hotter conditions and drought causing algal blooms, combined with intensification of agricultural land uses in some areas, and heavy rainfall and sea level rise (SLR) causing flooding and sedimentation of water sources and health impacts (11.3.6; Box 11.5). Some towns are only partially metered or not metered at all, which exacerbates the adaptation challenge ( [[#Hendy--2018|Hendy et al., 2018]] ; [[#WaterNz--2018|WaterNz, 2018]] ; Paulik 2019a). Unregulated or absent water supplies accentuate risks to vulnerable groups of people ( [[#MfE--2020b|MfE, 2020b]] ). Māori view water as the essence of all life, which makes any impacts on water a governance and stewardship concern, and increasingly, the subject of legal claims ( [[#MfE--2020a|MfE, 2020a]] ; [[#MfE--2020b|MfE, 2020b]] ; [[#MfE--2020c|MfE, 2020c]] ) (11.4.2). Māori understanding of time can also open up new spaces for rethinking freshwater management in a climate change context that does not reinforce or rearticulate multiple environmental injustices ( [[#Parsons--2021|Parsons et al., 2021]] ).

Water resource adaptation in New Zealand is variable across local government and water authorities but they all actively monitor water availability, demand and quality, and most have drought management plans. The 2019/2020 drought led to water shortages in the most populated areas of Waikato, Auckland and Northland, resulting in water reduction advisories and 5 to 8 weeks’ waiting time for water tank refills and water rationing. The Havelock North water supply contamination, which arose after an extreme rainfall event ( [[#DIA--2017a|DIA, 2017a]] ; [[#DIA--2017b|DIA, 2017b]] ), was exacerbated by fragmented governance and led to passage of the Taumata Arawai-Water Services Regulator Act of 2020 and the Water Services Bill of 2020 aimed at the protection of source water. The 2017 update to the National Policy Statement for Freshwater Management contains guidelines for implementation at the regional level ( [[#MfE--2017b|MfE, 2017b]] ), including consideration of climate change, which creates opportunities for adaptation. However, there remain tensions between land, water and people which are exacerbated by climate changes and have yet to be addressed (Box 11.5). The first National Adaptation Plan and the Resource Management law reform have the potential to help resolve these tensions (11.7.1) ( [[#CCATWG--2017|CCATWG, 2017]] ; [[#MfE--2020a|MfE, 2020a]] ).

'''Table 11.7 |''' Cities, settlements and infrastructure: key risks and adaptation options.

{| class="wikitable"
|-
! Sector

! Key Risks

! Adaptation Options

! Inter-Sector Dependencies

! Sources

|-
| Road

| Heat, SLR, coastal surges, floods and high-intensity rainfall impacts on road foundations

| Re-routing, coastal protection, improved drainage

| Ports (fuel supply), rail (fuel supply), electricity

| ( [[#NCCARF--2013|NCCARF, 2013]] ; [[#CoA--2018a|CoA, 2018a]] ; [[#MfE--2020a|MfE, 2020a]] )

|-
| Rail

| Extreme temperatures, flooding, SLR, high-intensity rainfall impacts on track foundations

| Drainage and ventilation improvements, systematic risk assessments, overhead wire and rail/sleeper upgrades, re-routing

| Electricity, telecommunications, fuel supply (transport, ports)

| ( [[#CoA--2018a|CoA, 2018a]] ; [[#MfE--2020a|MfE, 2020a]] )

|-
| Urban and Rural Built Environment 1

| Extreme temperatures, floods, extreme weather events, wildfire (at urban–rural interface), SLR

| Multiple options from the building-to-city scale to reduce heat impacts and improve climate resilience, behavioural change, coastal defences and managed retreat

| Road, rail, electricity, air and seaports, telecommunications, water and wastewater

| ( [[#CoA--2018a|CoA, 2018a]] ; [[#Newton--2018|Newton et al., 2018]] ; [[#Haddad--2019|Haddad et al., 2019]] ; [[#MfE--2020a|MfE, 2020a]] ; [[#Paulik--2020|Paulik et al., 2020]] ; [[#Tapper--2021|Tapper, 2021]] )

(Box 11.1)

(Box 11.4)

|-
| Electricity

| High-wind/ temperature events, wildfire, lightning, dust storms, drought (hydro)

| Demand management, re-engineering and new technology, network intelligence, smart metering, improved planning for outages

| Road, rail, water

| ( [[#CoA--2017|CoA, 2017]] ; [[#MfE--2020a|MfE, 2020a]] )

(11.3.10.)

|-
| Ports: Air and Sea

| SLR, coastal surges, wind, heat, extreme weather events

| Air: improved coastal, pluvial and fluvial flood protection, on-site services; sea: widening operational limits, raising wharfs, roads and breakwaters

| Electricity, road, rail, water

| ( [[#McEvoy--2014|McEvoy and Mullett, 2014]] ; [[#MfE--2020a|MfE, 2020a]] )

|-
| Telecommunications

| Floods, wildfires, extreme wind

| Protect, place underground, wireless systems

| Electricity, digital connectivity, all sectors serviced, rural communities

| ( [[#NCCARF--2013|NCCARF, 2013]] )

|-
| Stormwater Wastewater and Water supply a.

| High-intensity rainfall, increased and extreme temperatures, flooding, drought, SLR

| Large investments in upgrading centralised infrastructure and capacity, increasing investment in decentralised infrastructure and capacity (e.g., water-sensitive urban design), demand management, fewer options in smaller communities, governance at scale

| Electricity, telecommunications, urban and rural built environment

| ( [[#White--2017|White et al., 2017]] ; [[#CoA--2018a|CoA, 2018a]] ; [[#Gilpin--2020|Gilpin et al., 2020]] ; [[#MfE--2020a|MfE, 2020a]] ; [[#Wong--2020|Wong et al., 2020]] ; [[#Hughes--2021|Hughes et al., 2021]] )

(Box 11.4)

|}

Notes:

(a) Water supply safety and security and exposure of buildings have been identified as the most significant risks for New Zealand in terms of urgency and consequence ( [[#MfE--2020a|MfE, 2020a]] ). No such ranking of risk has been done for Australia.

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'''Box 11.3 | Drought, Climate Change and Water Reform in the Murray-Darling Basin'''
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The MDB is Australia’s largest, most economically important and politically complex river system (Figure Box 11.3.1). The MDB supports agriculture worth AUD$24 billion/year, 2.6 million people in diverse rural communities and important environmental assets including 16 Ramsar listed wetlands (DAWE, 2012). Climate change is projected to substantially reduce water resources in the MDB ( ''high confidence'' ), with the median projection indicating a 20% decline in average annual runoff under 2.2°C average global warming (Figure 11.3) ( [[#Whetton--2020|Whetton and Chiew, 2020]] ). This reduction, plus increased demand for water in hot and dry conditions, would increase the already intense competition for water ( ''high confidence'' ) ( [[#CSIRO--2008|CSIRO, 2008]] ; [[#Hart--2016|Hart, 2016]] ).

[[File:0651bc37944e998398b59c5f31aa1ee6 IPCC_AR6_WGII_Figure_11_Box_11_3_1.png]]

'''Figure Box 11.3.1 |''' '''(A) The Murray-Darling Basin, and (B) average annual river flows in the basin under pre-development conditions (from ( [[#CSIRO--2008|CSIRO, 2008]] ) showing that most of the runoff comes from the southeastern highlands.''' The borders show key drainage basins.

[https://www.ipcc.ch/figures/chapter-11/figure-11-box-11-3-1 ]

The economic, environmental and social impacts of the 1997–2009 Millennium Drought in the MDB ( [[#Chiew--2011|Chiew and Prosser, 2011]] ; [[#Leblanc--2012|Leblanc et al., 2012]] ; [[#van%20Dijk--2013|van Dijk et al., 2013]] ) and projections of a drier future under climate change have accelerated significant water policy reforms costing more than AUD$12 billion ( [[#Bark--2014|Bark et al., 2014]] ; [[#Docker--2014|Docker and Robinson, 2014]] ; [[#Hart--2016|Hart, 2016]] ). These reforms included the development of a Basin Plan ( [[#MDBA--2011|MDBA, 2011]] ; [[#MDBA--2012|MDBA, 2012]] ) requiring consistent regional water resource plans ( [[#MDBA--2011|MDBA, 2011]] ; [[#MDBA--2012|MDBA, 2012]] ; [[#MDBA--2013|MDBA, 2013]] ) and environmental watering strategies ( [[#MDBA--2014|MDBA, 2014]] ) across the MDB. Despite contestation, the reforms have resulted in some substantive achievements, including returning an equivalent of about one-fifth of consumptive water to the environment through the purchase of irrigation water entitlements and infrastructure projects ( ''medium confidence'' ) ( [[#Hart--2016|Hart, 2016]] ; [[#Gawne--2020|Gawne et al., 2020]] ; [[#MDBA--2020|MDBA, 2020]] ). However, the overall impacts of these water management initiatives are difficult to measure due to hydroclimatic variability, time lags and environmental, social and institutional complexity ( [[#Crase--2011|Crase, 2011]] ; [[#Bark--2014|Bark et al., 2014]] ; [[#Docker--2014|Docker and Robinson, 2014]] ; [[#MDBA--2020|MDBA, 2020]] ).

Reform initiatives such as water markets, improving agriculture water use efficiency ( [[#Koech--2018|Koech and Langat, 2018]] ), and increasing environmental water are helping buffer the system against droughts ( ''medium confidence'' ) ( [[#Moyle--2017|Moyle et al., 2017]] ), but they can also be maladaptive by perpetuating unsustainable water and land use under ongoing climate change. While water markets can allow users to adapt and shift water to higher value uses, they can also have adverse impacts unless supported by wider policy goals and planning processes ( [[#Wheeler--2013|Wheeler et al., 2013]] ; [[#Kirby--2014|Kirby et al., 2014]] ; [[#Grafton--2016|Grafton et al., 2016]] ; [[#Qureshi--2018|Qureshi et al., 2018]] ).

Adapting MDB management to climate risks is an escalating challenge, with the projected decline in runoff being potentially greater than the water recovered for the environment ( [[#Chiew--2017|Chiew et al., 2017]] ). While the Basin Plan includes mechanisms for climate risk management ( [[#Neave--2015|Neave et al., 2015]] ), it does not require altering pre-existing rules that distribute the impacts of anticipated reductions in water resources between users ( [[#Hart--2016|Hart, 2016]] ; Capon and Capon, 2017; [[#Alexandra--2020|Alexandra, 2020]] ). The intense drought conditions in 2017–2019 (BoM, 2021b), the South Australian Royal Commission investigation into the MDB reforms ( [[#SA%20Government--2019b|SA Government, 2019b]] ) and major fish kills in the lower Darling River in the summer of 2018/2019 ( [[#AAS--2019|AAS, 2019]] ; [[#Vertessy--2019|Vertessy et al., 2019]] ) have increased concerns about the Basin Plan’s climate adaptation deficit ( ''medium confidence'' ). Consequently, the MDB Authority (MDBA) is undertaking an assessment of climate change risks and developing adaptation mechanisms ( [[#MDBA--2019|MDBA, 2019]] ) that can feed into the revisions to the Basin Plan scheduled for 2026. The MDB reforms to date illustrate the difficulties in integrating climate change science and projections into management ( [[#Alexandra--2018|Alexandra, 2018]] ; [[#Alexandra--2020|Alexandra, 2020]] ). Anticipatory and participatory governance and adaptive management approaches supported by structural and institutional reforms would support the effectiveness of the reforms ( [[#Abel--2016|Abel et al., 2016]] ; [[#Alexandra--2019|Alexandra, 2019]] ; [[#Hassenforder--2019|Hassenforder and Barone, 2019]] ; Marshall and Lobry de Bruyn, 2021).

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'''Box 11.4 | Changing Flood Risk'''
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Pluvial (flash flood from high intensity rainfall) and fluvial (river) flooding are the most costly natural disasters in Australia, averaging AUD$8.8 billion per year ( [[#Deloitte--2017b|Deloitte, 2017b]] ). In New Zealand, insured damages for the 12 costliest flood events from 2007 to 2017 exceeded NZD$472 million, of which NZD$140 million has been attributed to anthropogenic climate change ( [[#Frame--2020|Frame et al., 2020]] ). Extreme rainfall intensity in northern Australia and New Zealand has been increasing, particularly for shorter (sub-daily) duration and more extreme high rainfall ( ''high confidence'' ) (Westra and Sisson, 2011; [[#Griffiths--2013|Griffiths, 2013]] ; [[#Laz--2014|Laz et al., 2014]] ; [[#Rosier--2015|Rosier et al., 2015]] ) (Table 11.2b). Changes are also occurring in spatial and temporal patterns and seasonality ( [[#Wasko--2015|Wasko and Sharma, 2015]] ; Zheng et al., 2015; Wasko et al., 2016).

Extreme rainfall is projected to become more intense ( ''high confidence'' ), but the magnitude of change is uncertain (Evans and McCabe, 2013; [[#Bao--2017|Bao et al., 2017]] ) (Table 11.3). The insured damage in New Zealand from more intense extreme rainfall under RCP8.5 is projected to increase 25% by 2080–2100 ( [[#Pastor-Paz--2020|Pastor-Paz et al., 2020]] ). In urban areas, extreme rainfall intensity is projected to increase pluvial flood risk ( ''high confidence'' ). In New Zealand, 20,000 km 2 of land, 675,000 people, and 411,000 buildings with a NZD$135 billion replacement value are exposed to flood risk (Paulik et al., 2019a).

In non-urban areas, where the flood response is also dependent on antecedent catchment conditions ( [[#Johnson--2016|Johnson et al., 2016]] ; Sharma et al., 2018), there is no evidence of increasing flood magnitudes in Australia ( [[#Ishak--2013|Ishak et al., 2013]] ; [[#Zhang--2016|Zhang et al., 2016]] ; [[#Bennett--2018|Bennett et al., 2018]] ), except for the most extreme events (Sharma et al., 2018; [[#Wasko--2019|Wasko and Nathan, 2019]] ). Modelling studies project increases in flood magnitudes in northern and eastern Australia and in western and northern New Zealand ( ''high confidence'' ) ( [[#Hirabayashi--2013|Hirabayashi et al., 2013]] ; Collins et al., 2018a; [[#Do--2020|Do et al., 2020]] ). The change in flood magnitude in southern Australia is uncertain because of the compensating effect of more intense extreme rainfall versus projected drier antecedent conditions ( [[#Johnson--2016|Johnson et al., 2016]] ; [[#Pedruco--2018|Pedruco et al., 2018]] ; [[#Wasko--2019|Wasko and Nathan, 2019]] ). Higher rainfall intensity and peak flows also increase erosion and sediment and nutrient loads in waterways (Lough et al., 2015) and exacerbate problems from ageing stormwater and wastewater infrastructure ( [[#Jollands--2007|Jollands et al., 2007]] ; [[#WSAA--2016|WSAA, 2016]] ; [[#Hughes--2021|Hughes et al., 2021]] ).

There is some recognition of the need for flood management and planning to adapt to climate change ( ''medium confidence'' ) ( [[#COAG--2011|COAG, 2011]] ; [[#CCATWG--2018|CCATWG, 2018]] ; [[#CoA--2020d|CoA, 2020d]] ). Australian flood estimation guidelines recommend a 5% increase in design rainfall intensity per degree global average warming ( [[#Bates--2015|Bates et al., 2015]] ). In New Zealand, the recommended increase ranges from 5% to more than 10% for shorter-duration and longer-return-period storms ( [[#MfE--2010|MfE, 2010]] ; Carey- [[#Smith--2018|Smith et al., 2018]] ). Both guidelines also indicate the potential for higher increases in extreme rainfall intensity.

Adaptation to reduce flooding and its impacts have included improved flood forecasting ( [[#Vertessy--2013|Vertessy, 2013]] ; [[#BoM--2016|BoM, 2016]] ) and risk management ( [[#AIDR--2017|AIDR, 2017]] ), accommodating risk through raising floor levels and sealing external doors ( [[#Queensland%20Government--2011|Queensland Government, 2011]] ; Wang et al., 2015), deploying temporary levee structures and reducing risk through spatial planning and relocation. Adaptation options in urban areas include improved stormwater management (Hettiarachchi et al., 2019; [[#Matteo--2019|Matteo et al., 2019]] ), ecosystem-based approaches such as maintaining floodplains, restoring wetlands and retrofitting existing flood control systems to attenuate flows, and water-sensitive urban design ( [[#WSAA--2016|WSAA, 2016]] ; [[#Radcliffe--2017|Radcliffe et al., 2017]] ; [[#Radhakrishnan--2017|Radhakrishnan et al., 2017]] ; [[#Rogers--2020b|Rogers et al., 2020b]] ).

Adaptation to changing flood risks is currently mostly reactive and incremental in response to flood and heavy rainfall events ( ''high confidence'' ). For example, the 2010–2011 flooding in eastern Australia resulted in changes to reservoir operations to mitigate floods ( [[#QFCI--2012|QFCI, 2012]] ) and insurance practice to cover flood damages ( [[#Phelan--2011|Phelan, 2011]] ; [[#Phelan--2011|Phelan et al., 2011]] ; [[#QFCI--2012|QFCI, 2012]] ; [[#Schuster--2013|Schuster, 2013]] ). Nevertheless, adaptation planning that is pre-emptive and incorporates uncertainties into flood projections is emerging ( ''medium confidence'' ) ( [[#Schumacher--2020|Schumacher, 2020]] ). Examples from New Zealand include the use of Dynamic Adaptive Pathways Planning (DAPP) ( [[#Lawrence--2017|Lawrence and Haasnoot, 2017]] ) with Real Options assessment ( [[#Infometrics%20and%20PSConsulting--2015|Infometrics and PSConsulting, 2015]] ) and designing decision signals and triggers to monitor changes before physical and coping thresholds are reached (Stephens et al., 2018). Implementing adaptive flood risk management relies upon an understanding of how such risks change in uncertain and ambiguous ways necessitating adaptive and robust decision-making processes. These can enable learning through participatory adaptive pathways approaches ( [[#Lawrence--2017|Lawrence and Haasnoot, 2017]] ; [[#Bosomworth--2019|Bosomworth and Gaillard, 2019]] ) and through coordination across different levels of government and statutory mandates, adaptation funding and individual and community adaptations (Glavovic et al., 2010; [[#Boston--2018|Boston and Lawrence, 2018]] ; [[#McNicol--2021|McNicol, 2021]] ).

Box 11.4

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