Editing IPCC:AR6/WGII/Chapter-10 (section)

=== 10.4.4 Freshwater Resources ===

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In Asia, freshwater resources, an important component of ecosystem services, are widely used for agriculture, domestic, irrigation, navigation, energy and industry. Freshwater availability is changing at the global scale because of unsustainable use of surface water and groundwater, pollution and other environmental changes. These changes in space and time, directly or indirectly, affect water-use sectors and services ( [[#Wheater--2015|Wheater and Gober, 2015]] ; [[#Rodell--2018|Rodell et al., 2018]] ). About 82% of the global population served by freshwater provisions from upstream areas are exposed to high threat ( [[#Green--2015|Green et al., 2015]] ). Given that some of the fastest-growing economies in the world are in Asia, and the geographies of development are highly uneven, both CIDs and non-climate drivers, such as socioeconomic changes, have contributed to water stress conditions in both water supply and demand in diverse sub-regions of Asia. In the case of Asia, therefore, the entanglement between the non-climate drivers and CIDs makes it difficult to attribute environmental changes—both present and projected—neatly and exclusively to CIDs.

[[#Immerzeel--2020|Immerzeel et al. (2020)]] have ranked all mountain-dependent water towers according to their water-supplying role and the downstream dependence of ecosystems, societies and economies. The resulting Global Water Tower index indicates that the upper Indus basin is both the most important and the most vulnerable water tower unit (WTU) in the world. A WTU is defined as ‘the intersection between major river basins and a topographic mountain classification based on elevation and surface roughness’. Whereas all important transboundary WTUs in Asia remain highly vulnerable, it is the Indus WTU (inhabited by approximately 235 million people in the basin in 2016 (which is projected to increase by 50% by the middle of 21st century) where the average annual temperature is projected to increase by 1.9°C between 2000 and 2050, with wide-ranging consequences and trans-sectoral spillovers. The Indus WTU faces a deep risk produced by a combination of factors including water stress, ineffective governance, hydropolitical tensions, population growth and density, urbanisation and social transformations, with a significant bearing on SDG 6 on water, SDG 2 on food and SDG 7 on energy.

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==== 10.4.4.1 Key Drivers ====

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Across Asia and its various sub-regions, the key drivers behind an increasingly inadequate supply of freshwater resources, affecting the livelihood security of millions, are varied, complex and intersect with multiple social, cultural, economic and environmental stressors ( [[#Luo--2017|Luo et al., 2017]] ; [[#Tucker--2015|Tucker et al., 2015]] ; [[#Kongsager--2016|Kongsager et al., 2016]] ). Water stress has been defined as the situation ‘when the demand for water exceeds its supply, during a certain period of time, or when poor quality restricts its use’( [[#Felberg--1999|Felberg et al., 1999]] ; see also Figure 4.32 in Lee et al., 2021a).

Freshwater resources in Asia, which include both surface water and groundwater, are considerably strained, and changing climate is ''likely'' to act as a major stress multiplier ( [[#Dasgupta--2015|]] [[#Dasgupta--2015|Dasgupta et al., 2015]] ; [[#Fant--2016|Fant et al., 2016]] ; [[#Gao--2018c|Gao et al., 2018c]] ; [[#Mack--2018|Mack, 2018]] ). In Southern and Eastern Asia (SEA) nearly 200 million people are at risk of serious water-stressed conditions. Effective mitigation might reduce the additional population under threat by 30% (60 million people), but still there is a 50% chance that 100 million people across SEA might face a 50% increase in water stress and a 10% chance that water stress will almost double in the absence of wide-ranging, multi-scalar adaptive measures ( [[#Gao--2018c|Gao et al., 2018c]] ). With Millennium Development Goal 7c, which aimed to halve the population that had no sustainable access to water and basic sanitation before 2015, not having been fully realised, and Sustainable Development Goal 6 on water and sanitation not having been effectively operationalised, the water stress is ''likely'' to increase by the end of 2030 ( [[#Weststrate--2019|Weststrate et al., 2019]] ).

In Asia and elsewhere the interplay between the challenge of sustainability and climate change poses major policy challenges ( [[#von%20Stechow--2016|von Stechow et al., 2016]] ). The pursuit of SDG 6—protection and restoration of water-related ecosystems, universal and equitable access to safe and affordable drinking water for all, improvement in water quality by reducing pollution, elimination of dumping and significant reduction in release of hazardous chemicals and materials, and treatment of waste water through recycling and safe reuse globally—could be directly or indirectly challenged and undermined by climate change ( [[#Parkinson--2019|Parkinson et al., 2019]] ). Dissolved organic materials from sewage can enhance CO 2 emissions, especially in rapidly urbanising river systems which receive untreated waste water and/or sewage across developing countries ( [[#Kim--2019b|Kim et al., 2019b]] ). Conversely, policy interventions aimed at significant augmentation in water-use efficiency across all sectors, ensuring sustainable withdrawals and supply of freshwater to address water scarcity and a significant reduction in the number of victims of water scarcity, especially the poor and marginalised, could mitigate vulnerabilities caused by climate change. More interdisciplinary research is needed on highly precarious future pathways and the intersection between CIDs and non-climate drivers in order to anticipate and mitigate diverging and uncertain outcomes.

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==== 10.4.4.2 Sub-regional Diversity ====

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According to a quantitative scenario assessment for future water supply and demand in Asia to 2050, based on global climate change and socioeconomic scenarios ( [[#Satoh--2017|Satoh et al., 2017]] ), water demand in sectors such as irrigation, industry and households will increase by 30–40% around 2050 in comparison with 2010. Water stress is ''likely'' to be more pronounced in Pakistan, and northern parts of India and China. By mid-21st Century, the international transboundary river basins of Amu Darya, Indus, Ganges could face severe water scarcity challenges due to climatic variability and changes acting as stress multipliers ( ''high confidence'' ). Within a country as well, the water scarcity could be exacerbated, such as in India and China, due to various drivers like population increase and climate change. Research on the differentiated impacts of climate change on freshwater sources across the Asian sub-regions remains inconclusive and requires assessment at the sub-regional scale ( [[#IPCC--2014b|IPCC, 2014b]] ; [[#Wester--2019|Wester et al., 2019]] ).

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==== 10.4.4.3 Observed Impact ====

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The climate-change impact on different parts of freshwater ecosystems ( [[#10.4.2|Section 10.4.2]] ) has affected water supply in various sub-regions of Asia. While headwater zones are susceptible to change in snow cover, permafrost and glaciers, the downstream plain areas of these river systems are vulnerable to the increasing high demand of freshwater which will affect water availability in space and time. The observed impact of climate change has also been seen in direct physical losses such as precipitation (Mekong Delta), floods (Vietnam) and saltwater intrusion leading to low agricultural productivity ( [[#Mora--2018|Mora et al., 2018]] ; [[#Almaden--2019a|Almaden et al., 2019a]] ; [[#Pervin--2020|Pervin et al., 2020]] ).

The HKH region extends 3,500 km from Afghanistan in the west to Myanmar in the east. It is a source of major river systems originating in Asia, supporting livelihoods, energy, agriculture and the ecosystem for 240 million people in the mountains and hills and 1.65 billion people in the plains ( [[#Sharma--2019|Sharma et al., 2019]] ). The HKH region stores about half of the ice mass in HMA, provisioning freshwater to almost 869 million people in the Indus, Tarim, Ganges and Brahmaputra river basins. While the warming climate increases the melt-water runoff enhancing water supply, it is indeed at the cost of glacier-mass reduction that would eventually reduce melt water and impact the people’s livelihood downstream in the future ( [[#Nie--2021|Nie et al., 2021]] ). The melt runoff from the region plays an important role in downstream agriculture such as in the case of Indus where two-thirds of total irrigation withdrawal is from melt runoff in the pre-monsoon season ( [[#Biemans--2019|Biemans et al., 2019]] ). Changes in cryosphere and other environmental changes have already impacted people living in high-mountain areas and are ''likely'' to introduce new challenges for water, energy and food security in the future ( [[#Borodavko--2018|Borodavko et al., 2018]] ; [[#Adler--2019|Adler et al., 2019]] ; [[#Bolch--2019|Bolch, 2019]] ; [[#Hoelzle--2019|Hoelzle et al., 2019]] ; [[#Rasul--2019|Rasul and Molden, 2019]] ; [[#Shen--2020|Shen et al., 2020]] ).

With climate-change impacts resulting in the shrinking and melting of snow, ice, glacier and permafrost, and correspondingly causing an increase in melt water, the incidences of flash floods, debris flow, landslides, snow avalanches, livestock diseases and other disasters in the HKH region have become more frequent and intense. Some of the key factors that get in the way of assigning confidence levels to climate-change impacts include lack of sufficient observed data on factors such as river discharges, precipitation and glacier melt ( [[#You--2017|You et al., 2017]] ). Climate-change impacts cryospheric water sources in the Hindu Kush, Karakoram and Himalayan ranges which, in turn, carry consequences for the Indus, Ganges and Brahmaputra basins.

The combined impacts of climate change and non-climate drivers on hydrological processes and water resources in transboundary rivers in diverse regions of Asia were well noted in AR5. In Central Asia, withdrawal is approximately equal to water availability, with Turkmenistan and Uzbekistan as the most water-stressed countries in the region ( [[#Karthe--2017|Karthe et al., 2017]] ; Russell, 2018). A study on water availability in mainland South Asia has pointed in the direction of decreasing precipitation trends in recent years, which have also contributed to the increasing incidence and severity of droughts ( [[#Liu--2018b|Liu et al., 2018b]] ). There are reports of increase in occurrence and severity of different forms of droughts in the Koshi River basin (Central Himalaya) ( [[#Wu--2019a|Wu et al., 2019a]] ; [[#Hamal--2020|Hamal et al., 2020]] ; [[#Dahal--2021|Dahal et al., 2021]] ; [[#Nepal--2021|Nepal et al., 2021]] ). Figure 10.5 shows the water stress in the HKH region. The water stress is relatively higher in the western region compared with the central and eastern regions.

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[[File:136ae2baf51f825017c0165043125ac4 IPCC_AR6_WGII_Figure_10_005.png]]

'''Figure 10.5 |''' '''Water stress in the Hindu Kush Himalaya (HKH) region according to Wester et al.''' '''(2019) and [[#Hu--2018|Hu and Tan (2018)]] .'''

Climate change is also having an impact on stream flows. The changes in snowmelt water can explain 19% of the variations in rivers of arid regions like Xinjiang, China ( [[#Bai--2018|Bai et al., 2018]] ) ( ''medium confidence'' ), and the 10.6% of the runoff of the upper Brahmaputra River was contributed by snow during 2003–2014 ( [[#Chen--2017c|Chen et al., 2017c]] ) ( ''medium confidence'' ). A recent study (Chen et al., 2018 f) has shown that with the average temperature after 1998 being 1.0°C higher than that during 1960–1998 in the Tienshan Mountains, the process of glacier shrinkage and decreases in snow cover are causing earlier peak runoff and aggravated extreme hydrological events, affecting regional water availability and adding to the future water crisis in Central Asia. The magnitude and frequency of flooding have increased across the Himalayan region, such as in the Tarim basin in China ( [[#Zhang--2016c|Zhang et al., 2016c]] ) and the higher Indus, Ganges and Brahmaputra, in the past six decades ( [[#Elalem--2015|Elalem and Pal, 2015]] ). The latter also reported the highest number of flood disasters and greater spatial coverage in recent decades as compared with previous decades. In the Middle Yellow River basin, which has become much warmer and drier, climate variability accounts for 75.8% of streamflow decrease during 1980–2000, whereas during 2001–2016, change in land use and cover was the main factor in streamflow decrease, accounting for 75.5% of the decline ( [[#Bao--2019|Bao et al., 2019]] ). The changes in hydrological regime and extreme floods cause changes in river morphology and the river channel system which impact water availability.

In China, a quantitative assessment based on a multi-model dataset (six global hydrological models driven by three observation-based global forcings) during 1971–2010 suggested that climate variability dominated the changes in streamflow in more the 80% of river segments, while direct human impact dominated changes mostly in northern China ( [[#Liu--2019b|Liu et al., 2019b]] ). In the Lancang-Mekong River basin, climate variability would have contributed 45% more flood occurrences in the middle of the basin, while reservoir operation reduced it by 36% during 2008–2016 as compared with 1985–2007 ( [[#Yun--2020|Yun et al., 2020]] ).

In western China, the total annual snow mass declines at a rate of 3.3 × 10 9 pg per decade ( ''p'' &lt; 0.05), which accounts for approximately 0.46% of the mean of annual snow mass (7.2 × 10 11 pg). The loss could be valued in terms of replacement cost at 0.1 billion CN¥ (at the present value) every year (1 USD = 7 CN¥) compounded over the past 40 years ( [[#Wu--2021|Wu et al., 2021]] ). In the Mekong River Delta in Vietnam, climate-change impacts include a 30% annual increase in rainfall, shifting rainfall patterns, an average temperature increase of 0.5°C over the past 30 years and an average SLR of 3 mm yr –1 over the past three decades, resulting in a greater flooding threat ( [[#Wang--2021a|Wang et al., 2021a]] ).

A recent study ( [[#Wang--2021b|Wang et al., 2021b]] ) has shown that during 1936–2019, due largely to intensified precipitation induced by a warming climate, the streamflow of the Ob, Yenisei and Lena rivers has increased by ∼ 7.7, 7.4 and 22.0%, respectively. While rising temperatures can reduce streamflow via evapotranspiration, it can enhance groundwater discharge to rivers due to permafrost thawing. In permafrost-developed basins, the thawing permafrost will continue to result in increased streamflow. However, with further permafrost degradation in the future, the positive effect of permafrost thaw on streamflow would probably be offset by the negative effect of the increase in basin evapotranspiration. This could result in a situation where runoff reaches threshold level and then declines. This is clearly marked in the Ob River basin, which is characterised by the highest precipitation, whereas in the case of the Yenisei and Lena rivers, further research is needed.

The HKH region is susceptible to floods and related hazards caused by a cloud burst and other landscape-based processes such as glacial lake outburst floods, which can seriously damage property, lives and infrastructure ( [[#Shrestha--2010|Shrestha et al., 2010]] ). The likely increased frequency of hazards caused by abnormal glacier changes, such as the glacier collapses happened on two glaciers in western Tibetan Plateau in 2016 ( [[#Kääb--2018|Kääb et al., 2018]] ), and also surges which were frequently found in this vast region (e.g. [[#Bhambri--2017|Bhambri et al., 2017]] ; [[#Mukherjee--2017|Mukherjee et al., 2017]] ; [[#Ding--2018|Ding et al., 2018]] ), threatening the security of the local and down streaming societies (high confidence).The total amount and area of glacier lakes increased during last decade ( [[#Zhang--2015|Zhang et al., 2015]] ; [[#Chen--2017c|Chen et al., 2017c]] ) ( ''high confidence'' ). Himalayan rivers are frequently hit by catastrophic floods caused by the failure of glacial lakes ( [[#Cook--2018|Cook et al., 2018]] ; [[#Ahluwalia--2016|Ahluwalia et al., 2016]] ). In Kedarnath, India (western Himalaya), a flash flood was triggered by glacier lake outburst flood (GLOF) released from the Chorabari glacial lake in June 2013 which caused extensive flooding, erosion of riverbanks and damage to downstream villages and towns, as well as the loss of several thousand lives ( [[#Rafiq--2019|Rafiq et al., 2019]] ; [[#Das--2015|Das et al., 2015]] ). Nepal has experienced 24 GLOF events which have caused considerable loss of life and damage to property and infrastructure (Icimod, 2011). There is ''high confidence'' that current glacier shrinkages have caused more glacial lakes to form in most mountainous regions, including HMA, but there is limited evidence that the frequency of GLOF has changed ( [[#Hock--2019|Hock et al., 2019]] ). [[#Veh--2018|Veh et al. (2018)]] reported no clear trend of increasing GLOF events in the Himalayan region, although the southern Himalaya was identified as a hotspot region compared with the western Himalaya. Research has shown a decrease in glacier area of 24% in Nepal between 1980 and 2010 ( [[#Bajracharya--2014|Bajracharya et al., 2014]] ).

Climate-change impacts on both the quantity and quality of freshwater resources will hinder the attainment of SDG-6 ( [[#Water--2020|Water, 2020]] ). Contamination of drinking water is caused by wildfires and drought that contribute to elevated levels of nutrients (nitrogen, phosphorus and sulphates), heavy metals (lead, mercury, cadmium and chromium), salts (chloride and fluorides), hydrocarbons, pesticides and even pharmaceuticals. Heavy rains and flooding also increase nutrients, heavy metals and pesticides, as well as turbidity and faecal pathogens in water supplies–especially when sewage treatment plants are overwhelmed by runoff ( [[#Mora--2018|Mora et al., 2018]] ). Pharmaceuticals and personal-care products (from source to disposal) are contributing to the vulnerability of urban waters. A study of vulnerability assessment of urban waters in highly populated cities in India and Sri Lanka, through analysing the concurrence of Pharmaceuticals and Personal Care Products (PPCPs), enteric viruses, antibiotic-resistant bacteria, metals, faecal contamination and antibiotic resistance genes (ARGs), also underlines the need for a resilience strategy and action plan ( [[#Rafiq--2019|Rafiq et al., 2019]] ).

Adequate water supply for various uses is crucial for millions of people living in the mountains of Asia. Particularly in the HKH region, mountain springs play an important role in generating stream flow for non-glaciated catchments and in maintaining dry-season flows across many watersheds ( [[#Scott--2019|Scott et al., 2019]] ; [[#Stott--2014|Stott and Huq, 2014]] ). There is a good deal of evidence that the springs are drying up or yielding less discharge ( [[#Tambe--2012|Tambe et al., 2012]] ; Tiwari and [[#Joshi--2014|Joshi, 2014]] ; [[#Sharma--2016|Sharma et al., 2016]] ), threatening local communities who depend on spring water for their lives and livelihoods. Some of the main reasons for drying springs include anthropogenic impacts (deforestation, exploitative land use), infrastructure (road construction), socioeconomic changes (increasing demand and modernisation of facilities) and climatic changes (changes in rainfall regime and higher temperature) ( [[#Stott--2014|Stott and Huq, 2014]] ; Tiwari and [[#Joshi--2014|Joshi, 2014]] ; [[#Sharma--2016|Sharma et al., 2016]] ).

The Ganges–Brahmaputra region also faces the threat increased frequency of flood events ( [[#Lutz--2019|Lutz et al., 2019]] ). Floods and extreme events can impact river channel systems ( [[#Grainger--2014|Grainger and Conway, 2014]] ). One of the challenges in South Asia is the shifting boundaries of river channels. For instance, the major floods on the Indus in July 2010 altered the river’s course in Pakistan, moving it closer to the Indian district of Kutch ( [[#Grainger--2014|Grainger and Conway, 2014]] ). In the eastern tributary of the Ganges system, the alluvial fan of the Koshi River basin has shifted by more than 113 km to the west in the past two centuries ( [[#Chakraborty--2010|Chakraborty et al., 2010]] ), which may be due to heavy sediment load from the Himalayan rivers in which about 50 million tons of sediment is deposited annually in the alluvial plains ( [[#Sinha--2019|Sinha et al., 2019]] ; [[#Chakraborty--2010|Chakraborty et al., 2010]] ).

Asia is no exception to the global trend of lake ecosystems, which provide drinking water to millions of people, being degraded ( [[#Jenny--2020|Jenny et al., 2020]] ) and severely threatened at the same time by climate change ( [[#Mischke--2020|Mischke, 2020]] ). Lake surface conditions, such as ice cover, surface temperature, evaporation and water level react dramatically to this threat, and there are negative implications for water quantity and quality, food provisioning, recreational opportunities and transportation ( [[#Woolway--2020|Woolway et al., 2020]] ). Due to substantial regional variability, the quantum of future changes in lake water storage remains uncertain. A recent study ( [[#Liu--2019a|Liu et al., 2019a]] ) using Moderate Resolution Imaging Spectroradiometer 500-m spatial resolution global water product data, and applying the least squares method to analyse changes in the area of 14 lakes in Central Asia from 2001 to 2016, has shown that the area-shrinkage changes for all plains lakes in the study region could be attributed to climate change and human activities.

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==== 10.4.4.4 Projections ====

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Asian and global water demands for irrigation, despite geographic variation in terms of water availability, are ''very likely'' to surpass supply by 2050 ( [[#Chartres--2014|Chartres, 2014]] ). A regional quantitative assessment ( [[#Lutz--2019|Lutz et al., 2019]] ) of the impacts of 1.5°C versus 2°C global warming for a major global climate-change hotspot–the Indus, Ganges and Brahmaputra river basins (IGB) in South Asia–shows adverse impacts of climate change on agricultural production, hydropower production and human health. A global temperature increase of 1.5°C with respect to pre-industrial levels would imply a ≈ 2.1°C temperature increase for IGB, whereas under a 2.0°C global temperature increase scenario, these river basins would warm by ≈ 2.7°C. Future warming is expected to further increase rain-on-snow events that can cause snowmelt flood during winter ( [[#Ohba--2020|Ohba and Kawase, 2020]] ), affecting hydropower and resulting in river flooding, avalanches and landslides.

In the Mekong River Delta (in Vietnam), with an area of 40,500 km 2 and home to 17.8 million people in 2018, climate change is projected to increase the average temperature by 1.1–3.6°C, and the maximum and minimum monthly flow are projected to increase and decrease, respectively, and are ''likely'' to result in a high risk of food during the wet season and water shortages during the dry season ( [[#Wang--2021a|Wang et al., 2021a]] ).

Researchers have found that the southern Tibetan Plateau has been consistently melting from 1998–2007 and is projected to continue melting until 2050 ( [[#Lutz--2014b|Lutz et al., 2014b]] ) ( ''high confidence'' ). In HMA, glacier ice is projected to decrease by 49 ± 7% and 64 ± 5% by the end of the century under RCP4.5 and RCP8.5 scenarios, respectively ( [[#Kraaijenbrink--2017|Kraaijenbrink et al., 2017]] ). Local- and regional-scale projections suggest that peak water will generally be reached around the middle of the century, followed by steadily declining glacier runoff thereafter ( [[#Hock--2019|Hock et al., 2019]] ). A global-scale projection suggests that a decline in glacier runoff by 2100 (RCP8.5) may reduce basin runoff by about 10% for at least 1 month of the melt season ( [[#Huss--2018|Huss and Hock, 2018]] ). Significantly, research on climate change and its impact across Asia remains inconclusive and requires an assessment at the sub-regional scale ( [[#IPCC--2014a|IPCC, 2014a]] ; [[#Wester--2019|Wester et al., 2019]] ).

There is a projection of an increase in runoff until the 2050s mainly due to an increase in precipitation in the upper Ganges, Brahmaputra, Salween and Mekong basins, where it could be due to accelerated melting in the upper Indus basin. The runoff could increase in the range of 3–27% (7–12% in Indus, 10–27% in Ganges and 3–8% in Brahmaputra) by mid-century compared with the reference period (1998–2008) for Himalayan river basins depending on the different RCP scenarios ( [[#Lutz--2014a|Lutz et al., 2014a]] ). Likewise, [[#Khanal--2021|Khanal et al. (2021)]] suggest contrasting responses to climate change for HMA rivers in which, on the seasonal scale, the earlier onset of melting causes a shift in magnitude and peak of water availability, whereas on the annual scale, total water availability increases for the headwaters. The future flow would increase in Nepal’s Central Himalaya region ( [[#Nepal--2016|Nepal, 2016]] ; [[#Ragettli--2016|Ragettli et al., 2016]] ; [[#Bajracharya--2018|Bajracharya et al., 2018]] ). These changes in water availability in space and time will have serious consequences in downstream water availability for various sectoral uses and ecosystem functioning in Asia ( [[#Nepal--2014|Nepal et al., 2014]] ; [[#Green--2015|Green et al., 2015]] ; [[#Arfanuzzaman--2018|Arfanuzzaman, 2018]] ; [[#Wijngaard--2018|Wijngaard et al., 2018]] ; [[#Rasul--2019|Rasul and Molden, 2019]] ); however, future water availability is largely uncertain due to significant variation in climate-change projections among different global climate models ( [[#Nepal--2015|Nepal and Shrestha, 2015]] ; [[#Lutz--2016|Lutz et al., 2016]] ; [[#Li--2019a|Li et al., 2019a]] ).

A recent study ( [[#Didovets--2021|Didovets et al., 2021]] ) covering eight river catchments having diverse natural conditions within Central Asia, where water availability or scarcity is also a major developmental concern, and using the eco-hydrological model SWIM (including scenarios from five bias-corrected GCMs under RCP4.5 and RCP8.5) has show an increase in mean annual temperature in all catchments for both RCPs to the end of the 21st century. The projected changes in annual precipitation indicate a clear trend to increase in the Zhabay and decrease in the Murghab catchments, and for other catchments, they were smaller. Both the projected trends for river discharge and precipitation show an increase in the northern and decrease in the southern parts of the study region, whereas seasonal changes include a shift in the peak of river discharge up to one month, a shortening of the snow accumulation period and a reduction in discharge during the summer months.

The intensity and frequency of extreme discharges are ''very likely'' to increase towards the end of the century. The future of the upper Indus basin water availability is highly uncertain in the long term due to uncertainty surrounding precipitation projections ( [[#Lutz--2016|Lutz et al., 2016]] ). The future hydrological extremes of the upper Indus, Ganges and Brahmaputra river basins suggest an increase in the magnitude of extremes towards the end of the 21st century by applying RCP4.5 and RCP8.5 scenarios, mainly due to an increase in precipitation extremes ( [[#Wijngaard--2017|Wijngaard et al., 2017]] ). In the Brahmaputra, Ganges and Meghna, including the downstream component, the runoff is projected to increase by 16, 33 and 40%, respectively, under the climate-change scenarios by the end of the century during which the changes in runoff are larger in the wet seasons than the dry seasons ( [[#Masood--2015|Masood et al., 2015]] ). In the Mekong River basin also, extremely high-flow events are ''likely'' to increase in both magnitude and frequency, which can exacerbate flood risk in the basin ( [[#Hoang--2016|Hoang et al., 2016]] ); however, uncertainty is high regarding future hydrological response due to large variation in precipitation projections, modelling approaches and bias-correction methods ( [[#Nepal--2015|Nepal and Shrestha, 2015]] ; [[#Lutz--2016|Lutz et al., 2016]] ; [[#Li--2019a|Li et al., 2019a]] ).

Current research on the adverse relationship between climate change and river flows suggests that there is a high possibility that some of the river basins affected by floods could be Brahmaputra, Congo, Ganges, Lena and Mekong, with a return period of 10 years ( [[#Best--2018|Best, 2018]] ).

In most parts of the upper Ganges and Brahmaputra rivers, the 50-year return level flood is ''likely'' to increase and to a lesser degree in Indus River. Similarly, the extreme precipitation events are also expected to increase to a higher degree in the Indus than the Ganges and Brahmaputra basins ( [[#Wijngaard--2017|Wijngaard et al., 2017]] ). Increase in extreme precipitation events is ''likely'' to cause more flash-flood events in the future ( ''medium confidence'' ). In the case of the Indus, increasing temperature trend in the future may lead to accelerated snow and ice melting which may increase the frequency and intensity of floods in the downstream areas ( [[#Hayat--2019|Hayat et al., 2019]] ). The Ganges–Brahmaputra region also faces the threat of increased frequency of flood events ( [[#Lutz--2019|Lutz et al., 2019]] ). Additionally, the Ganges basin also shows a higher sensitivity to changes in temperature and precipitation ( [[#Mishra--2016|Mishra and Lilhare, 2016]] ).

Assessing the impact of climate change on water resources in nine alpine catchments in arid and semiarid Xinjiang of China ( [[#Li--2019a|Li et al., 2019a]] ), it has been noted that even though the total discharge revealed an overall increasing trend in the near future, the impact of climate change on different hydrological components indicated significant spatio-temporal heterogeneity in terms of the area, elevation and slope of catchments, which could be usefully factored into climate-adaptation strategies.

It was noted early on ( [[#Singh--2011|Singh et al., 2011]] ) that the main drivers that influence the provisioning of ecosystem services and human well-being in the HKH region are a mix of environmental change in general and climate change in particular, but much more data and knowledge on the HKH region are needed in order to develop either a regional or global understanding of climate-change processes.

Climate change impacts cryospheric water sources in the Hindu Kush, Karakoram and Himalayan ranges which in turn carry consequences for the Indus, Ganges and Brahmaputra basins. The impact of climate change on spring-fed rivers in the HKH is under-researched and therefore makes projections difficult. Further research is needed for understanding the impact of deforestation, urbanisation, development and introduction of water infrastructures, such as tube wells, in the hill region ( [[#Aayog--2017|Aayog, 2017]] ). This in turn calls for greater investment in research and development for the HKH by both the national and regional organisations. There is ''high confidence'' that due to global warming, Asian countries could experience an increase in drought conditions (5–20%) by the end of this century ( [[#Prudhomme--2014|Prudhomme et al., 2014]] ; [[#Satoh--2017|Satoh et al., 2017]] ).

Soil erosion in high-mountain areas is particularly sensitive to climate change. A recent study ( [[#Wang--2020|Wang et al., 2020]] ) that focused on the mid-Yarlung Tsangpo River, located in the southern part of the Tibetan Plateau, has revealed dramatic land surface environment changes due to climate change during recent decades. It has further shown that increasing precipitation and temperature would lead to increasing soil-erosion risk in ~2050 based on the Coupled Model Intercomparison Project (CMIP5) and RUSLE models.

High-resolution climate-change simulations suggest that due to deadly heatwaves projected in some of the densely populated agricultural regions of South Asia (i.e., the Ganges and Indus river basins), those regions are ''likely'' to exceed the critical threshold of wet-bulb temperature of 35°C under the business-as-usual scenario of future GHG emissions ( [[#Im--2017|Im et al., 2017]] ).

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==== 10.4.4.5 Climate Vulnerability and Adaptation: Interfaces and Interventions ====

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In Asia and its diverse sub-regions, the challenge of adaptation to climate change at diverse sectors, sites and scales of vulnerability in the domain of freshwater resources is compounded by the nexus between long-standing non-climatic vulnerabilities and climatic impacts, both observed and projected. Water insecurities in Asia are increasing due to excessive freshwater withdrawals ( [[#Satoh--2017|Satoh et al., 2017]] ), economic and population growth ( [[#Gleick--2018|Gleick and Iceland, 2018]] ), urbanisation and peri-urbanisation ( [[#Roth--2019|Roth et al., 2019]] ), food insecurity ( [[#Demin--2014|Demin, 2014]] ) and lack of access to clean and safe drinking water ( [[#Cullet--2016|Cullet, 2016]] ), which mostly affects the health of the most vulnerable members of society.

Significantly, climate change will add to already existing vulnerabilities. In the case of the Yellow River basin in China, underlining the interface between future water scarcity and hydroclimatic and anthropogenic drivers, a recent study expects moderate-to-severe water scarcity over six Yellow River sub-catchments under the RCP4.5 scenario, and anticipates that human influences on water scarcity will be worse than that of climate change, with water availability in the downstream being impacted by concurrent changes in land use and high temperature ( [[#Omer--2020|Omer et al., 2020]] ). Nearly 8% of internationally shared or transboundary aquifers (TBAs), ensuring livelihood security for millions of people through sustaining drinking water supply and food production, are currently overstressed due to human overexploitation ( [[#Wada--2013|Wada and Heinrich, 2013]] ). The Asia Pacific region has the highest annual water withdrawal due to its geographic size, growing population and irrigation practices, and water for agriculture continues to consume 80% of the region’s resources ( [[#Taniguchi--2017b|Taniguchi et al., 2017b]] ; [[#Visvanathan--2018|Visvanathan, 2018]] ).

In South Asia, surface water and groundwater resources are already under stress (both in terms of quality and quantity) due to population growth, economic development, poor governance and management, and poor efficiency of use in economic production. In the past 40 years, there has been an increasing reliance on groundwater in South Asia for irrigation ( [[#Rodell--2009|Rodell et al., 2009]] ; [[#Tiwari--2009|Tiwari et al., 2009]] ; [[#Surie--2015|Surie and Prasai, 2015]] ; [[#Bhanja--2016|Bhanja et al., 2016]] ; [[#Shrestha--2016|Shrestha et al., 2016]] ; [[#Mukherjee--2018|Mukherjee, 2018]] ; [[#Shah--2018|Shah et al., 2018]] ). It is noteworthy that India, Bangladesh, Pakistan and China together account for more than 50% of the world’s groundwater withdrawals ( [[#Scott--2019|Scott et al., 2019]] ). A study conducted in the Shahpur and Maner districts of Bihar, India, in which drinking water sourced from the groundwater of 388 households was tested, showed that 70–90% of the sampled household’s drinking water contained either arsenic or iron, or both ( [[#Thakur--2019|Thakur and Gupta, 2019]] ). Given the nexus between CIDs and non-climate drivers, an effective adaptation to the impacts of climate change would also demand sustainable development and management of shared aquifer resources, which in turn require reliable TBA inventories and improved knowledge production and knowledge sharing on the shared groundwater systems ( [[#Lee--2018a|Lee et al., 2018a]] ).

A study of peri-urban spaces involving four South Asian cities, Khulna (Bangladesh) ( [[#Pervin--2020|Pervin et al., 2020]] ), Gurugram and Hyderabad (India), and Kathmandu (Nepal), has shown the nexus between intensifying use and deteriorating quality of water and the impact of climate change, resulting in peri-urban water insecurity and conflict ( [[#Roth--2019|Roth et al., 2019]] ). The challenge of ensuring access to water resources and their (re)allocation and prioritisation for marginalised communities remains on the agenda of policy-oriented interdisciplinary research and demands effective implementation of its findings at the grassroots level by the administrative agencies. Taking water security as a key CCA goal at the urban-city scale of Bangkok, a study ( [[#Babel--2020|Babel et al., 2020]] ) has shown the usefulness of a generic framework with 5 dimensions, 12 indicators and a set of potential variables to support national-level initiatives and plans in diverse climatic and socioeconomic conditions across various sub-regions of Asia.

In the Kathmandu valley in Nepal, where groundwater resources are under immense pressure from multiple stresses, including overextraction and climate change, mapping groundwater resilience to climate change has been demonstrated as a useful tool to understand the dynamics of groundwater systems, and thereby facilitate the development of strategies for sustainable groundwater management ( [[#Shrestha--2020|Shrestha et al., 2020]] ).

In the Mekong Delta, the groundwater storage is projected to decline by more than 120 and 160 million m 3 under RCP4.5 and RCP8.5 scenarios, respectively, by the end of the 21st century, in conjunction with land subsidence and SLR. This in turn calls for proactive planning and implementation of adaptation strategies that address multiple stresses in order to ensure sustainable utilisation of groundwater resources in the Mekong Delta in the context of future climatic conditions and associated uncertainties ( [[#Wang--2021a|Wang et al., 2021a]] ). Proposed CCA strategies for the Mekong River basin include a better understanding of the complex linkages between climate change, technological interventions, land-use change, water-use change and socioeconomic developments both in the upstream and downstream riparian countries ( [[#Evers--2018|Evers and Pathirana, 2018]] ).

While South Asian countries have done well in attaining Goal 6 of Sustaining Development Goals, access to safe and clean drinking water remains a challenge. Taking Indian rivers as an example, it is suggested that participatory river protection and rehabilitation, based on comprehensive knowledge of the river-system dynamics, and local awareness at the community level, may act as a multiplier for river conservation measures ( [[#Nandi--2016|Nandi et al., 2016]] ).

Hydroclimatic extremes in the HKH region could adversely impact the Ganga, Brahmaputra and Meghna basins ( [[#Wijngaard--2017|Wijngaard et al., 2017]] ; [[#Acharya--2019|Acharya and Prakash, 2019]] ). Studies have recommended watershed or basin analysis to address the challenge of adaptation in urban spaces ( [[#Lele--2018|Lele et al., 2018]] ). A study of northern Bangladesh that focused on encouraging traditional ways of cultivation suggests that rural women have Indigenous knowledge and their participation can play a useful role ( [[#Kanak%20Pervez--2015|Kanak Pervez et al., 2015]] ). The knowledge pertains to agriculture, soil conservation, fish and animal production, irrigation and water conservation. There has also been a focus on gendered construction of local flood-forecasting knowledge in rural communities in India living in the Gandak River basin ( [[#Acharya--2019|Acharya and Prakash, 2019]] ). While designing the adaptation options, understanding the water–energy–food (WEF) nexus among different water-use sectors is crucial ( [[#10.5.3|Section 10.5.3]] ). Understanding of the WEF nexus could be beneficial for achieving water security in developing countries in Asia ( [[#Nepal--2019|Nepal et al., 2019]] ).

AR5 identified a number of adaptation challenges and options facing the stakeholders in the wake of climate-change-induced vulnerabilities, uncertainties and risks in the freshwater sector, and underlined the importance of an integrated management approach as well as acknowledging diverse socioeconomic contexts, differentiated capacities and the uneven pace of impacts. Further validated by recent research in terms of their usefulness, these adaptation options include building and improving capital-intensive physical water infrastructure such as irrigation channels, flood-control dams and water storage ( [[#Nüsser--2017|Nüsser and Schmidt, 2017]] ). Drawing upon customary institutions and combining Indigenous knowledge systems with scientific knowledge, innovative structures, including artificial glaciers, ice stupas and snow barrier bands, have been built by local communities in Ladakh, Zanskar and Himachal Pradesh in India ( [[#Hock--2019|Hock et al., 2019]] ; [[#Nüsser--2019|Nüsser et al., 2019]] ). Communities in Solukhumbu, Nepal, in response to depleting water flow in snow-fed rivers, have chosen adaptation through changing practices by collecting water from distant sources for domestic consumption ( [[#McDowell--2013|McDowell et al., 2013]] ). Taking the IPCC concept of climate risk as a basis for adaptation planning, a pilot study of flood risk in Himachal Pradesh, India ( [[#Allen--2018|Allen et al., 2018]] ), integrating assessment of hazard, vulnerability and exposure in the complementary domains of CCA and DRR, has identified stakeholder consultation, knowledge exchange and institutional capacity building as key steps in adaptation planning. Aquifer storage and recovery has been proposed as an ‘alternative climate-proof freshwater source’ for deltaic regions in Asia, particularly those with a history of saline groundwater aquifers ( [[#Hoque--2016|Hoque et al., 2016]] ). It is further argued ( [[#Hadwen--2015|Hadwen et al., 2015]] ) that water, sanitation and hygiene objectives would need to be addressed as a component of a wider integrated water resource management (IWRM) framework.

Ensuring sustainability of the rivers and ecosystems requires coordinated and collaborative action on the part of all countries, with the long-term goal of synergising political, social, cultural and ecological facets associated with the riverine system. Daunting as this challenge is, evidence suggests that a long-term view of transboundary basins is not very optimistic as big rivers of Asia contribute heavily towards urban and agricultural activities, and are experiencing challenges of increasing sedimentation, large-scale damming and pollution, among others ( [[#Best--2018|Best, 2018]] ). In the case of China, [[#Sun--2016|Sun et al. (2016)]] have shown that the localised vulnerabilities within the Yangtze River basin prompt an ‘integrated basin-wide approach’ that is able to account for the specific needs of each of its sub-basins.

In HMA, factors that undermine effective adaptation to climate change include both sudden-onset and slow-paced disasters along with the knowledge deficit regarding cryospheric change and its adverse impacts on water resources and also the agriculture and hydropower sectors. Other key barriers include a sectoral approach, overemphasis on structural approaches and the lack of context-sensitive, community-centric understanding of how these changes influence perceptions, options and decisions about migration, relocation and resettlement ( [[#Rasul--2020|Rasul et al., 2020]] ; [[#Hock--2019|Hock et al., 2019]] ). More interdisciplinary research is needed on highly precarious future pathways and the intersection between CIDs and non-climate drivers in order to anticipate and mitigate diverging and uncertain outcomes.

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'''Box 10.4 | Case Study on Climate Vulnerability and Cross-Boundary Adaptation in Central Asia'''
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In Central Asia, water scarcity has been ranked in the top five global risks ( [[#Gleick--1993|Gleick, 1993]] ; [[#Zhupankhan--2018|Zhupankhan et al., 2018]] ). Cross-boundary adaptation remains critically important in this region with abundant glaciers in the Pamir Plateau of Tajikistan ( [[#Hu--2017|Hu et al., 2017]] ) and areas with severe glacier retreat in the Tianshan Mountains ( [[#Liu--2015|Liu and Liu, 2015]] ). The spatial variations of glacier and other climate variables have added to uncertainty related to the dynamic of the water cycle. The headwater regions, such as Pamir area, would be significantly affected by the climate parameters, such as the stronger rainfall intensity, more frequent rainfall and higher temperature ( [[#Luo--2019|Luo et al., 2019]] ). The water resources in the Pamir Plateau will range from −0.48 to 5.6% ( [[#Gulakhmadov--2020|Gulakhmadov et al., 2020]] ), and the crop phenological period in Tajikistan and Kyrgyzstan will be about 1–2 weeks earlier. The threat of agricultural water stress is increasing as well. The oasis in downstream areas will face more complex water resource fluctuations, water crisis and desertification. In particular, rain-fed agriculture in northern Kazakhstan, Uzbekistan and western Turkmenistan is particularly dependent on water resources. Under the RCP2.6 and RCP4.5 scenarios, considering CO 2 fertilisation effects and land-use projections, the increase in CO 2 atmospheric concentration and accumulated temperature can contribute to a 23% increase in cotton yield in Central Asia ( [[#Tian--2019|Tian and Zhang, 2019]] ), but extreme climate, such as drought, heatwaves and rainstorms, will have a 10% negative impact on agricultural production and the ecological environment ( [[#Zhang--2017|Zhang and Ren, 2017]] ). High-efficiency water-saving technology will help the upstream and downstream water resource management in Central Asian countries to adapt to the variation in water resources quantity, frequency and spatial pattern.

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