Editing IPCC:AR6/WGII/Chapter-4 (section)

=== 4.2.6 Observed Changes in Groundwater ===

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AR5 concluded that the extent to which groundwater abstractions are affected by climate change is not well known due to the lack of long-term observational data ( [[#Jiménez%20Cisneros--2014|Jiménez Cisneros et al., 2014]] ). AR6 ( [[#Douville--2021|Douville et al., 2021]] ) confirmed that, despite considerable progress since AR5, limitations in the spatio-temporal coverage of groundwater monitoring networks, abstraction data and numerical representations of groundwater recharge processes continue to constrain understanding of climate change impacts on groundwater.

Globally, groundwater use has societal and economic benefits, providing a critical buffer against precipitation variability. Groundwater irrigation has ensured food security, livelihood support and poverty alleviation, for example, in India ( [[#Sekhri--2014|Sekhri, 2014]] ), Bangladesh ( [[#Salem--2018|Salem et al., 2018]] ) and sub-Saharan Africa ( [[#Taylor--2013a|Taylor et al., 2013a]] ; [[#Cuthbert--2019b|Cuthbert et al., 2019b]] ). Groundwater is a safe drinking water source during natural hazard-induced disasters ( [[#Richts--2016|Richts and Vrba, 2016]] ). However, groundwater over-exploitation leads to the attenuation of societal benefits, including reduced agricultural production ( [[#Asoka--2020|Asoka and Mishra, 2020]] ; [[#Jain--2021|Jain et al., 2021]] ), decrease in adaptive capacity of communities ( [[#Blakeslee--2020|Blakeslee et al., 2020]] ) and water quality deterioration ( [[#Mas-Pla--2019|Mas-Pla and Menció, 2019]] ). Loss of traditional water systems based on groundwater, such as ''foggara'' in Tunisia ( [[#Mokadem--2018|Mokadem et al., 2018]] ), ''qanat'' in Pakistan ( [[#Mustafa--2008|Mustafa and Usman Qazi, 2008]] ), ''aflaj'' in Oman ( [[#Remmington--2018|Remmington, 2018]] ) and spring boxes in the Himalayas ( [[#Kumar--2018|Kumar and Sen, 2018]] ), also leads to loss of cultural values for local communities.

Even though global groundwater abstraction (789 ± 30 km3 yr−1) is just about 6% of the annual recharge (~13,466 km 3 ) ( [[#Hanasaki--2018|Hanasaki et al., 2018]] ), a few hotspots of groundwater depletion have emerged at local to regional scales since the end of 20th century to the beginning of the 21st century due to intensive groundwater use for irrigation. The variability in groundwater storage is a function of human abstraction and natural recharge, which is in turn controlled by local geology ( [[#Green--2016|Green, 2016]] ). In humid regions, precipitation influences recharge, and linear associations between precipitation and recharge are often observed ( [[#Kotchoni--2019|Kotchoni et al., 2019]] ); for example, over humid locations in sub-Saharan Africa ( [[#Cuthbert--2019b|Cuthbert et al., 2019b]] ).

A global review ( [[#Bierkens--2019|Bierkens and Wada, 2019]] ) of groundwater storage changes highlights that estimates of depletion rates at the global scale are variable. These estimates range from approximately 113 to 510 km 3 yr −1 and variation in estimates is due to methods and spatio-temporal scales considered ( ''high confidence'' ). Global hydrological models ( [[#Herbert--2019|Herbert and Döll, 2019]] ) show that human-induced groundwater depletion at rates exceeding 20 mm yr –1 (2001–2010) is occurring in the major aquifers systems such as the High Plains and California Central Valley aquifers (USA), Arabian aquifer (Middle East), North-Western Sahara Aquifer System (North Africa), Indo-Gangetic Basin (India) and North China Plain (China) ( ''high confidence'' ). Groundwater depletion at lower rates (&lt;10 mm yr –1 ) is taking place in the Amazon Basin (Brazil) and Mekong River Basin (South East Asia), primarily due to climate variability and change ( ''high confidence'' ). A global-scale analysis ( [[#Shamsudduha--2020|Shamsudduha and Taylor, 2020]] ) of GRACE satellite measurements (2002–2016) for the 37 world’s large aquifer systems reveals that trends in groundwater storage are mostly nonlinear and declines are not secular ( ''high confidence'' ). There are strong statistical associations between changes in groundwater storage and extreme annual precipitation from 1901 to 2016 in the Great Artesian Basin (Australia) and the California Central Valley aquifer (USA). Groundwater recharge of high magnitudes can be generated from intensive precipitation events. On the other hand, recharge can become more episodic, mostly in arid to semiarid locations ( ''robust evidence, medium agreement'' ). For example, in central Tanzania, seven rainfall events between 1955 and 2010 generated 60% of total recharge ( [[#Taylor--2013b|Taylor et al., 2013b]] ). Similarly, in southern India ( [[#Asoka--2018|Asoka et al., 2018]] ) and the southwestern USA ( [[#Thomas--2016|Thomas et al., 2016]] ), focused recharge via losses from ephemeral river channels, overland flows, and floodwaters is documented ( [[#Cuthbert--2019b|Cuthbert et al., 2019b]] ).

In cold regions, where snowmelt dominates the local hydrological processes, Irannezhad et al. (2016) and Vincent et al. (2019) show high recharge to aquifers from glacial meltwater, while [[#Nygren--2020|Nygren et al. (2020)]] report a decrease in groundwater recharge due to a shift in main recharge period from spring (snowmelt) to winter (rainfall). In Finland, a sustained reduction (almost 100 mm in 100 years) of long-term snow accumulation combined with early snowmelt has reduced spring recharge ( [[#Irannezhad--2016|Irannezhad et al., 2016]] ) ( ''medium confidence'' ).

Data from ground-based long-term records in the Indo-Gangetic Basin reveals that sustainable groundwater supplies are constrained more by extensive contamination (e.g., arsenic, salinity) than depletion ( [[#MacDonald--2016|MacDonald et al., 2016]] ). Many low-lying coastal aquifers are contaminated with increased salinity due to land use change, rising sea levels, reduced stream flows and increased storm surge inundation ( [[#Lall--2020|Lall et al., 2020]] ). Nearly 26 million people are currently exposed to very high (&gt;1500 μ S cm –1 ) salinity in shallow groundwater in coastal Bangladesh ( [[#Shamsudduha--2020|Shamsudduha and Taylor, 2020]] ).

Groundwater-dependent ecosystems (GDEs), such as terrestrial wetlands, stream ecosystems and estuarine and marine ecosystems ( [[#Kløve--2014|Kløve et al., 2014]] ), support wetlands and biodiversity, provide water supply and baseflows to rivers, offer recreational services and help control floods ( [[#Rohde--2017|Rohde et al., 2017]] ). Globally, 10–23% of the watersheds have reached the environmental flow limits due to groundwater pumping ( [[#de%20Graaf--2019|de Graaf et al., 2019]] ). A recent study of 4.2 million wells across the USA shows that induced groundwater recharge in nearly two thirds of these wells could reduce stream discharges, thereby threatening GDEs ( [[#Jasechko--2021|Jasechko et al., 2021]] ). [[#Work--2020|Work (2020)]] found reduced spring flow due to increased groundwater abstraction in 26 out of 56 springs studied in Florida (USA). GDEs in semiarid and arid regions tend to have much longer groundwater response times and may be more resilient to climate change than those in humid areas where groundwater occurrence is mostly at shallow levels ( [[#Cuthbert--2019a|Cuthbert et al., 2019a]] ; [[#Opie--2020|Opie et al., 2020]] ). However, groundwater depletion impacts on the full range of ecosystem services remain understudied ( [[#Bierkens--2019|Bierkens and Wada, 2019]] ).

A better understanding of and incorporating subsurface storage dynamics into ESMs will improve climate–groundwater interactions under global warming ( [[#Condon--2020|Condon et al., 2020]] ). Long-term groundwater-level monitoring data are of critical importance ( [[#Famiglietti--2014|Famiglietti, 2014]] ) for understanding the sensitivity of recharge processes to climate variability and, more critically, calibration and validation of hydrological models ( [[#Goderniaux--2015|Goderniaux et al., 2015]] ). GRACE satellite-derived groundwater storage estimates provide important insights at a regional scale ( [[#Rodell--2018|Rodell et al., 2018]] ) but overlook more localised depletion or short-term storage gains. Low- and middle-income countries such as central Asia and sub-Saharan Africa lack such monitoring networks, which is a significant knowledge gap.

In summary, groundwater storage has declined in many parts of the world, most notably since the beginning of the 21st century, due to the intensification of groundwater-fed irrigation ( ''high confidence'' ). Groundwater in aquifers across the tropics appears to be more resilient to climate change as enhanced recharge is observed to occur mostly episodically from intense precipitation and flooding events ( ''robust evidence, medium agreement'' ). In higher altitudes, warmer climates have altered groundwater regimes and may have led to reduced spring recharge due to reduced duration and snowmelt discharges ( ''medium confidence'' ).

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