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

===== 8.3.1.7.4 Groundwater =====

<div id="h4-4-siblings" class="h4-siblings"></div>

As the world’s most widespread store of freshwater (R.G. [[#Taylor--2013|Taylor et al., 2013]] a), groundwater is estimated to supply between a quarter and a third of the world’s annual freshwater withdrawals to meet agricultural, industrial and domestic demands ( [[#Döll--2012|Döll et al., 2012]] ; [[#Wada--2014|Wada et al., 2014]] ; [[#Hanasaki--2018|Hanasaki et al., 2018]] ).

Attribution of changes in groundwater storage, observed locally through piezometry (Figure 8.10; R.G. [[#Taylor--2013|Taylor et al., 2013]] a) or estimated from GRACE satellite measurements ( [[#Rodell--2018|Rodell et al., 2018]] ) at regional scales (&gt;100,000 km <sup>2</sup> ), is often complicated by non-climate influences that include land-use change ( [[#Favreau--2009|Favreau et al., 2009]] ) and human withdrawals ( [[#Bierkens--2019|Bierkens and Wada, 2019]] ).

<div id="_idContainer033" class="Basic-Text-Frame"></div>

[[File:0f716d182560d9bc8edaeede0f45dd62 IPCC_AR6_WGI_Figure_8_10.png]]

'''Figure 8.10 |''' '''Trends in Terrestrial Water Storage (TWS; in centimetres per year, cm y''' '''r''' <sup>–1</sup> ''') obtained on the basis of GRACE observations from April 2002 to March 2016.''' The cause of the trend in each outlined study region is briefly explained and colour-coded by category. The trend map was smoothed with a 150 km radius Gaussian filter for the purpose of visualization. However, all calculations were performed at the native 3° resolution of the data product. Figure from [[#Rodell--2018|Rodell et al. (2018)]] . Further details on data sources and processing are available in the chapter data table (Table 8.SM.1).

Following a global review of groundwater and climate change (R.G. [[#Taylor--2013|Taylor et al., 2013]] a) and AR5 WGII, evidence of an association between heavy or extreme precipitation and groundwater recharge has continued to grow, especially in tropical ( [[#Asoka--2018|Asoka et al., 2018]] ; [[#Cuthbert--2019a|Cuthbert et al., 2019a]] ; [[#Kotchoni--2019|Kotchoni et al., 2019]] ) and subtropical regions ( [[#Meixner--2016|Meixner et al., 2016]] ). Stable-isotope ratios of oxygen and hydrogen at 14 of 15 sites across the tropics trace groundwater recharge to intensive monthly rainfall, commonly exceeding the 70th intensity percentile, approximately ( [[#Jasechko--2015|Jasechko and]] [[#Taylor--2015|Taylor, 2015]] ). Further, heavy rainfall recharging groundwater resources is often influenced by climate variability such as ENSO and PDO (R.G. [[#Taylor--2013|Taylor et al., 2013]] b; [[#Kuss--2014|Kuss and Gurdak, 2014]] ; [[#Asoka--2017|Asoka et al., 2017]] ; [[#Cuthbert--2019b|Cuthbert et al., 2019b]] ; [[#Kolusu--2019|Kolusu et al., 2019]] ; [[#Shamsudduha--2020|Shamsudduha and Taylor, 2020]] ). Additionally, increases in groundwater storage estimated from GRACE for 37 of the world’s large-scale aquifer systems from 2002 to 2016 are generally found to result from episodic recharge associated with extreme (&gt;90th percentile) annual precipitation.

The overall underestimation of precipitation intensities in global climate models ( [[#Wehner--2010|Wehner et al., 2010]] , 2020; [[#Goswami--2017|Goswami and Goswami, 2017]] ) and of their sensitivity to warming temperatures ( [[#Borodina--2017|Borodina et al., 2017]] ) may lead to underestimates of their recharging effect on groundwater ( [[#Mileham--2009|Mileham et al., 2009]] ; [[#Cuthbert--2019b|Cuthbert et al., 2019b]] ). The limited ability of global climate models to represent key controls on regional rainfall variability like ENSO (Technical ( [[IPCC:Wg1:Chapter:Annex-vi|Annex VI]] and [[IPCC:Wg1:Chapter:Chapter-3#3.7.3|Section 3.7.3]] ; R. [[#Chen--2020|]] [[#Chen--2020|Chen et al., 2020]] ) may also underestimate observed recharge from such events that are of particular importance in drylands (R.G. Taylor et al. , 2013b; Cuthbert et al. , 2019b) . Numerical representations of the impact of precipitation intensification on groundwater recharge in large-scale models remain constrained by the challenges of including key recharge pathways that consider preferential flowpaths in soils ( [[#Beven--2018|Beven, 2018]] ) and focused recharge through leakage from surface waters ( [[#Döll--2014|Döll et al., 2014]] ).

Increasing global freshwater withdrawals, primarily associated with the expansion of irrigated agriculture in drylands, have led to global groundwater depletion that has an estimated range of about 100 and about 300 km <sup>3</sup> yr <sup>–1</sup> from hydrological models and volumetric-based calculations ( [[#Bierkens--2019|Bierkens and Wada, 2019]] ). The magnitude of this change is such that its estimated contribution to global sea level rise is in the order of 0.3 to 0.9 mm yr <sup>−1</sup> (Wada et al. , 2010; [[#Konikow--2011|Konikow, 2011]] ; Döll et al. , 2014; Pokhrel et al. , 2015; de Graaf et al. , 2017; Hanasaki et al. , 2018) . Groundwater depletion has been observed regionally in The USA High Plains, California’s Central Valley ( [[#Scanlon--2012|Scanlon et al., 2012]] ), north-west India (Rodell et al. , 2009; Asoka et al. , 2017), Upper Ganges in India ( [[#MacDonald--2016|MacDonald et al., 2016]] ), North China Plain ( [[#Feng--2013|Feng et al., 2013]] ), north-central Middle East region of Tigris–Euphrates–Western Iran ( [[#Voss--2013|Voss et al., 2013]] ), Central Asia ( [[#Hu--2019|Hu et al., 2019]] ), and North Africa ( [[#Bouchaou--2013|Bouchaou et al., 2013]] ). The regional contribution of agricultural irrigation to groundwater depletion was previously highlighted by SRCCL but no formal assessment of observed changes in global or regional groundwater featured in AR5.

Quantification of changes in groundwater storage from GRACE is currently constrained by uncertainty in the estimation of changes in other terrestrial water stores using uncalibrated, global-scale Land Surface Models (Döll et al. , 2014; Scanlon et al. , 2018) and the limited duration of the period of GRACE observations (2002 to 2016). Centennial-scale piezometry in north-west India reveals that recent groundwater depletion traced by GRACE ( [[#Rodell--2009|Rodell et al., 2009]] ; [[#Chen--2014|Chen et al., 2014]] ), follows more than a century of groundwater accumulation through canal leakage ( [[#MacDonald--2016|MacDonald et al., 2016]] ). Further, groundwater depletion is often localized occurring below the footprint (200,000 km <sup>2</sup> ) of GRACE, as has been well demonstrated by detailed modelling studies in the California Central Valley ( [[#Scanlon--2012|Scanlon et al., 2012]] ) and North China Plain ( [[#Cao--2016|Cao et al., 2016]] ).

Climate variability and drought affect groundwater depletion mainly due to amplified groundwater withdrawals. For instance, the depletion rate in Central Valley aquifer in the USA from 2006 to 2010 is estimated to range from 6 to 8 km <sup>3</sup> yr <sup>–1</sup> using GRACE data ( [[#Scanlon--2012|Scanlon et al., 2012]] ). In India, [[#Asoka--2017|Asoka et al. (2017)]] show contrasting trends in groundwater storage in the north (declining at 2 cm yr <sup>–1</sup> ) and south (increasing at 1–2 cm yr <sup>–1</sup> ) that is explained by variations in human withdrawals and precipitation linked to Indian Ocean sea surface temperature variability.

Changes in meltwater regimes from glaciers and seasonal snow packs tend to reduce the seasonal duration and magnitude of recharge ( [[#Tague--2009|Tague and Grant, 2009]] ). Aquifers in mountain valleys show shifts in the timing and magnitude of: (i) peak groundwater levels due to an earlier spring melt; and (ii) low groundwater levels associated with lower baseflow periods ( [[#Allen--2010|Allen et al., 2010]] ; [[#Dierauer--2018|Dierauer et al., 2018]] ; [[#Hayashi--2020|Hayashi, 2020]] ). The effects of receding alpine glaciers on groundwater systems are not well understood but long-term loss of glacier storage is estimated to reduce summer baseflow ( [[#Gremaud--2009|Gremaud et al., 2009]] ). In permafrost regions, coupling between surface water and groundwater systems may be particularly enhanced by warming ( [[#Lamontagne-Hallé--2018|Lamontagne-Hallé et al., 2018]] ; [[#Lemieux--2020|Lemieux et al., 2020]] ). In areas of seasonal or perennial ground frost, increased recharge is expected despite a decrease in absolute snow volume (Okkonen and Kløve, 2011; Walvoord and Kuryl yk, 2016) .

Coastal aquifers are the interface between the oceanic and terrestrial hydrological systems. Global sea level rise (SLR) causes interfaces between freshwater and saline-water to move inland. The extent of seawater intrusion into coastal aquifers depends on a variety of factors including coastal topography, recharge, and groundwater abstraction from coastal aquifers ( [[#Comte--2016|Comte et al., 2016]] ). Modelling results suggest that the impact of SLR on seawater intrusion is negligible compared to that of groundwater abstraction (Ferguson and Gleeson, 2012; [[#Yu--2019|Yu and Michael, 2019]] ) . Coastal aquifers under very low hydraulic gradients, such as the Asian mega-deltas, are theoretically sensitive to SLR but, according to evidence from [[#Akter--2019|Akter et al. (2019)]] in the Ganges-Brahmaputra-Megna basin, may be more severely and widely affected by changes in upstream river discharge. They argue further that saltwater inundation from storm surges will have the greatest localized effects.

In summary, there is ''medium confidence'' that increased precipitation intensities, partly due to human influence, have enhanced groundwater recharge, most notably in the tropics. There is ''high confidence'' that groundwater depletion has occurred since at least the start of the 21st century as a consequence of groundwater withdrawals for irrigation in some of the world’s most productive agricultural areas in drylands (e.g., southern High Plains and California Central Valley in the USA, the North China Plain, north-west India).

<div id="8.3.2" class="h2-container"></div>

<span id="observed-variations-in-large-scale-phenomena-and-regional-variability"></span>