Ground water and climate change

TLDR: In this paper, the authors critically review recent research assessing the impacts of climate on ground water through natural and human-induced processes as well as through groundwater-driven feedbacks on the climate system, and highlight the possible opportunities and challenges of using and sustaining groundwater resources in climate adaptation strategies.

Abstract: As the world's largest distributed store of fresh water, ground water plays a central part in sustaining ecosystems and enabling human adaptation to climate variability and change. The strategic importance of ground water for global water and food security will probably intensify under climate change as more frequent and intense climate extremes (droughts and floods) increase variability in precipitation, soil moisture and surface water. Here we critically review recent research assessing the impacts of climate on ground water through natural and human-induced processes as well as through groundwater-driven feedbacks on the climate system. Furthermore, we examine the possible opportunities and challenges of using and sustaining groundwater resources in climate adaptation strategies, and highlight the lack of groundwater observations, which, at present, limits our understanding of the dynamic relationship between ground water and climate.

Figures

Table 1 | Estimates of global- and continental-scale groundwater depletion.

Figure 2 | Conceptual representation of key interactions between ground water and climate.

Figure 3 | Global map of anthropogenic groundwater recharge rates in areas with substantial irrigation by surface water. Rates are estimated from the difference between the return flow of irrigation water to ground water and total groundwater withdrawals for the period 1998 to 20022.

Full text

NATURE CLIMATE CHANGE | ADVANCE ONLINE PUBLICATION | www.nature.com/natureclimatechange 1
G
round water is an almost ubiquitous source of generally
high-quality fresh water. ese characteristics promote its
widespread development, which can be scaled and local-
ized to demand, obviating the need for substantial infrastructure
1
.
Globally, ground water is the source of one third of all fresh-
water withdrawals, supplying an estimated 36%, 42% and 27%
of the water used for domestic, agricultural and industrial pur-
poses, respectively
2
. In many environments, natural groundwater
discharges sustain baseow to rivers, lakes and wetlands during
periods of low or no rainfall. Despite these vital contributions to
human welfare and aquatic ecosystems, a paucity of studies on the
relationship between climate and ground water severely restricted
the ability of the Intergovernmental Panel on Climate Change
(IPCC) to assess interactions between ground water and climate
change in both its third
3
and fourth
4
assessment reports. ere has
since been a marked rise in published research
5–8
applying local-
to global-scale modelling, as well as ground-based and satellite
monitoring, which has considerably enhanced our understanding
of interactions between ground water and climate. Here we build
on an earlier broad-based overview
8
of the topic, and examine
substantial recent advances. ese include emerging knowledge
of the direct and indirect (through groundwater use) eects of
climate forcing — including climate extremes — on groundwater
resources, as well as feedbacks between ground water and climate,
such as the contribution of groundwater depletion to global sea-
level rise. Furthermore, we identify critical gaps in our under-
standing of the interactions between ground water and climate.
Influence of climate on groundwater systems
Climate variability and change inuences groundwater systems
both directly through replenishment by recharge and indirectly
through changes in groundwater use. ese impacts can be modi-
ed by human activity such as land-use change (LUC).
Palaeohydrological evidence. e long-term responses of ground
water to climate forcing, largely independent of human activity,
can be detected from palaeohydrological evidence from regional
aquifer systems in semi-arid and arid parts of the world (Fig.1).
Much of the ground water owing in large sedimentary aquifers
of the central United States (High Plains aquifer), Australia (Great
Artesian basin), southern Africa (Kalahari sands) and North Africa
(Nubian sandstone aquifer system) was recharged by precipitation
thousands of years ago
10–13
. As evaporation and plant transpira-
tion consume soil moisture but leave chloride behind, substantial
Ground water and climate change
Richard G. Taylor et al.*
As the world’s largest distributed store of fresh water, ground water plays a central part in sustaining ecosystems and enabling
human adaptation to climate variability and change. The strategic importance of ground water for global water and food secu-
rity will probably intensify under climate change as more frequent and intense climate extremes (droughts and floods) increase
variability in precipitation, soil moisture and surface water. Here we critically review recent research assessing the impacts of
climate on ground water through natural and human-induced processes as well as through groundwater-driven feedbacks on
the climate system. Furthermore, we examine the possible opportunities and challenges of using and sustaining groundwater
resources in climate adaptation strategies, and highlight the lack of groundwater observations, which, at present, limits our
understanding of the dynamic relationship between ground water and climate.
accumulations of chloride in unsaturated soil proles within these
basins indicate that little (≤5mmyr
−1
) or no recharge has since
taken place
14
; which is the case across many of the basins. Stable
isotopes of oxygen and hydrogen, together with concentrations of
noble gases, suggest that recharge occurred under cooler climates
(≥5 °C cooler) before and occasionally during Late Pleistocene
glaciation, with further local additions during the Early Holocene.
Ground water that was recharged during cooler, wetter climates of
the Late Pleistocene and Early Holocene (≥5,000years ) is com-
monly referred to as ‘fossil ground water’. As current groundwater
recharge rates are responsible for at most a tiny fraction of total
groundwater storage, fossil aquifers are storage dominated rather
than recharge-ux dominated
15
. As such, their lifespan is deter-
mined by the rate of groundwater abstraction relative to exploit-
able storage. In these systems, robust estimates of groundwater
storage and accurate records of groundwater withdrawals are of
critical importance. Although fossil aquifers provide a reliable
source of ground water that is resilient to current climate variabil-
ity, this non-renewable groundwater exploitation is unsustainable
and is mined in a manner similar to oil
16
.
Direct impacts. Natural replenishment of ground water occurs
from both diuse rain-fed recharge and focused recharge via leak-
age from surface waters (that is, ephemeral streams, wetlands or
lakes) and is highly dependent on prevailing climate as well as
on land cover and underlying geology. Climate and land cover
largely determine precipitation and evapotranspiration, whereas
the underlying soil and geology (Fig. 1) dictate whether a water
surplus (precipitation minus evapotranspiration) can be transmit-
ted and stored in the subsurface. Modelled estimates of diuse
recharge globally
17,18
range from 13,000to 15,000km
3
yr
−1
, equiva-
lent to ~30% of the world’s renewable freshwater resources
19
or a
mean per capita groundwater recharge of 2,100to 2,500m
3
yr
−1
.
ese estimates represent potential recharge uxes as they are
based on a water surplus rather than measured contributions to
aquifers. Furthermore, these modelled global recharge uxes do
not include focused recharge, which, in semi-arid environments,
can be substantial
14,20
.
Spatial variability in modelled recharge is related primarily to
the distribution of global precipitation
17,18
. Over time, recharge
is strongly inuenced by climate variability — including climate
extremes (droughts and oods) that are oen related to modes of
climate variability such as the El Niño/Southern Oscillation (ENSO)
at multiyear timescales and the Pacic Decadal Oscillation, Atlantic
*A full list of author names and their aliations appears at the end of the review article.
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Multidecadal Oscillation (AMO) and others at longer timescales
21,22
.
During the recent multi-annual Millennium Drought in Australia,
groundwater storage in the Murray–Darling basin declined sub-
stantially and continuously by ~100±35km
3
from 2000to 2007in
response to a sharp reduction in recharge
23
. In tropical Africa,
heavy rainfall has been found to contribute disproportionately to
recharge observed in borehole hydrographs
21,24
. Recharge in semi-
arid environments is oen restricted to statistically extreme (heavy)
rainfall
17,25
that commonly generates focused recharge beneath
ephemeral surface water bodies
20,21,26
. Recharge from heavy rain-
fall events is also associated with microbial contamination of shal-
low groundwater-fed water supplies and outbreaks of diarrhoeal
diseases in both low- and high-income countries
27
. Wetter condi-
tions do not, however, always produce more groundwater recharge.
Incidences of greater (×2.5) winter precipitation in the southwest
United States during ENSO years give rise to enhanced evapotran-
spiration from desert blooms that largely or entirely consume the
water surplus
28
.
At high latitudes and elevations, global warming changes the
spatial and temporal distribution of snow and ice. Warming results
in less snow accumulation and earlier melting of snow, as well as
in more winter precipitation in the form of rain and an increased
frequency of rain-on-snow events. e aggregate impact of these
eects on recharge is not well resolved, but preliminary evidence
29,30
indicates that changes in snowmelt regimes tend to reduce the sea-
sonal duration and magnitude of recharge. Aquifers in mountain
valleys show shis in the timing and magnitude of: (1) peak ground-
water levels due to an earlier spring melt; and (2) low groundwater
levels associated with longer and lower baseow periods
31
(Fig.2).
Summer low ows in streams may be exacerbated by declining
groundwater levels, so that stream ow becomes inadequate to
meet domestic and agricultural water requirements and to maintain
ecological functions such as in-stream habitats for sh and other
aquatic species
31
. e eects of receding alpine glaciers on ground-
water systems are also not well understood, yet the long-term loss of
glacial storage is estimated to reduce similarly summer baseow
32
.
In the glaciated watersheds of the Himalayas, the impacts of large
reductions in glacial mass and increased evaporation on groundwa-
ter recharge are projected to be oset by a rise in precipitation
33
. In
permafrost regions, where recharge is at present ignored in global
analyses
17
, coupling between surface-water and groundwater sys-
tems may be particularly enhanced by warming
34
. In areas of sea-
sonal or perennial ground frost, increased recharge is expected even
though the absolute snow volume decreases
35
.
Human and indirect climate impacts. Links between climate and
ground water in the modern era are complicated by LUC, which
includes, most pervasively, the expansion of rain-fed and irrigated
agriculture. Managed agro-ecosystems do not respond to changes
in precipitation in the same manner as natural ecosystems. Indeed,
LUC may exert a stronger inuence on terrestrial hydrology than
climate change. During multi-decadal droughts in the West
African Sahel in the latter half of the twentieth century, groundwa-
ter recharge and storage rose rather than declined owing to a coin-
cidental LUC from savannah to cropland that increased surface
runo through soil crusting and focused recharge via ephemeral
ponds
36
. Much earlier in the twentieth century, LUC from natu-
ral ecosystems to rain-fed cropland in southeast Australia and the
southwest United States similarly increased groundwater storage
through increased recharge, but also degraded groundwater qual-
ity through the mobilization of salinity accumulated in unsatu-
rated soil proles
14
. In both regions, recharge rates under cropland
increased by one to two orders of magnitude
37–39
compared with
native perennial vegetation.
North West
Sahara
aquifer system
Nubian
sandstone
aquifer system
North China
plains aquifer
High Plains
aquifer
Guarani
aquifer
California
Central Valley
aquifer
Indo-
Gangetic
plain
Great
Artesian
basin
Major regional aquifer systems
Areas with some important but complex aquifers
Areas of generally low permeability with local minor aquifers
Figure 1 | Simplified version of a global groundwater resources map
9
, highlighting the locations of regional aquifers systems.
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Humans have also had large-scale impacts on the terrestrial
water system through irrigation (Fig. 2). In 2000, irrigation
accounted for ~70% of global freshwater withdrawals and ~90%
of consumptive water use
2
. is large-scale redistribution of fresh
water from rivers, lakes and ground water to arable land (Fig.2)
has led to: (1) groundwater depletion in regions with primarily
groundwater-fed irrigation; (2) groundwater accumulation as a
result of recharge from return ows from surface-water-fed irriga-
tion; and (3) changes in surface-energy budgets associated with
enhanced soil moisture from irrigation. Irrigation has depleted
groundwater storage in several semi-arid and arid environments
including the North China Plain
40
, northwest India
41
and the US
High Plains
42,43
, but also in humid environments in Brazil
44
and
Bangladesh
45
(Fig. 1) where abstraction is especially intense.
During a recent (2006to 2009) drought in the California Central
Valley (Fig.1), large-scale groundwater depletion occurred when
the source of irrigation water shied from surface water to predom-
inantly ground water. Gravity Recovery and Climate Experiment
(GRACE) satellite data and ground-based observations revealed
that groundwater storage declined by between 24and 31km
3
, a
volume that is equivalent to the storage capacity of Lake Mead, the
largest surface reservoir in the United States
46,47
. us, the indi-
rect eects of climate on ground water through changes in irriga-
tion demand and sources can be greater than the direct impacts
of climate on recharge. Global-scale modelling
2
highlights areas
of recent (1998 to 2002) groundwater accumulation through
irrigation return ows from surface-water-fed irrigation in the
Nile basin of Egypt, Tigris–Euphrates basin of Iraq, Syria and
Turkey, the lower Indus basin in Pakistan, and southeastern China
(Fig. 3). In parts of the California Central Valley, surface-water
irrigation since the 1960s has increased groundwater recharge by
a factor of approximately seven, replenishing previously depleted
aquifers and raising groundwater levels by up to 100m (ref.48).
Increased recharge may not only degrade groundwater quality
through the mobilization of salinity in soil proles (discussed
earlier) but also ush natural contaminants such as arsenic from
groundwatersystems
49,50
.
Future climate impacts on groundwater systems. As irrigation
dominates current groundwater use and depletion, the eects of
future climate variability and change on ground water may be great-
est through indirect eects on irrigation-water demand. Substantial
uncertainty persists about the impacts of climate change on mean
precipitation from general circulation models (GCMs)
51
, but there
is much greater consensus on changes in precipitation and tem-
perature extremes, which are projected to increase with intensica-
tion of the global hydrological system
52,53
. Longer droughts may be
interspersed with more frequent and intense rainfall events. ese
changes in climate may aect ground water initially and primar-
ily through changes in irrigation demand, in addition to changes
in recharge and discharge. A global analysis of the eects of climate
change on irrigation demand suggests that two thirds of the irri-
gated area in 1995 will be subjected to increased water requirements
for irrigation by 2070 (ref. 54). Projected increases in irrigation
demand in southern Europe will serve to stress limited groundwater
resources further
55
. Persistent droughts projected in the California
Central Valley over the latter half of the twenty-rst century may
trigger a shi from a predominantly surface-water to a predomi-
nantly groundwater supply for agriculture
56
. Increased groundwater
abstraction combined with reduced surface-water ows associated
with intermittent droughts during the rst half of the twenty-rst
century may, however, induce secondary eects (for example, land
subsidence) that severely constrain this future adaptation strategy.
Projections of the direct impacts of climate change on ground-
water systems are highly uncertain. e dominant source of
uncertainty lies in climate projections derived from GCMs, which
typically translate the same emissions scenarios into very dierent
climate scenarios, particularly for precipitation
51
. Nevertheless,
GCM projections of global precipitation for the twenty-rst cen-
tury broadly indicate a ‘rich get richer’ pattern in which regions of
moisture convergence (or divergence) are expected to experience
increased (or decreased) precipitation
52,57
. ere are no published
studies applying a large ensemble of GCMs and greenhouse-gas
emissions scenarios to generate recharge projections at the global
scale. Global simulations using output from two climate models
(ECHAM4, HadCM3) under two emissions scenarios (A2, B2)
project: (1) decreases in potential groundwater recharge of more
than 70% by the 2050s in northeast Brazil, southwest Africa and
along the southern rim of the Mediterranean Sea; and (2) increases
in potential recharge of more than 30% in the Sahel, Middle East,
northern China, Siberia and the western United States
17
. Baseline
recharge rates in many of these areas are, however, very low, so that
small changes in projected recharge can result in large percentage
changes. For most of the areas with high population densities and
high sensitivity to groundwater recharge reductions, model results
indicated that groundwater recharge is unlikely to decrease by more
than 10% until the 2050s
19
.
Groundwater recharge projections are closely related to pro-
jected changes in precipitation. Regional simulations using 16
GCMs in Australia project potential recharge decreases in the west,
central and south, and increases in the north based on the ensemble
median
57
. In Europe, potential recharge projections derived from an
ensemble of four GCMs demonstrate strong latitudinal dependence
on the direction of the climate change signal
58
. Substantial reduc-
tions in potential groundwater recharge are projected in southern
Europe (Spain and northern Italy) whereas increases are consist-
ently projected in northern Europe (Denmark, southern England,
northern France). Current uncertainty about the impacts of climate
on recharge derive not only from the substantial uncertainty in
GCM projections of precipitation but also from that associated with
the downscaling of GCM projections and the hydrological models
used
59
. For a chalk aquifer in England, for example, application of
an ensemble of 13 GCMs resulted in projected changes in ground-
water recharge for the 2080s of between −26% and +31% (ref.60).
Increased seasonality in
groundwater–surface-
water interactions
Return flows from
surface-water-fed
irrigation recharges
ground water
Groundwater-fed
irrigation depletes
groundwater storage
in dry areas
Groundwater
abstraction from
coastal aquifers
drives saline intrusion
Irrigated land increases evapotranspiration
Declining snow an
d
ice extent
Groundwater depletion contributes
to sea-level rise
Figure 2 | Conceptual representation of key interactions between ground
water and climate.
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In southern British Columbia, recharge projections for the 2080s
range from −10% to +23% relative to historical recharge
61
. At three
Australian sites, the choice of GCMs was found to be the great-
est source of uncertainty in future recharge projections, followed
by that of downscaling and then the applied hydrological model,
amounting to 53, 44and 24% of historical recharge, respectively
62
.
Uncertainty from downscaling can be greater than uncertainty due
to the choice of applied emissions scenarios
63,64
.
Current projections of groundwater recharge under climate
change commonly do not consider the intensication of precipita-
tion and physiological forcing of carbon dioxide (CO
2
). Although
precipitation intensity is of critical importance to recharge, his-
torical daily rainfall distributions are typically used to downscale
monthly rainfall projections to a daily timestep. Evidence from
the tropics
65
, where the intensication of precipitation is expected
to be especially strong, reveals that failure to consider changes in
daily rainfall distributions can systemically underestimate future
recharge. Transformation of the rainfall distribution to account for
changes in rainfall intensity reversed a projected 55% decline in
potential recharge to a 53% increase. Recent multi-model simu-
lations that account for precipitation intensication
66
represent a
critical advance in assessing climate change impacts on groundwa-
ter recharge and terrestrial water balances. Under higher atmos-
pheric CO
2
concentrations, terrestrial plants open their stomata
less; this response is projected to reduce evapotranspiration and
increase continental runo
67
. Recent analyses in Australia
68
show
that: (1) greater plant growth (and consequently greater leaf area)
can oset reductions in evapotranspiration through stomatal
closure; (2) reduced leaf area due to unfavourable climate condi-
tions can result in an increase of groundwater recharge even with
slightly decreased rainfall; and (3) changes in rainfall intensity can
have a greater impact on recharge uxes than rising atmospheric
CO
2
concentrations.
Groundwater impacts on the climate system
Ground water inuences climate through contributions to soil
moisture and global sea-level rise (SLR). Recent eors to describe
and quantify these eects are described below.
Groundwater-fed irrigation and soil moisture. Irrigation
can transform areas from moisture-limited to energy-limited
evapotranspiration, thereby inuencing water and energy budgets.
A modelling study
69
estimated that during the growing season,
averaged over the continental United States, irrigation increases
evapotranspiration by 4%. Simulations show that rising ground-
water-fed irrigation in the High Plains (Fig.1) over the twentieth
century increased downwind precipitation by ≤15% to 30% in the
month of July
70
, with associated increases in groundwater storage
and streamow observed from August to September
71
. Irrigation
in California’s Central Valley has strengthened the southwestern
US monsoon, increasing precipitation by 15% and discharge of the
Colorado River by 30% (ref.72). Similar impacts of groundwater-
fed irrigation on evapotranspiration and downwind precipitation
have been demonstrated in the Indian monsoon region using a
regional climate model
73
.
Ground water in land-surface models. Land-surface models
(LSMs), embedded in GCMs, have neglected hydrological pro-
cesses below the root zone such as lateral groundwater ow, as
these have been assumed to be disconnected from the atmosphere.
LSMs were subsequently retrotted with a simplied formulation
of unconned groundwater storage changes
74
. ere have also
been attempts to represent subsurface processes better in LSMs
75
or to couple more complete groundwater models to LSMs
76
.
ese eorts led to the discovery of a critical zone of water-table
depths from 2to 7m, where groundwater exerts the most inu-
ence on land-energy uxes
77
. Coupling of an integrated hydro-
logical model to mesoscale atmospheric models
78
revealed clear
connections between water-table depth and development of the
atmospheric boundary layer
79
. Representing groundwater ow in
atmospheric models at larger scales and longer time frames aects
land-surface moisture states that feed back into regional climate
where water tables are relatively shallow
80
. Without a prognostic
groundwater reservoir and explicit groundwater–surface-water
exchanges in LSMs, we remain unable to represent the integrated
response of the water cycle to human perturbations and climate
change. One key groundwater process missing from LSMs is lat-
eral groundwater ow. is ow occurs at multiple spatial scales
81
but is fundamentally important at hill-slope (or small model grid)
scales in a humid climate or at basin scales in semi-arid and arid
climates with regional aquifers where discharges can be remote
from sources of recharge
82
. Lateral groundwater ow supports per-
sistently wetter river valleys in humid climates, and regional wet-
lands and oases in arid climates
80
, aecting land-surface moisture
states and evapotranspiration uxes. Ground water also acts as an
important store and vehicle for carbon, although studies account-
ing for groundwater interactions and feedbacks in the global car-
bon budget are still in their infancy
83
.
Groundwater and sea-level rise. Coastal aquifers form the inter-
face between the oceanic and terrestrial hydrological systems and
provide a source of water for the more than one billion people liv-
ing in coastal regions
84
. Global SLR of 1.8mmyr
−1
over the second
half of the twentieth century
85
is expected to have induced fresh–
saline-water interfaces to move inland. e extent of seawater intru-
sion into coastal aquifers depends on a variety of factors including
coastal topography, recharge, and groundwater abstraction from
coastal aquifers
86,87
. Analytical models suggest that the impact of
SLR on seawater intrusion is negligible compared to that of ground-
water abstraction
87
. e eects of seawater intrusion have been
observed most prominently in association with intensive ground-
water abstraction around areas with high population densities (for
example, Bangkok, Jakarta, Gaza)
88,89
. Coastal aquifers under very
low hydraulic gradients, such as the Asian mega-deltas, are theo-
retically sensitive to SLR but, in practice, are expected in coming
decades to be more severely aected by saltwater inundation from
storm surges than SLR
87
.
1 3 10
20
mm yr
−1
50 250
77
6
Figure 3 | Global map of anthropogenic groundwater recharge rates in
areas with substantial irrigation by surface water. Rates are estimated
from the dierence between the return flow of irrigation water to ground
water and total groundwater withdrawals for the period 1998 to 2002
2
.
Note that in areas with predominantly groundwater-fed irrigation or
significant water withdrawals for domestic and industrial purposes, no
anthropogenic groundwater recharge occurs; a net abstraction of ground
water leads to groundwater depletion in regions with insucient natural
groundwater recharge.
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Groundwater depletion contributes to SLR through a net transfer
of fresh water from long-term terrestrial groundwater storage to
active circulation near the earth’s surface and its eventual transfer
to oceanic stores. e contribution of groundwater depletion to SLR
has, however, been a subject of debate. In the IPCC fourth assess-
ment report
90
, the contribution of non-frozen terrestrial waters,
including groundwater depletion, to sea-level variation was not
specied owing to its perceived uncertainty. Recently, there has
been a series of studies estimating the contribution of groundwa-
ter depletion to SLR
18,91–93
. Current estimates of global groundwater
depletion derived from ux-based (year 2000) and volume-based
(period, 2001–2008) methods are summarized in Table1. Global
groundwater depletion (204±30km
3
yr
−1
) estimated by the ux-
based method
91
derives from the dierence between grid-based
simulated groundwater recharge and net abstraction (that is
groundwater withdrawals minus return ows). is approach over-
estimates depletion as it does not account for increased capture due
to decreased groundwater discharge and long-distance surface-
water transfers. e volume-based method
92
combines evidence
of groundwater storage changes for the United States and another
ve aquifer systems (Indo-Gangetic plain, North China plain, Saudi
Arabia, Nubian sandstone and North West Sahara) (Fig.1), and then
extrapolates groundwater depletion elsewhere using the average
ratio of depletion to abstraction observed in the United States. is
approach produces a lower global estimate of groundwater deple-
tion (145±39km
3
yr
−1
) than the ux-based approach. Both meth-
ods reveal that groundwater depletion is most pronounced in Asia
(China, India) and North America (Table1). e dierent estimates
of global groundwater depletion produce variable estimates of its
current contribution to SLR (34% or 0.57±0.09mmyr
−1
versus 23%
or 0.4±0.1mmyr
−1
). Direct observations of groundwater depletion
continue to be hampered by a dearth of ground-based observations,
which not only limits our understanding of localized groundwa-
ter storage changes but also our ability to constrain evidence from
GRACE satellite observations at larger scales (≥150,000km
2
).
A look forward
Ground water can enhance the resilience of domestic, agricultural
and industrial uses of fresh water in the face of climate variability
and change. As the only perennial source of fresh water in many
regions, ground water is of vital importance to the water security
of many communities, including — most critically — rural dwell-
ers in low-income countries. Groundwater-fed irrigation provides
a buer against climate extremes and is consequently essential
to global food security. Furthermore, it alleviates poverty in low-
income countries by reducing crop failure and increasing yields
94
.
e value of ground water is expected to increase in coming decades
as temporal variabilities in precipitation, soil moisture and surface
water are projected to increase under more frequent and intense cli-
mate extremes associated with climate change
53
. Indeed, in light of
the resilience of groundwater resources to hydrological extremes,
ground water could have a strategic role in sustaining drinking-
water supplies under emergency conditions
95
.
As detailed earlier, substantial doubt remains about the projected
impacts of climate change on diuse groundwater recharge, which
is associated with the inherent uncertainties in climate projections
96
and terrestrial responses to changing precipitation and land cover.
More certain are rises in groundwater abstraction in absolute terms
and as a proportion of total water withdrawals, which threaten to
overexploit groundwater resources. is risk is particularly acute in
semi-arid regions where projected increases in the frequency and
intensity of droughts, combined with rising populations and stand-
ards of living as well as the projected expansion of irrigated land,
will intensify groundwater demand. To sustain groundwater use
under these conditions will require careful aquifer management
97
that: (1) is informed by integrated models able to consider the range
of interactions between ground water, climate and human activity
(summarized in Fig.2); and (2) exploits opportunities for enhanced
groundwater recharge associated with less frequent but heavier
rainfall events and changing meltwaterregimes.
Comprehensive management approaches to water resources
that integrate ground water and surface water may greatly reduce
human vulnerability to climate extremes and change, and promote
global water and food security. Conjunctive uses of ground water
and surface water that use surface water for irrigation and water
supply during wet periods, and ground water during drought
48
,
are likely to prove essential. Recognition of current uncertainty
in water resource projections and the longer residence time (dec-
adal to multigenerational) of fresh water in groundwater systems
will be critical in setting sustainability goals
97
. Managed aqui-
fer recharge wherein excess surface water, desalinated water and
treated waste water are stored in depleted aquifers could also sup-
plement groundwater storage for use during droughts
43,98
. Indeed,
the use of aquifers as natural storage reservoirs avoids many of the
problems of evaporative losses and ecosystem impacts associated
with large, constructed surface-water reservoirs. In South Asia, for
example, intensive groundwater abstraction for dry-season irriga-
tion has induced greater recharge in areas with permeable soils by
increasing available groundwater storage during the subsequent
monsoon
99
. In northern Europe, capture of projected increases in
groundwater recharge during winter may help to sustain anticipated
increases in summer demand
58
. Explicit representation in GCMs of
groundwater storage, its interactions with surface-water stores, and
anthropogenic perturbations — such as large-scale groundwater-
fed irrigation — is required to advance our understandingof both
the inuence of ground water on climateand the impact of climate
change on global freshwaterresources.
A fundamental impediment to using the adaptation strategies
discussed earlier is the lack of groundwater observations to inform
them. Since 2002, GRACE satellite observations have provided
Table 1 | Estimates of global- and continental-scale groundwater depletion.
Flux-based method
91
* Volume-based method
92
†
Region Groundwater depletion Sea-level rise Groundwater depletion Sea-level rise
World 204±30 0.57±0.09 145±39 0.40±0.11
Asia 150±25 0.42±0.07 111±30 0.31±0.08
Africa 5.0±1.5 0.014±0.004 5.5±1.5 0.015±0.004
North America 40±10 0.11±0.03 26±7 0.07±0.02
South America 1.5±0.5 0.0042±0.0014 0.9±0.5 0.002±0.001
Australia 0.5±0.2 0.0014±0.0006 0.4±0.2 0.001±0.0005
Europe 7±2 0.02±0.006 1.3±0.7 0.004±0.002
Flux-based and volume-based estimates of global and continental-scale groundwater depletion (km
3
yr
−1
) and their contributions to global sea-level rise (mmyr
−1
). *Year 2000. †Period between 2001and 2008.
REVIEW ARTICLE
NATURE CLIMATE CHANGE DOI: 10.1038/NCLIMATE1744
© 2012 Macmillan Publishers Limited. All rights reserved

Frequently asked questions

Q1. What contributions have the authors mentioned in the paper "Ground water and climate change" ?

Despite these vital contributions to human welfare and aquatic ecosystems, a paucity of studies on the relationship between climate and ground water severely restricted the ability of the Intergovernmental Panel on Climate Change ( IPCC ) to assess interactions between ground water and climate change in both its third3 and fourth4 assessment reports. Here the authors build on an earlier broad-based overview8 of the topic, and examine substantial recent advances. Furthermore, the authors identify critical gaps in their understanding of the interactions between ground water and climate. 

Q2. What is the role of ground water in the resilience of domestic, agricultural and industrial uses of fresh?

Ground water can enhance the resilience of domestic, agricultural and industrial uses of fresh water in the face of climate variability and change. 

Q3. What is the important factor in the forecast of recharge?

Although precipitation intensity is of critical importance to recharge, historical daily rainfall distributions are typically used to downscale monthly rainfall projections to a daily timestep. 

Q4. What is the role of ground water in sustaining ecosystems?

As the world’s largest distributed store of fresh water, ground water plays a central part in sustaining ecosystems and enabling human adaptation to climate variability and change. 

Q5. What is the common type of recharge in semiarid environments?

Recharge in semiarid environments is often restricted to statistically extreme (heavy) rainfall17,25 that commonly generates focused recharge beneath ephemeral surface water bodies20,21,26. 

Q6. What are the main characteristics of groundwater systems?

In these systems, robust estimates of groundwater storage and accurate records of groundwater withdrawals are of critical importance. 

Q7. What is the role of groundwater in the assessment of freshwater resources?

An expansion of groundwater monitoring, together with increased contributions of data to the GGMN, are necessary to improve access to groundwater data globally and promote the inclusion of ground water in the assessment and management of freshwater resources under climate change. 

Q8. What is the impact of the glaciation on recharge in the Himalayas?

In the glaciated watersheds of the Himalayas, the impacts of large reductions in glacial mass and increased evaporation on groundwater recharge are projected to be offset by a rise in precipitation33. 

Q9. What is the role of groundwater in sustaining food security?

Comprehensive management approaches to water resources that integrate ground water and surface water may greatly reduce human vulnerability to climate extremes and change, and promote global water and food security. 

Q10. What is the role of groundwater in the global carbon budget?

Ground water also acts as an important store and vehicle for carbon, although studies accounting for groundwater interactions and feedbacks in the global carbon budget are still in their infancy83. 

Q11. What is the effect of warming on recharge in permafrost regions?

In permafrost regions, where recharge is at present ignored in global analyses17, coupling between surface-water and groundwater systems may be particularly enhanced by warming34. 

Q12. Where is the intensification of rainfall expected to be especially strong?

Evidence from the tropics65, where the intensification of precipitation is expected to be especially strong, reveals that failure to consider changes in daily rainfall distributions can systemically underestimate future recharge. 

Q13. How much increase in evapotranspiration can a groundwater-fed irrigation cause?

A modelling study69 estimated that during the growing season, averaged over the continental United States, irrigation increases evapotranspiration by 4%. 

Q14. What is the extent of seawater intrusion into coastal aquifers?

The extent of seawater intrusion into coastal aquifers depends on a variety of factors including coastal topography, recharge, and groundwater abstraction from coastal aquifers86,87. 

Q15. What determines whether a water surplus can be transmitted and stored in the subsurface?

Climate and land cover largely determine precipitation and evapotranspiration, whereas the underlying soil and geology (Fig.  1) dictate whether a water surplus (precipitation minus evapotranspiration) can be transmitted and stored in the subsurface. 

Q16. What is the risk of groundwater depletion in semi-arid regions?

This risk is particularly acute in semi-arid regions where projected increases in the frequency and intensity of droughts, combined with rising populations and standards of living as well as the projected expansion of irrigated land, will intensify groundwater demand. 

Q17. What is the impact of snowmelt regimes on recharge?

The aggregate impact of these effects on recharge is not well resolved, but preliminary evidence29,30 indicates that changes in snowmelt regimes tend to reduce the seasonal duration and magnitude of recharge. 

Q18. What is the effect of receding alpine glaciers on groundwater systems?

The effects of receding alpine glaciers on groundwater systems are also not well understood, yet the long-term loss of glacial storage is estimated to reduce similarly summer baseflow32. 

Q19. What is the role of rainfall in the recharge of groundwater?

In tropical Africa, heavy rainfall has been found to contribute disproportionately to recharge observed in borehole hydrographs21,24. 

Q20. What are the impacts of rainfall intensification on groundwater?

Recent multi-model simulations that account for precipitation intensification66 represent a critical advance in assessing climate change impacts on groundwater recharge and terrestrial water balances.