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Groundwater is an invaluable global resource, but its long-term viability as a resource for consumption, agriculture, and ecosystems depends on precipitation recharging aquifers. How much precipitation recharges groundwaters varies enormously across Earth's surface, yet recharge rates often remain uncertain. Here we use a global synthesis of field-estimated recharge across six continents to show that globally recharge first-order follows a simple function of climatic aridity. We use this relationship to estimate long-term recharge outside of permafrost regions. Our aridity-based recharge estimates are consistent with the global field data but, on average, double previous estimates of global models. The spatial and process-averaging conveyed by the recharge rates generated by hydrological models results in predictions that are underestimations at both high and low recharge rates. Our recharge estimates suggest that more groundwater must contribute to evapotranspiration and streamflow than previously represented by global hydrological models and global water cycle diagrams.
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1. Introduction
Groundwater constitutes almost all of Earth's liquid fresh water (Abbott etal.,2019; Gleeson etal.,2016) and is
extensively extracted, with global withdrawals of hundreds of cubic kilometers per year (Döll etal.,2014; Margat
& Van der Gun,2013; Sutanudjaja et al., 2018). Groundwater provides approximately 2 billion people with
drinking water (Morris etal.,2003) and supplies almost 40% of irrigated lands worldwide (Siebert etal.,2010).
Groundwater also shapes ecosystems and landscapes as rivers and vegetation can source their waters from aqui-
fers (Berghuijs & Kirchner,2017; Evaristo & McDonnell,2017; Fan etal.,2017; Jasechko etal.,2016).
The dynamic roles of groundwater are not always apparent, but aquifers must be sufficiently recharged for
groundwater to sustain ecosystems and water resources into the future (Alley etal.,2002; Gleeson etal.,2012).
Earth's diversity of landscapes and climates results in groundwater recharge rates that vary by orders of magni-
tudes globally (MacDonald etal.,2021; Moeck etal.,2020; Scanlon etal.,2006). Yet, for most of Earth's surface,
groundwater recharge rates remain uncertain because measurements are sparse (Moeck etal., 2020; Scanlon
etal.,2006), and large-scale modeled recharge remains mostly unvalidated (de Graaf etal.,2015,2019; Döll &
Fiedler,2008; Li etal.,2021; Müller Schmied etal.,2021; Reinecke etal.,2021). In addition, upscaling recharge
estimates derived from extensively studied sites to other locations is challenging because many landscape, vege-
tation, and surface properties can affect recharge (Crosbie etal., 2018; De Vries & Simmers, 2002; Moeck
etal.,2020). These issues are problematic because accurate recharge estimates are needed to assess the sustain-
ability of groundwater use and the role of groundwater in supporting ecosystems and surface waters (Gleeson
Abstract Groundwater is an invaluable global resource, but its long-term viability as a resource
for consumption, agriculture, and ecosystems depends on precipitation recharging aquifers. How much
precipitation recharges groundwaters varies enormously across Earth's surface, yet recharge rates often remain
uncertain. Here we use a global synthesis of field-estimated recharge across six continents to show that globally
recharge first-order follows a simple function of climatic aridity. We use this relationship to estimate long-term
recharge in energy-limited systems outside of permafrost regions. Our aridity-based recharge estimates are
consistent with the global field data but, on average, double previous estimates of global models. Our higher
recharge estimates are likely caused by preferential groundwater recharge and discharge occurring at grid
scales finer than global models. The higher recharge estimates suggest that more groundwater contributes to
evapotranspiration and streamflow than previously represented by global hydrological models and global water
cycle diagrams.
Plain Language Summary Groundwater is an essential resource for societies and ecosystems.
The rate at which rainfall and snow replenish groundwater storage is important as it dictates the upper limit
of sustainable groundwater use. Here we use measurements of groundwater recharge to show how climate
determines groundwater recharge rates. Measured recharge rates, on average, strongly exceed those of
models. This suggests there is more recharge globally than currently acknowledged. Consequently, also more
groundwater recharge must get back to Earth's surface via river flow or water use of vegetation.
© 2022 The Authors.
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Global Recharge Data Set Indicates Strengthened
Groundwater Connection to Surface Fluxes
Wouter R. Berghuijs1 , Elco Luijendijk2,3 , Christian Moeck4 , Ype van der Velde1 , and
Scott T. Allen5
1Department of Earth Sciences, Free University Amsterdam, Amsterdam, The Netherlands, 2Bundesgesellschaft Für
Endlagerung, Peine, Germany, 3Department of Earth Science, University of Bergen, Bergen, Norway, 4Department Water
Resources and Drinking Water, Eawag - Swiss Federal Institute of Aquatic Science and Technology, Dübendorf, Switzerland,
5Department of Natural Resources and Environmental Science, University of Nevada, Reno, NV, USA
Key Points:
A global recharge data set indicates
that climate strongly shapes the
fraction of precipitation that will
recharge groundwaters
This recharge data set indicates more
recharge globally than existing global
hydrological models suggest
Thus, more groundwater must
contribute to evaporation and
streamflow than represented by
current global models and water cycle
Supporting Information:
Supporting Information may be found in
the online version of this article.
Correspondence to:
W. R. Berghuijs,
Berghuijs, W. R., Luijendijk, E., Moeck,
C., van der Velde, Y., & Allen, S. T.
(2022). Global recharge data set indicates
strengthened groundwater connection
to surface fluxes. Geophysical Research
Letters, 49, e2022GL099010. https://doi.
Received 6 APR 2022
Accepted 18 NOV 2022
Author Contributions:
Conceptualization: Wouter R. Berghuijs,
Christian Moeck, Ype van der Velde,
Scott T. Allen
Data curation: Wouter R. Berghuijs
Formal analysis: Wouter R. Berghuijs,
Elco Luijendijk
Investigation: Wouter R. Berghuijs
Methodology: Wouter R. Berghuijs
Resources: Wouter R. Berghuijs
Software: Wouter R. Berghuijs
Validation: Wouter R. Berghuijs
Visualization: Wouter R. Berghuijs
Writing – original draft: Wouter R.
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Regional analysis across carbonate rock landscapes indicates that many widely used hydrological models seem
to underestimate recharge (Hartmann etal.,2017). However, it remains unclear how widespread this model bias
is, as the enhanced recharge rates were attributed to the strong preferential flows in karst landscapes (Hartmann
etal., 2017). Yet, other evidence also suggests that global models overestimate the sensitivity of recharge to
climate change across arid regions in Africa because recharge induced by intense rainfall can lead to focused
recharge through losses from ephemeral overland flows, which are often not captured by large-scale models
(Cuthbert etal.,2019). Such discrepancies between models and observations are based on recharge and ground-
water observations across specific landscapes and climate conditions. Thus, it remains unclear how widespread
such issues are across other parts of Earth.
A recent global synthesis of recharge measurements from 5237 sites globally (Moeck etal.,2020) may alleviate
this issue. This synthesis data set provides a basis to investigate how observation-based recharge values vary
globally and to what extent there may be a widespread recharge bias in existing models. However, such investi-
gations are hampered by the large unquantified uncertainty associated with observation-based recharge estimates
(Crosbie etal.,2010,2018; Moeck etal.,2020). In addition, the exact spatial scale and period these measure-
ments represent remain uncertain and will never exactly overlap with those of models. However, the large number
of sites in the data set still allows to investigate the primary controls on global patterns of recharge and quantify
to what extent there could be a systematic recharge bias in existing models.
Here we show that climate aridity (Trabucco & Zomer,2009)—the ratio of potential evapotranspiration to
precipitation—strongly controls the fraction of precipitation that becomes groundwater. We parameterize a func-
tion that captures this relationship using the synthesis of groundwater recharge estimates (Moeck etal.,2020). We
show that the synthesis of groundwater recharge estimates (Moeck etal.,2020) indicates that existing hydrologi-
cal models, that have been previously used to predict recharge across the globe, underestimate recharge.
2. Methods and Data
2.1. Recharge Data
We obtain recharge rates from a recent global synthesis of groundwater recharge rates of 5237 sites located across
all continents but Antarctica (Moeck etal., 2020). The compiled data primarily originate from tracer methods
(∼80%) but are also derived from water table fluctuations, water balance methods, lysimeters, heat tracers, and
geophysical methods. This large variety of methods can affect estimated recharge rates at individual sites. The
recharge estimation studies cover the period from 1968 to 2018. The mean recharge rate is 234mmyr
−1, but over
40% of data points have rates between 0 and 25mmyr
−1 (median 51.3mmyr
−1). The data set contains recharge
rate estimates based on datasets that exceed at least 1year to avoid bias in the rates due to seasonal effects and
incomplete annual recharge values. The 5237 sites are assumed to represent naturally occurring recharge, as
recharge rates presumed to be affected by irrigation or managed aquifer recharge were already omitted by Moeck
etal.(2020). Study sites where rivers and streams dominate the estimated recharge were also omitted by Moeck
etal.(2020). Almost all these measurements will fall on recharge zones of the landscape, which in surface area
strongly dominate over the discharge zones near rivers (e.g., O'loughlin,1981). The global data has no quality
flags or uncertainties on recharge estimates because these estimates are also typically absent in many of the past
reports. For more information on the data, see Moeck etal.(2020) and the references therein. Most of the obser-
vations (n=4,386) originate from Australia (Crosbie etal.,2010) but these data have a similar relationship of
recharge fractions with aridity as the other data in the data set (Figure S1 in Supporting InformationS1).
2.2. Climate Data
We use temperature, aridity, precipitation, and FAO Penman-Monteith potential evapotranspiration data from
WorldClim (Fick & Hijmans,2017) and the global aridity and potential evapotranspiration database (Trabucco &
Zomer,2009). We define the aridity index as the ratio of mean potential evapotranspiration to mean precipitation.
Accordingly, high aridity index values reflect drier climates, whereas low values reflect humid climates. Regions
are classified as likely to have permafrost conditions when the mean annual temperature is below −2°C.
Writing – review & editing: Wouter
R. Berghuijs, Elco Luijendijk, Christian
Moeck, Ype van der Velde, Scott T. Allen
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2.3. Relationship Between Climate and Recharge
We use a mathematical expression that describes the global relationship between climate aridity and groundwater
recharge fractions:
1− ln
is groundwater recharge (mmyr
−1), P is precipitation (mmyr
is aridity (dimensionless), defined as
the ratio of potential evapotranspiration to precipitation (EP/P), and
(dimensionless) is a constant equating to the
fraction of precipitation that becomes recharge for
(i.e., humid conditions).
is the characteristic exponent
(dimensionless) of the aridity index. We calibrate the
using a least absolute residuals fit. The sigmoidal
equation was selected because it is among the simplest equations that enforce physically realistic upper and lower
limits for recharge fractions. It closely follows the exponential decrease of recharge fraction with increasing arid-
ity visible in global recharge data. We reorganize the equation to estimate total recharge (mm yr
−1) using global
precipitation and aridity data:
1− ln
2.4. Groundwater Recharge Estimates From Global Models
We obtained simulated diffuse recharge estimates from the PCR-GLOB hydrological model (de Graaf
etal.,2015,2019) and the WaterGAP Global Hydrology Model (versions v2.1f and v2.2d) (Döll & Fiedler,2008;
Müller Schmied etal.,2021), and machine learning (Mohan etal.,2018). Also considering recharge from surface
water bodies did not change the overall results significantly. For the 5237 stations with recharge data, we compare
the observed recharge with the simulated recharge (Figure3). The simulated recharge values from the global
hydrological models represent mean annual recharge over a period that ranges from the year 1960 to 2001 (Döll
& Fiedler,2008), 1957 to 2002 (De Graaf etal.,2015), 1960 to 2010 (de Graaf etal.,2019) and 1901 to 2016
(Müller Schmied etal.,2021), respectively.
3. Results and Discussion
3.1. The Relationship Between Aridity and Recharge
The observation-based recharge estimates from sites spanning most regions of the globe show that recharge
fractions are strongly controlled by climate aridity (Figure1), despite many other factors also affecting ground-
water recharge globally (e.g., Moeck etal.,2020). In humid climates, typically, larger fractions of precipitation
recharge groundwater. This recharge fraction shrinks with increasing aridity, often approaching almost zero in
very arid sites. This relationship is nonlinear, and the empirical data show substantial variation for a given aridity,
reflecting an influence of other environmental conditions. However, the pattern is sufficiently monotonic to yield
a highly significant correlation between climate aridity and the fraction of precipitation that recharges groundwa-
ters (Spearman ρ=−0.674; p<0.001). This relationship is consistent with past work, which indicated that both
precipitation and potential evapotranspiration can strongly affect groundwater recharge (e.g., Moeck etal.,2020;
Scanlon etal., 2006), but goes beyond these past works by also showing how the partitioning of precipitation
changes. This quantified partitioning pattern is important as it substantiates the relative importance of recharge
across different climates.
The vast majority (i.e., 99%) of the observation-based recharge values are from regions with climate aridi-
ties exceeding 0.75. These aridities cover most of Earth's surface aside from several of Earth's wettest regions
(e.g., Congo Basin, Amazonia, Southeastern Asia), which largely fall outside the observational range. The
observation-based recharge values (Figure1b) suggest recharge fractions can shrink again at very low aridities,
but this remains uncertain because only few of the recharge-measurement sites fall in energy-limited systems with
aridity below one. In addition, the observation-based recharge sites fall outside regions underlain by permafrost
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(Obu,2021), where recharge processes often differ from non-permafrost regions (Walvoord & Kurylyk,2016). It
is good to note that a large part of the observation-based global data set (n=4,386) originates from the Australian
continent, mostly synthesized by another study (Crosbie etal.,2010). These Australian data have higher recharge
rates (mean=244mmyr
−1) compared to the remainder of the global data set (mean=188mmyr
−1) (Figure S1 in
Supporting InformationS1), but both parts of this global data set have a similar pattern of recharge fractions that
shrink with aridity according to the worldwide trend (Figure1; Figure S1 in Supporting InformationS1). There-
fore, the aridity-recharge relationship is likely representative for large parts of Earth's surface, but how recharge
in very humid and permafrost regions evolves with aridity cannot be directly constrained by the existing data.
Much of the variations in recharge can be described by a sigmoidal function of climate aridity (Equation1;
Figure1b). Calibrated on all data, this function describes how recharge exceeds 50% of precipitation (α=0.72,
with 95% confidence bounds 0.71, 0.73) when aridity approaches one (i.e., precipitation equals potential evapo-
transpiration), and decreases with increasing aridity (β=15.11, with 95% confidence bounds 14.91, 15.30). The
relationship seems inaccurate at low aridities, where both high and low recharge rates can occur.
Large but unquantified uncertainties associated with recharge measurements can limit the correspondence
between our model and the observations. However, although this parameterization is simple, it captures the
observed global trend in the fraction of precipitation that becomes groundwater recharge more accurately than
widely used global hydrological models (Figure3), which underestimate recharge in both arid and humid regions
(Extended Data Figure S2 in Supporting InformationS1). The parsimony of our aridity-recharge relationship
(Figure1b) may limit its predictive power but using more predictor variables does not substantially improve its
predictive capacity (Figure S3 in Supporting InformationS1). A split-sample test using 80% of the data for cali-
bration and the remaining 20% for validation still yields relatively narrow confidence bounds of the fitted param-
eters (95% confidence intervals α=0.69–0.75, β=14.0–16.2, not displayed), thus also subsets of the empirical
data effectively constrain the relationship (Figure S3 in Supporting InformationS1). Thus, the seemingly overly
simple predictions of groundwater recharge based on only climate aridity appear surprisingly effective compared
to the status quo, despite excluding many other factors that may also affect groundwater recharge.
3.2. Global Recharge Pattern
The parameterized relationship between climate aridity and recharge fraction (Equation1; Figure1b) enables
estimating the global distributions of groundwater recharge fractions (Figure2a) and total groundwater recharge
(Figure2b) using global aridity and precipitation data (see Methods). The estimated global pattern of groundwa-
ter recharge fractions shows large regional differences in how much precipitation recharges groundwater, broadly
Figure 1. Groundwater recharge fractions vary with aridity. Recharge fractions (the ratio between long-term recharge and
long-term precipitation) at the 5237 sites and the global pattern of climate aridity (a), whereby recharge negatively correlates
with aridity (b). The gray markers indicate the recharge fractions of individual groundwater recharge sites, whereas dark
markers average across 2% of the sites, removing most local site-to-site variability. The pink shading indicates a 25–75th
percentile over 100 data points. The red line depicts the calibrated sigmoid function Equation1. These data show how a
distinct trend of groundwater recharge fractions decreasing with aridity. The relationship is least constrained at low aridities,
where both high and low recharge rates can occur. There is substantial site-to-site recharge variability that is not explained by
aridity which is caused by other conditions that affect the physical recharge rates, and the large uncertainties associated with
recharge measurements.
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Figure 2. Estimated global patterns of groundwater recharge outside of permafrost regions. Estimates of groundwater recharge fractions vary regionally (a) and are
based on global climate data and Equation1. The absolute groundwater recharge values show high spatial variation because both the precipitation amount and the
fraction of precipitation that becomes recharge are correlated with aridity (b) (note the logarithmic color scales). Markers indicate the observations at the 5237 sites (b).
Permafrost regions are classified by having a mean annual temperature below −2°C. The estimates exclude regions with mean temperatures below −2°C because these
regions lack observations, whereas regions with aridity below 1 are excluded indicating that the data in these very humid regions is limited.
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consistent with the data set comprising observations from the 5237 sites
(Figure2a; Figure 1b). We exclude regions that can have permafrost (i.e.,
mean temperature below −2°C). Energy-limited regions with aridity below
1 are excluded from Figure1 to indicate that the empirical climate aridity
function poorly fits observations in these regions.
Estimated groundwater recharge fractions are low (<0.1) across roughly half
of Earth's surface (excluding permafrost regions) (Figure2a), as drylands
are very prevalent across all continents but Europe (Figure 1a) (Berg &
McColl,2021). Recharge fractions increase across more humid parts of Earth
such as most of Europe, eastern North America, central Africa, Southern
Asia, and most of South America. These regional patterns are both pres-
ent in observations and the estimated global pattern. Absolute recharge rates
show largely similar regional patterns (Figure2b), but the differences in esti-
mated recharge are even greater between humid and arid regions. Estimated
recharge would be highest in the equatorial wet regions and coastal regions
of Central and North America, Europe, and Oceania (consistent with earlier
global estimates), but large parts of these areas have aridities below 1 which
means recharge estimates are hard to constrain because few data exist at these
locations and the function starts to poorly fit the data. Nevertheless, even
when recharge fractions are low, the potential of high absolute recharge rates
will remain substantial in these regions as they experience high precipitation
Observation-based recharge values more than double those of several previous
global model estimates (Figure3; Figure S4 in Supporting InformationS1).
Such model estimates have not been systematically evaluated with observed
recharge data but rather with proxies such as streamflow measurements and
groundwater levels, or only with a small amount of field data. If we compare
the recharge rates from the widely used PCR-GLOB and WaterGAP global
hydrological models with the recharge observations at the 5237 sites, we find
that these models on average have 50% less recharge than the empirical data
(Figure3; Figures S4a–S4d in Supporting InformationS1). A similar but even more substantial difference is pres-
ent in another global recharge estimate based on 715 sites with recharge data (Figure3; Figure S4e in Supporting
Information S1). Split-sample tests do not show any such biases resulting from our aridity-recharge fraction
relationship (Figure S2 in Supporting InformationS1). An example realization of our relationship (Figure S4f in
Supporting InformationS1) shows how much better it explains observed recharge than other global hydrologi-
cal models (Figures S4a–S4e in Supporting InformationS1). The biases of the hydrological models arise from
underestimations at both high and low recharge rates. The difference in modeled and field-estimated recharge
may partly arise from the difference in the scales they represent. Global hydrological models simulate hydrologi-
cal behavior at multiple km
2 per grid-cell scale, thereby covering both recharge and discharge zones. In contrast,
most observations will be in recharge zones (Moeck etal.,2020), but discharge zones tend to cover only a small
part of the Earth's surface (e.g., O'loughlin,1981).
4. Implications and Conclusions
Aquifer storages are governed by the balance between recharge and discharge of groundwater to surface waters
and vegetation, in addition to human abstractions (Alley etal.,2002). Where observations are available, field
observations of recharge more than double most previous model estimates (Figure3; Figure S4 in Supporting
InformationS1). This enhanced recharge does not counter the current understanding of regional groundwater
overuse and its threats to global water security (Famiglietti,2014) because groundwater overuse results in storage
depletion and declining water levels that have been robustly documented in many more arid areas across the globe
(e.g., Rodell etal.,2018).
Most recharge will resurface as evapotranspiration or river flow (Alley etal.,2002). Thus, higher recharge rates
imply that groundwater's role in evapotranspiration and surface water fluxes is larger than previously modeled.
Figure 3. Comparison of observed versus predicted recharge for several
global recharge predictions. Moving averages of recharge predicted by
global models such as PCR-GLOB, WATER-GAP, and machine learning are
systematically lower than recharge of the 5237 observation sites (as indicated
by lines above the 1:1-line). The predictions by global models underestimate
recharge by more than 50% compared to the recharge measurement. Using
the sigmoid function (Equation1) largely removes this bias and produces
an overall average recharge of a very similar magnitude as global recharge
estimates. The presented recharge rates are moving averages over 10% of
the data. More detailed comparisons of modeled and observed recharge are
presented in Figure S4 in Supporting InformationS1.
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This also suggest that global hydrological models have overestimated (near) surface fluxes, such as overland flow,
shallow subsurface flows through the unsaturated zone, and soil-moisture-fed evapotranspiration.
The implied greater role of groundwater in supplying streamflow and evapotranspiration is consistent with
global observations that have shown that vegetation can source substantial parts of their water from groundwa-
ter, and vegetation disproportionally occurs near zones where it can access groundwater as a water source (Fan
etal., 2017; Koirala etal.,2017). It is also consistent with the observation that most precipitation is stored in
landscapes for at least several months before being observed in rivers (Jasechko etal.,2016), but note that older
water can also have other sources as water also can reside in soils and reservoirs for months before being meas-
ured as streamflow (Messager etal.,2016; Sprenger etal.,2019). These dynamic connections with vegetation and
streams likely predominantly occur in the upper layers of groundwater as deeper groundwaters mostly exchange
slowly with the Earth's surface (Berghuijs & Kirchner,2017; Jasechko etal.,2016,2017).
Recharge and its main potential fates (i.e., streamflow vs. evapotranspiration) depend strongly on climate arid-
ity (Budyko,1974). How much precipitation becomes streamflow shrinks with increasing aridity, whereas the
evaporative fraction grows with increasing aridity (Budyko,1974) (Figure S5 in Supporting InformationS1).
In humid areas, which typically have substantial recharge, both streamflow and evapotranspiration can have
groundwater contributions as streams typically have water levels below adjacent groundwater levels (Jasechko
et a.,2021). Losing rivers are more common in drier climates (Jasechko et a.,2021), suggesting a smaller role
for recharge in their streamflow and probably more recharge ultimately going to evapotranspiration. The relative
contribution of groundwater for transpiration is also reported to grow with aridity (Evaristo & McDonnell,2017)
though conservation of mass dictates that groundwater will typically only be a small component of total evap-
otranspiration across arid landscapes (i.e., recharge<< evapotranspiration). In mesic regions, the fraction of
precipitation that recharges groundwater derived from the synthesis recharge data set tends to exceed the fraction
that typically becomes streamflow (Figure S5 in Supporting InformationS1), which suggests that also a part of
evapotranspiration is supplied by groundwater. The gradients of recharge fraction with climate aridity may also
help to assess the impacts of climate changes on groundwater recharge. The effects of climate change on recharge
are currently highly uncertain and mostly unquantified (IPCC,2021).
A strong connection of groundwater with surface water and plant transpiration remains absent from most
diagrams of the global water cycle (Abbott etal.,2019; Dorigo etal.,2021; Oki & Kanae,2006). Although such
water cycle diagrams may not be intended as complete representations of the hydrological cycle, they often play
an important role in teaching, research, communication, and policymaking (Abbott etal.,2019). Therefore, we
need to consider revising those diagrams by increasing the rate at which groundwater is being replenished and
discharged and strengthening the link of groundwater with incoming precipitation, surface waters, and vegetation
(e.g., Miguez-Macho & Fan,2021).
The underrepresentation of groundwater as a key contributor to evapotranspiration and river flows may be
pervasive in hydrological models. Recharge is an internal flux that accumulates uncertainties and errors of
other components of the budget (Reinecke etal.,2021), and models are often not designed to treat groundwa-
ter recharge as a main source of streamflow and evapotranspiration. Preferential flow paths that can recharge
groundwaters are important in virtually any landscape (Beven & Germann,2013; Nimmo,2012) and contribute
disproportionally to fluxes such as recharge (Berghuijs & Kirchner,2017). Many of these pathways are absent in
global hydrological models. Connections of groundwaters with streamflow and evapotranspiration could also be
strengthened by including lateral groundwater flows (Maxwell & Condon,2016). Many of these lateral connec-
tions between surface water and groundwater likely occur at scales smaller than the grid-cells of most models
and thus require implicit sub-grid parameterizations (Fan etal.,2019). Strengthening the groundwater connection
to surface fluxes in these models is essential, given that models are the foundation of our understanding of our
planet, and underpin present-day environmental science and policymaking.
Data Availability Statement
All data used in this study are available via the cited sources. Precipitation data are available at https://www. Potential evapotranspiration and aridity data are available at https:// Recharge data are available at https://opendata.eawag.
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... No global dataset on groundwater pumping exists, and abstractions can only be estimated 28,55 . Currently available global groundwater recharge estimates are highly spatially biased 56 , and modeled recharge is highly uncertain 57 . ...
... Multiple reasons contribute to the differences between the four models investigated here, including (1) uncertainties in groundwater recharge estimates, (2) spatial resolution of the models, (3) model choices concerning the model parameterization, and (4) conceptual choices in model implementation (e.g., subsurface layering and assigned permeabilities). Groundwater recharge estimates (1) are highly uncertain 56,57,64,69 , and their evaluation is challenging due to sparse observations associated with significant uncertainties 72 . The original spatial resolution (2) of Reinecke and de Graaf is similar (5 and 6 arcmins), whereas Verkaik and Fan use a higher resolution (30 arcsec). ...
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Groundwater is essential for maintaining healthy ecosystems and securing human access to freshwater. Here we show that current estimates of global groundwater accessibility by ecosystems and humans are highly uncertain. To quantify this uncertainty, we define three categories of accessibility and investigate four global groundwater models. Averaged across these models, we estimate that 23% [most deviating model: 71%] of the land area contains groundwater accessible to ecosystems and humans, 57% [29%] is accessible to humans only, and 20% [0.01%] is costly to access or inaccessible. We find that the uncertainty in estimating water table depth severely affects our ability to assess groundwater's crucial role in ecosystem health, global water supplies associated to food security, and human health, with possible implications for achieving multiple Sustainable Development Goals. To reduce this uncertainty, we outline three pathways towards (1) better global datasets, (2) alternative strategies for model evaluation, and (3) greater cooperation with regional experts.
... Estimates of global recharge rates from large-scale hydrologic models range from 5,900 to 24,500 km 3 /yr [24][25][26][27][28][29] (Fig. 2). An analysis examining a global compilation of field estimates of recharge suggested a value near the upper end of this range 30 . Discharge from groundwater as pumping (734 km 3 /yr 29 ) and submarine groundwater discharge (78 km 3 /yr 31 ) are comparatively small and within the range of uncertainty of global recharge estimates. ...
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Groundwater is one of the largest reservoirs of water on Earth but has relatively small fluxes compared to its volume. This behavior is exaggerated at depths below 500 m, where the majority of groundwater exists and where residence times of millions to even a billion years have been documented. However, the extent of interactions between deep groundwater (>500 m) and the rest of the terrestrial water cycle at a global scale are unclear because of challenges in detecting their contributions to streamflow. Here, we use a chloride mass balance approach to quantify the contribution of deep groundwater to global streamflow. Deep groundwater likely contributes <0.1% to global streamflow and is only weakly and sporadically connected to the rest of the water cycle on geological timescales. Despite this weak connection to streamflow, we found that deep groundwaters are important to the global chloride cycle, providing ~7% of the flux of chloride to the ocean.
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The importance of soil moisture in triggering river floods is increasingly recognized. However, soil moisture represents only a fraction of the water stored in the unsaturated zone. In contrast, groundwater from the deeper, saturated zone, may contribute a significant proportion of river flow, but its effects on flooding are poorly understood. Here we analyze hydroclimatic records of thousands of North American watersheds spanning 1981-2018 to show that baseflow (i.e., groundwater-sustained river flows) affects the magnitude of annual flooding at time scales from days to decades. Annual floods almost always arise through the co-occurrence of high precipitation (rainfall + snowmelt) and baseflow. Flood magnitudes are often more strongly related to variations in antecedent baseflow than antecedent soil moisture and short-term (≤3-day) extreme precipitation. In addition, multi-decadal trends in flood magnitude and decadal flood variations tend to better align with groundwater storage and baseflow trends than with changing precipitation extremes and soil moisture. This reveals the importance of groundwater in shaping North American river floods and often decouples the spatial patterns of flood trends from those of shifting precipitation extremes and soil moisture.
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Vegetation modulates Earth’s water, energy and carbon cycles. How its functions might change in the future largely depends on how it copes with droughts1–4. There is evidence that, in places and times of drought, vegetation shifts water uptake to deeper soil5–7 and rock8,9 moisture as well as groundwater10–12. Here we differentiate and assess plant use of four types of water sources: precipitation in the current month (source 1), past precipitation stored in deeper unsaturated soils and/or rocks (source 2), past precipitation stored in groundwater (source 3, locally recharged) and groundwater from precipitation fallen on uplands via river–groundwater convergence toward lowlands (source 4, remotely recharged). We examine global and seasonal patterns and drivers in plant uptake of the four sources using inverse modelling and isotope-based estimates. We find that (1), globally and annually, 70% of plant transpiration relies on source 1, 18% relies on source 2, only 1% relies on source 3 and 10% relies on source 4; (2) regionally and seasonally, source 1 is only 19% in semi-arid, 32% in Mediterranean and 17% in winter-dry tropics in the driest months; and (3) at landscape scales, source 2, taken up by deep roots in the deep vadose zone, is critical in uplands in dry months, but source 4 is up to 47% in valleys where riparian forests and desert oases are found. Because the four sources originate from different places and times, move at different spatiotemporal scales and respond with different sensitivity to climate and anthropogenic forces, understanding the space and time origins of plant water sources can inform ecosystem management and Earth system models on the critical hydrological pathways linking precipitation to vegetation.
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More than 80% of studies reporting the area on Earth underlain by permafrost refer to the extent of the permafrost region, which accounts for around 21 million km2 (22% of the Northern Hemisphere’s exposed land surface). Since each permafrost zone (continuous, discontinuous, sporadic, and isolated, which combined represent the permafrost region) is not entirely underlain by permafrost, the actual area underlain by permafrost (permafrost area) accounts for approximately 14 million km2 (15% of the exposed land surface area in the Northern Hemisphere). Such overstatements create a general impression that there is 6 million km2 more permafrost than currently estimated. I highlight the known differences between the terms permafrost region and permafrost area, along with explaining the possible reasons leading to these overstatements. Recent studies estimating permafrost area extent are summarized to provide a baseline for the geoscientific community and general public about the actual area underlain by permafrost for different parts of the globe.
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Drylands, comprising land regions characterized by water-limited, sparse vegetation, have commonly been projected to expand globally under climate warming. Such projections, however, rely on an atmospheric proxy for drylands, the aridity index, which has recently been shown to yield qualitatively incorrect projections of various components of the terrestrial water cycle. Here, we use an alternative index of drylands, based directly on relevant ecohydrological variables, and compare projections of both indices in Coupled Model Intercomparison Project Phase 5 climate models as well as Dynamic Global Vegetation Models. The aridity index overestimates simulated ecohydrological index changes. This divergence reflects different index sensitivities to hydroclimate change and opposite responses to the physiological effect on vegetation of increasing atmospheric CO2. Atmospheric aridity is thus not an accurate proxy of the future extent of drylands. Despite greater uncertainties than in atmospheric projections, climate model ecohydrological projections indicate no global drylands expansion under greenhouse warming, contrary to previous claims based on atmospheric aridity. Model projections of future drylands distribution using a proxy based on atmospheric aridity show expansion under climate change, but may not be an accurate representation. An alternative index based on ecohydrological variables such as water limitation shows no global expansion of drylands.
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WaterGAP is a global hydrological model that quantifies human use of groundwater and surface water as well as water flows and water storage and thus water resources on all land areas of the Earth. Since 1996, it has served to assess water resources and water stress both historically and in the future, in particular under climate change. It has improved our understanding of continental water storage variations, with a focus on overexploitation and depletion of water resources. In this paper, we describe the most recent model version WaterGAP 2.2d, including the water use models, the linking model that computes net abstractions from groundwater and surface water and the WaterGAP Global Hydrology Model (WGHM). Standard model output variables that are freely available at a data repository are explained. In addition, the most requested model outputs, total water storage anomalies, streamflow and water use, are evaluated against observation data. Finally, we show examples of assessments of the global freshwater system that can be achieved with WaterGAP 2.2d model output.
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Billions of people rely on groundwater as being an accessible source of drinking water and for irrigation, especially in times of drought. Its importance will likely increase with a changing climate. It is still unclear, however, how climate change will impact groundwater systems globally and, thus, the availability of this vital resource. Groundwater recharge is an important indicator for groundwater availability , but it is a water flux that is difficult to estimate as uncertainties in the water balance accumulate, leading to possibly large errors in particular in dry regions. This study investigates uncertainties in groundwater recharge projections using a multi-model ensemble of eight global hydrological models (GHMs) that are driven by the bias-adjusted output of four global circulation models (GCMs). Pre-industrial and current groundwater recharge values are compared with recharge for different global warming (GW) levels as a result of three representative concentration pathways (RCPs). Results suggest that projected changes strongly vary among
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Groundwater forms the basis of water supplies across much of Africa and its development is rising as demand for secure water increases. Recharge rates are a key component for assessing groundwater development potential, but have not been mapped across Africa, other than from global models. Here we quantify long-term average (LTA) distributed groundwater recharge rates across Africa for the period 1970–2019 from 134 ground-based estimates and upscaled statistically. Natural diffuse and local focussed recharge, where this mechanism is widespread, are included but discrete leakage from large rivers, lakes or from irrigation are excluded. We find that measurable LTA recharge is found in most environments with average decadal recharge depths in arid and semi-arid areas of 60 mm (30–140 mm) and 200 mm (90–430 mm) respectively. A linear mixed model shows that at the scale of the African continent only LTA rainfall is related to LTA recharge—the inclusion of other climate and terrestrial factors do not improve the model. Kriging methods indicate spatial dependency to 900 km suggesting that factors other than LTA rainfall are important at local scales. We estimate that average decadal recharge in Africa is 15 000 km³ (4900–45 000 km³), approximately 2% of estimated groundwater storage across the continent, but is characterised by stark variability between high-storage/low-recharge sedimentary aquifers in North Africa, and low-storage/high-recharge weathered crystalline-rock aquifers across much of tropical Africa. African water security is greatly enhanced by this distribution, as many countries with low recharge possess substantial groundwater storage, whereas countries with low storage experience high, regular recharge. The dataset provides a first, ground-based approximation of the renewability of groundwater storage in Africa and can be used to refine and validate global and continental hydrological models while also providing a baseline against future change.
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Groundwater is a crucial resource for current and future generations, but it is not being sustainably used in many parts of the world. The objective of this review is to provide a clear portrait of global-scale groundwater sustainability, systems, and resources in the Anthropocene to inspire a pivot toward more sustainable pathways of groundwater use. We examine groundwater from three different but related perspectives of sustainability science, natural resource governance and management, and Earth System science. An Earth System approach thus highlights the connections between groundwater and the other parts of the system and how these connections are impacting, or are impacted by, groundwater pumping. Groundwater is the largest store of unfrozen freshwater on Earth and is heterogeneously connected to many Earth System processes on different timescales. We propose a definition of groundwater sustainability that has a direct link with observable data, governance, and management as well as the crucial functions and services of groundwater. ▪ Groundwater is depleted or contaminated in some regions; it is ubiquitously distributed, which, importantly, makes it broadly accessible but also slow and invisible and therefore challenging to govern and manage. ▪ Regional differences in priorities, hydrology, politics, culture, and economic contexts mean that different governance and management tools are important, but a global perspective can support higher level international policies in an increasingly globalized world that require broader analysis of interconnections and knowledge transfer between regions. ▪ A coherent, overarching framework of groundwater sustainability is more important for groundwater governance and management than the concepts of safe yield, renewability, depletion, or stress. Full access link:
Life on Earth vitally depends on the availability of water. Human pressure on freshwater resources is increasing, as is human exposure to weather-related extremes (droughts, storms, floods) caused by climate change. Understanding these changes is pivotal for developing mitigation and adaptation strategies. The Global Climate Observing System (GCOS) defines a suite of Essential Climate Variables (ECVs), many related to the water cycle, required to systematically monitor the Earth's climate system. Since long-term observations of these ECVs are derived from different observation techniques, platforms, instruments, and retrieval algorithms, they often lack the accuracy, completeness, resolution, to consistently to characterize water cycle variability at multiple spatial and temporal scales. Here, we review the capability of ground-based and remotely sensed observations of water cycle ECVs to consistently observe the hydrological cycle. We evaluate the relevant land, atmosphere, and ocean water storages and the fluxes between them, including anthropogenic water use. Particularly, we assess how well they close on multiple temporal and spatial scales. On this basis, we discuss gaps in observation systems and formulate guidelines for future water cycle observation strategies. We conclude that, while long-term water-cycle monitoring has greatly advanced in the past, many observational gaps still need to be overcome to close the water budget and enable a comprehensive and consistent assessment across scales. Trends in water cycle components can only be observed with great uncertainty, mainly due to insufficient length and homogeneity. An advanced closure of the water cycle requires improved model-data synthesis capabilities, particularly at regional to local scales.
Most rivers exchange water with surrounding aquifers1,2. Where groundwater levels lie below nearby streams, streamwater can infiltrate through the streambed, reducing streamflow and recharging the aquifer³. These ‘losing’ streams have important implications for water availability, riparian ecosystems and environmental flows4–10, but the prevalence of losing streams remains poorly constrained by continent-wide in situ observations. Here we analyse water levels in 4.2 million wells across the contiguous USA and show that nearly two-thirds (64 per cent) of them lie below nearby stream surfaces, implying that these streamwaters will seep into the subsurface if it is sufficiently permeable. A lack of adequate permeability data prevents us from quantifying the magnitudes of these subsurface flows, but our analysis nonetheless demonstrates widespread potential for streamwater losses into underlying aquifers. These potentially losing rivers are more common in drier climates, flatter landscapes and regions with extensive groundwater pumping. Our results thus imply that climatic factors, geological conditions and historic groundwater pumping jointly contribute to the widespread risk of streams losing flow into surrounding aquifers instead of gaining flow from them. Recent modelling studies¹⁰ have suggested that losing streams could become common in future decades, but our direct observations show that many rivers across the USA are already potentially losing flow, highlighting the importance of coordinating groundwater and surface water policy.
Estimating diffuse recharge of precipitation is fundamental to assessing groundwater sustainability. Diffuse recharge is also the process through which climate and climate change directly affect groundwater. In this study, we evaluated diffuse recharge over the conterminous U.S. simulated by a suite of land surface models (LSMs) that were forced using a common set of meteorological input data. Simulated annual recharge exhibited spatial patterns that were similar among the LSMs, with the highest values in the eastern U.S. and Pacific Northwest. However, the magnitudes of annual recharge varied significantly among the models and were associated with differences in simulated ET, runoff and snow. Evaluation against two independent datasets did not answer the question of whether the ensemble mean performs the best, due to inconsistency between those datasets. The amplitude and timing of seasonal maximum recharge differed among the models, influenced strongly by model physics governing deep soil moisture drainage rates and, in cold regions, snowmelt. Evaluation using in situ soil moisture observations suggested that true recharge peaks 1-3 months later than simulated recharge, indicating systematic biases in simulating deep soil moisture. However, recharge from lateral flows and through preferential flows cannot be inferred from soil moisture data, and the seasonal cycle of simulated groundwater storage actually compared well with in situ groundwater observations. Long-term trends in recharge were not consistently correlated with either precipitation trends or temperature trends. This study highlights the need to employ dynamic flow models in LSMs, among other improvements, to enable more accurate simulation of recharge.