Rapid Communication/

Long-Term Groundwater Depletion
in the United States
by Leonard F. Konikow

Abstract
The volume of groundwater stored in the subsurface in the United States decreased by almost 1000 km3
during 1900–2008. The aquifer systems with the three largest volumes of storage depletion include the High
Plains aquifer, the Mississippi Embayment section of the Gulf Coastal Plain aquifer system, and the Central Valley
of California. Depletion rates accelerated during 1945–1960, averaging 13.6 km3 /year during the last half of the
century, and after 2000 increased again to about 24 km3 /year. Depletion intensity is a new parameter, introduced
here, to provide a more consistent basis for comparing storage depletion problems among various aquifers by
factoring in time and areal extent of the aquifer. During 2001–2008, the Central Valley of California had the
largest depletion intensity. Groundwater depletion in the United States can explain 1.4% of observed sea-level
rise during the 108-year study period and 2.1% during 2001–2008. Groundwater depletion must be confronted
on local and regional scales to help reduce demand (primarily in irrigated agriculture) and/or increase supply.

Introduction
Removal of water from storage in porous media is a
natural consequence of well withdrawals of groundwater
(see Theis 1940). The decrease in volume of stored
groundwater is termed “depletion.” Groundwater storage
depletion is becoming recognized as an increasingly
serious global problem that threatens the sustainability
of water supplies and critical ecosystems (e.g., Konikow
and Kendy 2005; Gleeson et al. 2010; Schwartz and
Ibaraki 2011; Gleeson et al. 2012). While long-term
groundwater depletion is driven largely by overexploitation (i.e., large and unsustainable withdrawals by wells),
shorter term local to regional trends in depletion may be
dominated by natural variability over months to years in
precipitation and recharge. Thus, establishing long-term
trends in depletion over periods of many decades and
climate variability cycles is required to demonstrate the
anthropogenic linkage and to provide a sound basis for
431 National Center, U.S. Geological Survey, Reston, VA 20192;
703-648-5878; fax: 703-648-5274; lkonikow@usgs.gov
There are no conflicts of interest and no financial disclosures.
Received October 2014, accepted October 2014.
Published 2014. This article is a U.S. Government work and is
in the public domain in the USA.
doi: 10.1111/gwat.12306

2

extrapolating depletion trends into the future. This then
can serve as a basis for deciding whether management
intervention is needed and which types of actions are
feasible and most beneficial relative to costs.
Groundwater depletion can have a number of detrimental effects. These include reduced well yields,
increased pumping costs, needs to drill deeper wells, irreversible land subsidence, reduced base flow to springs,
streams, and other surface water bodies, and loss of
wetlands (Alley et al. 1999; Bartolino and Cunningham 2003; Konikow and Kendy 2005). Depletion effects
can, in turn, lead to land-use changes (e.g., Harrington
et al. 2007). Reduced groundwater discharge can damage aquatic ecosystems. Land subsidence can cause costly
infrastructure damage (Galloway et al. 1999). The quality
of groundwater in the presence of substantial depletion
can also deteriorate because of sea water intrusion and/or
induced leakage from low-permeability confining units
that contain poorer quality groundwater. The effects of
continued depletion combine to make groundwater supplies unsustainable in the long term (e.g., Custodio 2005;
Van der Gun and Lipponen 2010).
Groundwater depletion can be characterized or measured in two ways. First, water-level declines are a direct
consequence of depletion and their magnitude provides

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 evidence for the seriousness of a depletion problem.
Water-level declines in the United States from predevelopment through 2007 were mapped by Reilly et al.
(2008) and show that substantial depletion occurs, at
least locally, throughout the nation. However, by definition, depletion represents a change in the volume (and
mass) of water stored in the subsurface, so the second
approach is to characterize groundwater depletion in terms
of volumetric changes. Because volumetric assessments
are difficult to integrate over the area of an aquifer, they
have rarely been done or documented, except for some
regional systems where transient three-dimensional flow
models have been well calibrated for time periods starting
with early in development or those where the recognition of the problem has led to comprehensive monitoring
programs.
Large changes in groundwater volume can also be
detected as a change in mass using a time series of
accurate gravity measurements. Where all (or a wellcharacterized part) of the regional change in mass can be
attributed solely to changes in the volume of groundwater
in storage, groundwater depletion can be estimated from
these gravity measurements (e.g., Tiwari et al. 2009;
Famiglietti et al. 2011; Castle et al. 2014). However, these
gravity methods cannot look back before the start of the
time series of gravity measurements, which started in 2002
for the GRACE satellites.
This study evaluates long-term changes in groundwater storage in the United States for 1900–2008,
and expands on the results of Konikow (2011, 2013).
In aquifer systems not explicitly evaluated herein, the
changes in storage were either small or indeterminate
based on available data. The analysis brings together
information from the literature and from new analyses,
directly estimating net depletion using calibrated groundwater models, analytical approaches, and/or volumetric
budget analyses for multiple aquifer systems. Methods
applied to individual aquifer systems are described in
more detail by Konikow (2013).

estimate the annual and total cumulative depletion during the study period. Most assessed areas are west of the
Mississippi River. The analyses indicate that the cumulative depletion volume during the 20th century was about
800 km3 . The total depletion increased to almost 1000 km3
by the end of 2008—a 25% increase in just 8 years!
During 1950–2005, about 15% of the total U.S. withdrawals were derived from (or balanced by) groundwater
storage depletion (Konikow and Leake 2014). For just
2005, the depletion component had increased to 19%.
The magnitudes of depletion volume vary greatly
across the United States (Konikow 2013; Figure S1). The
three largest volumes of groundwater storage depletion
occur in the High Plains aquifer, the Mississippi Embayment section of the Gulf Coastal Plain aquifer system, and
the Central Valley of California. Combined, the depletion
in these three systems accounts for two-thirds of the total
depletion in the United States. In contrast, two western
volcanic aquifer systems (Snake River Plain and Columbia
Plateau aquifer systems) have shown long-term cumulative increases in the volume of groundwater stored (i.e.,
negative depletion), mostly because of increased recharge
arising from infiltration of irrigation water derived from
imported surface water sources.
The total long-term depletion volume is equivalent to
about twice the volume of water contained in Lake Erie.
Extracted groundwater can transfer through any number of
pathways through the hydrologic cycle. Regardless of the
pathway and travel time, however, the ultimate sink for
the great majority of the depletion volume is the oceans
(Wada et al. 2010; Konikow 2011). The long-term net
depletion volume is large enough that it can represent a
significant transfer of water mass from land to sea. If the
U.S. depletion volume is spread over the surface area of
the oceans, it alone would account for a sea-level rise
(SLR) of 2.8 mm. This represents about 1.4% of observed
SLR during the 108-year time period.

Rates of Depletion
Volumetric Groundwater Depletion
Groundwater withdrawals in the United States have
increased dramatically during the 20th century—more
than doubling from 1950 through 1975 (Hutson et al.
2004). In 2005, total groundwater withdrawals were about
114 km3 /year and the cumulative withdrawals from 1950
to 2005 were approximately 5340 km3 (Kenny et al.
2009). With increasing groundwater use, one might expect
increased groundwater depletion.
The long-term volumetric depletion in the United
States during 1900–2008 was estimated for 40 separate
aquifers and/or subareas, as well as for one broad diffuse land-use category representing agricultural and land
drainage where the water table has been permanently lowered (Konikow 2013; Table S1, Supporting Information).
The areas selected included systems where data indicated
that substantial changes in the volume of groundwater
stored were occurring and where data were sufficient to
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Groundwater storage depletion can also be assessed
and compared in terms of rates—volumetric changes per
unit time. The volumetric rates of groundwater depletion
in the 40 study areas during the 20th century were
consistently less than 3.0 km3 /year (Figure 1) in individual
systems, but totaled about 8.0 km3 /year for the entire
United States (in this and subsequent maps, Hawaii
and Alaska are not shown because of the relatively
small magnitude of depletion in Hawaii and negligible
depletion in Alaska). The U.S. depletion rate increased
from 2.4 km3 /year during the first half of the 20th century
to 13.6 km3 /year during the last half of the century
(Figure S4).
The highest depletion rate in a single system during
this 100-year period is in the High Plains aquifer (about
2.6 km3 /year). However, there was a large variation over
time and space. For example, in the Nebraska part
of the northern High Plains, small water-table rises
occurred in parts of this area and the net depletion was

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 Figure 1. Average groundwater depletion rate during 1900–2000 in 40 assessed aquifer systems or subareas in the
conterminous 48 states.

negligible. In contrast, in the Texas part of the southern
High Plains, development of groundwater resources
was more extensive and the depletion rate averaged
1.6 km3 /year. Similarly, the majority of depletion in the
Central Valley occurs in the southern third of the area
in the Tulare Basin (San Joaquin Valley). The three
next highest rates of depletion were observed in the
Mississippi embayment (1.2 km3 /year), the Central Valley
of California (1.1 km3 /year), and the alluvial basins of
Arizona (1.05 km3 /year). Of the two volcanic systems
showing net water-table rises during the 20th century, the
Snake River Plain had the larger negative depletion rate
(−0.4 km3 /year), whereas the Columbia Plateau aquifers
averaged −0.05 km3 /year).
During the last 8 years of the study period
(2001–2008), some marked changes occurred and
the time-averaged depletion rate for the United States
increased to 23.9 km3 /year. The highest rates occur in
the High Plains aquifer (10.2 km3 /year), the Mississippi
embayment (8.0 km3 /year), and the Central Valley
of California (3.9 km3 /year) (Figure 2). Depletion in
the alluvial basins of Arizona—on the whole—was
reversed, and averaged about −0.4 km3 /year during this
most recent 8-year period. These substantial changes in
Arizona most likely arose from a combination of factors,
including changes in water management and water use
practices after 1980, importation of Colorado River water
since 1985, and the implementation of artificial recharge
programs (Galloway et al. 1999). During 2001–2008,
the trends of increasing groundwater storage in the
two volcanic aquifer systems was reversed; both now
experienced net decreases in the volumes of groundwater
in storage (a depletion rate averaging about 0.2 km3 /year
4

in both the Columbia Plateau and Snake River Plain
aquifer systems).
During 1900–2000, the average groundwater depletion rate in the United States of about 8.0 km3 /year can
explain 0.022 mm/year of SLR. During 2001–2008, the
average groundwater depletion rate in the United States
was 23.9 km3 /year, which can explain 0.066 mm/year of
SLR. The observed rate of SLR increased from about
1.7 mm/year during the 20th century (Church and White
2006) to about 3.1 mm/year during 1993–2003 (Bindoff
et al. 2007). Thus, although the rate of SLR has increased,
the relative growth of groundwater depletion in the United
States has increased even more, so that the percentage of
SLR that can be explained by groundwater depletion in
the United States has increased from 1.3% during the 20th
century to 2.1% during 2001–2008.

Depletion Intensity
Another way to assess the magnitude of aquifer
depletion, as well as to provide a better basis of comparisons between different areas, is to normalize the
depletion volume by time and for the areal extent of
the aquifer. This new measure is termed the depletion
intensity and has dimensions of L/T. The depletion intensity differs from the groundwater footprint introduced
by Gleeson et al. (2012) in that the latter is essentially
a water balance that relates groundwater withdrawals
to recharge and base flow discharge without directly
assessing changes in storage. However, depletion intensity
assesses changes in storage without assessing the imbalance between recharge and discharge that leads to storage
depletion. Both measures are useful in their own way.

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 Figure 2. Average groundwater depletion rate during 2001-2008 in 40 assessed aquifer systems or subareas in the
conterminous 48 states.

The normalization by aquifer area reduces the
apparent variability in groundwater depletion; depletion
intensity in the United States during the 20th century
(Figure 3) is generally more uniform than either depletion
volumes or depletion rates. For 1900–2000, the highest
depletion intensities were in three relatively small basins
located in southern California.
Depletion intensities changed markedly after 2000
(Figure 4). During the beginning of the 21st century,
the greatest depletion intensity occurs in the Central
Valley of California, where the aquifer-wide depletion
intensity averaged 0.075 m/year. The next highest values
occur in the Coachella Valley of California and the
Pahvant Valley of Utah. The Mississippi embayment
aquifer system exhibits the next largest depletion intensity,
followed by the High Plains aquifer, where the high
volume of depletion storage depletion is moderated
by the large areal extent of this aquifer system. The
depletion intensity during 2001–2008 in the High Plains
aquifer system was 0.028 m/year—only about one-third
of that in the Central Valley. Consideration needs to be
given to developing improved water management and
depletion mitigation strategies in critical areas having high
depletion intensities.

Discussion
Groundwater depletion is manifested by water-level
declines. Thus, well yields in unconfined aquifers should
decrease over time with continuing depletion because
decreased saturated thickness translates into decreased
aquifer transmissivity. Furthermore, in all types of
aquifers reduced heads (and lowered water levels in
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wells) mean that more energy (and cost) is required to
lift water, and for a given pump the same expenditure of
energy for a greater lift will yield less water. Maintaining
steady levels of groundwater extraction, even temporarily,
may require a substantial expense for deepening existing
wells or drilling new deeper replacement wells. Together
with other costs and consequences of groundwater
depletion, these physical and economic factors should
result in an eventual reduction in groundwater withdrawals in depleting aquifers. Although there are some
local examples where storage depletion and persistent
water-level declines are at least a contributing factor to
reduced pumpage, which in turn should slow down the
rate of depletion, for the United States as a whole there
is little indication that this self-limitation on continued
depletion is yet in force. Although groundwater depletion
affects the sustainability of groundwater development,
ultimately groundwater depletion itself is unsustainable.
If groundwater depletion persists in an aquifer, at
some time the ability of the aquifer to supply water
will be adversely affected. In some areas where groundwater depletion has continued for decades, well yields
have decreased so much that agricultural production is
adversely affected because farmers must reduce their
irrigated acreage, reduce the seasonal irrigation volumes,
or cease irrigation altogether (Scanlon et al. 2012;
Steward et al. 2013).
What does the future hold? Substantial population growth over the next several decades seems
highly likely, and this would represent a major driving
force for increasing the demand for food, energy, and
water supply. This is a water security issue of both
a national and global scope (e.g., see Braga et al.

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 Figure 3. Groundwater depletion intensity during 1900–2000 in 40 assessed aquifer systems or subareas in the conterminous
48 states.

Figure 4. Groundwater depletion intensity during 2001–2008 in 40 assessed aquifer systems or subareas in the conterminous
48 states.

2014). The sustainability of groundwater supplies is
key in continuing to meet the present demand and in
meeting future increased demands, and such sustainability is threatened by continued groundwater storage
depletion.
Can we, and should we, take steps to control
and/or limit future groundwater depletion or to reverse
historical depletion? This is a complex issue because the
scientific and technical issues controlling groundwater
storage changes are intertwined with political, legal,
management, and socioeconomic issues and constraints
6

(also see Van der Gun and Lipponen 2010). Success
in controlling or mitigating groundwater depletion will
require a comprehensive and integrative approach to
these many factors. Furthermore, the hydrology and
hydrodynamics of aquifers dictates that aquifers respond
to stresses at local to regional scales, and optimal
management will require local and regional cooperation
and action (also see Llamas and Martinez-Santos 2005).
Any changes in national policies must recognize that
local groundwater conditions are highly variable, as is
groundwater governance, and a single technical approach

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 may not be suitable or optimal for all aquifers. From the
perspective of technical approaches to mediating depletion problems, water managers will have to take actions
to directly or indirectly (1) reduce demand (primarily in
irrigated agriculture) through increased efficiencies, tax
or cost incentives (or disincentives), well permitting, and
by fostering changes in land use, industry, and population, and (2) increase supply through managed aquifer
recharge, desalination, developing alternative sources, and
other means.
Groundwater flow is slow, especially relative to
surface water systems or the atmosphere. Similarly, propagation of stresses and responses in groundwater systems
are generally much slower than in surface water systems or the atmosphere. Moreover, groundwater residence
times are typically much greater than surface water residence times as well. These general characteristics mean
that critical problems in groundwater systems are relatively slow to spread, slow to be noticed, and slow to
be remedied (e.g., see Alley et al. 2002; Bredehoeft and
Durbin 2009; Walton 2011). Therefore, planning, policies, and actions must take a long-term view, recognize
the hydraulic interconnection between surface water and
groundwater, and strive for beneficial results over periods
of many years or decades.

Conclusions
Data from detailed analyses of 40 aquifer systems
having substantial groundwater storage changes during
the 108-year time period of 1900–2008 showed a
variation in responses. Only two aquifer systems (the
Columbia Plateau and Snake River Plain aquifer systems
in the northwestern United States) experienced substantial
increases in water stored, primarily because of increased
irrigation using diverted surface water, which caused
groundwater recharge to increase above natural rates.
However, most aquifer systems with changes showed
a net depletion, often a substantial one. From 1900 to
2008, the volume of groundwater stored in aquifers in
the United States has decreased by about 1000 km3 ,
a magnitude indicating that groundwater depletion is
a serious problem. The aquifer systems with the three
largest volumes of storage depletion include the High
Plains aquifer, the Mississippi Embayment section of the
Gulf Coastal Plain aquifer system, and the Central Valley
of California.
Rates of groundwater depletion increased most
notably after the mid-1940s, mostly driven by increased
use of groundwater as a water source for irrigated
agriculture, which in turn is related to rural electrification, availability of more efficient submersible pumps,
better well drilling technology, and general economic
growth at that time. The trend in the rates of depletion
more or less stabilized at relatively high rates averaging about 14 km3 /year from about 1960 through 2000.
After 2000, the average rate of groundwater depletion in
the United States increased again—to an average rate of
almost 24 km3 /year during 2001–2008 inclusive. During
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2001–2008, the depletion rate in the High Plains aquifer
increased to 10.2 km3 /year. The two volcanic aquifer systems in the northwestern United States (Columbia Plateau
and Snake River Plain), which had experienced substantial water-level rises during the 20th century, had a trend
reversal during 2001–2008, when they both experienced
net depletion rates of about 0.2 km3 /year.
Depletion intensity is a new parameter—introduced
in this study—to better characterize groundwater storage
depletion. It normalizes the depletion volume by time
and aquifer area, thereby offering a consistent measure
to compare depletion magnitude among various aquifer
systems. The greatest depletion intensity in the United
States during 2001–2008 occurs in the Central Valley
of California, indicating that, on the basis of this
measure, this may be the system having the most serious
groundwater depletion problem in the United States.
Because of its large areal extent, the High Plains aquifer
system, often cited as one of the most impacted, would
rank only fifth according to this measure.
Long-term groundwater depletion represents a large
transfer of water from the continents to the oceans.
Thus, groundwater depletion represents a small but
nontrivial contributor to SLR. Depletion in the United
States alone can explain more than 2% of observed SLR
during 2001–2008, and global depletion would explain
substantially more than that. Groundwater depletion needs
to be accounted for when estimating water budgets to
explain past sea-level change or predicting future sea-level
change.
Aquifers can serve as large reservoirs to provide
a reliable long-term source of water supply—either as a
primary source or to supplement surface water sources
in times of increased climate uncertainty and resource
variability. Actions to limit or even reverse groundwater
storage depletion at a minimum would extend the useful
life of an aquifer as a source of water supply and with
greater effect perhaps even create a new sustainable
balance. Thus, with increasing demands for water likely
to occur in the future, such actions would seem to
be highly desirable in many currently depleted aquifer
systems throughout the nation. Management actions can
only focus on some combination of reducing demand and
increasing supply, and there are a number of ways—both
politically and technologically—to achieve both goals.
The important thing is that beneficial actions to offset or
remediate the substantial national problem of groundwater
storage depletion be taken to assure our future water
security.

Acknowledgments
E.A. Achey, S.M. Feeney, D.P. McGinnis, and J.J.
Donovan assisted with analyses and calculations for
some of the aquifer systems. The author benefited from
insightful discussions with numerous USGS colleagues
and with the late Prof. T.N. Narasimhan. The author also
appreciates the helpful review comments and suggestions
of C.C. Faunt, H.M. Haitjema, and K.A. Uhlman.

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7

 This work was supported by funding from the U.S.
Geological Survey’s National Research Program, Office
of Groundwater, and Groundwater Resources Program.

Supporting Information
Additional Supporting Information may be found in the
online version of this article:
Table S1. Groundwater storage depletion in aquifer
systems, subareas, or by land-use category, United States
(1900–2008).
Figure S1. Map of the United States (excluding Alaska)
showing cumulative groundwater depletion, 1900–2008,
in 40 assessed aquifer systems or subareas.
Figure S2. Annual cumulative groundwater depletion in
the United States, 1900–2008.
Figure S3. Change in the average groundwater depletion
rates from 1961–1980 to 2001–2008 in 40 assessed
aquifer systems or subareas in the conterminous 48 states.
Figure S4. Five-year averaged rate of groundwater
depletion in the U.S., 1900–2008.
Figure S5. Change in groundwater depletion intensity
from 1900–2000 to 2001–2008 in 40 assessed aquifer
systems or subareas in the conterminous 48 states.

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