Rapid Communication/

Long-Term Groundwater Depletionin the United Statesby Leonard F. Konikow

AbstractThe 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 HighPlains aquifer, the Mississippi Embayment section of the Gulf Coastal Plain aquifer system, and the Central Valleyof California. Depletion rates accelerated during 1945–1960, averaging 13.6 km3/year during the last half of thecentury, and after 2000 increased again to about 24 km3/year. Depletion intensity is a new parameter, introducedhere, to provide a more consistent basis for comparing storage depletion problems among various aquifers byfactoring in time and areal extent of the aquifer. During 2001–2008, the Central Valley of California had thelargest depletion intensity. Groundwater depletion in the United States can explain 1.4% of observed sea-levelrise during the 108-year study period and 2.1% during 2001–2008. Groundwater depletion must be confrontedon local and regional scales to help reduce demand (primarily in irrigated agriculture) and/or increase supply.

IntroductionRemoval of water from storage in porous media is a

natural consequence of well withdrawals of groundwater(see Theis 1940). The decrease in volume of storedgroundwater is termed “depletion.” Groundwater storagedepletion is becoming recognized as an increasinglyserious global problem that threatens the sustainabilityof water supplies and critical ecosystems (e.g., Konikowand Kendy 2005; Gleeson et al. 2010; Schwartz andIbaraki 2011; Gleeson et al. 2012). While long-termgroundwater depletion is driven largely by overexploita-tion (i.e., large and unsustainable withdrawals by wells),shorter term local to regional trends in depletion may bedominated by natural variability over months to years inprecipitation and recharge. Thus, establishing long-termtrends in depletion over periods of many decades andclimate variability cycles is required to demonstrate theanthropogenic 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

extrapolating depletion trends into the future. This thencan serve as a basis for deciding whether managementintervention is needed and which types of actions arefeasible and most beneficial relative to costs.

Groundwater depletion can have a number of detri-mental effects. These include reduced well yields,increased pumping costs, needs to drill deeper wells, irre-versible land subsidence, reduced base flow to springs,streams, and other surface water bodies, and loss ofwetlands (Alley et al. 1999; Bartolino and Cunning-ham 2003; Konikow and Kendy 2005). Depletion effectscan, in turn, lead to land-use changes (e.g., Harringtonet al. 2007). Reduced groundwater discharge can dam-age aquatic ecosystems. Land subsidence can cause costlyinfrastructure damage (Galloway et al. 1999). The qualityof groundwater in the presence of substantial depletioncan also deteriorate because of sea water intrusion and/orinduced leakage from low-permeability confining unitsthat contain poorer quality groundwater. The effects ofcontinued depletion combine to make groundwater sup-plies unsustainable in the long term (e.g., Custodio 2005;Van der Gun and Lipponen 2010).

Groundwater depletion can be characterized or mea-sured in two ways. First, water-level declines are a directconsequence 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 prede-velopment through 2007 were mapped by Reilly et al.(2008) and show that substantial depletion occurs, atleast locally, throughout the nation. However, by defi-nition, depletion represents a change in the volume (andmass) of water stored in the subsurface, so the secondapproach is to characterize groundwater depletion in termsof volumetric changes. Because volumetric assessmentsare difficult to integrate over the area of an aquifer, theyhave rarely been done or documented, except for someregional systems where transient three-dimensional flowmodels have been well calibrated for time periods startingwith early in development or those where the recogni-tion of the problem has led to comprehensive monitoringprograms.

Large changes in groundwater volume can also bedetected as a change in mass using a time series ofaccurate gravity measurements. Where all (or a well-characterized part) of the regional change in mass can beattributed solely to changes in the volume of groundwaterin storage, groundwater depletion can be estimated fromthese gravity measurements (e.g., Tiwari et al. 2009;Famiglietti et al. 2011; Castle et al. 2014). However, thesegravity methods cannot look back before the start of thetime series of gravity measurements, which started in 2002for the GRACE satellites.

This study evaluates long-term changes in ground-water storage in the United States for 1900–2008,and expands on the results of Konikow (2011, 2013).In aquifer systems not explicitly evaluated herein, thechanges in storage were either small or indeterminatebased on available data. The analysis brings togetherinformation from the literature and from new analyses,directly estimating net depletion using calibrated ground-water models, analytical approaches, and/or volumetricbudget analyses for multiple aquifer systems. Methodsapplied to individual aquifer systems are described inmore detail by Konikow (2013).

Volumetric Groundwater DepletionGroundwater withdrawals in the United States have

increased dramatically during the 20th century—morethan doubling from 1950 through 1975 (Hutson et al.2004). In 2005, total groundwater withdrawals were about114 km3/year and the cumulative withdrawals from 1950to 2005 were approximately 5340 km3 (Kenny et al.2009). With increasing groundwater use, one might expectincreased groundwater depletion.

The long-term volumetric depletion in the UnitedStates during 1900–2008 was estimated for 40 separateaquifers and/or subareas, as well as for one broad dif-fuse land-use category representing agricultural and landdrainage where the water table has been permanently low-ered (Konikow 2013; Table S1, Supporting Information).The areas selected included systems where data indicatedthat substantial changes in the volume of groundwaterstored were occurring and where data were sufficient to

estimate the annual and total cumulative depletion dur-ing the study period. Most assessed areas are west of theMississippi River. The analyses indicate that the cumula-tive depletion volume during the 20th century was about800 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. with-

drawals were derived from (or balanced by) groundwaterstorage depletion (Konikow and Leake 2014). For just2005, the depletion component had increased to 19%.

The magnitudes of depletion volume vary greatlyacross the United States (Konikow 2013; Figure S1). Thethree largest volumes of groundwater storage depletionoccur in the High Plains aquifer, the Mississippi Embay-ment section of the Gulf Coastal Plain aquifer system, andthe Central Valley of California. Combined, the depletionin these three systems accounts for two-thirds of the totaldepletion in the United States. In contrast, two westernvolcanic aquifer systems (Snake River Plain and ColumbiaPlateau aquifer systems) have shown long-term cumula-tive increases in the volume of groundwater stored (i.e.,negative depletion), mostly because of increased rechargearising from infiltration of irrigation water derived fromimported surface water sources.

The total long-term depletion volume is equivalent toabout twice the volume of water contained in Lake Erie.Extracted groundwater can transfer through any number ofpathways through the hydrologic cycle. Regardless of thepathway and travel time, however, the ultimate sink forthe great majority of the depletion volume is the oceans(Wada et al. 2010; Konikow 2011). The long-term netdepletion volume is large enough that it can represent asignificant transfer of water mass from land to sea. If theU.S. depletion volume is spread over the surface area ofthe oceans, it alone would account for a sea-level rise(SLR) of 2.8 mm. This represents about 1.4% of observedSLR during the 108-year time period.

Rates of DepletionGroundwater storage depletion can also be assessed

and compared in terms of rates—volumetric changes perunit time. The volumetric rates of groundwater depletionin the 40 study areas during the 20th century wereconsistently less than 3.0 km3/year (Figure 1) in individualsystems, but totaled about 8.0 km3/year for the entireUnited States (in this and subsequent maps, Hawaiiand Alaska are not shown because of the relativelysmall magnitude of depletion in Hawaii and negligibledepletion in Alaska). The U.S. depletion rate increasedfrom 2.4 km3/year during the first half of the 20th centuryto 13.6 km3/year during the last half of the century(Figure S4).

The highest depletion rate in a single system duringthis 100-year period is in the High Plains aquifer (about2.6 km3/year). However, there was a large variation overtime and space. For example, in the Nebraska partof the northern High Plains, small water-table risesoccurred 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 theconterminous 48 states.

negligible. In contrast, in the Texas part of the southernHigh Plains, development of groundwater resourceswas more extensive and the depletion rate averaged1.6 km3/year. Similarly, the majority of depletion in theCentral Valley occurs in the southern third of the areain the Tulare Basin (San Joaquin Valley). The threenext highest rates of depletion were observed in theMississippi embayment (1.2 km3/year), the Central Valleyof California (1.1 km3/year), and the alluvial basins ofArizona (1.05 km3/year). Of the two volcanic systemsshowing net water-table rises during the 20th century, theSnake River Plain had the larger negative depletion rate(−0.4 km3/year), whereas the Columbia Plateau aquifersaveraged −0.05 km3/year).

During the last 8 years of the study period(2001–2008), some marked changes occurred andthe time-averaged depletion rate for the United Statesincreased to 23.9 km3/year. The highest rates occur inthe High Plains aquifer (10.2 km3/year), the Mississippiembayment (8.0 km3/year), and the Central Valleyof California (3.9 km3/year) (Figure 2). Depletion inthe alluvial basins of Arizona—on the whole—wasreversed, and averaged about −0.4 km3/year during thismost recent 8-year period. These substantial changes inArizona most likely arose from a combination of factors,including changes in water management and water usepractices after 1980, importation of Colorado River watersince 1985, and the implementation of artificial rechargeprograms (Galloway et al. 1999). During 2001–2008,the trends of increasing groundwater storage in thetwo volcanic aquifer systems was reversed; both nowexperienced net decreases in the volumes of groundwaterin storage (a depletion rate averaging about 0.2 km3/year

in both the Columbia Plateau and Snake River Plainaquifer systems).

During 1900–2000, the average groundwater deple-tion rate in the United States of about 8.0 km3/year canexplain 0.022 mm/year of SLR. During 2001–2008, theaverage groundwater depletion rate in the United Stateswas 23.9 km3/year, which can explain 0.066 mm/year ofSLR. The observed rate of SLR increased from about1.7 mm/year during the 20th century (Church and White2006) to about 3.1 mm/year during 1993–2003 (Bindoffet al. 2007). Thus, although the rate of SLR has increased,the relative growth of groundwater depletion in the UnitedStates has increased even more, so that the percentage ofSLR that can be explained by groundwater depletion inthe United States has increased from 1.3% during the 20thcentury to 2.1% during 2001–2008.

Depletion IntensityAnother way to assess the magnitude of aquifer

depletion, as well as to provide a better basis of com-parisons between different areas, is to normalize thedepletion volume by time and for the areal extent ofthe aquifer. This new measure is termed the depletionintensity and has dimensions of L/T. The depletion inten-sity differs from the groundwater footprint introducedby Gleeson et al. (2012) in that the latter is essentiallya water balance that relates groundwater withdrawalsto recharge and base flow discharge without directlyassessing changes in storage. However, depletion intensityassesses changes in storage without assessing the imbal-ance between recharge and discharge that leads to storagedepletion. 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 theconterminous 48 states.

The normalization by aquifer area reduces theapparent variability in groundwater depletion; depletionintensity in the United States during the 20th century(Figure 3) is generally more uniform than either depletionvolumes or depletion rates. For 1900–2000, the highestdepletion intensities were in three relatively small basinslocated 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 CentralValley of California, where the aquifer-wide depletionintensity averaged 0.075 m/year. The next highest valuesoccur in the Coachella Valley of California and thePahvant Valley of Utah. The Mississippi embaymentaquifer system exhibits the next largest depletion intensity,followed by the High Plains aquifer, where the highvolume of depletion storage depletion is moderatedby the large areal extent of this aquifer system. Thedepletion intensity during 2001–2008 in the High Plainsaquifer system was 0.028 m/year—only about one-thirdof that in the Central Valley. Consideration needs to begiven to developing improved water management anddepletion mitigation strategies in critical areas having highdepletion intensities.

DiscussionGroundwater depletion is manifested by water-level

declines. Thus, well yields in unconfined aquifers shoulddecrease over time with continuing depletion becausedecreased saturated thickness translates into decreasedaquifer transmissivity. Furthermore, in all types ofaquifers reduced heads (and lowered water levels in

wells) mean that more energy (and cost) is required tolift water, and for a given pump the same expenditure ofenergy for a greater lift will yield less water. Maintainingsteady levels of groundwater extraction, even temporarily,may require a substantial expense for deepening existingwells or drilling new deeper replacement wells. Togetherwith other costs and consequences of groundwaterdepletion, these physical and economic factors shouldresult in an eventual reduction in groundwater with-drawals in depleting aquifers. Although there are somelocal examples where storage depletion and persistentwater-level declines are at least a contributing factor toreduced pumpage, which in turn should slow down therate of depletion, for the United States as a whole thereis little indication that this self-limitation on continueddepletion is yet in force. Although groundwater depletionaffects the sustainability of groundwater development,ultimately groundwater depletion itself is unsustainable.

If groundwater depletion persists in an aquifer, atsome time the ability of the aquifer to supply waterwill be adversely affected. In some areas where ground-water depletion has continued for decades, well yieldshave decreased so much that agricultural production isadversely affected because farmers must reduce theirirrigated acreage, reduce the seasonal irrigation volumes,or cease irrigation altogether (Scanlon et al. 2012;Steward et al. 2013).

What does the future hold? Substantial popu-lation growth over the next several decades seemshighly likely, and this would represent a major drivingforce for increasing the demand for food, energy, andwater supply. This is a water security issue of botha 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 conterminous48 states.

Figure 4. Groundwater depletion intensity during 2001–2008 in 40 assessed aquifer systems or subareas in the conterminous48 states.

2014). The sustainability of groundwater supplies iskey in continuing to meet the present demand and inmeeting future increased demands, and such sustain-ability is threatened by continued groundwater storagedepletion.

Can we, and should we, take steps to controland/or limit future groundwater depletion or to reversehistorical depletion? This is a complex issue because thescientific and technical issues controlling groundwaterstorage changes are intertwined with political, legal,management, and socioeconomic issues and constraints

(also see Van der Gun and Lipponen 2010). Successin controlling or mitigating groundwater depletion willrequire a comprehensive and integrative approach tothese many factors. Furthermore, the hydrology andhydrodynamics of aquifers dictates that aquifers respondto stresses at local to regional scales, and optimalmanagement will require local and regional cooperationand action (also see Llamas and Martinez-Santos 2005).Any changes in national policies must recognize thatlocal groundwater conditions are highly variable, as isgroundwater governance, and a single technical approach

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may not be suitable or optimal for all aquifers. From theperspective of technical approaches to mediating deple-tion problems, water managers will have to take actionsto directly or indirectly (1) reduce demand (primarily inirrigated agriculture) through increased efficiencies, taxor cost incentives (or disincentives), well permitting, andby fostering changes in land use, industry, and popula-tion, and (2) increase supply through managed aquiferrecharge, desalination, developing alternative sources, andother means.

Groundwater flow is slow, especially relative tosurface water systems or the atmosphere. Similarly, prop-agation of stresses and responses in groundwater systemsare generally much slower than in surface water sys-tems or the atmosphere. Moreover, groundwater residencetimes are typically much greater than surface water res-idence times as well. These general characteristics meanthat critical problems in groundwater systems are rela-tively slow to spread, slow to be noticed, and slow tobe remedied (e.g., see Alley et al. 2002; Bredehoeft andDurbin 2009; Walton 2011). Therefore, planning, poli-cies, and actions must take a long-term view, recognizethe hydraulic interconnection between surface water andgroundwater, and strive for beneficial results over periodsof many years or decades.

ConclusionsData from detailed analyses of 40 aquifer systems

having substantial groundwater storage changes duringthe 108-year time period of 1900–2008 showed avariation in responses. Only two aquifer systems (theColumbia Plateau and Snake River Plain aquifer systemsin the northwestern United States) experienced substantialincreases in water stored, primarily because of increasedirrigation using diverted surface water, which causedgroundwater recharge to increase above natural rates.However, most aquifer systems with changes showeda net depletion, often a substantial one. From 1900 to2008, the volume of groundwater stored in aquifers inthe United States has decreased by about 1000 km3,a magnitude indicating that groundwater depletion isa serious problem. The aquifer systems with the threelargest volumes of storage depletion include the HighPlains aquifer, the Mississippi Embayment section of theGulf Coastal Plain aquifer system, and the Central Valleyof California.

Rates of groundwater depletion increased mostnotably after the mid-1940s, mostly driven by increaseduse of groundwater as a water source for irrigatedagriculture, which in turn is related to rural electrifica-tion, availability of more efficient submersible pumps,better well drilling technology, and general economicgrowth at that time. The trend in the rates of depletionmore or less stabilized at relatively high rates averag-ing about 14 km3/year from about 1960 through 2000.After 2000, the average rate of groundwater depletion inthe United States increased again—to an average rate ofalmost 24 km3/year during 2001–2008 inclusive. During

2001–2008, the depletion rate in the High Plains aquiferincreased to 10.2 km3/year. The two volcanic aquifer sys-tems in the northwestern United States (Columbia Plateauand Snake River Plain), which had experienced substan-tial water-level rises during the 20th century, had a trendreversal during 2001–2008, when they both experiencednet depletion rates of about 0.2 km3/year.

Depletion intensity is a new parameter—introducedin this study—to better characterize groundwater storagedepletion. It normalizes the depletion volume by timeand aquifer area, thereby offering a consistent measureto compare depletion magnitude among various aquifersystems. The greatest depletion intensity in the UnitedStates during 2001–2008 occurs in the Central Valleyof California, indicating that, on the basis of thismeasure, this may be the system having the most seriousgroundwater depletion problem in the United States.Because of its large areal extent, the High Plains aquifersystem, often cited as one of the most impacted, wouldrank only fifth according to this measure.

Long-term groundwater depletion represents a largetransfer of water from the continents to the oceans.Thus, groundwater depletion represents a small butnontrivial contributor to SLR. Depletion in the UnitedStates alone can explain more than 2% of observed SLRduring 2001–2008, and global depletion would explainsubstantially more than that. Groundwater depletion needsto be accounted for when estimating water budgets toexplain past sea-level change or predicting future sea-levelchange.

Aquifers can serve as large reservoirs to providea reliable long-term source of water supply—either as aprimary source or to supplement surface water sourcesin times of increased climate uncertainty and resourcevariability. Actions to limit or even reverse groundwaterstorage depletion at a minimum would extend the usefullife of an aquifer as a source of water supply and withgreater effect perhaps even create a new sustainablebalance. Thus, with increasing demands for water likelyto occur in the future, such actions would seem tobe highly desirable in many currently depleted aquifersystems throughout the nation. Management actions canonly focus on some combination of reducing demand andincreasing supply, and there are a number of ways—bothpolitically and technologically—to achieve both goals.The important thing is that beneficial actions to offset orremediate the substantial national problem of groundwaterstorage depletion be taken to assure our future watersecurity.

AcknowledgmentsE.A. Achey, S.M. Feeney, D.P. McGinnis, and J.J.

Donovan assisted with analyses and calculations forsome of the aquifer systems. The author benefited frominsightful discussions with numerous USGS colleaguesand with the late Prof. T.N. Narasimhan. The author alsoappreciates the helpful review comments and suggestionsof C.C. Faunt, H.M. Haitjema, and K.A. Uhlman.

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

Supporting InformationAdditional Supporting Information may be found in theonline version of this article:

Table S1. Groundwater storage depletion in aquifersystems, 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 inthe United States, 1900–2008.Figure S3. Change in the average groundwater depletionrates from 1961–1980 to 2001–2008 in 40 assessedaquifer systems or subareas in the conterminous 48 states.Figure S4. Five-year averaged rate of groundwaterdepletion in the U.S., 1900–2008.Figure S5. Change in groundwater depletion intensityfrom 1900–2000 to 2001–2008 in 40 assessed aquifersystems or subareas in the conterminous 48 states.

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