U.S. Department of the Interior

U.S. Geological Survey

Groundwater Resources Program

Groundwater Availability of the 

Mississippi Embayment

Professional Paper 1785

Mississippi 

 River

Groundwater

flow

Well
Front cover. Block diagram view of the Mississippi embayment land surface, major rivers, and hydrogeologic units below land 

surface from the southeast looking northwest.
Groundwater Availability of the 

Mississippi Embayment

By Brian R. Clark, Rheannon M. Hart, and Jason J. Gurdak

Groundwater Resources Program

Professional Paper 1785

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2011

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Suggested citation:

Clark, B.R., Hart, R.M., and Gurdak, J.J., 2011, Groundwater availability of the Mississippi embayment: U.S. Geological Survey Professional Paper 1785, 62 p.

Library of Congress Cataloging-in-Publication Data

Clark, Brian R.

 Groundwater availability of the Mississippi Embayment / by Brian R. Clark, Rheannon M. Hart, and Jason J. Gurdak.

 p. cm. -- (Professional paper ; 1785)

 ISBN 978-1-4113-3134-1 (soft cover)

 1. Groundwater--Mississippi Embayment. 2. Groundwater--Southern States. 3. Water table--Mississippi Embayment. 4. Water table--Southern States. 5. Aquifers--Mississippi Embayment. 6. Aquifers--Southern States. I. Hart, 

Rheannon M. II. Gurdak, Jason J. III. Title. IV. Series: U.S. Geological Survey professional paper ; 1785. 

 GB1018.C53 2011

 551.490975--dc23

 2011021667
iii

Foreword

Although often overlooked, groundwater is increasingly important to all our lives. Groundwater 

is the Nation’s principal reserve of freshwater. It provides half our drinking water and is essential to U.S. food production while facilitating business and industry in promoting economic wellbeing. Groundwater is also an important source of water for sustaining the ecosystem health of 

rivers, wetlands, and estuaries throughout the country. 

Large-scale development of groundwater resources with accompanying declines in groundwater 

levels and other effects of pumping have led to concerns about the future availability of groundwater to meet all our Nation’s needs. The depletion of groundwater to satisfy the country’s thirst 

and the compounding effects of recent droughts emphasize the need for an updated status of 

the Nation’s groundwater resources. Assessments of groundwater resources provide the science 

and information needed by the public and decision makers to evaluate water availability and 

its effects on the water supply, as well as, to manage and use the water resources responsibly. 

Adding to this already complex task of resource assessment is the analysis of potential future 

effects due to climate variability, which can further exacerbate an already challenging situation.

The U.S. Geological Survey’s (USGS) Groundwater Resources Program is conducting largescale multidisciplinary regional studies of groundwater availability, such as this study of the 

Mississippi embayment aquifer system. These regional studies are intended to provide 

citizens, communities, and natural resource managers with clearer knowledge of the status of 

the Nation’s groundwater resources and how changes in land use, water use, and climate have 

affected those resources, and to develop tools to forecast how these resources may change 

in the future. Over time, the findings from these individual regionally integrated groundwater 

assessments of principal aquifers will be combined to provide a national assessment. Results 

derived from these studies will provide much needed answers to basic questions about the 

Nation’s ability to meet current and future demands for groundwater.

William H. Werkheiser, 

Associate Director for Water
iv

Contents

Foreword ........................................................................................................................................................iii

Executive Summary.......................................................................................................................................1

Background and Major Issues ...........................................................................................................1

Groundwater Budget............................................................................................................................2

Climate Trend Evaluation.....................................................................................................................2

Groundwater Monitoring Evaluation .................................................................................................3

Introduction.....................................................................................................................................................3

Purpose and Scope ..............................................................................................................................4

Study Area Characteristics.................................................................................................................4

Climate...........................................................................................................................................4

Population .....................................................................................................................................8

Land Use........................................................................................................................................8

Water Use .....................................................................................................................................8

Geologic Setting...........................................................................................................................8

Hydrogeologic Units..................................................................................................................10

Previous Investigations......................................................................................................................13

Methods Overview.......................................................................................................................................13

Numerical Model ................................................................................................................................13

Data Compilation.................................................................................................................................14

Modifications to the Numerical Model ...........................................................................................14

Development of the Hydrologic System...................................................................................................14

Predevelopment Conditions..............................................................................................................14

Postdevelopment Conditions ............................................................................................................15

Groundwater Availability, a Water-Budget Perspective.......................................................................17

Regional Water Budget......................................................................................................................20

Principal Aquifer Water Budgets .....................................................................................................25

Local Area Water Budgets................................................................................................................28

Climate Trends and Potential Future Effects on Groundwater Availability ...............................32

Hydroclimate Data Processing and Analysis........................................................................32

Climate Simulation Development ............................................................................................35

Climate Trend Scenarios and Simulated Water Levels .......................................................36

Evaluation of Regional Groundwater-Level Monitoring Network...............................................37

Existing Monitoring Networks .................................................................................................37

Network Analysis.......................................................................................................................37

Groundwater-Flow Model Improvements.......................................................................................40

Challenges for Future Groundwater Availability Determinations—Limitations and 

Lessons Learned.............................................................................................................................40

Scaling Regional to Local Issues ..............................................................................................................42

Acknowledgments.......................................................................................................................................43

References....................................................................................................................................................43

Appendixes 1–3............................................................................................................................................49
v

Appendix 1. Effects of Natural-Climate Trends on Interannual to Multidecadal Timescales......50

Appendix 2. Singular Spectrum Analysis .............................................................................................52

Appendix 3. Extrapolated 30-Year Precipitation Values Interpolated Spatially across the 

Mississippi Embayment Regional Aquifer Study Area.............................................................58

Figures

1. Map showing location of the study area and Coastal Plain physiographic province

sections ..........................................................................................................................................5

2. Map showing streams simulated in the study area................................................................6

3. Graphs showing cumulative departure from average precipitation within the study

area and at selected locations, showing dry and wet periods for the study area............7

4. Map showing land use within the Mississippi embayment...................................................9

5. Graph showing cumulative groundwater pumpage by aquifer used in the

Mississippi Embayment Regional Aquifer Study model.......................................................10

6. Cross-sectional view of hydrogeologic units within the study area with

predevelopment conceptual water movement......................................................................11

7. Map showing simulated predevelopment streamflow leakage .........................................16

8. Diagram showing predevelopment groundwater-flow budget of the alluvial and

Tertiary systems, in acre-feet per year...................................................................................17

9. Maps showing comparison of A, 1929 and B, 2006 potentiometric surface of the

Grand Prairie area ......................................................................................................................18

10. Cross-sectional view of hydrogeologic units within the study area with

postdevelopment flow (2006) conceptual water movement................................................19

11. Diagram showing postdevelopment groundwater-flow budget of the alluvial and

Tertiary systems in acre-feet per year....................................................................................20

12. Graphs showing groundwater-flow budget of the MERAS model A, seasonal after

1986, spring/summer depicted by large net pumpage and fall/winter depicted by

small net pumpage; B, average annual...................................................................................21

13. Graph showing cumulative change in groundwater storage by aquifer ..........................22

14. Map showing water-level change from predevelopment to 2007 in the 

A, Mississippi River Valley alluvial aquifer and B, the middle Claiborne aquifer ............23

15. Pie charts showing percent area for water-level change groups from

predevelopment to 2007 in the Mississippi River Valley alluvial aquifer and the

middle Claiborne aquifer ...........................................................................................................25

16. Graphs showing groundwater-flow budget of the alluvial aquifer A, seasonal after

1986, spring/summer depicted by large net pumpage and fall/winter depicted by

small net pumpage; B, average annual...................................................................................26

17. Graphs and pie chart showing groundwater-flow budget of the middle Claiborne

aquifer...........................................................................................................................................27

18. Graphs showing groundwater-flow budgets of local areas................................................29

19. Pie charts showing predevelopment groundwater-flow interactions with adjacent

hydrogeologic units within local areas...................................................................................30

20. Pie charts showing postdevelopment groundwater-flow interactions with

adjacent hydrogeologic units within local areas ..................................................................31

21. Graphs showing demonstration of hydroclimate data-processing steps using 

A, total monthly precipitation time series, B, monthly cumulative departure (MCD),

and C, normalized departure series.........................................................................................33
vi

22. Graph showing average annual (2-year or more) precipitation values used in

the MERAS model and selected meteorological stations including extrapolated

precipitation data from 2008 to 2038........................................................................................34

23. Graph showing specified net recharge at selected locations including

extrapolated net recharge from 2008 to 2038.........................................................................34

24. Graph showing relation between fractional change in pumping from 2005 and

fractional change in precipitation at Memphis, Tennessee, from 2005.............................35

25. Pie charts showing percent area for water-level change groups from

predevelopment to 2038 in the Mississippi River Valley alluvial aquifer and the

middle Claiborne aquifer (dry and wet scenarios)................................................................36

26. Maps showing water-level change of the dry scenario from predevelopment to

2038 in the A, Mississippi River Valley alluvial aquifer and B, the middle Claiborne

aquifer...........................................................................................................................................38

27. Map showing results of OPR-PPR showing the relative importance of spring 2007

water-level observations in the middle Claiborne aquifer...................................................41

1–1. Graphs showing interactions between the positive and negative phases of the 

A, multivariate El Niño/Southern Oscillation (ENSO) index, B, Pacific Decadal

Oscillation (PDO) index, and C, Atlantic Multidecadal Oscillation (AMO) index

cumulatively affect United States climate and, in turn, surface and groundwater

resources .....................................................................................................................................50

2–1. Graphs showing statistically significant reconstructed components based on the

Chi-squared significance test from the singular spectrum analysis for time series

of A, precipitation at meteorological stations, B, streamflow at streamflow-gaging

stations, and C, groundwater levels at wells from the study area indicate that all

time series contain variations that are consistent with El Niño Southern

Oscillation (2 to 6 years), Pacific Decadal Oscillation (10 to 25 years), and greater

than Pacific Decadal Oscillation (greater than 25 years) periods, which are

indicated by the gray shading. Additional variations with periodicities of 6 to 10

years were observed in all three types of hydrologic time series .....................................57

3–1. Maps showing the extrapolated 30-year precipitation values interpolated spatially

across the Mississippi Embayment Regional Aquifer Study area are shown as 

A, 2-year average extrapolated annual precipitation (in inches) and as B, deviation of

the 2-year average extrapolated annual precipitation from the average observed

annual precipitation (in percent) for the time period 2008 to 2038 .....................................58

Tables

1. Correlation of hydrogeologic units across States within the Mississippi 

Embayment Regional Aquifer Study ........................................................................................12

2–1. Summary of the period and percent variance for reconstructed components 

from the singular spectrum analysis .......................................................................................53
vii

Conversion Factors

Inch/Pound to SI

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square mile (mi2

) 259.0 hectare (ha)

Volume

acre-foot (acre-ft) 1,233 cubic meter (m3

)

Flow rate

acre-foot per year (acre-ft/yr) 1,233 cubic meter per year (m3

/yr)

cubic foot per second (ft3

/s) 0.02832 cubic meter per second (m3/s)

gallon per minute (gal/min) 0.06309 liter per second (L/s)

gallon per day (gal/d) 0.003785 cubic meter per day (m3

/d)

million gallons per day (Mgal/d) 0.04381 cubic meter per second (m3

/s)

billion gallons per day (Ggal/d) 43.81 cubic meter per second (m3

/s)

inch per year (in/yr) 25.4 millimeter per year (mm/yr)

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

 °F=(1.8×°C)+32

Temperature in degrees Fahrenheit (°F) may be converted to degrees Celsius (°C) as follows:

 °C=(°F-32)/1.8

Vertical coordinate information is referenced to the National Geodetic Vertical Datum of 1929 

(NGVD 29).

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD 83).

Altitude, as used in this report, refers to distance above the vertical datum.

*Transmissivity: The standard unit for transmissivity is cubic foot per day per square foot times 

foot of aquifer thickness [(ft3

/d)/ft2]ft. In this report, the mathematically reduced form, foot 

squared per day (ft2/d), is used for convenience.

Specific conductance is given in microsiemens per centimeter at 25 degrees Celsius (μS/cm at 

25 °C).

Concentrations of chemical constituents in water are given either in milligrams per liter (mg/L) 

or micrograms per liter (μg/L).
viii

Acronyms

AMO Atlantic Multidecadal Oscillation

ENSO El Niño Southern Oscillation

ET Evapotranspiration

GCRASA Gulf Coast Regional Aquifer-System Analysis

GCM Global Climate Model

NAMS North American Monsoon System

MERAS Mississippi Embayment Regional Aquifer Study

MNW Multi-Node Well

PDO Pacific Decadal Oscillation

RASA Regional Aquifer-System Analysis

SFR Streamflow Routing

USACE U.S. Army Corps of Engineers

USGS U.S. Geological Survey

3D Three Dimensional
Groundwater Availability of the Mississippi Embayment

By Brian R. Clark, Rheannon M. Hart, and Jason J. Gurdak

Executive Summary

The study described in this report, initiated by the 

U.S. Geological Survey in 2006, was designed to evaluate 

available fresh groundwater reserves within the Mississippi 

embayment, South-Central United States, as an area within 

a broader national assessment. The goals of the national 

assessment are to document effects of human activities on 

water levels and groundwater storage, explore climate variability effects on the regional water budget, and evaluate the 

adequacy of data networks at a regional scale. The Mississippi 

embayment was chosen as a study area in the national assessment because of the substantial dependency on groundwater 

for agriculture and municipal needs. To provide information 

to stakeholders addressing the groundwater-availability issues, 

the U.S. Geological Survey Groundwater Resources Program 

supported a detailed assessment of groundwater availability 

through the Mississippi Embayment Regional Aquifer Study 

(MERAS). This assessment included (1) an evaluation of how 

these resources have changed over time through the use of 

groundwater budgets, (2) development of a numerical modeling tool to assess system responses to stresses from future 

human uses and climate trends, and (3) application of statistical tools to evaluate the importance of individual observations 

within a groundwater-monitoring network.

This study focuses on multiple spatial and temporal 

scales to examine changes in groundwater pumping, storage, 

water-level declines, and sources. The regional scale provides 

a broad view of the sources and demands on the system with 

time. The principal aquifer scale provides an evaluation of the 

changes invoked on the system by differing demands. Lastly, 

the local scale provides a closer examination of the interaction 

between different aquifers and confining units and the changes 

in these interactions under current pumping conditions. By 

focusing on multiple scales, water-resource managers may 

utilize this study to understand system response to changes as 

they affect the system as a whole.

Background and Major Issues

The MERAS area encompasses approximately 

78,000 square miles in an area known as the Mississippi 

embayment. The study area includes parts of 8 States and 

10 primary hydrogeologic units. These hydrogeologic units 

include two primary aquifers—the Mississippi River Valley 

alluvial aquifer and the middle Claiborne aquifer.

Arkansas ranks first in the Nation for rice and third for 

cotton production, with both crops dependent on groundwater as a major source of irrigation requirements. Multiple 

municipalities rely on the groundwater resources to provide 

water for industrial and public use, which includes the city 

of Memphis, Tennessee. The demand for the groundwater 

resource has resulted in groundwater availability issues in the 

Mississippi embayment including: (1) declining groundwater levels of 50 feet or more in the Mississippi River Valley 

alluvial aquifer in parts of eastern Arkansas from agricultural 

pumping, (2) declining groundwater levels of over 360 feet 

over the last 90 years in the confined middle Claiborne aquifer 

in southern Arkansas and northern Louisiana from municipal 

pumping, and (3) litigation between the State of Mississippi 

and a Memphis water utility over water rights in the middle 

Claiborne aquifer.

Population density within the Mississippi embayment 

is greatest in several large urban areas such as Memphis, 

Tennessee, Pine Bluff, Arkansas, and Jackson, Mississippi. 

Increases in population and the movement of people from 

rural to urban areas have increased the need for large pumping centers. Because of continuing groundwater demand and 

the resulting concentration of withdrawals, groundwater levels 

have declined sharply in selected areas. Population increased 

2.4 percent in the study area from 2000 to 2005; the total 

population for 2005 was approximately 5,206,000.

An estimated 12 million acre-feet per year (11 billion 

gallons per day) of groundwater was pumped in 2005 from 

aquifers in the Mississippi embayment. Irrigation constitutes 

the largest groundwater use, accounting for approximately 

10 million acre-feet per year (9 billion gallons per day) in 

2000 from the Mississippi River Valley alluvial aquifer in 

Arkansas, Louisiana, Mississippi, and Missouri, and to a lesser 

extent in Illinois, Kentucky, and Tennessee. The proportion 

of groundwater use in the Mississippi embayment is reflected 

in the pumpage amounts used in a groundwater-flow model 

of the MERAS area. Cumulative groundwater pumpage from 

predevelopment through 2007 from the Mississippi River 

Valley alluvial aquifer amounts to over 280 million acre-feet 

(enough to cover the entire study area with more than 5 feet 

of water), or approximately 87 percent of the total cumulative 
2 Groundwater Availability of the Mississippi Embayment

amount pumped. The amount pumped from the Mississippi 

River Valley alluvial aquifer is more than nine times that from 

the middle Claiborne aquifer, which is the second largest producer of groundwater in the Mississippi embayment. Cumulative groundwater pumped from the middle Claiborne aquifer 

made up approximately 9 percent of the total, while combined 

pumping from remaining aquifers made up 4 percent of the 

total cumulative amount.

Groundwater Budget

Predevelopment groundwater flow is represented in the 

MERAS model as a steady-state stress period, assumed to 

be prior to 1870. The simulated groundwater-flow budget 

indicates the largest predevelopment inflow to the system is 

net recharge to the alluvial aquifer. This inflow is balanced 

by outflow to gaining streams. Overall, water enters as net 

recharge to the alluvial aquifer or through outcrop areas of the 

various hydrogeologic units. Away from the outcrop areas, 

groundwater flow in the deeper formations is primarily upward 

into overlying units, ultimately discharging to streams through 

the alluvial aquifer.

Total net recharge and discharge (sum of inflows or outflows) for the model ranged from about 0.66 million acre-feet 

per year during predevelopment to 20.16 million acre-feet per 

year by the end of the simulation (final simulated irrigation 

period in summer of 2006). This change in the model budget 

reflects increases in withdrawals compared to predevelopment 

conditions. Cumulative storage within aquifers simulated in 

the MERAS model indicates overall depletion of 140 million 

acre-feet (equivalent to 2.8 feet of water covering the entire 

study area). An estimate of the volume of freshwater stored 

in the alluvial aquifer can be made by calculating the thickness of the saturated zone (simulated spring 2007 water table 

minus bottom of the alluvial aquifer as defined by the MERAS 

model) and multiplying by the specific yield (ranging from 

0.1 to 0.3). The volume of freshwater currently (2007) stored 

in the alluvial aquifer was estimated to be approximately 

536 million acre-feet (equivalent to 10.7 feet of water covering the entire study area). The total amount of water removed 

from aquifer storage to date in the Mississippi embayment 

equals approximately 26 percent of the amount stored only 

in the alluvial aquifer. This comparison is provided to gain 

perspective on the quantity of water pumped from aquifers in 

the Mississippi embayment. It is unrealistic to assume that all 

water may be removed by pumping. To do so would substantially affect the performance of wells (the efficiency of pumping is reduced as water levels decline), flow to streams, and 

likely would affect water quality.

In the Mississippi embayment, groundwater pumping 

has produced water-level declines across large areas. The 

change in water level from the predevelopment condition was 

calculated using the simulated water levels of the alluvial 

and middle Claiborne aquifers. The water levels representing 

the predevelopment conditions for the alluvial and middle 

Claiborne aquifers were subtracted from the respective water 

levels at the end of each stress period. Changes in the water 

level in the alluvial aquifer are noticeable throughout Arkansas 

and parts of Mississippi; by 2007, 0.7 percent of the alluvial 

area (216 square miles) exhibit declines of more than 100 feet. 

Changes in water level in the middle Claiborne aquifer are 

more dramatic than changes in the alluvial aquifer, with more 

than 13.3 percent of the aquifer area (7,529 square miles) 

exhibiting water-level declines of more than 100 feet by 2007. 

The largest declines of more than 300 feet in water level in 

the middle Claiborne aquifer occur in southern Arkansas and 

northern Louisiana. Most areas within the middle Claiborne 

aquifer boundary are simulated as confined, and therefore, 

given a relatively small amount of pumping compared to the 

alluvial aquifer, water levels decline dramatically. However, 

because of the smaller amounts of removed water and the 

confined nature of the aquifer, a rebound of water levels would 

be likely if pumping ceased in these areas. This is the situation 

in the middle Claiborne aquifer in southern Arkansas where a 

concerted effort to reduce pumping has resulted in water-level 

increases of almost 50 feet since 2004. The reverse is typically true of unconfined aquifers, like the alluvial aquifer, in 

that although pumping has not resulted in water-level changes 

as great as those seen in the middle Claiborne aquifer, much 

more water has been removed. Therefore, a much longer 

period of time would be required to replenish the aquifer if 

pumping ceased in the alluvial aquifer.

Postdevelopment inflow to the system is still through net 

recharge to the alluvial aquifer and the outcrop areas of the 

several hydrogeologic units, however, the flow between each 

unit is no longer upward to the alluvial aquifer. Groundwater 

flow during the summer of 2006 was primarily downward to 

offset demand from pumping. Early in the model simulation 

(1870–1920s), the primary components of the water budget 

were simulated as outflow from stream leakage and inflow 

from net recharge. As pumpage increased through time, water 

that would otherwise flow to streams reversed, and net stream 

leakage became an inflow to the system. The largest reversals 

began in the mid-1980s, but indications of the reversal began 

in the early 1960s with a trend in loss of streamflow leakage 

coupled with the first consistent inflow from storage. While 

groundwater pumped out of the alluvial aquifer was derived 

primarily from storage, pumpage out of the middle Claiborne 

aquifer was derived primarily from other aquifers (up to 

15 percent from the alluvial aquifer), followed by flow from 

storage and net recharge.

Climate Trend Evaluation

The potential consequences of climate change have been 

identified as a major concern facing the sustainability of the 

Nation’s groundwater resources. To address this concern, 

two climate simulations were developed through the use of 

the MERAS model by extending the simulation period by 

30 years to the year 2038 using extrapolated precipitation 
Introduction 3

based on frequency analysis of historic climate cycles. For the 

climate simulations, grids of extrapolated precipitation were 

input into the MERAS model and converted into extrapolated 

net recharge.

The first climate scenario (dry scenario) simulated with 

the MERAS model was designed to represent a primarily dry 

period with respect to precipitation. The change in precipitation at Memphis, Tennessee, was more highly correlated to 

change in pumping than other locations, indicating that the 

amount of pumping may be estimated based on changes in 

precipitation. Based on this relation, pumping was estimated 

from 2005 through 2038 that would result from an extrapolated change in precipitation.

The second climate scenario (wet scenario) was constructed to provide a contrast to the dry period simulated with 

the first scenario by incorporating precipitation from one of 

the wettest periods (1991) occurring within the study area. 

Precipitation from 1991 was used as the input for the calculation of net recharge while maintaining the pumping used in 

the dry scenario so that a direct comparison of the effect of 

changes in precipitation could be made.

There is little difference between the dry and wet scenarios in terms of percent water-level change. Both scenarios 

resulted in 14.6 to 13.9 percent of the area containing more 

than 100 feet of decline, 14.5 to 13.8 percent containing 

between 75 and 100 feet of decline, and 15.8 to 15.7 percent 

containing 51 to 75 feet of decline in the alluvial aquifer. 

The middle Claiborne aquifer water-level changes also were 

similar between the two scenarios. These scenarios indicate 

that even with a 25-percent increase in precipitation from that 

of the dry scenario, there is little difference in the resultant 

water levels. This is in large part because of the magnitude 

of differences between changes in net recharge and changes 

in pumping. When compared to the volume of water pumped 

out of the system, the effect of this change in net recharge is 

negligible.

Groundwater Monitoring Evaluation

The groundwater-level monitoring network used to 

construct the 2007 middle Claiborne aquifer potentiometric 

surface was used as an example case to demonstrate statistical technique and to evaluate the importance of individual 

groundwater-level observations. To calculate the importance 

of each water-level observation to a prediction, predictions 

were specified as water-level altitudes near the end of the 

dry scenario simulation. These predictions were located near 

the center of cones of depression. Many of the observations 

that have a high importance are in close proximity to stressed 

areas of the aquifer. Some, however, rank higher than might 

be expected based on location, such as the high relative 

importance of observations in Mississippi in the southeastern part of the study area. The reasons for the high relative 

importance of these observations in Mississippi are two-fold: 

(1) the observations and the nearby predictions share high 

sensitivity to common parameters, and (2) the observations 

are in an area of sparse observations making the sensitivities 

higher than observations in areas where multiple observations 

exist. The results also indicate relative low importance of 

many of the observations in southern Arkansas and northern 

Louisiana. However, the ranking of these observations are 

based solely on the predictions used in this evaluation. It is 

important to note that this analysis reflects the importance 

of observations under the current stress conditions in this 

particular model. The addition of high volume pumping 

centers in a relatively unstressed area of the middle Claiborne 

aquifer may alter these results. This underscores the ability of 

the technique to evaluate a groundwater-monitoring network 

under a variety of situations.

Introduction

Water resources in the Mississippi embayment area have 

a profound effect on the economy, which is based largely on 

agriculture. Regarding agricultural commodity acreages within 

the Mississippi embayment, Arkansas ranks first in the Nation 

for rice and third for cotton with a total value of over $2.7 billion, and Mississippi ranks first for aquaculture and fourth 

for cotton in 2007 with a total value of over $600 million 

(U.S. Department of Agriculture, 2010). These commodities 

rely on groundwater from the Mississippi River Valley alluvial 

aquifer, as well as other regional aquifers in the Mississippi 

embayment. Pumping from the Mississippi River Valley alluvial aquifer ranked third in the Nation for total withdrawals or 

12 percent of the total groundwater withdrawals (Maupin and 

Barber, 2005). As with other areas within the United States, 

the populace of the Mississippi embayment is concerned with 

declining water levels and depletion of regionally important 

aquifers. The greatest issue of concern is the availability of 

the resource in particular areas within the Mississippi Embayment Regional Aquifer Study (MERAS), which occasionally 

has brought on litigation. Availability issues in the Mississippi embayment include: (1) declining groundwater levels of 

50 ft or more in the Mississippi River Valley alluvial aquifer 

in parts of eastern Arkansas (Schrader, 2008), (2) declining 

groundwater levels of more than 360 ft over the last 90 years 

in the confined middle Claiborne aquifer in southern Arkansas 

and northern Louisiana (Freiwald and Johnson, 2007), and 

(3) litigation between the State of Mississippi and a Memphis 

water utility over water rights in the middle Claiborne aquifer 

(Charlier, 2010).

A holistic analysis of groundwater-flow systems is 

increasingly important. Previous studies included the use of 

models to simulate the groundwater-flow system on a local 

scale, such as a part of an aquifer, an entire aquifer, or even 

parts of multiple aquifers. While it is useful to examine parts 

of the system at local scales, there is a need to look at the 

larger regional and aquifer scale system to better understand how all the parts interact. To provide information to 
4 Groundwater Availability of the Mississippi Embayment

stakeholders addressing the groundwater-availability issues, 

the U.S. Geological Survey Groundwater Resources Program supported a detailed assessment of groundwater availability through MERAS. This assessment included (1) an 

evaluation of how these resources have changed over time 

through the use of groundwater budgets, (2) development 

of a numerical modeling tool to assess system responses to 

stresses from future human uses and climate trends, and (3) 

application of statistical tools to evaluate the importance of 

individual observations within a groundwater-monitoring 

network.

Purpose and Scope

The purpose of this Professional Paper is to describe 

groundwater availability and factors that affect its sustainability in the Mississippi embayment. This is one of a series 

of reports documenting results of the MERAS. A companion 

report by Hart and others (2008) describes the hydrogeologic framework while Clark and Hart (2009) describes 

the construction and calibration of the groundwater-flow 

(MERAS) model developed for this study. This Professional 

Paper includes a description of the historical background of 

the hydrologic system, analyses of transient water budgets, 

effects of climate on the groundwater system, and evaluation 

of a groundwater-monitoring network. The water budgets, 

effects of climate, and evaluation of the monitoring network 

were interpreted through use of the groundwater-flow model 

developed for this study (Clark and Hart, 2009). The assessment builds on previous work that has taken place across the 

area for many years by Federal, State, and local agencies at 

a variety of different scales. The focus of analysis is directed 

primarily at two principal aquifers over the 137 years of recent 

history.

Study Area Characteristics

The study area encompasses approximately 78,000 mi2 in 

an area known as the Mississippi embayment and corresponds 

to the MERAS model area (fig. 1). The study area boundary 

encompasses parts of 8 States and includes approximately 

6,900 linear miles of simulated streams, 70,000 well locations, 

and 10 primary hydrogeologic units. These hydrogeologic 

units include two primary aquifers—the Mississippi River 

Valley alluvial aquifer and the middle Claiborne aquifer (Hart 

and others, 2008). The study area lies within parts of three 

physiographic sections (West Gulf Coastal Plain, East Gulf 

Coastal Plain, and Mississippi Alluvial Plain) of the Coastal 

Plain physiographic province (fig. 1).

Climate

Climate within the study area ranges from humid, 

temperate in the northern part to humid, subtropical in the 

southern part. Precipitation is usually greater in the southern 

part of the Mississippi embayment (approximately 56 in/yr) 

than in the northern part (approximately 48 in/yr). Precipitation is distributed fairly evenly throughout the year with the 

greatest amounts generally occurring in April and the least 

in October (Kleiss and others, 2000). The average temperature ranges from 58oF in the north to 66oF in the south 

(Cushing and others, 1970; National Oceanographic and 

Atmospheric Administration, 2009a). Much of the precipitation is consumed by evapotranspiration. Another large 

part runs off to the many streams in the study area (fig. 2). 

Climate within the study area is known to contain modes of 

natural climate variability including seasonal, interannual, 

interdecadal, and multidecadal climate variability (McCabe 

and others, 2004). Some of these modes of variability are 

likely manifested in a variety of responses in the surface and 

subsurface hydrology.

Natural-climate patterns that are known to have substantial effect on the hydrology across the United States include 

the El Niño-Southern Oscillation (ENSO), North American 

Monsoon System (NAMS), Pacific Decadal Oscillation 

(PDO), and Atlantic Multidecadal Oscillation (AMO), with the 

ENSO having the strongest effects on seasonal and year-toyear variations in precipitation in the southeast (U.S. Global 

Change Research Program, 2010). In the Southeastern United 

States, above-normal precipitation and below-normal temperatures occurred in approximately 80 percent of the summer events during ENSO years (National Drought Mitigation 

Center, 2010). These climate patterns can augment or diminish 

human stresses on groundwater, causing dramatic responses 

in groundwater levels and storage (Hanson and others, 2004). 

Much of the previous research of hydrologic response to 

ENSO, PDO, and AMO have centered on the Western United 

States, and relatively little is known about the groundwater 

responses in aquifers of the Southeastern United States or the 

study area.

Cumulative departure from average precipitation within 

the MERAS model area indicates that climate can vary spatially and temporally. Temporally, there are five distinct wet 

periods and six distinct dry periods that occurred from 1895 

to 2008 (fig. 3). There is an overall drying trend starting 

at 1895 and continuing through 1943 with a slight 5-year 

wet period from 1918 to 1923. Between 1923 and 1970, the 

climate fluctuates slightly with periods of both wet and dry; 

however, after 1970 there is an overall 30-year wet period 

until 2004 where the climate seems to be shifting into a drier 

period. Spatially, there are differences between the north 

and south, as well as, the east and west. A dry period exists 

between 1961 and 1970 for the northeastern site (Memphis, 

Tennessee) and the western site (El Dorado, Arkansas) and 

a wet and dry period for the southeastern site (Jackson, Mississippi) for the same time period. A wet period exists from 

1983 to 1987 for the southeastern site (Jackson, Mississippi) 

and the northeastern site (Memphis, Tennessee), while the 

southwestern (El Dorado, Arkansas) site fluctuates with 

periods of both wet and dry with an overall wetting trend 

(fig. 3).
Introduction 5

Physiographic province sections from Fenneman and Johnson, 1946

City populations from U.S. Census Bureau, 2009

! !

!

ARKANSAS !

MISSISSIPPI

MISSOURI

LOUISIANA

ALABAMA

TENNESSEE

KENTUCKY

ILLINOIS

TEXAS

94° 93° 92° 91° 90° 89° 88° 87°

38°

37°

36°

35°

34°

33°

32°

31°

Mississippi River

EXPLANATION

Sections of Coastal Plain Physiographic Province

 West Gulf Coastal Plain

 Mississippi Alluvial Plain

 East Gulf Coastal Plain

Map area

Location map

TEXAS

OKLAHOMA

KANSAS MISSOURI

ILLINOIS

KENTUCKY

LOUISIANA

ALABAMA

TENNESSEE MISSISSIPPI

ARKANSAS

0 50 100 MILES

0 50 100 KILOMETERS

Study area boundary—Corresponds to boundary

 of Mississippi embayment

Facies transition zone—Lower Claiborne aquifer 

 and lower Claiborne confining unit in south;

 middle Claiborne aquifer in north

!

!

!

Memphis

(650,100)

Pine Bluff

(55,085)

Monroe

(53,107)

Jackson

(184,256)

El Dorado

(21,530)

Little Rock

(183,133)

Jonesboro

(55,515)

! City location and 2000 population (U.S. Census Bureau, 2009) (55,515)

HOLMES

COUNTY

Base from U.S. Geological Survey digital data, 1:100,000

Figure 1. Location of the study area and Coastal Plain physiographic province sections.
6 Groundwater Availability of the Mississippi Embayment

River

ARKANSAS

MISSISSIPPI

MISSOURI

LOUISIANA

ALABAMA

TENNESSEE

KENTUCKY

TEXAS

ILLINOIS

94° 93° 92° 91° 90° 89° 88° 87°

38°

37°

36°

35°

34°

33°

32°

31°

0 50 100 MILES

0 50 100 KILOMETERS

Red River

Bayou

Bayou Macon

Bogue River

Black River

St. Francis River

Obion

River

Fork

North S Fork Obion R

North Fork Deer R

Forked Deer River

Hatchie River

Middle Forked Deer R

Wolf

River

River

Mississippi River

Mississippi River

Right Hand Chute Little River

Cache River

L’Anguille River

Coldwater

Little Tallahatchie River

Yalobusha River

Sunflower River

Tallahatchie

River

Yazoo River

Big Black River

Pearl River

Chickasawhay River

Leaf River

Tombigbee River

Alabama River

Smackover Creek

Bayou D’Arbonne

Red River

Bodcau

Dorcheat Bayou

Dugdemona River

Bouef River

Arkansas River

Bayou Meto

White River

Saline River

Bayou Bartholomew

Cornie Bayou

Loosahatchie

Nonconnah R

Ouachita River

Study area boundary

Stream simulated in model

EXPLANATION

Base from U.S. Geological Survey digital data, 1:100,000

Figure 2. Streams simulated in the study area.
Introduction 7

Year

Jackson, Mississippi

El Dorado, Arkansas

Memphis, Tennessee

Little Rock, Arkansas

Jonesboro, Arkansas

Dry Wet Dry Wet Wet Dry Dry Wet Dry Wet Dry

0

-150

-100

-50

50

100

150

200

0

-150

-100

-50

50

100

150

200

1890 1910 1930 1950 1970 1990 2010

1890 1910 1930 1950 1970 1990 2010

Year

Dry Wet Dry Wet Wet Dry Dry Wet Dry Wet Dry

Cumulative departure, in inches Cumulative departure, in inches

Study area

EXPLANATION

EXPLANATION

Figure 3. Cumulative departure from average precipitation within the study area and at selected locations, showing dry and wet 

periods for the study area.
8 Groundwater Availability of the Mississippi Embayment

Population

Population density within the Mississippi embayment is 

greatest in four large urban areas. Based on 2000 U.S. Census 

data (Tele Atlas North American, Inc., 2008), 67 percent of the 

total population is located in urban areas. The most densely 

populated areas are Jonesboro, Arkansas, Monroe, Louisiana, 

Jackson, Mississippi, and Memphis, Tennessee (fig. 1). Population increased 2.4 percent in the study area from 2000 to 

2005; the total population for 2005 was approximately 5.2 million (Tele Atlas North American, Inc., 2008). The increase in 

population overall and the movement of people from rural to 

urban areas have expanded the occurrence of large pumping 

centers, which can result in declining groundwater levels with 

measureable cones of depression.

Land Use

Land use in the Mississippi embayment is primarily agricultural. Cultivated cropland accounts for approximately 45 percent of the study area, forested land 38 percent, 

water 14 percent, and urban land 3 percent of the total area 

(U.S. Geological Survey, 2008b) (fig. 4). The greatest density 

of cropland lies within the area covered by the Mississippi 

Alluvial Plain (fig. 1). About 7 percent of the Mississippi Alluvial Plain is used for rice, 22 percent for cotton, 35 percent for 

soybeans, 5 percent for corn and wheat, 10 percent for pasture, 

and the rest for other crops or nonagricultural land (Stuart and 

others, 1996). The largest urban area is the city of Memphis, 

Tennessee (fig. 4), which covers about 280 mi2 and historically 

has relied heavily on groundwater pumpage to meet municipal 

requirements.

Water Use

An estimated 12 million acre-ft/yr (11 Ggal/d) of 

groundwater was pumped in 2005 in the Mississippi embayment (U.S. Geological Survey, 2010a). Groundwater use for 

irrigation is the largest use, accounting for approximately 

10 million acre-ft/yr (9 Ggal/d) of groundwater pumped in 

2000 from the Mississippi River Valley alluvial aquifer in 

Arkansas, Louisiana, Mississippi, and Missouri, and to a 

lesser extent in Illinois, Kentucky, and Tennessee (Maupin 

and Barber, 2005). In Arkansas, the largest user of groundwater in the Mississippi embayment, 93 percent of groundwater 

withdrawals in 2005 were for irrigation. Surface irrigation is 

the predominant application method in Arkansas, Louisiana, 

Mississippi, and Missouri (Hutson and others, 2004; Kenny 

and others, 2009). Groundwater pumping for irrigation is 

seasonal, beginning as early as April and extending through 

September in some years. Public supply and commercial/

industrial water use can be provided by groundwater and 

surface-water sources. Approximately 89 percent of public 

supply water use in Mississippi is provided by groundwater, compared to 34 percent in Arkansas and 49 percent in 

Louisiana (Kenny and others, 2009). The estimated use of 

groundwater for public supply in Shelby County, Tennessee

(location of Memphis, Tennessee) was over 200,000 acre-ft/yr 

(180 Mgal/d) in 2005, and groundwater is the sole source for 

public supply in that county. Aquaculture is a large commercial business in Arkansas, Louisiana, and Mississippi. 

Of the total withdrawal of water for aquaculture use in these 

states, 82 percent is from groundwater (Kenny and others, 

2009). Some of the larger users of water for industrial use in 

Arkansas, Louisiana, and Mississippi include those manufacturing paper products and chemicals (Johnson, 1993; Holland, 

2007; Sargent, 2007). Arkansas and Mississippi use approximately 1.6 to 1.7 times more surface water than groundwater 

for industrial use in contrast to Louisiana, which uses over 

10 times more (Kenny and others, 2009).

Groundwater use in the Mississippi embayment is 

reflected in the pumpage amounts used in the MERAS 

model. A brief description of the water-use compilation 

for the MERAS model is included in the Data Compilation section. For additional information see Clark and Hart 

(2009). Cumulative groundwater pumpage to 2007 from the 

Mississippi River Valley alluvial aquifer amounts to over 

280 million acre-ft (enough to cover the entire study area in 

more than 5 ft of water), or approximately 87 percent of the 

total cumulative amount pumped (fig. 5). The amount from 

the Mississippi River Valley alluvial aquifer is more than 

nine times that of the middle Claiborne aquifer, which is the 

second largest producer of groundwater in the Mississippi 

embayment. Cumulative groundwater pumping from the 

middle Claiborne aquifer makes up approximately 9 percent 

of the total cumulative amount, while combined pumping 

from remaining aquifers makes up 4 percent of the total 

cumulative amount.

Geologic Setting

The Mississippi embayment aquifer system lies within 

a plunging syncline with the axis primarily trending southward roughly parallel to the Mississippi River (Hart and others, 2008). The plunging syncline is a result of downwarping 

and rifting related to the Ouachita orogeny, which formed a 

deep catch basin for subsequent sedimentation. Downwarping and downfaulting proceeded further as a response to 

the weight of sediment accumulation (Hosman, 1996). This 

deep catch basin started infilling with Jurassic sediments, 

followed by Cretaceous marine deposits, and finally Cenozoic deposits, which constitute the main water-bearing units 

of importance within the Mississippi embayment. Cyclic 

invasions by transgressing and regressing seas through the 

Cretaceous and Tertiary Periods created the synclinal shape 

resulting in older rock units cropping out on the periphery 

of the embayment (Arthur and Taylor, 1998). The units 

exposed within the study area (fig. 6) are Cenozoic in age 

and consist primarily of Tertiary and Quaternary sands and 

gravels, silts, and clays.
Introduction 9

0 50 100 MILES

0 50 100 KILOMETERS

! ARKANSAS

MISSISSIPPI

MISSOURI

LOUISIANA

ALABAMA

TENNESSEE

KENTUCKY

ILLINOIS

TEXAS

Base from U.S. Geological Survey digital data, 1:100,000 Land use from U.S. Geological Survey, 2008b

Memphis

38°

37°

36°

35°

34°

33°

32°

31°

94° 93° 92° 91° 90° 89° 88° 87°

EXPLANATION

Land use

Water

Urban

Barren

Natural forest

Cultivated crops

Wetlands

Study area boundary

Figure 4. Land use within the Mississippi embayment.
10 Groundwater Availability of the Mississippi Embayment

300

250

200

150

100

50

0

1870 1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2007

Cumulative groundwater pumpage,

in millions of acre-feet

Year

Mississippi River Valley alluvial aquifer

Upper Claiborne aquifer

Middle Claiborne aquifer

Winona-Tallahatta aquifer

Lower Claiborne aquifer

Middle and lower Wilcox aquifers

EXPLANATION

Figure 5. Cumulative groundwater pumpage by aquifer used in the Mississippi Embayment Regional Aquifer Study model.

Hydrogeologic Units

The primary hydrogeologic units simulated in the 

MERAS model include 10 units described by Hart and others (2008) and four additional minor hydrogeologic units 

described in Clark and Hart (2009). The units described 

by Hart and others consist of the Mississippi River Valley 

alluvial aquifer (hereafter referred to as the alluvial aquifer), 

the Vicksburg-Jackson confining unit, the upper Claiborne 

aquifer, the middle Claiborne confining unit, the middle 

Claiborne aquifer, the lower Claiborne confining unit, the 

lower Claiborne aquifer, the middle Wilcox aquifer, the lower 

Wilcox aquifer, and the Midway confining unit. Two of these 

primary hydrogeologic units are principal aquifers: the alluvial 

aquifer and the middle Claiborne aquifer (Renken, 1998). 

The alluvial aquifer is Quaternary and all other units simulated in the MERAS model are Tertiary (table 1). The alluvial 

aquifer consists primarily of gravel and sand deposits and 

generally contains groundwater under unconfined conditions. 

The middle Claiborne aquifer consists primarily of sand, silt, 

and clay deposits and generally contains groundwater under 

confined conditions. The minor hydrogeologic units described 

by Clark and Hart (2009) are the El Dorado confining unit, the 

El Dorado Sand, the Winona-Tallahatta aquifer, and the Old 

Breastworks confining unit. The El Dorado Sand is the lower 

part of the middle Claiborne aquifer in south-central Arkansas 

and north-central Louisiana. The El Dorado Sand is separated 

by a locally extensive confining unit, termed the El Dorado 

confining unit in this report, of up to 100 ft thick. The WinonaTallahatta aquifer is the lower part of the lower Claiborne 

confining unit throughout much of Mississippi and includes 

the Tallahatta and Winona Formations. Additionally, throughout most of Arkansas and Louisiana, the middle and lower 

Wilcox aquifers are undifferentiated; however, in areas of 

Tennessee and Mississippi, the lower Wilcox aquifer may be 

separated into two units, the lower Wilcox aquifer and the Old 

Breastworks confining unit. The lower Wilcox aquifer consists 

of the lower part of the Wilcox Formation and includes the 

Old Breastworks Formation in Missouri and Tennessee. It is 

the lowermost aquifer in Tertiary rocks within the Mississippi 

embayment (Lloyd and Lyke, 1995) (table 1).

The hydrogeologic units and model framework, as part of 

this study, were a refinement of the framework developed by 

the Gulf Coast Regional Aquifer-System Analysis (GCRASA) 

program. Through the GCRASA program, 1,000 geophysical 

logs were used to develop the hydrogeologic framework of the 

Mississippi embayment, Texas coastal uplands, and the coastal 

lowlands aquifer systems (Hosman and Weiss, 1991) (approximately 300 geophysical logs of the 1,000 used were within the 

Mississippi embayment). The MERAS, focused on the Mississippi embayment area, used over 2,600 geophysical logs to 

develop the hydrogeologic framework (Hart and others, 2008; 

Hart and Clark, 2008). This approximately 3-year effort utilized 

multiple hydrologists in four States and resulted in substantial 

improvement in the resolution of the hydrogeologic units over 

previous studies. Additionally, all geophysical logs used in the 

interpretation of the model framework were archived and placed 

online (http://pubs.usgs.gov/sir/2008/5192/) for the benefit 

of studies that may want to continue refining the knowledge 

gained through the MERAS effort. The same nomenclature 
Introduction 11

Recharge

Mississippi River

Recharge

Water movement

ARKANSAS

LOUISIANA

MISSISSIPPI

TENNESSEE

KENTUCKY MISSOURI

ALABAMA

A A'

A

A'

Mississippi River Valley aquifer aquiferVicksburg-Jackson confining unitUpper Claiborne aquiferMiddle Claiborne confining unitMiddle Claiborne aquiferLower Claiborne confining unitLower Claiborne aquiferMiddle and lower Wilcox aquifers

EXPLANATION

Figure 6. Cross-sectional view of hydrogeologic units within the study area with predevelopment conceptual water movement.
12 Groundwater Availability of the Mississippi Embayment Table 1. Correlation of hydrogeologic units across States within the Mississippi Embayment Regional Aquifer Study.

QUATERNARY

HOLOCENE PLEISTOCENEAlluvium and terrace deposits Alluvium and loess deposits

Vicksburg-Jackson

confining unit

Mississippi River 

Valley alluvial

aquifer

Alluvium,

terrace, and

loess

deposits

Alluvium and 

terrace deposits

Vicksburg Formation Not present in study area

ERATHEM

SYSTEM

EPOCHLOUISIANA ARKANSAS Southern Northeastern MISSOURI KENTUCKY TENNESSEE MISSISSIPPI ALABAMA Hydrogeologic units

Vicksburg

Formation

Vicksburg

OLIGOCENE

Midway confining 

unit

Middle

Claiborne

aquifer 

Lower

Claiborne

confining

unit 

Middle Claiborne 

confining unit 

Lower Wilcox 

aquifer

Upper Claiborne

aquifer

Cockfield

Formation

Cook

Mountain

Formation

Lisbon

Formation

Sparta

Sand

Tallahatta

Formation

Tallahatta

Formation

Tallahatta

Formation

Memphis

Sand

Sparta

Sand

Sparta

SandCaneRiver

Formation

Carrizo

Sand

Dolet

Hills

Formation Fort Pillow

Sand

No Wilcox

deposits

identified as 

being of 

Paleocene age

Wilcox

Formation

Old Breastworks

Formation

Undifferentiated

Undifferentiated

CENOZOIC

TERTIARY

Claiborne

EOCENE

Jackson

Midway Group

Meridian Sand

Member

Hatchetigbee

Formation

Modified from Hart and others, 2008

UPPER PALEOCENE

Wilcox

Gosport Sand

Bashi Formation

Tuscahoma Sand

Nanafalia Formation

Jackson Formation

Undifferentiated

Naborton Formation

Zilpha Clay

Winona Sand

Middle Wilcox 

aquifer

Flour Island

Formation

Memphis

Sand

Fort Pillow

Sand

Old Breastworks

Formation

Flour Island

Formation

GROUP Midway Lower Claiborne aquifer

1 Lower Claiborne aquifer includes the upper Wilcox aquifer in some parts of Mississippi.2 Winona and Tallahatta Formations are included with lower Claiborne confining unit in Hart and others (2008).3 Old Breastworks confining unit is included with middle Wilcox aquifer in Hart and others (2008).4 El Dorado confining unit and El Dorado Sand are included with middle Claiborne aquifer.

Model layer

number

2

34

5–78–9

1011

12–13

1

Base of  model 

2 4

1

3
Methods Overview 13

and hydrogeologic units were used between this study and the 

GCRASA study with an effort to adjust any inconsistencies 

between hydrogeologic units from the previous GCRASA 

study.

Previous Investigations

Previous investigations of groundwater flow in the 

embayment are numerous. Some early examples were the 

1906 investigation of the underground waters of northern 

Louisiana (Veach, 1906) and a 1928 investigation of groundwater resources of Mississippi (Stephenson and others, 1928). 

The U.S. Geological Survey (USGS) began an effort in 1978 

to “provide a regional understanding and assessment of the 

Nation’s ground-water resources” (Bennett, 1979). This 

endeavor, known as the Regional Aquifer-System Analysis 

(RASA) Program, provided studies on the Nation’s most 

important aquifer systems. The Mississippi embayment aquifer system was analyzed as part of the GCRASA study. The 

GCRASA study produced several USGS Professional Papers 

(Grubb, 1998; Hosman and Weiss, 1991; Weiss, 1992; Ackerman, 1996; Ryder and Ardis, 2002; Williams, 2001; Hosman, 1996; Martin and Whiteman, 1999; Arthur and Taylor, 

1998) from several authors. The GCRASA compiled data and 

simulated groundwater flow using groundwater-flow models in three main parts: the Mississippi River Valley alluvial 

aquifer, the Mississippi embayment aquifer system, and the 

Gulf Coastal lowland aquifer system (U.S. Geological Survey, 

2008a). Other reports summarize results of groundwater-flow 

simulations of parts of the Mississippi embayment including Reed (1972), Brahana and Mesko (1988), Fitzpatrick and 

others (1990), Mahon and Ludwig (1990), Sumner and Wasson (1990), McWreath and others (1991), Kilpatrick (1992), 

Mahon and Poynter (1993), Ackerman (1996), Arthur and Taylor (1998), Hays and others (1998), Arthur (2001), Brahana 

and Broshears (2001), McKee and Clark (2003), Stanton and 

Clark (2003), and Reed (2003).

Methods Overview

The primary method used to evaluate the conceptual 

groundwater flow of predevelopment and changes in postdevelopment groundwater flow, effects of climate variability, 

and a groundwater-monitoring network was the MERAS 

model (Clark and Hart, 2009). Groundwater-flow budgets 

were analyzed using the computer program ZONEBUDGET 

(Harbaugh, 1990). Additional analysis of the groundwaterflow system was through use of model scenarios to evaluate the effects of climate variability on groundwater levels. 

The scenarios extend the MERAS model simulation through 

2037 and use forecasted precipitation based on frequency 

analysis of historic climate cycles. The computer program 

OPR-PPR (Tonkin and others, 2007) was used to demonstrate an evaluation of the relative importance of individual 

groundwater-level observations in a monitoring network in 

the middle Claiborne aquifer. Such monitoring networks are 

critical indicators of current, past, and future conditions of a 

groundwater resource.

Numerical Model

The construction and calibration of the MERAS model 

developed for this study are documented in Clark and Hart 

(2009). A brief description of the model is provided herein. 

The model was constructed using the modular three-dimensional (3D) finite-difference USGS MODFLOW-2005 code 

(Harbaugh, 2005) along with the preconditioned conjugate 

gradient solver (Hill, 1990). The model is subdivided into a set 

of discrete blocks in space and time. The blocks represent cells 

(with uniform horizontal spacing of 1 mile on a side and variable thickness) of porous material within which the hydraulic 

properties are the same. A set of finite-difference equations 

describes groundwater flow through each cell. These equations 

can be solved to simulate either equilibrium (steady-state) 

conditions or transient conditions, which simulate changes in 

stresses over fixed periods of time. The finite-difference grid 

is oriented north-south and consists of 414 rows, 397 columns, 

and 13 layers. The model simulates 137 years (1870–2007) 

of system response to stress divided into 69 stress periods. 

The first stress period is simulated as steady state to represent predevelopment conditions. Stress periods 2 through 27 

are variable in length to reflect embayment-wide changes in 

groundwater withdrawals. Stress periods 28 (beginning in 

1986) through 69 are each 6 months in length to reflect springsummer (April–September) and fall-winter (October–March) 

conditions related to irrigation (Clark and Hart, 2009). Areal 

recharge is applied throughout the MERAS model area using 

the MODFLOW-2005 Recharge Package (Harbaugh, 2005) 

and represents net recharge (defined as “the entry into the saturated zone of water made available at the water-table surface, 

together with the associated flow away from the water table 

within the saturated zone” (Freeze and Cherry, 1979)) because 

evapotranspiration (ET) was not explicitly simulated. Pumpage from irrigation, municipal, and industrial wells was simulated using the Multi-Node Well (MNW) Package (Halford 

and Hanson, 2002). The MNW Package allows simulation of 

flow in wells that are completed in multiple aquifers or model 

layers. There are 43 streams included within the MERAS 

model. Each stream in the model area was represented using 

the Streamflow-Routing (SFR) Package of MODFLOW (Prudic and others, 2004). The initial criterion for the inclusion of 

streams in the model was a mean annual flow of 1,000 ft3/s 

or more. Other streams were added based on inclusion by 

previous model studies that demonstrated the interaction of 

the streams with surficial aquifers. The perimeter of the model 

area and the base of the flow system are represented as noflow boundaries. The perimeter of the model area represents 

an area where the hydrogeologic units do not exist or where 

flow into or out of the model area is assumed to be negligible. In model simulation the density of water was assumed 
14 Groundwater Availability of the Mississippi Embayment

to remain constant in time throughout the flow system. 

The downdip limit of each model layer is a no-flow boundary, which approximates the extent of water with less than 

10,000 mg/L dissolved solids. The downdip limits of parts of 

the middle Claiborne and lower aquifers terminate north of the 

10,000 mg/L dissolved solids line in an area that approximates 

the freshwater boundary delineated by Payne (1968).

Data Compilation

Data compilation for construction of the numerical 

groundwater-flow model used in this report began in 2006 

(Clark and Hart, 2009). Data collection efforts were focused 

on six main components: (1) the hydrogeologic framework, 

(2) groundwater pumpage (water use), (3) hydraulic-head 

observations, (4) surface-water flows, (5) aquifer properties, and (6) net recharge. The database of information used 

to construct the hydrogeologic framework represented in the 

MERAS model includes over 2,600 geophysical logs (Hart 

and Clark, 2008; Hart and others, 2008). Compilation of the 

groundwater-pumpage data involved the collaboration of 

USGS personnel in seven states. This compilation included 

reported, estimated, and trend analysis of groundwater pumpage for as much as 100 years of data distributed to approximately 70,000 groundwater-well locations. Hydraulic-head 

data for the MERAS model consisted of over 55,000 groundwater-level altitudes from the USGS National Water Information System (http://waterdata.usgs.gov/nwis) and computation 

of 14 streamflow or stream-leakage values. Surface-water 

flows were required in the MERAS model for each of the 

69 stress periods (spanning 137 years) at each point on the 

model boundary that a stream crosses into the active model 

area. Aquifer properties for each hydrogeologic unit were 

evaluated based on available aquifer test information, literature values for similar hydrogeologic units, or from previous 

groundwater-model studies. Net recharge to the outcrop areas 

of all hydrogeologic units was assigned by zone as a fraction 

of precipitation based on typical literature values and soil type 

or geology. These values were modified locally or regionally 

during calibration of the model. Additional information about 

the groundwater-flow model construction can be found in 

Clark and Hart (2009).

Modifications to the Numerical Model

The numerical model documented by Clark and Hart 

(2009) was modified slightly to improve the simulation of 

groundwater flow. These adjustments included decreasing 

the net recharge early in the simulation (predevelopment 

to the 1960s). This reduction in net recharge was necessary 

because it was noted in the model output that too much water 

as net recharge was applied to some aquifers, which caused 

an increase in water going into storage through time and 

higher than observed water levels early in the simulation. The 

simulated increase of water in storage early in the simulation 

is counter to the concept of the system being at steady state 

at that time. A possible justification for this change in the 

conceptual model of the groundwater system is that, prior 

to extensive development, recharge entered the aquifer and 

because water levels were likely within a few feet of land 

surface, ET removed much of the groundwater, maintaining steady-state conditions within the groundwater system. 

As pumping increased in the Mississippi embayment, water 

levels dropped and ET rates diminished, allowing greater 

amounts of net recharge to enter the aquifer. Because ET is 

not explicitly simulated in the MERAS model, the amount 

of net recharge was reduced during the early period of the 

simulation to account for the effect of ET on the groundwater 

system.

The modifications to the MERAS model for this report 

did not substantially change the calibration. For example, the 

root mean square error (RMSE) between all (55,786 comparisons) simulated and observed hydraulic heads is 23.24 ft 

compared to 23.18 ft in the original model (Clark and Hart, 

2009). The RMSE for the alluvial aquifer observations is 

16.94 ft and for the middle Claiborne aquifer observations 

is 35.14 ft in the modified model compared to 16.43 ft and 

35.78 ft (Clark and Hart, 2009). Similar to the original model, 

simulated streamflow is underpredicted for 18 observations 

and overpredicted for 10 observations in the modified model. 

In the modified model, 4.2 percent of streamflow was gained 

from the groundwater system into a segment of the White 

River for 1998, the same as the original model (Clark and 

Hart, 2009). The water budgets for the two models were similar during the summer 2006. Total flows (sum of inflows or 

outflows) were 20.16 million acre-ft/yr in the modified model 

compared to 20.39 million acre-ft/yr in the original model 

(Clark and Hart, 2009).

Development of the Hydrologic System

The following sections describe the conceptual model 

of predevelopment groundwater flow and postdevelopment 

history of the hydrologic system. Anthropogenic effects of 

pumping and large-scale engineering of the hydrologic system 

has reversed groundwater flow in some areas.

Predevelopment Conditions

Groundwater-flow direction and magnitude in the study 

area are influenced by structural and depositional characteristics, surface-water bodies, and anthropogenic requirements. 

The synclinal shape of the Mississippi embayment creates 

natural recharge areas where the deeper, confined hydrogeologic units crop out along the east and west side of the 

study area (fig. 6). Conceptual predevelopment groundwater 

movement was into the recharge areas, downdip toward the 

axis of the synclinal structure, and discharged upward through 

overlying units (fig. 6).
Development of the Hydrologic System 15

There are no known predevelopment (nonsimulated) 

potentiometric surfaces for the part of the alluvial aquifer simulated by the MERAS model. Williams and Williamson (1989) 

calculated an average depth to water of 25.7 ft in 6,825 wells 

less than 150 ft deep. Before development of the groundwater 

resource in the early 1900s, water levels in the alluvial aquifer 

were presumed to generally follow land surface and slope 

toward major rivers (Ackerman, 1989).

Predevelopment potentiometric surfaces for the middle 

Claiborne aquifer also are scarce, though some individual 

measurements exist from the first half of the 1900s. Reed 

(1972) presents a potentiometric surface of the middle 

Claiborne for 1886 “based on measurements made prior to 

extensive development.” Anecdotal information and measured 

data indicate artesian conditions in the middle Claiborne aquifer existed in predevelopment and postdevelopment in some 

areas. One example of the potential for artesian conditions in 

the middle Claiborne aquifer (potentiometric head above land 

surface) is in the Holmes County area of Mississippi (fig. 1). 

Wells placed in the lower Claiborne aquifer in this area had 

water levels over 100 ft above land surface in the 1940s to the 

1960s (U.S. Geological Survey, 2010b).

Many streams dissect the Mississippi embayment, ranging in size from the Mississippi River, the second longest 

river in the United States, to numerous small streams (most 

being part of the Mississippi River system). Other major 

streams in the Mississippi embayment include the Arkansas, 

White, Ouachita, Red, Yazoo, and St. Francis Rivers (fig. 2). 

Conceptually, all streams in predevelopment time would 

be gaining (receiving water from the groundwater system) 

because there are no anthropogenic stresses to remove water 

from the system. Therefore, all net recharge entering the 

system increases the water level, causing flow toward the only 

exits in the model (because ET is not explicitly simulated), 

which are the streams. Simulated predevelopment conditions 

indicate that not all streams or all parts of streams are gaining 

(fig. 7). Streams in some areas, including the higher topographical areas along the eastern side of the MERAS model, 

are simulated as losing water to the groundwater system. The 

losing part of streams accounts for approximately 32 percent 

of the steady-state simulated stream cells. The average flow 

from these losing stream cells is 1,400 acre-ft/yr (0.22 ft3

/sec). 

The average flow to gaining stream cells from the aquifer is 

2,000 acre-ft/yr (0.31 ft/3sec). This indicates that, in general, 

streams lose water to the aquifer in higher altitude areas 

and gain water from the aquifer downstream in lower altitude areas. The streams gain slightly more water than is lost 

because of additional net recharge that is added to the system. 

In terms of a predevelopment water budget, this means that 

the inflow of water from losing stream cells plus net recharge 

equals the outflow from gaining stream cells.

Predevelopment groundwater flow is represented in the 

MERAS model within a single steady-state stress period, 

assumed to be prior to 1870. As with the conceptual model 

of predevelopment groundwater flow, water enters as net 

recharge to the alluvial aquifer (Quaternary sediments) or 

through outcrop areas for the other hydrogeologic units. Away 

from the outcrop areas, groundwater flow is primarily upward 

into overlying units, ultimately discharging to streams through 

the alluvial aquifer (fig. 6). The simulated groundwater-flow 

budget indicates that the largest predevelopment inflow to 

the system is net recharge to the alluvial aquifer (fig. 8). This 

inflow is balanced primarily by outflow to gaining streams. 

Net predevelopment recharge to the alluvial aquifer totaled 

567,568 acre-ft/yr, almost six times more than the net predevelopment recharge to the Tertiary aquifers and confining 

units (table 1). Groundwater flow from the Tertiary system to 

the alluvial aquifer totaled an additional 160,649 acre-ft/yr. Most 

of the combined groundwater inflow to the alluvial aquifer 

of 728,217 acre-ft/yr was discharged to streams and a small 

amount (605 acre-ft/yr) discharged back into the Tertiary 

system.

Postdevelopment Conditions

Development of the groundwater system began just 

a few years after the end of the Civil War. Prior to the late 

1800s, most wells were developed in the Quaternary sands and 

gravels at depths ranging as much as 150 ft. Since that time, 

technological advances in deep well pumps and well-drilling 

technology allowed for the expansion of deep wells throughout the Mississippi embayment. As the necessity increased 

and costs decreased, plantations, cotton gins, ice factories, 

cotton-oil mills, and cotton factories justified the expenditure 

for deep wells. The importance of the deeper groundwater also 

began to be associated with the clean supply of potable water, 

or as Veatch (1906) described the “hygienic value of deep 

well waters.” The use of deep wells became popular to avoid 

malaria and typhoid fever carried in shallow wells. Veatch 

(1906) also mentions the dual dependency of some early public water supplies on groundwater and surface water and the 

importance of lumber mills in the development of deep wells. 

As cities became more populated, the rapid development and 

increased fire risks required systems to quickly and efficiently 

deliver groundwater. Finally, the railroads and lumber industry, associated with rapid development, required large amounts 

of a steady groundwater supply (Veatch, 1906).

Agricultural activities have increased since the early 

1920s and are important to the economy of the Mississippi 

embayment. Engler and others (1945) note commercial rice 

farming in the Grand Prairie area of Arkansas beginning in 

1904. However, the increased agricultural activities have 

necessitated a greater need for groundwater withdrawals. 

Since the early 1900s, water levels in many wells within the 

alluvial aquifer in the Grand Prairie area (Arkansas, Prairie, 

and Lonoke Counties, Arkansas) have declined substantially 

(Schrader, 2008). A comparison of two potentiometric-surface 

maps of the alluvial aquifer in the Grand Prairie from 1929 

and 2006 shows declines of as much as 43 ft in the center of 

the cone of depression and an apparent decline of 100 ft in 

central Lonoke County (fig. 9).
16 Groundwater Availability of the Mississippi Embayment

ARKANSAS

MISSISSIPPI

MISSOURI

LOUISIANA

ALABAMA

TENNESSEE

TEXAS

ILLINOIS

KENTUCKY

94° 93° 92° 91° 90° 89° 88° 87°

38°

37°

36°

35°

34°

33°

32°

31°

0 25 50 100 MILES

0 50 100 25 KILOMETERS

Base from U.S. Geological Survey digital data, 1:100,000

Streamflow leakage—in thousands of 

 acre-feet per year. Negative values are 

 from the aquifer to the stream, positive 

 values are from the stream to the aquifer.

1.1 to 4.5

0.3 to 1.0

0.1 to 0.2

-0.1 to 0.0

-0.9 to -0.2

-1.2 to -1.0

-4.0 to -1.3

High : 650

Low : 12

Land-surface altitude—in feet

 above National Geodetic

 Vertical Datum, 1929

EXPLANATION

Figure 7. Simulated predevelopment streamflow leakage.
Groundwater Availability, a Water-Budget Perspective 17

In areas of Mississippi, groundwater was considered 

abundant as late as the 1920s. Stephenson and others (1928) 

noted more than 50,000 acre-ft/yr (5 Mgal/d) supplied to 

three cities within Mississippi and the presence of six flowing wells. Stephenson and others (1928) also stated that 

groundwater within the alluvial aquifer in Mississippi “may 

be regarded as practically inexhaustible ...” with well yields 

as much as 4,000 gal/min. This statement is in contrast to a 

quote from a recent Yazoo Mississippi Delta report which 

reads “as water levels continue to decline in the Central Delta, 

the production and efficiency of irrigation wells are being 

threatened” (Yazoo Mississippi Delta Joint Water Management District, 2008).

The largest urban center in the Mississippi embayment 

is the area of Memphis, Tennessee, with a 2000 population 

of 650,100 (U.S. Census Bureau, 2009). The first well in the 

middle Claiborne aquifer in the Memphis, Tennessee, area was 

completed in 1886 (Criner and Parks, 1976). This well was 

artesian at that time. Criner and Parks (1976) also state that 

“Pumpage from [the middle Claiborne] aquifer in 1975 was 

about 188 Mgal/d which is about twice that in 1950, five times 

that in 1920, and ten times that in 1890.”

Groundwater resources of the Mississippi embayment 

have been tapped for human use for over 100 years. Through 

this period, changes in technology and agricultural practices 

have affected the natural groundwater flow and the interaction with surface-water bodies in the area (fig. 10). As the 

groundwater resource was developed for anthropogenic uses, 

agricultural demands for groundwater irrigation from the 

alluvial aquifer and municipal and industrial demands for 

groundwater from the middle Claiborne aquifer created large 

cones of depression in some areas. These cones of depression 

have resulted in a change in the direction of net discharge so 

that the postdevelopment flow is now downward (fig. 10). The 

simulated groundwater-flow budget indicates that the largest postdevelopment inflow to the system is net recharge to 

the alluvial aquifer (fig. 11). Net postdevelopment recharge 

to the alluvial aquifer totaled 2,513,094 acre-ft/yr, over six 

times more than the net postdevelopment recharge to the 

Tertiary aquifers and confining units (table 1). Groundwater 

flow from the alluvial aquifer to the Tertiary system totaled 

501,682 acre-ft/yr.

Streams within the Mississippi embayment generally 

have low gradients, and some have been hydrologically altered 

(channelized, impounded, and diverted) to suit agricultural 

purposes (Justus, 2003). In some instances, impoundments 

may limit streamflow downstream from the impoundment 

so that little if any water is available to recharge the aquifer. 

Surface-water diversions have been suggested as a means to 

alleviate the stress on the underlying aquifers. Projects such as 

the Grand Prairie Area Demonstration Irrigation Project have 

been established by the Memphis District, U.S. Army Corps 

of Engineers (USACE) (U.S. Army Corps of Engineers, 2009) 

to “protect and preserve the alluvial and Sparta aquifers.” The 

project aims to provide additional water to already established 

tailwater recovery systems and on-farm reservoirs through a 

series of canals to supplement irrigation needs (U.S. Army 

Corps of Engineers, 2009).

Groundwater Availability, a WaterBudget Perspective

Groundwater availability comprises many factors. These 

factors include water law, regulations, economics, infrastructure, quality of water, and quantity of water (U.S. Geological 

Survey, 2002). The water law factor involves the legality of 

pumping water that may affect a neighboring farm, State, or 

country. The regulation factor may be imposed where pumping 

from a water resource could negatively impact the sustained 

use of the resource by causing unacceptable environmental, 

Net

recharge

96,826

Streams

63,201

Net

recharge

567,568

Streams

727,608

Alluvial aquifer

Tertiary aquifers

160,649

605

Figure 8. Predevelopment groundwater-flow budget of the alluvial and Tertiary 

systems, in acre-feet per year.
18 Groundwater Availability of the Mississippi Embayment MISSOURI ARKANSAS TENNESSEE LOUISIANA TEXAS MISSISSIPPI ALABAMA KENTUCKY Grand Prairie boundary

A

B

ARKANSAS

JEFFERSON

LONOKE

LEE

MONROE

SALINE

PRAIRIE

PHILLIPS

PULASKI

GRANT

LINCOLN

DESHA Bolivar

CLEVELAND

COAHOMA

COAHOMA

91° 91°15' 91°30' 91°45' 92° 92°15'

34°45'34°30'34°15'

Grand Prairieboundary

Potentiometric map from Schrader, 2008

Potentiometric map from Engler and others, 1945 0 5 10 2.5 MILES

0 5 102.5 KILOMETERS

0 5 10 2.5 MILES

0 5 102.5 KILOMETERS

EXPLANATION

Study area boundary

—Corresponds to boundary

Potentiometric contours of the Mississippi River Valley

 of Mississippi embayment

alluvial aquifer—Shows altitude at which water level

 would have stood in tightly cased wells. Contour interval  10 feet. Datum is NGVD of 1929

160

140

190

150

180

170

160

130

200

210

160

PULASKI

ARKANSAS

JEFFERSON

LONOKE

LEE

MONROE

PRAIRIE

PHILLIPS

GRANT

LINCOLN

DESHA

BOLIVAR CLEVELAND

SALINE

COAHOMA

COAHOMA

91° 91°15' 91°30' 91°45' 92° 92°15'

34°45'34°30'34°15'

Grand Prairieboundary

130

140

120

170

160

150

180

110

190 200 210

100

90

80

90

100

150

220

140

150

Figure 9. Comparison of 

A, 1929 and 

B, 2006 potentiometric 

surface of the Grand Prairie area.
Development of the Hydrologic System 19

Mississippi River

A'

A

A'

Recharge

Recharge

Recharge

Water movement

A

Wells

Mississippi River Valley aquifer aquiferVicksburg-Jackson confining unitUpper Claiborne aquiferMiddle Claiborne confining unitMiddle Claiborne aquiferLower Claiborne confining unitLower Claiborne aquiferMiddle and lower Wilcox aquifers

EXPLANATION

ARKANSAS

LOUISIANA

MISSISSIPPI

TENNESSEE

KENTUCKY MISSOURI

ALABAMA

Pumping wells

Figure 10. Cross-sectional view of hydrogeologic units within the study area with postdevelopment flow (2006) conceptual water movement.
20 Groundwater Availability of the Mississippi Embayment

Net recharge

401,821

Streams

29,369

Net

recharge

2,513,094

Streams

697,029

Alluvial aquifer

Tertiary aquifers

0

501,682

Net storage loss 16,388,070

Net storage loss

126,891

Pumping

19,097,010

Pumping

1,059,782

Figure 11. Postdevelopment groundwater-flow budget of the alluvial and Tertiary systems in acre-feet per 

year.

economic, or social consequences. The infrastructure factor 

determines if a suitable network of pumps, pipes, valves, and 

power sources exist to transport the water for the required 

application. The acceptable quality of water is dependent 

on the intended use, rather it be for human consumption or 

other application such as industrial uses. Finally, the quantity 

factor affects the availability by the determination of how 

much water is actually present, regardless of the previous 

factors. The intent of the analysis in this Professional Paper 

is to examine the quantity factor or the amount of groundwater available in the Mississippi embayment. Groundwater 

availability in the MERAS is changing because of natural and 

anthropogenic events such as precipitation, land use, stream 

channelization, and pumping.

The groundwater budget indicates changes in storage, flow 

into (inflows), and out of (outflows) the aquifer system simulated by the MERAS model from the predevelopment period 

(1870) to 2007 (fig. 12) (Clark and Hart, 2009). Negative rates 

indicate outflows from the groundwater system, and positive 

rates indicate inflows to the groundwater system. In the latter 

part of the simulation (beginning in 1986), each stress period 

represents a 6-month time period of alternating spring/summer 

and fall/winter (fig. 12A). These seasonal stress periods were 

designed to simulate the growing season when water is pumped 

for irrigation and the winter season when most irrigation wells 

are not pumping. Annual averages, rather than seasonal, of all 

groundwater-flow components are shown in figure 12B.

Regional Water Budget

Total net recharge and discharge (sum of inflows or outflows) for the model ranged from about 0.66 million acre-ft/yr 

during predevelopment to 20.16 million acre-ft/yr by the end 

of the simulation (final simulated irrigation period in summer 2006). During predevelopment, the aquifer system was 

considered to be at steady state, resulting in a net change of 

zero in aquifer storage. Early in the model simulation (1870–

1920s), the primary components were simulated as outflow 

from stream leakage and inflow from net areal recharge. As 

pumpage increased through time, water that would otherwise 

flow to streams reversed and net stream leakage became an 

inflow to the system. Net areal recharge increased through the 

first 93 years of the simulation but dropped in dominance since 

the 1980s because of the increase in flow from aquifer storage. 

The largest reversals began in the mid 1980s, but indications 

of the reversal began in the early 1960s when a trend in loss of 

streamflow leakage as discharge and the first consistent inflow 

from storage began (fig. 12A). The seasonality of irrigation 

produces large net pumping values during the spring/summer 

stress periods, alternating with small net pumping values in 

the fall/winter stress periods (fig. 12A). Pumping from wells 

became the largest outflow component of the water budget 

with a net rate of 20.16 million acre-ft/yr (10.65 million 

acre-ft/yr annual average) at the end of the model simulation in summer 2006. The large pumpage amounts are offset 

primarily by movement of water from storage into the aquifer. 

In the fall/winter stress periods, the net storage is reversed to 

replenish some of the storage capacity that was lost during 

the spring/summer pumping. The net change in storage was 

negative (water removed from storage), as reflected by the 

measured mean annual decline of water levels in some areas 

of the alluvial aquifer from 0.5 to 1.0 ft/yr from 1982 to 2006 

(Schrader, 2008).

The storage component of the groundwater budget is 

important because the values of storage indicate the amount of 

water withdrawn from or added into the aquifers. The cumulative storage for aquifers simulated in the MERAS model 

(fig. 13) indicates overall depletion of 140 million acre-ft 

(equivalent to 2.8 ft of water covering the entire study area) 

and depletion from the alluvial aquifer of 149 million acre-ft 
Groundwater Availability, a Water-Budget Perspective 21

Outflow

Inflow

Outflow

Inflow

18702007

18801890

190019101920

19301940195019601970198019902000

Millions of acre-feet per year

1870 1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2007

EXPLANATION

Net pumpage from wells

Net withdrawal from or

 addition to storage

Net areal recharge

Net stream leakage

Modified from Clark and Hart, 2009

B

25

20

15

10

5

0

-5

-10

-15

-20

-25

Millions of acre-feet per year

A

EXPLANATION

Net pumpage from wells

Net withdrawal from or

 addition to storage

Net areal recharge

Net stream leakage

25

20

15

10

5

0

-5

-10

-15

-20

-25

Year

Year

Figure 12. Groundwater-flow budget of the MERAS model A, seasonal after 1986, spring/summer depicted by 

large net pumpage and fall/winter depicted by small net pumpage; B, average annual.
22 Groundwater Availability of the Mississippi Embayment

-20

0

20

40

60

80

100

120

140

160

Change in groundwater storage,

in millions of acre-feet

Year

Total of all hydrogeologic units in the Mississippi embayment

Mississippi River Valley alluvial aquifer

Upper Claiborne aquifer

Middle Claiborne aquifer

Winona-Tallahatta aquifer

Lower Claiborne aquifer

Middle and lower Wilcox aquifers

1870 1879 1889 1899 1909 1919 1929 1939 1949 1959 1969 1979 1989 1999 2007

Withdrawal from

storage

Addition to

storage

EXPLANATION

Figure 13. Cumulative change in groundwater storage by aquifer.

from predevelopment to 2007. The difference in overall depletion compared to depletion in the alluvial aquifer originates 

from additions of water from confining units not shown in 

figure 13. An estimate of the volume of freshwater stored 

in the alluvial aquifer can be made by calculating the thickness of the saturated zone (simulated spring 2007 water table 

minus bottom of the alluvial aquifer from the MERAS model) 

and multiplying by the specific yield (ranging from 0.1 to 0.3 

in the MERAS model; Clark and Hart, 2009). This calculation results in a value of approximately 536 million acre-ft 

(equivalent to 10.7 ft of water covering the entire study area) 

of water stored in the alluvial aquifer. The amount of water 

removed from aquifer storage is approximately 26 percent of 

the amount stored only in the alluvial aquifer. Alley (2007) 

notes “reported estimates of the volume of recoverable ground 

water in storage should be viewed with caution. Efforts to link 

such estimates with ground water availability may be highly 

misleading unless the full effects of ground water development 

are considered...” Therefore, the amount of water stored in 

the alluvial aquifer stated here is used to gain perspective on 

the quantity of water pumped from aquifers in the Mississippi 

embayment. It is unrealistic to assume that all water stored in 

the aquifers in the Mississippi embayment may be removed by 

pumping. To do so would substantially affect the performance 

of wells (the efficiency of pumping is reduced as water levels 

decline) and flow to streams, and would likely affect water 

quality.

In the Mississippi embayment, groundwater pumping 

has produced water-level declines across large areas. The 

change in water level from the predevelopment condition 

was calculated using the simulated water level of the alluvial 

and middle Claiborne aquifers. The water levels representing 

the predevelopment conditions for the alluvial and middle 

Claiborne aquifers were subtracted from the respective water 

levels at the end of each stress period. The time lapse result of 

this calculation may be seen in an animation at http://gallery.

usgs.gov/videos/330 and http://gallery.usgs.gov/videos/331. 

The result of the last (spring of 2007) stress period of the 

simulation is shown in figure 14. Changes in the water level 

in the alluvial aquifer are noticeable throughout Arkansas and 

parts of Mississippi; by 2007, 0.7 percent of the alluvial area 

(216 mi2

) exhibited declines of more than 100 ft (figs. 14A and 

15). Changes in water level in the middle Claiborne aquifer 

are more dramatic than changes in the alluvial aquifer, with 

over 13.3 percent of the aquifer area (7,529 mi2

) exhibiting 

water-level declines of over 100 ft by 2007 (figs. 14B and 15). 

The largest declines of over 300 ft in water level in the middle 

Claiborne aquifer occur in southern Arkansas and northern 

Louisiana (fig. 14B). The animations and figures 14 and 15 

also demonstrate differences in confined and unconfined aquifers. Most areas within the middle Claiborne aquifer boundary 

are simulated as confined and, therefore, given a relatively 

small amount of pumping compared to the alluvial aquifer, 

water levels decline dramatically. However, because of the 

smaller amounts of removed water and the confined nature 

of the aquifer, if pumping ceased in these areas, a rebound of 

water levels would be likely. This is the situation in southern 

Arkansas where a concerted effort to reduce pumping has 

resulted in water-level increases of almost 50 ft since 2004 in 

the middle Claiborne aquifer (Friewald and Johnson, 2007). 
Groundwater Availability, a Water-Budget Perspective 23

94° 93° 92° 91° 90° 89° 88° 87°

38°

37°

36°

35°

34°

33°

32°

31°

Crowleys Ridge

ARKANSAS

MISSISSIPPI

TENNESSEE

MISSOURI

LOUISIANA

KENTUCKY

ILLINOIS

ALABAMA

Study area boundary

50.1 to 100

0.1 to 50

-24.9 to 0

-49.9 to -25

-74.9 to -50

-99.9 to -75

-130 to -100

Water-level change from

 predevelopment to 2007,

 in feet

EXPLANATION

A

TEXAS

Base from U.S. Geological Survey digital data, 1:100,000

0 25 50 100 MILES

0 25 50 100 KILOMETERS

Figure 14. Water-level change from predevelopment to 2007 in the A, Mississippi River Valley alluvial aquifer and B, the 

middle Claiborne aquifer.
24 Groundwater Availability of the Mississippi Embayment

94° 93° 92° 91° 90° 89° 88° 87°

38°

37°

36°

35°

34°

33°

32°

31°

ARKANSAS

MISSISSIPPI

TENNESSEE

MISSOURI

LOUISIANA

KENTUCKY

ILLINOIS

ALABAMA

0 25 50 100 MILES

0 25 50 100 KILOMETERS

Study area boundary

50.1 to 100

0.1 to 50

-24.9 to 0

-49.9 to -25

-74.9 to -50

-99.9 to -75

-199.9 to -100

-299.9 to -200

-429.7 to -300

Water-level change from

 predevelopment to 2007,

 in feet

EXPLANATION

B

TEXAS

Base from U.S. Geological Survey digital data, 1:100,000

Figure 14. Water-level change from predevelopment to 2007 in the A, Mississippi River Valley alluvial aquifer and B, the 

middle Claiborne aquifer.—Continued
Groundwater Availability, a Water-Budget Perspective 25

Mississippi River Valley

alluvial aquifer

Middle Claiborne

aquifer

The thickness of the pie chart shows the relative amount of water pumped from each aquifer. 

The diameter of the pie chart indicates the relative areal extent of each aquifer.

Percentages in pie charts may not total 100 percent because of rounding.

Greater than 100

76 to 100

51 to 75

25 to 50

Less than 25

Percent of area of water-level decline

 from predevelopment to 2007, in feet

EXPLANATION

0.7

3.4

9.1

20.7

66.1

13.3

6.9

15.7 21.4

42.8

Figure 15. Percent area for water-level change groups from 

predevelopment to 2007 in the Mississippi River Valley alluvial 

aquifer and the middle Claiborne aquifer. 

The reverse is typically true of unconfined aquifers, like the 

alluvial aquifer, in that although pumping has not resulted in 

water-level changes as great as those seen in the middle Claiborne aquifer, much more water has been removed. Therefore, 

if pumping ceased in the alluvial aquifer, a much longer period 

of time would be required to replenish the aquifer.

The pattern of simulated groundwater flow in 2006 

(fig. 10) was in many ways a reversal of the predevelopment 

flow conditions (fig. 6). Whereas much of the inflow to the 

system in postdevelopment was still through net recharge 

to the alluvial aquifer and the outcrop areas of the several 

hydrogeologic units, the flow was no longer upward to the 

alluvial aquifer. Groundwater flow during the summer 2006 

was primarily downward to offset demand from pumping from 

the Tertiary system.

Principal Aquifer Water Budgets

The groundwater-flow budgets of the alluvial and 

middle Claiborne aquifers were extracted from model output 

to evaluate groundwater flow at the aquifer scale (figs. 16 

and 17). The groundwater-flow budget of the alluvial aquifer 

(fig. 16) appears almost identical in shape and magnitude to 

the diagram of the groundwater-flow budget for the overall 

system (fig. 12). This is because most water movement within 

the system is in the alluvial aquifer. Subtle differences include 

less pumpage during fall/winter stress periods and the inclusion of net flow to and from other aquifers in the MERAS 

model. Net vertical or horizontal flow into the alluvial aquifer 

from the other aquifers occurs primarily in the early part of the 

simulation period (prior to about 1960). The pattern of flow 

into the alluvial aquifer is also demonstrated in figure 6 that 

shows flow from most hydrogeologic units into the alluvial 

aquifer during predevelopment time. Net vertical or horizontal 

groundwater flow from other aquifers into the alluvial aquifer reversed and removed water from the alluvial aquifer in 

the 1980s (fig. 16B). One of the largest stream inflow events 

occurred in 1993, which corresponded to widespread flooding in the Mississippi River system. Whereas pumping from 

the alluvial aquifer (fig. 16A) is shown to increase dramatically from 1983 to 1986, this was actually the transition from 

annual averages of pumping in early stress periods to 6-month 

seasonal pumping in the last 20 years of the simulation. For 

example, during a given summer stress period, an average of 

20 million acre-ft/yr may be pumped over a 6-month irrigation 

season. During the other 6 months of the year, pumping is negligible (assume zero for this example). Therefore, the annual 

average value of pumping would be 10 million acre-ft/yr.

A comparison of groundwater-flow budgets indicates substantially less water in the middle Claiborne aquifer (fig. 17) 

than in the alluvial aquifer (fig. 16). While groundwater 

pumped out of the alluvial aquifer is derived primarily from 

storage (up to 70 percent, fig. 16), pumpage out of the middle 

Claiborne aquifer is derived primarily from other aquifers (up 

to 15 percent from the alluvial aquifer), followed by storage 
26 Groundwater Availability of the Mississippi Embayment

70

17

13

Percent distribution of sources of inflow,

near end of the simulation

Outflow

Inflow

Outflow

Inflow

18702007

18801890

190019101920

19301940195019601970198019902000

Millions of acre-feet per year

1870 1880 1890 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2007

EXPLANATION

Net pumpage from wells

Net withdrawal from or

 addition to storage

Net flow from adjacent

 aquifers and confining

 units

Net areal recharge

Net stream leakage

EXPLANATION

Net pumpage from wells

Net withdrawal from or

 addition to storage

Net flow from adjacent

 aquifers and confining

 units

Net areal recharge

Net stream leakage

B

25

20

15

10

5

0

-5

-10

-15

-20

-25

Millions of acre-feet per year

A

25

20

15

10

5

0

-5

-10

-15

-20

-25

Year

Year

Figure 16. Groundwater-flow budget of the alluvial aquifer A, seasonal after 1986, spring/summer depicted by 

large net pumpage and fall/winter depicted by small net pumpage; B, average annual.
Groundwater Availability, a Water-Budget Perspective 27

Percent distribution of sources of inflow,

near end of the simulation. Percentage in pie 

chart does not total 100 percent because of rounding

22

8

48 6

15

Outflow

Inflow

Millions of acre-feet per year

1870 1879 1889 1999 1909 1919 1929 1939 1949 1959 1969 1979 1989 1999 2007

B

1.00

0.80

0.60

0.40

0.20

0

-0.20

-0.40

-0.60

-0.80

-1.00

Year

1870 1879 1889 1999 1909 1919 1929 1939 1949 1959 1969 1979 1989 1999 2007

Year

Outflow

Inflow

Millions of acre-feet per year

EXPLANATION

Net pumpage from wells

Net withdrawal from or

 addition to storage

Net flow from adjacent

Net from Mississippi River

 Valley alluvial aquifer

 aquifers and confining units

Net areal recharge

Net stream leakage

EXPLANATION

Net pumpage from wells

Net withdrawal from or

 addition to storage

Net flow from adjacent

Net from Mississippi River

 Valley alluvial aquifer

 aquifers and confining units

Net areal recharge

Net stream leakage

A

25

20

15

10

5

0

-5

-10

-15

-20

-25

Figure 17. Groundwater-flow budget of the middle Claiborne aquifer.
28 Groundwater Availability of the Mississippi Embayment

and net areal recharge. Early in the simulation period, water 

was derived primarily from net areal recharge and stream 

leakage. Discharge from the middle Claiborne aquifer during 

this early period was because of pumping from the middle 

Claiborne aquifer, flow to the alluvial aquifer, or flow to other 

aquifers. Beginning in the 1950s, discharge from the middle 

Claiborne aquifer to the alluvial aquifer was reversed such 

that flow was induced from the alluvial aquifer to the middle 

Claiborne aquifer (fig. 17).

Local Area Water Budgets

Spatial variability in groundwater availability exists 

within the Mississippi embayment, even within the same aquifer. Spatial variability of groundwater budgets and interactions 

between the alluvial and middle Claiborne aquifers and interactions between the middle Claiborne aquifer with other minor 

units were evaluated in three areas using the ZONEBUDGET 

post-processing software (Harbaugh, 1990) (figs. 18 and 19). 

The locations of areas were based on an enclosed cone of 

depression in the Grand Prairie area of Arkansas (Schrader, 

2008) for the alluvial aquifer and stressed areas of the middle 

Claiborne aquifer in southern Arkansas and northern Louisiana 

(hereafter referred to as the Sparta recovery area) and Memphis, Tennessee (hereafter referred to as the Memphis area).

For the alluvial aquifer, pumpage is the dominant stress 

on the groundwater system in the Grand Prairie (fig. 18), 

which is similar to other stressed areas in the alluvial aquifer. 

Similarities within the alluvial aquifer are also evident in an 

evaluation of water-level declines. Schrader (2008) evaluated long-term water-level changes in the alluvial aquifer in 

Arkansas for the period 1982 to 2006. The annual water-level 

decline in a well in the Grand Prairie was approximately 

0.80 ft/yr. The saturated thickness of the alluvial aquifer near 

this well was approximately 30 ft in the spring of 2008. By 

comparison, three wells north of the Grand Prairie area had 

similar annual water-level declines of 0.62, 1.02, and 1.28 ft/yr. 

The saturated thickness of the alluvial aquifer near these wells 

north of the Grand Prairie area ranged from approximately 

38 to 81 ft in the spring of 2008. The annual water-level 

decline for a well in an area southeast of the Grand Prairie 

area in Mississippi was 0.69 ft/yr with a saturated thickness of 

approximately 80 ft in the spring of 2008. The Grand Prairie 

and Mississippi areas appear to be the most similar in terms 

of average pumping per square mile and annual water-level 

decline, though the saturated thickness in Mississippi is more 

than twice that of the Grand Prairie. Reasons for this may be 

that pumping in the Grand Prairie may have begun earlier in 

time and with greater intensity than in Mississippi. On average, the water-level decline for the three wells north of the 

Grand Prairie area is greater than the Grand Prairie or Mississippi areas. The water-level declines also correspond well 

with the greater average pumping north of the Grand Prairie 

area. If the water-level declines were to continue in a linear 

fashion, the estimated time required to lower the water level to 

the bottom of the alluvial aquifer ranges from 30 to 130 years. 

This simple calculation is useful to bracket the period of time 

for which water may be available, but it does not take into 

account many variables such as pumping rates, recharge, or 

the likely inability to continue to pump water from the bottom 

of the aquifer because of reduced pump or well efficiencies.

Pumpage from the middle Claiborne aquifer is one to 

two orders of magnitude less than pumpage from the alluvial 

system. The large difference in pumping is attributed to fewer 

wells, lower aquifer permeability, and lower storage than 

the alluvial aquifer. However, because the middle Claiborne 

aquifer is confined, the volumes of groundwater pumped 

produce much larger water-level declines. Average pumping in 

the Sparta recovery area is substantially less than that from the 

Memphis area. Yet, the pumping in the Sparta recovery area 

has resulted in far greater water-level declines (over 360 ft) 

(Freiwald and Johnson, 2007) than the declines in the Memphis area (over 60 ft). The differences in water-level decline 

between the Sparta recovery and Memphis areas are attributed 

primarily to the much less permeable properties of the middle 

Claiborne aquifer in the Sparta recovery area (Clark and Hart, 

2009).

Temporal variability throughout the system is driven primarily by increases and decreases in pumping from the alluvial aquifer related to irrigation demands and by changes in 

industrial and municipal pumpage from the middle Claiborne 

aquifer. Pumping amounts from the alluvial aquifer in the 

Grand Prairie area fluctuated with a slight trend in decreased 

pumping through the last 20 years of the simulation (late 

1980s to 2006) (fig. 18). There is anecdotal evidence that the 

water levels in some areas of the alluvial aquifer in the Grand 

Prairie area have declined to the point where pumping is not 

feasible in recent years (relative to 2010). Simulated pumping 

in the MERAS model can be restricted when the water level 

falls below a set criterion in an attempt to simulate deterrence 

in pumping during low water-level conditions. These conditions result in a decrease in pumping from the alluvial aquifer 

in the Grand Prairie (fig. 18). Pumping from the middle Claiborne aquifer in the Memphis area increased rapidly until the 

1970s and has since remained relatively constant.

Net interflow between adjacent units (excludes inflow 

from storage, recharge, and streams) also changes through 

time. In the Grand Prairie area, under predevelopment conditions, the small amount of water that entered from interflow 

from adjacent units was upward from the underlying Vicksburg-Jackson confining unit (fig. 19). This is consistent with 

the schematic in figure 6 that demonstrates groundwater discharging primarily upward to the alluvial aquifer. Near the end 

of the simulation, net interflow to the alluvial aquifer changes 

from upward flow from the Vicksburg-Jackson confining unit 

to primarily horizontal flow within the alluvial aquifer in the 

Grand Prairie (fig. 20). Net interflow to the middle Claiborne 

aquifer in the Grand Prairie area was primarily down from the 

alluvial aquifer under predevelopment conditions (fig. 19), 

which would occur near the western boundary of the Grand 

Prairie area, and secondarily from upward flow from the lower 
Groundwater Availability, a Water-Budget Perspective 29

Figure 18. Groundwater-flow budgets of local areas.

!

!

!

Memphis

Monroe

Middle Claiborne aquifer

Sparta recovery area

Middle Claiborne aquifer

Memphis area

Mississippi River

Valley alluvial aquifer

Grand Prairie area

Middle Claiborne aquifer

Grand Prairie area

0

-500

-400

-300

-200

-100

100

200

300

400

500

0

-500

-400

-300

-200

-100

100

200

300

400

500

Thousands of acre-feet per year

1870 1879 1889 1899 1909 1919 1929 1939 1949 1959 1969 1979 1989 1999 2007

Year

0

-500

-400

-300

-200

-100

100

200

300

400

500

Thousands of acre-feet per year

1870 1879 1889 1899 1909 1919 1929 1939 1949 1959 1969 1979 1989 1999 2007

Year

0

-500

-400

-300

-200

-100

100

200

300

400

500

Thousands of acre-feet per year

1870 1879 1889 1899 1909 1919 1929 1939 1949 1959 1969 1979 1989 1999 2007

Year

Thousands of acre-feet per year

1870 1879 1889 1899 1909 1919 1929 1939 1949 1959 1969 1979 1989 1999 2007

Year

EXPLANATION

Net pumpage from wells

Net withdrawal from or

 addition to storage

Net flow from adjacent

 aquifers and confining

 units

Net areal recharge

Net stream leakage

Boundary of Mississippi Embayment

Regional Aquifer Study

0 50 100 25 MILES

0 50 100 25 KILOMETERS

El Dorado
30 Groundwater Availability of the Mississippi Embayment

To the Mississippi River Valley alluvial aquifer

To the middle Claiborne aquifer

Interflow sources—Values in parenthesis represent the percent 

 of the total inflow from adjacent units. The height of the pie charts 

 are relative to the 2006 middle Claiborne inflow amount from adjacent 

 zones or confining units. Percentages in pie charts may not total 

 100 percent because of rounding

1

2

EXPLANATION

0 50 100 25 MILES

0 50 100 25 KILOMETERS

Net from Mississippi River Valley

alluvial aquifer (93.9)

Net horizontal flow (0.7)

Net from lower Claiborne

confining unit (5.5)

2

Net from Vicksburg

confining unit (1.0)

1

Net from Vicksburg

confining unit (1.0)

2

Net from lower Claiborne

confining unit (9.8)

Net horizontal flow (90.2)

Net from surficial

aquifer (55.3)

Net from Wilcox Group (1.6)

Net horizontal

flow (43.2)

2

!

!

Memphis

Monroe

!

El Dorado

Boundary of Mississippi Embayment

Regional Aquifer Study

Figure 19. Predevelopment groundwater-flow interactions with adjacent hydrogeologic units within local areas.
Groundwater Availability, a Water-Budget Perspective 31

Net from middle Claiborne

confining unit (3.8)

Net from Mississippi River Valley

alluvial aqufier (11.4)

Net horizontal flow (81.0)

Net from lower Claiborne

confining unit (3.8)

Net from Vicksburg

 confining unit (0.2)

Net horizontal flow (99.8)

2

2

2

1

Net from lower Claiborne

aquifer (1.4) Net from middle Claiborne

confining unit (5.6)

Net from surficial

aquifer (30.1)

Net from

upper

Claiborne

aquifer (13.7)

Net from Winona-Tallahatta

Formation (0.4)

Net horizontal

flow (48.7)

Net horizontal flow (72.5)

Net from lower Claiborne

confining unit (5.4)

Net from middle Claiborne

confining unit (22.2)

!

!

!Memphis

Monroe

El Dorado

Boundary of Mississippi Embayment

Regional Aquifer Study

0 50 100 25 MILES

0 50 100 25 KILOMETERS

To the Mississippi River Valley alluvial aquifer

To the middle Claiborne aquifer

Interflow sources—Values in parenthesis represent the percent 

 of the total inflow from adjacent units. The height of the pie charts 

 are relative to the 2006 middle Claiborne inflow amount from adjacent 

 zones or confining units. Percentages in pie charts may not total 

 100 percent because of rounding

1

2

EXPLANATION

Figure 20. Postdevelopment groundwater-flow interactions with adjacent hydrogeologic units within local areas.
32 Groundwater Availability of the Mississippi Embayment

Claiborne confining unit. This is also consistent with the schematic in figure 6, which describes groundwater movement into 

the recharge areas on the west and east side of the MERAS 

model and upward into overlying units.

The groundwater-budget analyses for the Sparta recovery and Memphis areas does not include the alluvial aquifer 

because it is not as extensive or is not in contact with the 

middle Claiborne aquifer in these zones. The simulated predevelopment budget indicates that flow into the Sparta recovery 

area is primarily horizontal from other parts of the middle 

Claiborne aquifer (net horizontal flow, fig. 19). While the primary interflow component in postdevelopment continues to be 

horizontal flow, 22.2 percent of the total inflow is now downward flow from the middle Claiborne confining unit, induced 

by a downward gradient in water level created by pumping 

(fig. 20). The simulated predevelopment budget for the middle 

Claiborne aquifer in the Memphis area indicates two primary 

sources, the first from the surficial aquifer and the second from 

horizontal flow from other parts of the middle Claiborne aquifer (fig. 19). The primary interflow components in postdevelopment in the Memphis area continues to be horizontal flow 

and flow from the surficial aquifer, though additional sources 

include downward (net) flow from the upper Claiborne aquifer 

and the middle Claiborne confining unit, and horizontal flow 

from the lower Claiborne aquifer (net from lower Claiborne 

aquifer, fig. 20). The Memphis area inflow derived from 

downward vertical leakage across confining units and from the 

surficial aquifer unit ranges from 40 to 55 percent of pumping for the period from 1980 to 2007. A study by Brahana and 

Broshears (2001) indicated that “more than 50 percent of the 

water withdrawn from the Memphis aquifer in 1980 is derived 

from vertical leakage across confining units, and the leakage 

from the shallow aquifer...”. The similarity of the percent 

of downward leakage simulated by the MERAS model, and 

the study by Brahana and Broshears indicates little change in 

groundwater flow into the middle Claiborne aquifer occurred 

over the last 27 years, which may indicate near steady-state 

conditions in the Memphis area.

Climate Trends and Potential Future Effects on 

Groundwater Availability

Climate trends affect various components of the global 

hydrologic cycle in the space, time, and frequency domains 

(Loáiciga and others, 1996; Sherif and Singh, 1999; Milly 

and others, 2005; Holman, 2006; Intergovernmental Panel on 

Climate Change, 2007). Components of the hydrologic cycle 

that may be affected include atmospheric water vapor content, 

precipitation and evapotranspiration patterns, snow cover and 

melting of ice and glaciers, soil temperature and moisture, 

and surface runoff and streamflow (Bates and others, 2008). 

Changes to these components of the global hydrologic cycle 

will in turn likely affect recharge, discharge, and groundwater storage of many aquifers worldwide (Gurdak and others, 

2009).

The potential effects of climate trends on water resources 

are well recognized, although there has been relatively little 

research relating to groundwater (Holman, 2006). Estimating the potential effects of climate trends on groundwater 

availability is difficult because changes in climate may affect 

hydrogeologic processes and groundwater availability directly 

and indirectly (Dettinger and Earman, 2007). The Intergovernmental Panel on Climate Change (IPCC) stated that a lack 

of necessary data currently (2007) makes it very challenging 

to determine the magnitude and direction of groundwater 

change resulting solely from human-induced climate change 

(Intergovernmental Panel on Climate Change, 2007; Kundzewicz and others, 2007). Groundwater availability has been 

affected since the mid-1900s by a number of other anthropogenic factors, such as contamination, reduction in streamflow 

because of pumping, lowering of the water table, and loss of 

storage because of groundwater mining (primarily for irrigated 

agriculture) (appendix 1) (Kundzewicz and others, 2007). 

Additionally, groundwater systems often respond more slowly 

and have a greater temporal lag to climate trends than surfacewater systems, further challenging a clear understanding of 

climate-trend effects on groundwater availability (Chen and 

others, 2004; Hanson and others, 2004, 2006; Gurdak and 

others, 2008; Kundzewicz and others, 2007; Gurdak, 2008). 

This study is the first to quantify the cumulative interactions 

of natural climate variability on interannual to multidecadal 

timescales and groundwater pumping on the availability of 

groundwater in the MERAS.

Hydroclimate Data Processing and Analysis

Frequency analysis of hydroclimate data enables 

improved understanding of subsurface hydrologic responses 

to climate trends. The hydroclimate data processing and 

analysis employed in this study generally followed methods 

described by Hanson and others (2004) and Gurdak and others (2007).

The hydroclimate data analysis included groundwaterlevel time series from 40 wells, precipitation time series 

from 34 meteorological stations, streamflow time series 

from 8 USGS streamflow-gaging stations (appendix 2), and 

groundwater pumping records compiled from a larger network 

of hydrologic time series in the study area (Clark and Hart, 

2009). The wells and meteorological stations were selected 

on the basis of the location of groundwater-level wells as well 

as the length and completeness of record. The water-level and 

streamflow data were obtained from USGS National Water 

Information System (U.S. Geological Survey, 2009). Monthly 

precipitation time-series data were obtained from the National 

Oceanic and Atmospheric Administration (2009b).

The first data-processing step (fig. 21) was designed to 

remove the influence of zero values common to precipitation 

time series and to integrate any incomplete groundwater-level 

records. This step was performed by transforming all hydrologic time series (fig. 21A) into monthly cumulative departure 
Groundwater Availability, a Water-Budget Perspective 33

series (fig. 21B) from the period of record using the monthly 

mean. Next, the residuals of the monthly cumulative departure 

series were obtained by subtracting a regression-fitted loworder (cubic) polynomial. The overall shape of the low-order 

polynomial represents interannual, multidecadal, or longer 

temporal trends (or responses) in the hydroclimate data to 

larger climatic cycles or periods of anthropogenic effects and 

tends to be dominated by the lowest frequency containing 

the most variance (Hanson and others, 2004). The residuals were finally normalized by the historic mean to facilitate 

statistical comparisons between various data types and are 

referred to as normalized departures (unitless) (fig. 21C). 

These data-processing steps effectively eliminated parts of the 

lowest-frequency cycles that would otherwise dominate the 

variance of the time-series analysis (Hanson and others, 2004). 

These steps were necessary to remove many of the long-term 

anthropogenic effects in the groundwater-level time series, 

such as implementation of improved irrigation technology, and 

annual anthropogenic effects, such as crop rotation and other 

irrigated-agricultural practices of the study area.

Using the computer software SSA-MTM Toolkit (Ghil 

and others, 2002; Dettinger and others, 1995) singular spectrum analysis (Vautard and others, 1992) was then applied 

during frequency analysis of the time-series residuals, using 

a modified approach developed by Hanson and others (2004) 

(appendix 2).

Extrapolations spanning 30 years of precipitation and 

streamflow inputs to the MERAS model were developed 

from the results of the singular spectrum analysis following a 

method described by Dettinger (2003). It should be stressed 

that the extrapolated precipitation and streamflow values 

generated for the MERAS model from this method are not 

climate predictions and likely will not prove to be precisely 

correct. As Dettinger (2003) describes, the method uses the 

singular spectrum analysis identified climate cycle 

reconstructed components and realistic randomness to generate extrapolations of precipitation that 

contain patterns of typical climate trends that may 

occur in the future. The method incorporated variability consistent with known climate cycles (such 

as AMO, PDO, and ENSO, appendix 1) and did not 

include climate cycles less than 2 years. The 30-year 

extrapolations of precipitation were generated using 

the computer software kSpectra Toolkit 2.23 by fitting autoregressive models of various orders to the 

reconstructed components from the singular spectrum analysis of each precipitation time series and 

allowing those autoregressive models to step forward 

to forecast the reconstructed components for an 

additional 30 years. The reconstructed components 

for each precipitation time series are then converted 

back to precipitation values by stepping through 

the data-processing steps in reverse order. A similar method was used to extrapolate streamflow for 

streams crossing the MERAS model boundary. The 

extrapolation method incorporates less emphasis on 

the high-frequency noise inherent to the precipitation data than 

the lower-frequency trends, and thus produces extrapolated 

precipitation values that best approximate the lower frequency, 

interannual to multidecadal trends in precipitation of the study 

area. The 2-year average observed and extrapolated annual 

precipitation for the study area and selected meteorological stations are shown in figure 22. The hydroclimatic analysis used 

the low-order (cubic) polynomial trend to smooth out noise in 

the precipitation values, while also smoothing maximum and 

minimum values. This creates a much smoother precipitation 

pattern through time compared to actual precipitation values 

while forecasting 30-year extrapolations (fig. 22). The extrapolated 30-year precipitation values from each meteorological 

station were interpolated spatially across the model area for the 

time period 2008 to 2038 (appendix 3), which were subsequently used to generate continuous net recharge grids for the 

groundwater-flow model (as described in the next section). 

For more information on the extrapolation method used in this 

analysis see Dettinger (2003).

Net recharge in the MERAS model is specified as a fraction of precipitation (Clark and Hart, 2009). Therefore, grids 

of extrapolated precipitation (appendix 3) from the processed 

time series information were input into the MERAS model. 

The model then converted the precipitation values into specified net recharge (fig. 23). A drying period is evident from 

approximately 2008–24, followed by a wetter period from 

2025 to the end of the simulation (fig. 22), which is the result 

of the forecasted future trends in the AMO and PDO phase 

variability. The forecasted shift from a drying to wetting trend 

in 2024 to 2025 is likely the result of a future shift from the 

negative PDO and positive AMO phase to a positive PDO 

and positive AMO phase. As discussed in the “Hydroclimate 

Data Processing and Analysis” section and depending on 

the future ENSO variability, the study area could experience 

A

B

C 3

 0

 -3

1900 1920 1940 1960 1980 2000

Year

Total

monthly

precipitation,

in inches

Monthly

cumulative

departures (MCD),

in inches

Normalized

departures,

unitless

MCD series

Polynomial fit

4

3

 2

 1

 0

 5

 0

 -5

 -10

 -15

 -20

Figure 21. Demonstration of hydroclimate data-processing steps using 

A, total monthly precipitation time series, B, monthly cumulative departure 

(MCD), and C, normalized departure series.
34 Groundwater Availability of the Mississippi Embayment

20

30

40

50

60

70

80

1890 1910 1930 1950 1970 1990 2010 2030

Year

Extrapolated

precipitation

Average precipitation, in inches

2-year average

Memphis

Jackson

Little Rock

El Dorado

Jonesboro

EXPLANATION

Figure 22. Average annual (2-year or more) precipitation values used in the MERAS model and selected meteorological 

stations including extrapolated precipitation data from 2008 to 2038.

2-year average

Memphis

Jackson

Little Rock

El Dorado

Jonesboro

EXPLANATION

1890 1910 1930 1950 1970 1990 2010 2030

Year

Extrapolated

net recharge

Net recharge, in inches per year

0

0.5

1

1.5

2

2.5

3

Figure 23. Specified net recharge at selected locations including extrapolated net recharge from 2008 to 2038.
Groundwater Availability, a Water-Budget Perspective 35

precipitation conditions between the forecasted 2008–24 

period similar to those from the drought of the 1950s based 

on PDO and AMO variability. Although the forecasted shift 

from a drying to wetting trend in 2024 to 2025 may bring 

additional precipitation to the study area, the general trend 

is expected to be less than average precipitation because of 

the historical drought frequency during the positive PDO and 

positive AMO phase. In general, historical precipitation in the 

study area is greatest in the south and southeast and least in 

the north and northwest. Extrapolated grids of precipitation 

(appendix 3) follow a similar trend spatially, as lower precipitation areas are shown to continually have less precipitation, and higher precipitation areas are shown to continually 

have more precipitation. The spatial patterns of extrapolated 

precipitation during the drying phase (2008–24) are very 

similar to the spatial patterns of drought frequency during the 

negative PDO and positive AMO (McCabe and others, 2004) 

and discussed in “Hydroclimate Data Processing and Analysis,” with relatively drier conditions in the west and northwest 

of the study area (appendix 3). The extrapolated precipitation during the drying phase (2008–24) represents a (–) 5 to 

25 percent deviation from the observed annual precipitation at 

each meteorological station (appendix 3). Following the peak 

drying conditions between 2024 and 2028 in the southwestern 

part of the study area (appendix 3), the relative wetting trend 

begins in the southern part of the study area and propagates 

northward between 2028 and 2038 (appendix 3), which is 

very similar to the spatial patterns during the positive PDO 

and positive AMO as reported by McCabe and others (2004). 

Although some areas in the southern parts of the study 

area are shown to have as much as a 20-percent increase in 

average annual precipitation based on the observed annual 

precipitation, these areas are isolated and the majority of the 

study area is shown to have less than 5 to –25 percent deviation from the observed annual precipitation (appendix 3). The 

modest wetting trend between 2028 and 2038 is consistent 

with the drier than average conditions during the positive 

PDO and positive AMO (McCabe and others, 2004).

Global Climate Models (GCMs) do not accurately predict local climate trends, but in general, 

most GCMs predict an overall increase in temperature, globally, with the greatest predicted change in 

temperature to occur in the polar regions, specifically the Arctic region (Intergovernmental Panel on 

Climate Change, 2007). Also, the greatest predicted 

increases in precipitation are in the high-latitude 

regions, and the greatest decreases in precipitation 

are in most subtropical land regions (Intergovernmental Panel on Climate Change, 2007). Based on 

the IPCC GCM predictions, temperatures in the Mis

sissippi embayment will increase slightly (1–1.5°C),

and precipitation will decrease slightly (approximately 5 percent) over the next 20 years (through 

2029) contrasted with the high-latitude regions having up to a 3°C increase in temperature and 20 percent increase in precipitation (Intergovernmental 

-

Panel on Climate Change, 2007). The GCM prediction of 

slight decreases in precipitation in the Mississippi embayment 

corresponds well with the precipitation inputs used in the 

climate scenario simulations in this professional paper. Each 

method appears to indicate that while climate trends likely will 

affect precipitation and temperature in the Mississippi embayment, the trends likely will be small compared to that in polar 

or subtropical land regions.

Climate Simulation Development

Two climate scenarios were simulated with the MERAS 

model by extending the simulation period by 30 years to 2038. 

Sixteen stress periods were used to represent the projected 

30-year period. The first stress period is 275 days in length 

and extends the MERAS model to the end of 2007. Each stress 

period following 2007 is 2 years in length.

The first climate scenario (dry scenario) simulated with the 

MERAS model used changes in pumping from existing pumping centers and extrapolated net recharge changes (fig. 23). The 

change in precipitation from each location listed in figure 22 was 

compared to the change in total site-specific irrigation pumping 

from the Arkansas Natural Resources Commission Water Use 

Database System from 1995 to 2005 (U.S. Geological Survey, 

2010c). The change in precipitation at Memphis, Tennessee, was 

more highly correlated to change in pumping with a square of 

the correlation coefficient (R2) of 0.77 (fig. 24) than other locations. Based on this linear relation, the equation of the best fit 

line was used to solve for the fractional change in pumping from 

2005 that would result from an extrapolated change in precipitation. The extrapolated precipitation at Memphis, Tennessee, 

resulted in a slight positive change in precipitation from 2005. 

The linear relation to pumping then dictates a negative change 

in irrigation pumping compared to 2005; a reduction of up to 

approximately 13 percent. Because 2005 is one of the driest 

years on record in the study area and pumping remains relatively 

high throughout the extrapolated part of the simulation, scenario 

1 is considered to span a primarily dry period.

Fractional change in precipitation from 2005

Fractional change in pumping from 2005

-0.4

-0.35

-0.3

-0.25

-0.2

-0.15

-0.1

-0.05

0

-0.2 0 0.2 0.4 0.6 0.8 1

Change in pumping = -0.3487x - 0.0459

Square of the correlation

 coefficient (R2) = 0.77

EXPLANATION

Figure 24. Relation between fractional change in pumping from 2005 and 

fractional change in precipitation at Memphis, Tennessee, from 2005.
36 Groundwater Availability of the Mississippi Embayment

The second climate scenario (wet scenario) was constructed to provide a contrast to the dry period simulated with 

the first scenario by incorporating precipitation from one of 

the wettest periods (1991) occurring within the study area. 

Precipitation from 1991 was used as the input for the calculation of net recharge while maintaining the pumping used in 

the dry scenario so that a direct comparison of the effect of 

changes in precipitation could be made. Extrapolated streamflow was used in the dry and wet scenarios.

Climate Trend Scenarios and Simulated Water 

Levels

Water level continued to decline in both of the climate 

scenarios from 2007 simulated levels. The area containing 

more than 100 ft of water-level decline in the alluvial aquifer 

increased from 2007 by 13.9 percent from 0.7 to 14.6 percent 

for the dry scenario (figs. 15 and 25). The area containing 

more than 100 ft of decline in the middle Claiborne aquifer 

Mississippi River Valley

alluvial aquifer

Middle Claiborne

aquifer

The thickness of the pie chart shows the relative amount of water pumped from each aquifer. 

The diameter of the pie chart indicates the relative areal extent of each aquifer.

Greater than 100

76 to 100

51 to 76

25 to 50

Less than 25

Percent of area of water-level decline

 from predevelopment to 2038, in feet

EXPLANATION

Dry scenario Wet scenario

Dry scenario Wet scenario

14.6 14.5

15.8

16

39.1

13.8 13.9

15.7

16

40.6

23.2

14.4 14.4

15.8

32.2

21.7

14.3 14.1

16.6

33.3

Figure 25. Percent area for water-level change groups from predevelopment to 2038 in the Mississippi River Valley alluvial 

aquifer and the middle Claiborne aquifer (dry and wet scenarios).
Groundwater Availability, a Water-Budget Perspective 37

increased from 2007 by 9.9 percent from 13.3 to 23.2 percent 

for the dry scenario (figs. 15 and 25). Spatially, the changes 

from 2007 to 2038 are most evident in the alluvial aquifer 

throughout almost all of Arkansas and a part of west-central 

Mississippi where the land area with declines greater than 

25 ft almost doubled (figs. 14A and 26A). The areas of greatest 

decline in the alluvial aquifer are in southeastern Missouri 

and an area just west of Crowleys Ridge. While water levels 

have declined in the area west of Crowleys Ridge in recent 

years, water levels in the area of southeastern Missouri have 

not. This simulated decline may be the result of the continual 

decrease in precipitation forecasted in the northern part of the 

study area. The greatest water-level declines in the middle 

Claiborne aquifer remained in the southern part of the study 

area, in southern Arkansas and northern Louisiana. However, 

the maximum water level changed only by about 10 ft, which 

may indicate the effectiveness of conservation practices and 

alternative water supplies in these areas. More noticeable 

changes in the middle Claiborne aquifer occur throughout the 

remainder of Arkansas and into the southwestern corner of 

Tennessee (figs. 14B and 26B). Water-level changes in this part 

of the middle Claiborne aquifer tend to affect large areas commensurate with the confined condition of the aquifer and are in 

parts of the aquifer that likely are more affected by pumping in 

the alluvial aquifer (fig. 26B).

There is little difference between the dry and wet scenarios in terms of percent water-level change. Both scenarios 

resulted in 14.6 to 13.9 percent of the area containing greater 

than 100 ft of decline, 14.5 to 13.8 percent containing between 

75 and 100 ft of decline, and 15.8 to 15.7 percent containing 

51 to 75 ft of decline in the alluvial aquifer (fig. 25). The middle Claiborne aquifer water-level changes were also similar 

between the two scenarios. These scenarios indicate that even 

with a 25-percent increase in precipitation from that of the dry 

scenario, there is little difference in the resultant water levels. 

This is in large part because of the magnitude of differences 

between changes in net recharge and changes in pumping. 

When compared to the volume of water pumped out of the 

system, the effect of this change in net recharge is negligible.

Whereas little difference exists between the dry and wet 

scenarios in terms of water-level change, this analysis is useful 

because it provides an example of the MERAS model as a 

tool to examine the sensitivity of the groundwater system to 

gradual long-term trends in precipitation. Analysis such as this 

using the MERAS model may be utilized to examine a host 

of scenarios and the resultant system response beyond those 

simulated by the scenarios presented here.

Evaluation of Regional Groundwater-Level 

Monitoring Network

Groundwater-level monitoring networks provide waterresource managers the ability to examine the current status 

and long-term trends in the hydrologic system. The location of 

wells in these monitoring networks and the timing of observed 

water-level measurements in these wells are critical to the 

management of the resource in a way that best suits anthropogenic and environmental needs. The MERAS model is one 

tool that can be used to evaluate the effectiveness of monitoring networks within the Mississippi embayment. The following evaluation documents a demonstration of one application 

using the MERAS model and the computer program OPR-PPR 

(Tonkin and others, 2007).

The groundwater-level monitoring network used to 

construct the 2007 middle Claiborne aquifer potentiometric 

surface (Schrader, 2007) was used to evaluate the importance 

of individual groundwater-level observations given current 

pumpage and recharge used in the dry climate scenario. The 

Observation-Prediction (OPR) statistic indicates the relative 

importance of an observation to a prediction (Tonkin and others, 2007). The MERAS model documented in Clark and Hart 

(2009) and the MERAS model simulation of the dry climate 

scenario provides a base model and a prediction model for 

evaluating the importance of observed water-level altitudes.

OPR-PPR was used in two modes, OPROMIT and 

OPRADD, to omit existing observations and add new potential observations, respectively. OPROMIT was used to calculate the importance of observations by measuring the percent 

change in prediction standard deviation caused by omitting the 

observations. OPRADD works in the same manner by adding 

new observations.

Existing Monitoring Networks

Multiple groundwater-level monitoring networks exist in 

the Mississippi embayment. In the middle Claiborne aquifer, 

a total of 748 water-level measurements were used to create 

an embayment-wide potentiometric-surface map (Schrader, 

2007). Of these monitoring networks, many are used routinely 

to develop potentiometric-surface maps within individual 

States or pairs of States (Stanton, 1997; Brantly and others, 

2002; Schrader, 2004; Schrader and Jones, 2007). A similar 

water-level network currently exists in some areas of the 

alluvial aquifer (Schrader, 2006, 2008). Several State and local 

agencies also collect water-level information.

Network Analysis

For the middle Claiborne aquifer, many of the water 

levels used to construct the potentiometric-surface representing 2007 conditions also were used as observations (for 

calibration purposes) in the MERAS model. In step one of 

this evaluation, all middle Claiborne aquifer water levels 

that also were used in the construction of the 2007 potentiometric-surface map were omitted using the OPROMIT 

mode of OPR-PPR to calculate their relative importance. In 

step two, any observed water levels at monitoring wells used 

in the 2007 potentiometric-surface map that had not been 

used in the MERAS model calibration were then added as 

new observations using the OPRADD mode of OPR-PPR to 
38 Groundwater Availability of the Mississippi Embayment

94° 93° 92° 91° 90° 89° 88° 87°

38°

37°

36°

35°

34°

33°

32°

31°

Crowleys Ridge

ARKANSAS

MISSISSIPPI

TENNESSEE

MISSOURI

LOUISIANA

KENTUCKY

ILLINOIS

ALABAMA

0 25 50 100 MILES

0 25 50 100 KILOMETERS

Study area boundary

50.1 to 100

0.1 to 50

-24.9 to 0

-49.9 to -25

-74.9 to -50

-99.9 to -75

-145 to -100

Water-level change from

 predevelopment to 2038,

 in feet

EXPLANATION

A

TEXAS

Base from U.S. Geological Survey digital data, 1:100,000

Figure 26. Water-level change of the dry scenario from predevelopment to 2038 in the A, Mississippi River Valley alluvial 

aquifer and B, the middle Claiborne aquifer.
Groundwater Availability, a Water-Budget Perspective 39

94° 93° 92° 91° 90° 89° 88° 87°

38°

37°

36°

35°

34°

33°

32°

31°

ARKANSAS

MISSISSIPPI

TENNESSEE

MISSOURI

LOUISIANA

KENTUCKY

ILLINOIS

ALABAMA

Study area boundary

50.1 to 100

0.1 to 50

-24.9 to 0

-49.9 to -25

-74.9 to -50

-99.9 to -75

-199.9 to -100

-299.9 to -200

-440 to -300

Water-level change from

 predevelopment to 2038,

 in feet

EXPLANATION

B

0 25 50 100 MILES

0 25 50 100 KILOMETERS

TEXAS

Base from U.S. Geological Survey digital data, 1:100,000

Figure 26. Water-level change of the dry scenario from predevelopment to 2038 in the A, Mississippi River Valley alluvial 

aquifer and B, the middle Claiborne aquifer.—Continued
40 Groundwater Availability of the Mississippi Embayment

evaluate the relative importance of observed water levels at 

these wells. It is important to note there are many more waterlevel observations in the MERAS model, both spatially and 

temporally, than the sum of those omitted and added as part 

of this evaluation.

To calculate the importance of each 2007 middle Claiborne aquifer observation to a prediction, OPR-PPR predictions were specified as water-level altitudes near the end of 

the dry scenario simulation. These water-level altitudes were 

located near the center of cones of depression described by 

Schrader (2007) and were based on the simulated 2038 water 

level of the middle Claiborne aquifer. Using these predictions 

(prediction locations) and the observations of the 2007 middle 

Claiborne aquifer monitoring network, a map was created 

representing the percent decrease of uncertainty (relative 

importance) related to each observation (fig. 27). Many of the 

observations that have a high importance are in close proximity to stressed areas of the aquifer. Some, however, rank higher 

than might be expected based on location, such as the high 

relative importance of observations in Mississippi in the southeastern part of the study area. The reasons for the high relative 

importance of these observations in Mississippi are two-fold: 

(1) the observations and the nearby predictions share high sensitivity to common parameters, and (2) the observations are in 

an area of sparse observations making the sensitivities higher 

than observations in areas where multiple observations exist. 

The results also indicate relative low importance of many of 

the observations in southern Arkansas and northern Louisiana. 

However, the ranking of these observations are based solely 

on the predictions used in this analysis.

It is important to note that this analysis reflects the importance of observations under the current stress conditions in this 

particular model. The addition of new, high-volume pumping 

centers in a relatively unstressed area of the middle Claiborne 

aquifer may alter these results. This underscores the ability of 

the technique to evaluate a groundwater-monitoring network 

under a variety of situations.

Groundwater-Flow Model Improvements

Improvements made in the development of the MERAS 

model include a refined hydrogeologic framework, the ability 

to simulate routed streamflow, the ability to simulate flow 

through wells screened in multiple aquifers, and the incorporation of available site-specific water use. The MERAS 

model incorporates data from more than 2,600 geophysical 

logs used to interpret the thickness and extent of hydrogeologic units in the study area that encompasses 78,000 mi2

. 

Though gaps in data exist, the dataset of geophysical information is valuable and is a refinement of the understanding 

gained through the Gulf Coast Regional Aquifer System 

Analysis program. Geophysical logs in the dataset can also 

be helpful in interpreting the quality of groundwater. At the 

time of writing this report, studies are underway to expand 

the knowledge of aquifers with elevated salinity values that, 

with treatment, could become sources of water acceptable for 

human use. The MERAS model is one of the first regional 

models in the Mississippi embayment to incorporate the 

SFR Package of MODFLOW. The use of SFR in the model 

removes the constraints of specifying a fixed hydraulic 

head within the streams allowing the model to simulate the 

interaction of the stream with the aquifer and route the water 

downstream to the next model cell. By using the MNW Package of MODFLOW, the model can simulate flow through the 

well bore from multiple layers. This may be useful in areas 

such as the Grand Prairie, where anecdotal evidence exists 

for multiscreened wells, but the effect of these wells on the 

system is not known.

Additionally, the MERAS model simulates all aquifers 

and confining units in the Mississippi embayment, including 

the alluvial aquifer, in a single groundwater-flow model. Previous models were forced to constrain the study area to a select 

set of aquifers or geographic region because of limitations of 

data or computer computational time. The holistic simulation 

by the MERAS model allows analysis of the larger regional 

and aquifer scale system to better understand how all the parts 

interact.

The demonstration of OPR-PPR used in this report 

provides an example to evaluate groundwater-level monitoring 

networks in this holistic view. The utilization of OPR-PPR in a 

multi-aquifer system such as simulated by the MERAS model 

not only provides information on the importance of observations relative to stresses in other aquifers in the system, but 

also may help optimize the placement and timing of waterlevel observations under tight fiscal budgets.

Challenges for Future Groundwater 

Availability Determinations—

Limitations and Lessons Learned

Groundwater models are a simplification of the real 

world. As such, improvements are always possible in many 

areas. As with most models, the parameter values and input 

data used in the MERAS model are considered nonunique, 

thus a different combination of values could produce similar 

results. Caution is needed when making groundwater management decisions based on the steady-state and transient simulations described in this report in lieu of the stated assumptions 

and limitations. For example, actual water-level declines in 

wells will differ from computed values, and declines in or 

near individual high-capacity pumping wells generally will be 

greater. The MERAS model could be improved with increased 

knowledge of recharge rates, water use, evapotranspiration, and stream leakage. Each of these limiting data sets is 

described in detail as follows.

Information on the amount of recharge entering the 

groundwater system and the amount of groundwater pumped 

from the various aquifers (water use) is crucial to the MERAS 
Challenges for Future Groundwater Availability Determinations—Limitations and Lessons Learned 41

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94° 93° 92° 91° 90° 89° 88° 87°

38°

37°

36°

35°

34°

33°

32°

31°

Memphis

Pine Bluff

El Dorado

Monroe

Jackson

ARKANSAS

MISSOURI

LOUISIANA

KENTUCKY

TENNESSEE

MISSISSIPPI

ALABAMA

)

Low

"

Low

"

"

"

" High

Relative importance

 of omitted observations

Relative importance

 of new observations

Prediction locations

High

Study area boundary

EXPLANATION

0 25 50 100 MILES

0 50 100 25 KILOMETERS

TEXAS

Base from U.S. Geological Survey digital data, 1:100,000

Figure 27. Results of OPR-PPR showing the relative importance of spring 2007 water-level observations in the middle 

Claiborne aquifer.
42 Groundwater Availability of the Mississippi Embayment

model. While some site-specific information on water use 

exists in some areas, particularly irrigation water use in Arkansas, the period of record for this information is within the last 

10 to 15 years, which is a relatively small amount of time 

compared to the simulation period of 137 years. Over much 

of the study area and simulation period, each of these datasets 

related to recharge and water use currently (2007) has little or 

no information based on actual measured values. Net recharge 

in the MERAS model is calibrated based on water level and 

streamflow observations. The range of net recharge values in 

the MERAS model are within values used by past models constructed in the study area and based on streamflow separation 

methods. However, field values to validate these ranges are 

sparse or difficult to compare.

ET is not currently explicitly simulated in the MERAS 

model, though it is possible to incorporate ET as a package 

within MODFLOW-2005. By the explicit simulation of ET, 

the modifications made to the net recharge early in the simulation (predevelopment to the 1960s) might not be necessary. 

The explicit simulation of ET would also help in removing 

groundwater at other periods during the simulation if water 

levels rise to the point that ET effects would be active.

Water use as simulated in the MERAS model was 

derived from a combination of trend analysis, water-use 

estimates, and reported data. Estimated pumpage amounts 

and distribution are evident in the middle Claiborne aquifer in the Grand Prairie area where there is an increase in 

pumping through 1958, then a sharp reduction around 1960, 

followed by a continued increase to 2006 (fig. 18). The sharp 

reduction around 1960 is an artifact of a change in pumping 

distribution based on a water-use trend analysis using State 

level data to a distribution based on county level data (Clark 

and Hart, 2009). At the regional scale, these water-use and 

pumping distribution discrepancies tend to be less noticeable (fig. 12). One of the primary reasons for this study is to 

evaluate and understand the availability of water for human 

(including agricultural) and environmental requirements. To 

understand the availability of water for human requirements, 

a reasonable estimate of past, current, and future amounts 

of water use need to be determined. For many areas of the 

Mississippi embayment, records of the amount of water 

for municipal and industrial use are maintained in paper or 

electronic format on a monthly basis. Well locations and 

water-use values in the MERAS model represent one possible 

configuration, but the real configuration (actual well locations and amounts of pumpage over the last 137 years) is to 

a large extent unknown. Reporting and validating of wateruse amounts are crucial to the development of groundwater 

models that can accurately represent groundwater flow in 

regional and local settings. It is notable that the total amount 

of estimated groundwater use in 2005 of almost 12 million acre-ft/yr is 1.27 million acre-ft/yr more than the total 

simulated by the MERAS model (10.73 million acre-ft/yr in 

2005, or 10.65 million acre-ft/yr in 2006). This difference 

is likely the result of drawdown constraints placed on each 

well in the MERAS model noted in Clark and Hart (2009). 

These constraints allowed the desired pumping from a well 

to be decreased or turned off based on the water level in the 

surrounding aquifer. Uncertainty in the simulation of water 

level in the MERAS model may consequently have caused 

the decreased pumping simulated by the model. This difference could also be the result of inaccuracies in the estimate of 

groundwater pumping.

It has long been assumed that there is a large amount 

of interaction between the many miles of streams and canals 

with the aquifers in the study area. To simulate stream leakage in the MERAS model, basic factors such as the thickness 

and hydraulic conductivity of the streambed are required. 

Very few studies exist to quantify these streambed factors. 

Studies are needed to examine the importance of the stream 

interaction on an annual time scale because these studies 

have indicated that over long time periods (annual or longer) 

there is a net zero exchange of water between streams and the 

aquifer system.

There are also limitations associated with the hydroclimate data processing and analysis. The analysis uses 

a low-order polynomial trend to smooth out noise in the 

precipitation values, which results in the inability to forecast 

episodic (high variability) precipitation events. It is possible 

that episodic events induce larger, though perhaps shortterm (less than 1-year) changes in the groundwater system. 

However, it is these short-term changes that may have a larger 

effect on the agriculturally based economy of the Mississippi 

embayment.

Scaling Regional to Local Issues

The MERAS model was designed to evaluate groundwater flow on a regional scale. However, the value of having 

regional perspective provides context such as well-established 

boundary conditions that enables smaller scale analysis. In 

some areas, local information may be available to better represent groundwater flow on a local scale. In these instances, 

the MERAS model may provide a useful structure to build 

the local-scale model and the groundwater-flow boundary 

conditions where hydrogeologic boundaries do not naturally 

exist. An example of this might include the use of data from 

geophysical logs that are pertinent to the local area of interest. These data may be added to the existing regional data to 

create interpretations of local hydrogeologic units for use in 

the local model. One suggestion to incorporate these new data 

is the use of Local Grid Refinement (Mehl and Hill, 2007), 

which allows the interaction of a local-scale model grid with 

the regional-scale model grid. A local-scale model may use 

smaller grid cells with different hydraulic properties and 

more refined vertical layers than the regional model to better 

simulate effects on flow and water levels from local stresses, 

such as pumping.
References 43

Acknowledgments

The Mississippi Embayment Regional Aquifer Study 

(MERAS) was conducted with support from the U.S. Geological Survey Groundwater Resources Program. The authors wish 

to thank many talented individuals that have contributed to 

this Professional Paper. Suggestions by Arlen Harbaugh, Tom 

Reilly, Kevin Dennehy, and Randall T. Hanson were invaluable throughout the modeling and report process, and Alyssa 

Dausman and Geoff Delin, who served as technical reviewers. 

Extensive assistance by Claire Teideman and Matt Tonkin 

were crucial for the analysis of OPR-PPR. The experience 

and knowledge in regional-scale models provided by Claudia 

Faunt was a huge benefit to this report, as well as the preparation of previous MERAS reports.

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Appendixes 1–3 49

Appendixes 1–3
50 Groundwater Availability of the Mississippi Embayment

Appendix 1. Effects of Natural-Climate Trends on Interannual to Multidecadal 

Timescales

To best understand the potential consequences of climate 

trends, research needs to address the effects from natural climate patterns on interannual to multidecadal timescales, which 

are major obstacles to the reliable characterization of global 

climate trends resulting from human activities (Ghil, 2002). 

Interannual to multidecadal natural climate trends across the 

United Staes are affected by various ocean-atmospheric phenomenon, including the El Niño Southern Oscillation (ENSO), 

Pacific Decadal Oscillation (PDO), and Atltantic Multidecadal 

Oscillation (AMO) (Gurdak and others, 2009). Climate variability on these timescales may have a substantial influence on 

spatiotemporal patterns in precipitation, drought, evapotranspiration, streamflow, and groundwater recharge and discharge 

(Hanson and others, 2004).

ENSO is a coupled ocean-atmospheric phenomenon that 

has interannual variability with irregular 2- to 6-year cycles 

between the positive (El Niño) and negative (La Niña) phases 

(fig. 1–1A) (Wolter and Timlin, 1993, 1998). The El Niño generally results in relatively heavier precipitation during winter 

for parts of the United States that include the MERAS study 

area, and the La Niña generally results in drier conditions for 

parts of the United States that include the MERAS study area 

(Wolter and Timlin, 1993, 1998).

PDO is an ENSO-like pattern of Pacific Ocean climate 

variability that affects winter air temperatures and precipitation in the United States and has interdecadal variability with 

irregular 10- to 25-year cycles (fig. 1–1B) (Mantua and Hare, 

2002). PDO was in the positive phase from 1925 to 1946 and 

1977 to 1999 and the negative phase from the early 1900s to 

1924 and 1947 to 1976 (fig. 1–1B) (Mantua and Hare, 2002). 

PDO is currently (2010) in the negative phase, which has been 

associated with drought conditions across much of the United 

States (McCabe and others, 2004).

Periods of climate variability greater than 25 years are 

referred to as greater than PDO (Hanson and others, 2004, 

2006; Gurdak and others, 2007). The greater than PDO cycles 

may be attributed to longer PDO fluctuations with periodicities of 50 to 70 years (Minobe, 1997) or variations in the 

AMO. However, the sources and stationarity of the greater 

than PDO forcings remain uncertain. The AMO is an index 

of sea-surface temperatures averaged over the North Atlantic 

Ocean with irregular 50- to 80-year cycles (fig. 1–1C) that 

has a strong influence on summer precipitation and drought 

frequency in the United States (Enfield and others, 2001). 

AMO was in the positive phases from 1860 to 1880 and 1930 

to 1960, and the negative phases from 1905 to 1925 and 1970 

3

2

1

0

–1

–2

–3

4

2

0

–2

–4

 1860 1880 1900 1920 1940 1960 1980 2000

0.6

0.3

0

–0.3

–0.6

A

B

C

Standardized departures, unitless

Year

Figure 1–1. Interactions between the positive (red) and negative (blue) phases of the A, multivariate El Niño/Southern 

Oscillation (ENSO) index (Wolter and Timlin, 1993, 1998), B, Pacific Decadal Oscillation (PDO) index (Mantua and Hare, 2002), and 

C, Atlantic Multidecadal Oscillation (AMO) index (Enfield and others, 2001) cumulatively affect United States climate and, in turn, 

surface and groundwater resources.
Appendix 1. Effects of Natural-Climate Trends on Interannual to Multidecadal Timescales 51

to 1990 (fig. 1–1C). Since 1995, AMO has been in the positive phase, which may result in above-normal frequencies of 

drought in the United States for the coming decades (McCabe 

and others, 2004).

Recent research has identified interactions between interannual and multidecadal natural climate trends that cumulatively affect recharge (Dickinson and others, 2004; Hanson 

and others, 2004, 2006; Gurdak and others, 2007). Groundwater levels can respond dramatically when climate trends from 

different cycles lie coincident in an increasing (positive or 

wet/warm) or decreasing (negative or dry/cool) phase of variability that affects recharge and discharge. To demonstrate this 

phenomenon, Hanson and others (2004) isolated statistically 

independent oscillatory signals (or cycles) within a synthetic 

hydrologic time-series record to illustrate the cumulative effect 

of the independent signals on the time series. In the example, 

Hanson and others (2004) identified 10-, 6-, and 1-year oscillatory signals that contributed 81.0, 13.1, and 3.2 percent of 

the variances of the synthetic time series, respectively (Hanson and others, 2004). The annual signal in the example is 

less evident during periods of decreasing (negative) variation 

of the 10-year signal; such as around 1907 (see figs. 2 and 3 

of Hanson and others, 2004). Furthermore, drought conditions may exist around 1917 in this example because all three 

signals are in decreasing (negative) phases (see figs. 2 and 3 

of Hanson and others, 2004). Conversely, the annual signal 

is magnified if the 10- and 6-year signals are in an increasing 

(positive) phase of variation, such as around 1913, and may 

result in flooding conditions (see figs. 2 and 3 of Hanson and 

others, 2004).

Gurdak and others (2007) present a real-world example 

of the cumulative effect of statistically independent oscillatory signals on groundwater levels in the High Plains aquifer, 

which covers parts of eight Western States in the Great Plains 

physiographic province of the United States. In the High 

Plains aquifer study, statistically independent ENSO and PDO 

signals in the groundwater levels were found to be partially 

dampened or accentuated by groundwater pumping and other 

climate forcings on interannual to multidecadal time scales, 

such as ENSO, PDO, and AMO (see fig. 9 of Gurdak and others, 2007). For example, the amplitude of ENSO signals in the 

groundwater levels generally was dampened during the negative PDO periods, especially from the mid-1940s to mid-1970s 

(see fig. 9 of Gurdak and others, 2007). Therefore, awareness 

of natural-climate spatiotemporal trends and interactions and 

human stresses on the aquifer system can play a crucial role 

in successful modeling and management of groundwater 

resources (Hanson and others, 2004).
52 Groundwater Availability of the Mississippi Embayment

Appendix 2. Singular Spectrum Analysis

The singular spectrum analysis method is a widely used 

form of principal-component analysis in lag-time domain that 

uses a data-adaptive signal-to-noise enhancement to detect 

periodic signals in short, noisy time series (Vautard and others, 1992). The data-adaptive enhancements help separate the 

time series into reconstructed components that are statistically 

independent and thus can be classified into trends, oscillatory patterns, and noise (Ghil and others, 2002). The singular 

spectrum analysis method has been shown to be optimal in 

the sense of capturing the maximum variance with the fewest independent reconstructed components (Shun and Duffy, 

1999). The variability in most hydrologic time series can be 

adequately described in terms of the first 10 reconstructed 

components and often takes the form of quasi-periodic or 

near-sinusoidal oscillations that are related to interannual to 

multidecadal natural climate variability (Hanson and others, 

2004). A Chi-squared significance test (Allen and Smith, 1996; 

Ghil and others, 2002) was performed on each of the first 

10 reconstructed components to test their individual significance against a red-noise null-hypothesis at the 95-percent 

confidence level. Statistically significant reconstructed components from time series at the wells, meteorological stations, 

and streamflow-gaging stations are listed in table 2–1.

The results of the singular spectrum analysis indicate that 

all the time series from the selected groundwater-level sites, 

meteorological stations, and streamflow-gaging stations contain variations within the ranges consistent with ENSO, PDO, 

and greater than PDO (fig. 2–1A–C and table 2–1). Other 

reconstructed components were identified between the known 

ENSO and PDO cycles with periodicities between 6 and 

10 years (fig. 2–1A–C and table 2–1). The specific cause of 

variations in hydrologic time series with periodicities of 6 to 

10 years remains uncertain. Hanson and others (2004) noted 

that these variations may be related to a periodic persistence 

of northerly movement of subtropical moisture from the Gulf 

of Mexico or from the Pacific Ocean, which may be caused 

by the North American Monsoon System (NAMS). However, 

Fye and others (2006) identified 7- to 8-year periodicities in 

precipitation variability across North America that may originate from the North Atlantic Oscillation. The 6- to 10-year 

cycles identified in the MERAS area have variance generally 

less than 20 and 30 percent in the precipitation and streamflow 

time series, respectively, but contain up to 60 percent of the 

variance in select reconstructed components of the groundwater levels (fig. 2–1A–C). The uncertain origin of the 6- to 

10-year forcings and the relative strength of these signals in 

groundwater levels indicates that further study is needed to 

better understand the processes causing these relatively important variations in MERAS groundwater levels. 

The majority of the variance in the precipitation time 

series was attributed to greater than PDO (greater than 

25 years) and PDO periods (10 to 25 years), capturing 

approximately 10 to 80 percent and 10 to 70 percent of the 

variance, respectively (fig. 2–1A). The greater than PDO and 

PDO climatic forcings apparently have more control on precipitation variability in the MERAS than the higher-frequency 

forcings, such as NAMS and ENSO. Lower-frequency periodicities in the precipitation time series cluster around 20.8, 27.8, 

and 41.7 years (fig. 2–1A), which are similar to the approximate 20- to 22- and 28-year periodicities previously indentified in precipitation time series of the High Plains aquifer of 

the central United States and identified as related to PDO and 

AMO variability (Gurdak and others, 2007). 

McCabe and others (2004) showed that spatio-temporal 

patterns in precipitation variability and drought frequency (in 

percent of years) across the conterminous United States can 

be attributed to the interactions between the PDO and AMO 

phases of variability. ENSO variability can also augment or 

diminish the cumulative effect of PDO and AMO on MERAS 

precipitation. McCabe and others (2004) found that much 

of the United States has a relatively low drought frequency 

during the positive PDO and negative AMO phases, which is 

observed as relatively high average annual precipitation in the 

MERAS such as during the positive PDO and negative AMO 

phases of the late 1970s and early 1980s (fig. 1–1B–C and 

fig. 22). The moderate El Niño of these periods may help to 

augment average annual precipitation in MERAS (fig. 1–1A). 

During the negative PDO and negative AMO phases, McCabe 

and others (2004) identified higher drought frequency across 

parts of the United States, including parts of the Great Plains 

physiographic province and the southwestern part of MERAS 

in the West Gulf Coastal Plain physiographic section (fig. 1), 

and relatively lower drought frequency across the eastern part 

of the MERAS in the East Gulf Coastal Plain physiographic 

section (fig. 1). The spatio-temporal response in precipitation 

to the negative PDO and negative AMO phases (fig. 21B–C) 

is illustrated during the mid-1960s and the early- to mid1970s as relatively low average annual precipitation at the 

El Dorado and western precipitation sites and relatively high 

average annual precipitation for the Jackson and Memphis 

precipitation sites in eastern MERAS (fig. 22). Conversely, the 

opposite spatio-temporal patterns were identified by McCabe 

and others (2004) during the positive PDO and positive AMO 

that results in lower drought frequency in the southwestern 

part of MERAS in the West Gulf Coastal Plain physiographic 

section (fig. 1), and relatively higher drought frequency across 

the eastern part of the MERAS in the East Gulf Coastal Plain 

physiographic section (fig. 1). The spatio-temporal response 

of precipitation in the MERAS (fig. 22) is not entirely consistent with McCabe and other’s (2004) United States drought 

frequency patterns during the positive PDO and positive AMO 

phases (fig. 1–1B–C). For example, during the positive PDO 

and positive AMO phases of the late-1950s to early-1960s, 

late-1990s, and around 2005, the average annual precipitation 
Appendix 2. Singular Spectrum Analysis 53

Table 2–1. Summary of the period and percent variance for reconstructed components from the singular spectrum analysis.

[--, reconstructed components were not estimated based on Chi-squared significance test; darker shaded cells have periods consistent with greater than Pacific 

Decadal Oscillation (greater than 25 years); lighter shaded cells have periods consistent with Pacific Decadal Oscillation (10 to 25 years); thicker bounded cells 

have periods between 6 and 10 years; thinner bounded cells have periods consistent with El Niño Southern Oscillation (2 to 6 years)]

Site 

number

Start 

year

End 

year

Number

of 

years

Period, in years (percent variance) of reconstructed components from the singular spectrum analysis

1 2 3 4 5 6 7 8 9 10

Precipitation sites

11084 1904 1994 90 27.8

(72.1)

 -- -- 6.0 

(0.5)

4.6 

(0.3)

3.6 

(0.3)

2.8 

(0.2)

2.6 

 (0.2)

2.2 

(0.1)

 --

12813 1918 1994 76 27.8 

(70.0)

20.8 

(19.6)

 -- 4.9 

(1.8)

4.0 

(1.3)

2.8 

(0.6)

2.5 

(0.6)

2.1 

(0.5)

 -- --

13160 1893 1994 101 27.8 

(52.1)

 -- 8.3 

(6.9)

6.9 

(3.7)

4.9 

(1.5)

4 

(1.4)

3.3 

(1.1)

3.0 

(0.5)

 -- --

15749 1940 1994 54 20.8 

(68.7)

 -- 5.2 

(2.9)

3.6 

 (1.6)

2.7 

 (1.1)

2.2 

 (0.7)

 -- -- 1.7 

 (0.4)

1.6 

 (0.3)

17366 1882 1994 112 27.8

(42.6)

20.8 

(35.5)

9.3 

(9.9)

7.6 

 (3.9)

5.6 

 (1.6)

4.4 

 (1.0)

3.6 

 (0.8)

3.2 

 (0.6)

2.8 

 (0.4)

2.5 

 (0.4)

18178 1891 1994 103 27.8 

(50.9)

16.7 

(27.8)

9.3 

(7.3)

6.4

 (2.5)

4.9

 (2.2)

4.0

 (2.0)

3.3

 (1.2)

3.0

 (0.6)

 -- --

31596 1884 1994 110 27.8 

(47.5)

 -- -- -- 5.6

 (1.9)

4.4

 (1.2)

 -- 3.2

 (0.5)

2.8

 (0.3)

2.5

 (0.2)

31632 1892 1994 102 20.8 

(42.9)

 -- 8.3

(10.4)

 -- 4.9

 (2.2)

4.2

 (1.0)

3.5

 (0.7)

3.0

 (0.4)

2.1

 (0.3)

2.3

 (0.3)

34572 1904 1994 90 16.7 

(28.1)

11.9

 (27.5)

7.6

(15.8)

5.6

 (6.8)

4.4

 (4.4)

3.6

 (3.4)

3.0

 (2.1)

2.6

 (1.3)

2.3

 (1.1)

2.0

 (1.0)

34756 1887 1994 107 41.7 

(56.8)

 -- 9.3 

(6.8)

 -- 5.2

 (1.9)

4.2 

 (1.3)

3.6 

 (0.9)

3.0 

 (0.5)

2.8 

 (0.4)

 --

35754 1884 1994 110 27.8 

(49)

20.8 

 (27.6)

8.3 

 (6.1)

 -- 4.9 

 (3.8)

 -- 3.6 

 (1.3)

3.2 

 (0.7)

2.8 

 (0.4)

2.5 

 (0.3)

35908 1882 1994 112 41.7 

(60.8)

16.7 

 (27.7)

10.4 

 (5.0)

7.6 

 (2.2)

5.6 

 (1.1)

4.6 

 (0.7)

3.8 

 (0.4)

3.2 

 (0.3)

2.8 

 (0.2)

2.5 

 (0.2)

36253 1894 1994 100 20.8 

(40.5)

 -- 8.3 

 (9.5)

 -- -- -- 3.3 

 (1.0)

3.0 

 (0.6)

2.5 

 (0.6)

 --

160098 1885 1994 109 41.7 

(78.2)

 -- 10.4 

 (2.5)

7.6 

 (1.3)

5.2 

 (0.6)

4.4 

 (0.5)

 -- 3.1 

 (0.2)

2.7 

 (0.1)

2.5 

 (0.1)

160205 1888 1994 106 27.8 

(52.7)

20.8 

 (28.2)

9.3 

 (5.6)

6.9 

 (3.6)

5.2 

 (2.5)

4.0 

 (1.6)

 -- 3.0 

 (0.7)

2.6 

 (0.4)

2.4 

 (0.4)

160537 1894 1994 100 27.8 

(40.6)

16.7 

 (28.3)

8.3 

 (10.5)

 -- -- -- -- 3.0 

 (0.8)

2.4 

 (0.6)

 --

162151 1892 1994 102 27.8 

(63.8)

 -- 9.3 

 (4.2)

6.9 

 (1.7)

4.9 

 (0.7)

4.2 

 (0.6)

3.3 

 (0.6)

2.8 

 (0.4)

 -- 2.3 

 (0.2)

168163 1890 1994 104 41.7 

(75)

 -- 10.4 

 (3.0)

6.9 

 (1.4)

5.2 

 (0.7)

4.2 

 (0.6)

3.5 

 (0.5)

 -- -- 3.0 

 (0.2)

231364 1882 1994 112 41.7 

(61.2)

 -- 10.4 

 (4.6)

7.6 

 (1.1)

5.6 

 (0.5)

4.6 

 (0.5)

3.8 

 (0.4)

3.1 

 (0.2)

2.8 

 (0.2)

2.5 

 (0.1)

232289 1904 1994 90 27.8 

 (46.5)

11.9 

 (22.0)

 -- -- -- -- -- 2.6 

 (0.9)

2.3 

 (0.7)

2.0 

 (0.5)

232809 1878 1994 116 20.8 

 (30.2)

 -- 9.3 

 (18.5)

 -- 5.6 

 (3.3)

4.6 

 (1.7)

3.8 

 (1.3)

3.3 

 (1.1)

2.9 

 (0.8)

2.0 

 (0.5)

235253 1893 1994 101 41.7 

 (60.1)

16.7 

 (22.6)

9.3 

 (7.7)

 -- 4.9 

 (1.6)

4.0 

 (0.8)

3.5 

 (0.5)

2.9 

 (0.4)

2.6 

 (0.2)

2.1 

 (0.2)
54 Groundwater Availability of the Mississippi Embayment

Table 2–1. Summary of the period and percent variance for reconstructed components from the singular spectrum analysis—

Continued 

[--, reconstructed components were not estimated based on Chi-squared significance test; darker shaded cells have periods consistent with greater than Pacific 

Decadal Oscillation (greater than 25 years); lighter shaded cells have periods consistent with Pacific Decadal Oscillation (10 to 25 years); thicker bounded cells 

have periods between 6 and 10 years; thinner bounded cells have periods consistent with El Niño Southern Oscillation (2 to 6 years)]

Site 

number

Start 

year

End 

year

Number

of 

years

Period, in years (percent variance) of reconstructed components from the singular spectrum analysis

1 2 3 4 5 6 7 8 9 10

Precipitation sites—Continued

220021 1886 1994 108 27.8 

 (46.0)

16.7 

 (25.7)

9.3 

 (11.7)

6.9 

 (5.4)

5.0 

 (2.5)

4.2 

 (1.2)

3.5 

 (1.2)

 -- -- --

220488 1882 1994 112 41.7 

 (62.2)

20.8 

 (25.1)

10.4 

 (5.8)

 -- 5.6 

 (1.3)

4.6 

 (0.8)

3.8 

 (0.4)

3.3 

 (0.3)

2.4 

 (0.2)

2.5 

 (0.2)

220955 1889 1994 105 41.7 

 (70.1)

 -- 10.4 

 (3.5)

 -- -- -- -- 3.1 

 (0.3)

 -- --

221094 1874 1994 120 41.7 

 (62.6)

 -- -- -- 6.4 

 (0.7)

 -- 4.2 

 (0.6)

3.3 

 (0.3)

3.1 

 (0.2)

 --

221865 1903 1994 91 27.8 

 (52.5)

 -- 8.3 

 (7.9)

6.0 

 (2.5)

4.4 

 (1.4)

3.8 

 (1.2)

3.1 

 (0.7)

2.5 

 (0.5)

2.3 

 (0.5)

2.0 

 (0.3)

223887 1890 1994 104 41.7 

 (68.0)

 -- 9.3 

 (3.3)

6.9 

 (1.2)

5.2 

 (0.6)

4.2 

 (0.4)

3.5 

 (0.3)

3.0 

 (0.2)

2.6 

 (0.2)

2.4 

 (0.1)

223975 1882 1994 112 41.7 

 (60.7)

20.8 

 (23.9)

10.4 

 (5.3)

7.6 

 (2.8)

5.6 

 (2.1)

4.4 

 (0.9)

3.8 

 (0.5)

3.3 

 (0.4)

2.7 

 (0.3)

2.5 

 (0.3)

225247 1888 1994 106 41.7 

 (57.9)

16.7 

 (21.0)

9.3 

 (7.0)

6.9 

 (3.8)

 -- -- -- -- -- --

229439 1888 1994 106 41.7 

 (69.7)

 -- 10.4 

 (5.1)

6.9 

 (1.8)

5.2 

 (0.7)

4.2 

 (0.5)

 -- 3.1 

 (0.2)

2.5 

 (0.1)

2.4 

 (0.1)

401790 1885 1994 109 41.7 

 (56.2)

20.8 

 (26.0)

9.3 

 (6.2)

 -- 5.2 

 (2.1)

4.4 

 (1.3)

3.6 

 (0.9)

3.2 

 (0.6)

2.7 

 (0.3)

2.5 

 (0.3)

402589 1898 1994 96 27.8 

 (50.4)

 -- 7.6 

 (6.9)

 -- -- -- 3.2 

 (1.1)

2.8 

 (0.5)

2.5 

 (0.4)

2.2 

 (0.3)

404561 1891 1994 103 41.7 

 (67.8)

 -- 9.3 

 (3.6)

 -- -- -- 3.5 

 (0.5)

3.0 

 (0.3)

2.6 

 (0.2)

2.3 

 (0.2)

Streamflow sites

02467000 1928 2009 81 20.8 

 (45.7)

11.9 

 (28.3)

6.9 

 (9.6)

5.2 

 (4.0)

3.8 

 (2.8)

 -- -- -- 2.6 

 (0.9)

2.3 

 (0.5)

07022000 1933 2009 76 20.8 

 (62.5)

 -- 6.9 

 (5.3)

4.9 

 (1.8)

3.8 

 (0.9)

3.1 

 (0.5)

 -- -- 2.5 

 (0.2)

2.2 

 (0.1)

07029500 1929 2009 80 27.8 

 (58.8)

11.9 

 (24.1)

7.0 

 (7.3)

 -- 3.8 

 (1.3)

 -- -- -- 2.6 

 (0.5)

2.3 

 (0.4)

07043500 1945 2009 64 20.8 

 (64.0)

10.4 

 (21.5)

5.6 

 (5.8)

 -- 3.1 

 (1.6)

2.5 

 (1.0)

2.1 

 (0.5)

1.9 

 (0.3)

 -- --

07063000 1936 2009 73 20.8 

 (57.0)

 -- -- -- -- -- -- -- -- --

07074500 1927 2009 82 27.8 

 (61.7)

 -- 6.9 

 (4.7)

 -- 4.0 

 (1.0)

3.3 

 (0.5)

2.8 

 (0.3)

 -- -- 2.3 

 (0.2)

07264000 1954 2009 55 13.9 

 (48.6)

8.3 

 (27.9)

4.6 

 (9.5)

3.5 

 (4.2)

2.7 

 (1.9)

 -- -- 2.2 

 (1.2)

1.8 

 (0.5)

1.5 

 (0.4)

07268000 1938 2009 71 20.8 

 (47.1)

11.9 

 (26.1)

6.0 

 (8.6)

4.4 

 (5.1)

3.3 

 (3.2)

2.8 

 (1.9)

2.3 

 (1.2)

1.0 

 (0.8)

 -- 1.9 

 (0.7)
Appendix 2. Singular Spectrum Analysis 55

Table 2–1. Summary of the period and percent variance for reconstructed components from the singular spectrum analysis—

Continued 

[--, reconstructed components were not estimated based on Chi-squared significance test; darker shaded cells have periods consistent with greater than Pacific 

Decadal Oscillation (greater than 25 years); lighter shaded cells have periods consistent with Pacific Decadal Oscillation (10 to 25 years); thicker bounded cells 

have periods between 6 and 10 years; thinner bounded cells have periods consistent with El Niño Southern Oscillation (2 to 6 years)]

Site number Start 

year

End 

year

Number 

of 

years

Period, in years (percent variance) of reconstructed components from the singular 

spectrum analysis

1 2 3 4 5 6 7 8 9 10

Groundwater-levels sites

321709092452401 1960 2008 48 13.9 

 (66.0)

6.9 

 (16.2)

 -- -- -- -- -- 1.3 

 (0.8)

1.2 

 (0.7)

1.0 

 (0.6)

312206093311001 1965 2008 43 9.26 

 (26.1)

7.0 

 (23.1)

 -- -- 3.1 

 (8.5)

2.7 

 (6.1)

2.0 

 (2.8)

1.2 

 (1.6)

 -- --

313510093282101 1954 1999 45 16.7 

 (30.1)

 -- -- 6.9 

 (11.5)

3.3 

 (6.7)

2.8 

 (6.0)

2.0 

 (3.3)

1.8 

 (2.4)

1.4 

 (1.9)

1.3 

 (1.8)

314708093385201 1955 1987 33 10.4 

 (24.3)

4.9 

 (14.6)

2.6 

 (9.8)

2.1 

 (8.7)

 -- -- 1.4 

 (6.8)

 -- -- -- 

315444092122801 1980 2008 28 6.9 

 (32.1)

5.6 

 (21.1)

 -- -- 1.9 

 (6.4)

1.6 

 (5.4)

0.8 

 (3.7)

 -- -- -- 

324626091543901 1955 2008 53 -- 8.3 

 (24.6)

4.6 

 (6.0)

3.3 

 (3.9)

2.5 

 (3.0)

2.0 

 (2.0)

1.7 

 (1.5)

1.5 

 (1.3)

1.0 

 (1.3)

1.0 

 (1.1)

315450092310102 1971 2007 36 12.0 

 (68.4)

6.4 

 (22.0)

 -- 1.0 

 (1.6)

 -- -- 0.8 

 (0.7)

1.4 

 (0.4)

0.7 

 (0.2)

0.5 

 (0.2)

320153093583601 1982 2008 26 5.6 

 (28.2)

3.8 

 (26.0)

2.3 

 (12.8)

 -- -- 1.6 

 (4.4)

1.4 

 (2.4)

0.8 

 (1.3)

 -- 0.6 

 (0.9)

320316093114201 1981 2008 27 -- -- 3.2 

 (16.9)

 -- -- 1.6 

 (6.2)

0.8 

 (4.4)

0.8 

 (3.2)

0.7 

 (2.0)

0.6 

 (1.5)

320541092291601 1978 2008 30 13.9 

 (77.3)

6.0 

 (13.4)

2.7 

 (2.7)

 -- -- -- 1.5 

 (0.6)

0.8 

 (0.3)

0.7 

 (0.2)

0.7 

 (0.2)

315521093343801 1977 2008 31 13.9 

 (75.7)

7.6 

 (14.7)

2.5 

 (2.3)

2.0 

 (1.8)

 -- -- -- -- -- -- 

315948092300301 1979 2007 28 10.4 

 (66.0)

5.2 

 (20.0)

2.5 

 (4.4)

1.8 

 (2.4)

 -- -- -- -- 0.7 

 (0.4)

0.6 

 (0.3)

320555092043501 1989 2008 19 -- -- -- -- 1.6 

 (6.8)

0.7 

 (3.7)

0.6 

 (1.9)

0.5 

 (1.6)

0.5 

 (1.0)

 -- 

320731093561901 1955 1983 28 9.3 

 (61.2)

 -- 2.5 

 (5.4)

 -- -- -- -- -- -- -- 

321709092452401 1960 2008 48 -- 10.4 

 (16.2)

 -- -- -- -- 1.2 

 (0.9)

1.1 

 (0.8)

 -- 0.9 

 (0.6)

321714092041402 1965 2007 42 12.0 

 (27.0)

6.4 

 (17.1)

 -- -- 3.3 

 (8.5)

2.5 

 (6.3)

2.0 

 (4.7)

1.5 

 (3.0)

1.4 

 (2.8)

1.2 

 (1.7)

321714092041401 1965 2007 42 12.0 

 (35.0)

6.9 

 (18.8)

 -- -- 3.6 

 (6.8)

2.3 

 (3.8)

2.0 

 (3.5)

1.5 

 (3.2)

1.3 

 (2.5)

 -- 

322357092341701 1974 2008 34 12.0 

 (61.7)

7.6 

 (19.6)

3.0 

 (3.9)

2.3 

 (3.1)

 -- -- 1.5 

 (1.5)

1.4 

 (1.5) 

0.8 

 (0.6)

 -- 

322422092020701 1965 2007 42 16.7 

 (52.0)

16.7 

 (16.4)

 -- -- 1.8 

 (4.6)

 -- -- -- -- -- 

322518093381701 1955 1987 32 7.6 

 (44.7)

 -- 2.8 

 (8.5)

2.1 

 (3.9)

 -- -- -- -- -- -- 
56 Groundwater Availability of the Mississippi Embayment

Table 2–1. Summary of the period and percent variance for reconstructed components from the singular spectrum analysis—

Continued 

[--, reconstructed components were not estimated based on Chi-squared significance test; darker shaded cells have periods consistent with greater than Pacific 

Decadal Oscillation (greater than 25 years); lighter shaded cells have periods consistent with Pacific Decadal Oscillation (10 to 25 years); thicker bounded cells 

have periods between 6 and 10 years; thinner bounded cells have periods consistent with El Niño Southern Oscillation (2 to 6 years)]

Site number Start 

year

End 

year

Number 

of 

years

Period, in years (percent variance) of reconstructed components from the singular 

spectrum analysis

1 2 3 4 5 6 7 8 9 10

Groundwater-levels sites—Continued

322531092053901 1965 2005 40 10.4 

 (46.3)

6.4 

 (28.8)

3.5 

 (8.2)

2.7 

 (4.0)

 -- -- 2.0 

 (1.8)

1.5 

 (1.5)

1.3 

 (1.2)

0.9 

 (0.7)

322649092063101 1946 1989 43 10.4 

 (42.1)

6.9 

 (30.0)

4.0 

 (8.4)

 -- -- 2.8 

 (2.6)

2.1 

 (1.8)

1.7 

 (1.4)

1.3 

 (0.9)

1.3 

 (0.8)

322841092063301 1946 1974 28 7.6 

 (37.3)

4.6 

 (24.7)

2.5 

 (8.4)

1.9 

 (4.4)

1.2 

 (3.6)

1.1 

 (3.6)

0.6 

 (2.4)

 -- 0.8 

 (1.9)

0.8 

 (1.6)

322901092091501 1946 1990 44 -- -- 3.6 

 (7.0)

 -- 1.0 

 (3.1)

1.0 

 (2.9)

 -- -- -- -- 

323029091430001 1969 2008 39 8.3 

 (41.0)

 -- 3.2 

 (9.1)

 -- -- -- 1.9 

 (2.2)

1.5 

 (1.1)

1.3 

 (0.8)

 -- 

323030091554801 1965 2007 42 13.9 

 (65.7)

8.3 

 (23.8)

4.0 

 (3.3)

2.8 

 (1.5)

 -- -- 2.1 

 (0.7)

0.9 

 (0.4)

1.4 

 (0.3)

1.4 

 (0.3)

323100092165802 1969 2008 39 10.4 

 (29.8)

11.9 

 (18.5)

 -- -- -- 2.5 

 (5.2)

 -- 1.5 

 (3.3)

1.8 

 (3.0)

0.8 

 (1.5)

323251092551901 1940 1989 49 -- 10.4 

 (20.6)

 -- -- 1.0 

 (2.6)

1.0 

 (2.6)

 -- 1.2 

 (1.7)

1.2 

 (1.5)

 -- 

323458092275101 1968 2008 40 11.9 

 (46.5)

5.6 

 (25.1)

 -- -- 1.6 

 (1.9)

1.6 

 (1.8)

 -- -- 1.2 

 (1.4)

 -- 

323505092535001 1970 2008 38 11.9 

 (34.0)

5.6 

 (17.8)

3.1 

 (9.6) 

2.5 

 (6.7)

 -- -- 1.9 

 (2.5)

 -- -- 1.4 

 (2.1)

324508091252301 1955 2008 53 13.9 

 (43.6)

9.3 

 (26.9)

4.4 

 (6.9)

 -- -- 3.5 

 (4.0)

2.5 

 (1.7)

2.1 

 (1.0)

1.2 

 (0.8)

1.2 

 (0.7)

324707093025001 1955 2008 53 11.9 

 (38.1)

7.6 

 (32.7)

4.6 

 (8.6)

 -- -- -- 0.9 

 (0.9)

1.0 

 (0.9)

1.0 

 (0.8)

0.7 

 (0.8)

324753091471202 1968 2008 40 -- 9.3 

 (18.7)

 -- 4.9 

 (13.6)

 -- 2.3 

 (5.8)

1.8 

 (4.4)

1.5 

 (2.3)

0.8 

 (1.7)

1.3 

 (1.6)

325100091132401 1955 2008 53 11.9 

 (16.3)

 -- -- 5.2 

 (13.6)

4.0 

 (10.4)

3.2 

 (7.1)

2.5 

 (2.9)

1.4 

 (2.7) 

1.3 

 (2.2)

2.1 

 (1.9)

325518093221901 1979 2008 29 -- 83.3 

 (18.6)

 -- 2.9 

 (15.4)

2.4 

 (10.2)

1.6 

 (6.0)

1.4 

 (3.9)

 -- 0.7 

 (1.8)

0.7 

 (1.2)

325752093271801 1937 1989 52 20.8 

 (67.7)

10.4 

 (21.7)

 -- -- -- 2.1 

 (0.7)

1.5 

 (0.6)

1.5 

 (0.5)

1.2 

 (0.4)

1.1 

 (0.3)

330002092445901 1978 2008 30 10.4 

 (40.2)

4.6 

 (16.9)

2.5 

 (9.1)

 -- -- -- 1.4 

 (4.2)

0.8 

 (2.0)

 -- -- 

331438092411901 1942 2008 66 20.8 

 (64.0)

 -- 5.9 

 (4.3)

4.2 

 (2.1)

3.2 

 (1.1)

2.5 

 (0.6)

2.3 

 (0.6)

 -- -- 1.8 

 (0.4)

331609093144902 1953 2008 55 -- 10.4 

 (14.7)

 -- -- -- 1.2 

 (2.9)

1.2 

 (2.7)

 -- -- -- 

341138091551601 1958 2008 50 11.9 

 (36.8)

7.6 

 (31.6)

4.0 

 (9.0)

3.1 

 (5.0)

2.3 

 (3.1)

1.9 

 (2.0)

1.6 

 (1.4)

1.0 

 (1.1)

1.0 

 (1.0)

1.3 

 (0.8)
Appendix 2. Singular Spectrum Analysis 57

was relatively high for much of the MERAS, except for the 

El Dorado, Jonesboro, and other western sites during late1990s, and around 2005 (fig. 22). During these periods, ENSO 

was in the El Niño phase, which may help to diminish the 

drought frequency patterns across parts of the MERAS as 

identified by McCabe and others (2004). Similar to the positive 

PDO and positive AMO patterns, McCabe and others (2004) 

observed that during the negative PDO and positive AMO most 

of the United States has a relatively higher drought frequency 

except for the southeastern corner of MERAS area, which was 

observed to have a lower frequency of drought. The spatiotemporal response in precipitation to the negative PDO and 

positive AMO phases (fig. 1–1B–C) is illustrated during the 

1950s and from about 2007 to 2010 as having relatively low 

average annual precipitation at the MERAS sites (fig. 22). During these periods ENSO was in the La Niña phase, which may 

have augmented the drought frequency patterns across parts of 

the MERAS as identified by McCabe and others (2004).

The majority of the variance in the streamflow time series 

was attributed to PDO periods (10 to 25 years), capturing 

approximately 20 to 70 percent of the variance and having 

periodicities that cluster around 10.4, 11.9, and 20.8 years 

(table 2–1 and fig. 2–1B). Two streamflow time series had 

reconstructed components consistent with greater than PDO 

(greater than 25 years) capturing approximately 60 percent of 

the variance and having periodicities at 27.8 years (table 2–1 

and fig. 2–1B). Higher-frequency periodicities also are consistent with the 6- to 10-year period and ENSO (2 to 6 years), but 

capture less than 25 percent of the variance in the streamflow 

time series.

The majority of the variance in the groundwater level 

time series was attributed to PDO periods (10 to 25 years), 

capturing approximately 20 to 80 percent of the variance, and 

having periodicities that cluster around 10.4, 11.9, 13.9, 16.7, 

and 20.8 years (table 2–1 and fig. 2–1C). One groundwater 

level time series had a reconstructed component consistent 

with greater than PDO (greater than 25 years) capturing 

approximately 20 percent of the variance and having a periodicity at 83.3 years (table 2–1 and fig. 2–1C). Higher-frequency 

periodicities also are consistent with the 6- to 10-year period 

and ENSO (2 to 6 years) and capture between less than 10 and 

60 percent of the variance in the groundwater level time series.

Period, in years

0 10 20 30 40 50 60 70 80 90 

100

80

60

40

20

0

Variance, in percent100

80

60

40

20

0

100

80

60

40

20

0

A

C

B

El Nino

Southern

Oscillation

Pacific

Decadal

Oscillation

Greater than Pacific Decadal Oscillation

Figure 2–1. Statistically significant reconstructed components based on the Chi-squared significance test from the singular 

spectrum analysis for time series of A, precipitation at meteorological stations, B, streamflow at streamflow-gaging stations, 

and C, groundwater levels at wells from the study area indicate that all time series contain variations that are consistent with El 

Niño Southern Oscillation (2 to 6 years), Pacific Decadal Oscillation (10 to 25 years), and greater than Pacific Decadal Oscillation 

(greater than 25 years) periods, which are indicated by the gray shading. Additional variations with periodicities of 6 to 10 years 

were observed in all three types of hydrologic time series.
58 Groundwater Availability of the Mississippi Embayment

Appendix 3. Extrapolated 30-Year Precipitation Values Interpolated Spatially 

across the Mississippi Embayment Regional Aquifer Study Area

Figure 3–1. The extrapolated 30-year 

precipitation values interpolated 

spatially across the Mississippi 

Embayment Regional Aquifer Study 

area are shown as A, 2-year average 

extrapolated annual precipitation 

(in inches) and as B, deviation of 

the 2-year average extrapolated 

annual precipitation from the average 

observed annual precipitation (in 

percent) for the time period 2008 to 

2038.

A B

2008 to 2010

2010 to 2012

2012 to 2014

92° 90° 88° 92° 90° 88°

34°

38°

36°

32°

34°

38°

36°

32°

34°

38°

36°

32°

34°

38°

36°

32°

34°

38°

36°

32°

34°

38°

36°

32°

0 50 100 MILES

0 50 100 KILOMETERS

EXPLANATION

Deviation of 2-year average 

 extrapolated annual precipitation 

 from the average observed annual 

 precipitation, in percent 

2-year average extrapolated 

 annual precipitation, in inches 

75

65

55

45

35

25

Meteorological

 station

.

30

20

5

–5

–25

–40

–60
Appendix 3. Extrapolated 30-Year Precipitation 59

A B

2014 to 2016

2016 to 2018

2018 to 2020

92° 90° 88° 92° 90° 88°

34°

38°

36°

32°

34°

38°

36°

32°

34°

38°

36°

32°

34°

38°

36°

32°

34°

38°

36°

32°

34°

38°

36°

32°

0 50 100 MILES

0 50 100 KILOMETERS

EXPLANATION

Deviation of 2-year average 

 extrapolated annual precipitation 

 from the average observed annual 

 precipitation, in percent 

2-year average extrapolated 

 annual precipitation, in inches 

75

65

55

45

35

25

Meteorological

 station

.

30

20

5

–5

–25

–40

–60

Figure 3–1. The extrapolated 30-year 

precipitation values interpolated 

spatially across the Mississippi 

Embayment Regional Aquifer Study 

area are shown as A, 2-year average 

extrapolated annual precipitation 

(in inches) and as B, deviation of 

the 2-year average extrapolated 

annual precipitation from the average 

observed annual precipitation (in 

percent) for the time period 2008 to 

2038.—Continued
60 Groundwater Availability of the Mississippi Embayment

A B

2020 to 2022

2022 to 2024

2024 to 2026

92° 90° 88° 92° 90° 88°

34°

38°

36°

32°

34°

38°

36°

32°

34°

38°

36°

32°

34°

38°

36°

32°

34°

38°

36°

32°

34°

38°

36°

32°

0 50 100 MILES

0 50 100 KILOMETERS

EXPLANATION

Deviation of 2-year average 

 extrapolated annual precipitation 

 from the average observed annual 

 precipitation, in percent 

2-year average extrapolated 

 annual precipitation, in inches 

75

65

55

45

35

25

Meteorological

 station

.

30

20

5

–5

–25

–40

–60

Figure 3–1. The extrapolated 30-year 

precipitation values interpolated 

spatially across the Mississippi 

Embayment Regional Aquifer Study 

area are shown as A, 2-year average 

extrapolated annual precipitation 

(in inches) and as B, deviation of 

the 2-year average extrapolated 

annual precipitation from the average 

observed annual precipitation (in 

percent) for the time period 2008 to 

2038.—Continued
Appendix 3. Extrapolated 30-Year Precipitation 61

A B

2026 to 2028

2028 to 2030

2030 to 2032

92° 90° 88° 92° 90° 88°

34°

38°

36°

32°

34°

38°

36°

32°

34°

38°

36°

32°

34°

38°

36°

32°

34°

38°

36°

32°

34°

38°

36°

32°

0 50 100 MILES

0 50 100 KILOMETERS

EXPLANATION

Deviation of 2-year average 

 extrapolated annual precipitation 

 from the average observed annual 

 precipitation, in percent 

2-year average extrapolated 

 annual precipitation, in inches 

75

65

55

45

35

25

Meteorological

 station

.

30

20

5

–5

–25

–40

–60

Figure 3–1. The extrapolated 30-year 

precipitation values interpolated 

spatially across the Mississippi 

Embayment Regional Aquifer Study 

area are shown as A, 2-year average 

extrapolated annual precipitation 

(in inches) and as B, deviation of 

the 2-year average extrapolated 

annual precipitation from the average 

observed annual precipitation (in 

percent) for the time period 2008 to 

2038.—Continued
62 Groundwater Availability of the Mississippi Embayment

A B

2032 to 2034

2034 to 2036

2036 to 2038

92° 90° 88° 92° 90° 88°

34°

38°

36°

32°

34°

38°

36°

32°

34°

38°

36°

32°

34°

38°

36°

32°

34°

38°

36°

32°

34°

38°

36°

32°

0 50 100 MILES

0 50 100 KILOMETERS

EXPLANATION

Deviation of 2-year average 

 extrapolated annual precipitation 

 from the average observed annual 

 precipitation, in percent 

2-year average extrapolated 

 annual precipitation, in inches 

75

65

55

45

35

25

Meteorological

 station

.

30

20

5

–5

–25

–40

–60

Figure 3–1. The extrapolated 30-year 

precipitation values interpolated 

spatially across the Mississippi 

Embayment Regional Aquifer Study 

area are shown as A, 2-year average 

extrapolated annual precipitation 

(in inches) and as B, deviation of 

the 2-year average extrapolated 

annual precipitation from the average 

observed annual precipitation (in 

percent) for the time period 2008 to 

2038.—Continued
Appendix 3. Extrapolated 30-Year Precipitation 63

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 Lafayette and Rolla Publishing Service Centers 

For additional information concerning this publication, contact:

 Director, USGS Arkansas Water Science Center

 401 Hardin Road

 Little Rock, AR 72211

 (501) 228–3600

Or visit the Arkansas Water Science Center Web site at:

http://ar.water.usgs.gov
64 Groundwater Availability of the Mississippi Embayment

Groundwater Availability of 

the Mississippi Embayment

Clark and others—Groundwater Availability of the Mississippi Embayment—Professional Paper 1785

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