U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2012–5154

Revised November 2012

Prepared in cooperation with the Harris–Galveston Subsidence District, 

the Fort Bend Subsidence District, and 

the Lone Star Groundwater Conservation District

Hydrogeology and Simulation of Groundwater Flow and 

Land-Surface Subsidence in the Northern Part of the 

Gulf Coast Aquifer System, Texas, 1891–2009

TEXAS LOUISIANA

Houston Area

Groundwater Model

study area

West Fork San Jacinto River

Colorado River

 Addicks

Reservoir

 Lake

Houston

Galveston

 Bay

 Reservoir

Barker

Trinity River

 Lake

Livingston

 Lake

Conroe

Brazos

River

Neches River

Sabine River

Sam

 Rayburn

 Reservoir

Toledo

 Bend

Reservoir

B A Steinhagen

 Lake

Calcasieu

 Lake

Sabine

 Lake

Matagorda

 Bay

Somerville

 Lake

Lake

 Texana

Lavaca River

GULF OF MEXICO

West Fork San Jacinto River

Colorado River

 Addicks

Reservoir

 Lake

Houston

Galveston

 Bay

 Reservoir

Barker

Trinity River

 Lake

Livingston

 Lake

Conroe

Brazos

River

Neches River

Sabine River

Sam

 Rayburn

 Reservoir

Toledo

 Bend

Reservoir

B A Steinhagen

 Lake

Calcasieu

 Lake

Sabine

 Lake

Matagorda

 Bay

Somerville

 Lake

Lake

 Texana

Lavaca River

GULF OF MEXICO

POLK

SAN

 JACINTO

LIBERTY

GRIMES

MONTGOMERY

WALLER

HARRIS

HARDIN

JEFFERSON

CHAMBERS

AUSTIN

COLORADO

WHARTON

WASHINGTON

WALKER

TRINITY

MATAGORDA

BRAZORIA

FORT BEND

JASPER

TYLER

NEWTON VERNON

BEAUREGARD

CAMERON

CALCASIEU

JEFF DAVIS

ALLEN

ORANGE

SABINE

NACOGDOCHES

SHELBY

 SAN

 AUGUSTINE

JACKSON

GONZALES

BURLESON

BRAZOS

BASTROP

LEE

CALDWELL

LAVACA

GALVESTON

ANGELINA

CALHOUN

FAYETTE

SABINETEXAS

LOUISIANA

Huntsville

Houston

Beaumont

 Port 

Arthur

Baytown

Evadale

Hydrogeology and Simulation of 

Groundwater Flow and Land-Surface 

Subsidence in the Northern Part of the 

Gulf Coast Aquifer System, Texas, 1891–2009

By Mark C. Kasmarek

Prepared in cooperation with the Harris–Galveston Subsidence District, 

the Fort Bend Subsidence District, and 

the Lone Star Groundwater Conservation District

Scientific Investigations Report 2012–5154

Revised November 2012

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: 2012

Revised: November 2012

This and other USGS information products are available at http://store.usgs.gov/

U.S. Geological Survey

Box 25286, Denver Federal Center

Denver, CO 80225

To learn about the USGS and its information products visit http://www.usgs.gov/

1-888-ASK-USGS

Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the 

U.S. Government.

Although this report is in the public domain, permission must be secured from the individual copyright owners to 

reproduce any copyrighted materials contained within this report.

Suggested citation:

Kasmarek, M.C., 2012, Hydrogeology and simulation of groundwater flow and land-surface subsidence in the northern 

part of the Gulf Coast aquifer system, Texas, 1891–2009: U.S. Geological Survey Scientific Investigations Report 

2012–5154, 55 p. Revised November 2012.
iii

Acknowledgments

The author thanks Robert Gabrycsh (U.S. Geological Survey, retired) and Stan A. Leake 

(U.S. Geological Survey) for contributing their insights and expertise on modeling and the 

mechanism of land-surface subsidence, as well as Jeffery W. East, Michaela R. Johnson, and 

Barclay W. Shoemaker (U.S. Geological Survey) for assisting with computer programming, 

modeling, and geographic information system applications. Mike Reedy of Freese and 

Nichols Inc., John Siefert of LBG-Guyton Associates, and Scott Marr from Fugro–McClelland 

(Southwest), Inc., provided information pertaining to historical water use, subsidence-related 

data, and other information helpful during the modeling process. The author also recognizes 

the Harris–Galveston Subsidence District, the Fort Bend Subsidence District, the Lone Star 

Groundwater Conservation District, the Texas Water Development Board, and the San Jacinto 

River Authority for their updated water-use data used in the Houston Area Groundwater 

Model. Finally, the author would like to thank James Rumbaugh and Doug Rumbaugh of 

Environmental Simulations, Inc., for their timely updates and modification to their graphical 

user interface used with the MODFLOW-2000 Subsidence and Aquifer-System Compaction 

package that made the calibration process more efficient.

v

Contents

Acknowledgments .......................................................................................................................................iii

Abstract ..........................................................................................................................................................1

Introduction ....................................................................................................................................................1

Purpose and Scope .............................................................................................................................2

Previous Studies ..................................................................................................................................2

Description of Study Area for the Houston Area Groundwater Model ......................................2

Hydrogeology .................................................................................................................................................4

Hydrogeologic Units and Geologic Setting .....................................................................................4

Hydraulic Properties ..........................................................................................................................10

Groundwater Flow Conditions, Recharge, and Discharge .........................................................12

Groundwater Development ..............................................................................................................13

Potentiometric Surfaces and Land-Surface Subsidence ...........................................................13

Simulation of Groundwater Flow and Land-Surface Subsidence ......................................................14

Model Description .............................................................................................................................14

Mathematical Representation ................................................................................................14

Grid Design .................................................................................................................................15

Boundaries .................................................................................................................................15

Recharge and Discharge ................................................................................................15

Initial Conditions .......................................................................................................................16

Land-Surface Subsidence and Storage in Clays ........................................................16

Withdrawals ...............................................................................................................................17

Model Calibration ...............................................................................................................................17

Model Results .....................................................................................................................................21

Simulated Hydraulic Properties Associated with Groundwater Flow and 

Subsidence ...................................................................................................................21

Simulated and Measured Hydrographs ................................................................................35

Simulated and Estimated Water-Budget Components .......................................................35

Simulated and Measured Land-Surface Subsidence ........................................................35

Sensitivity Analysis ............................................................................................................................44

Model Limitations ...............................................................................................................................44

Assumption ................................................................................................................................48

Input Data ...................................................................................................................................49

Scale of Application .................................................................................................................50

Summary .......................................................................................................................................................50

References ...................................................................................................................................................52

Figures

1. Map showing location of the Houston Area Groundwater Model study area and 

finite-difference grid, southeastern Texas and southwestern Louisiana ..........................3

2. Hydrogeologic section of the Gulf Coast aquifer system in Harris County and 

adjacent counties, Texas ............................................................................................................5
vi

3. Correlation of stratigraphic and hydrogeologic units in the Houston Area 

Groundwater Model study area ................................................................................................6

4. Map showing extent and outcrop area of the Chicot aquifer in the Houston Area 

Groundwater Model study area ................................................................................................7

5. Map showing extent, outcrop area, and subcrop area of the Evangeline aquifer in 

the Houston Area Groundwater Model study area ................................................................8

6. Map showing extent, outcrop area, and subcrop area of the Burkeville confining 

unit in the Houston Area Groundwater Model study area ....................................................9

7. Map showing extent, outcrop area, and subcrop area of the Jasper aquifer in the 

Houston Area Groundwater Model study area ....................................................................11

8. Graph showing total groundwater withdrawals used during transient Houston 

Area Groundwater Model simulations, by stress periods, 1891–2009 ..............................18

9. Map showing simulated hydraulic conductivity of the Chicot aquifer in the Houston 

Area Groundwater Model study area ....................................................................................22

10. Map showing simulated hydraulic conductivity of the Evangeline aquifer in Houston 

Area Groundwater Model study area ....................................................................................23

11. Map showing simulated hydraulic conductivity of the Jasper aquifer in the Houston 

Area Groundwater Model study area ....................................................................................24

12. Map showing simulated sand storativity of the Chicot aquifer in the Houston Area 

Groundwater Model study area ..............................................................................................25

13. Map showing simulated sand storativity of the Evangeline aquifer in the Houston 

Area Groundwater Model study area ....................................................................................26

14. Map showing simulated sand storativity of the Jasper aquifer in the Houston Area 

Groundwater Model study area ..............................................................................................27

15. Map showing simulated inelastic-clay storativity of the Chicot aquifer in the 

Houston Area Groundwater Model study area ....................................................................28

16. Map showing simulated inelastic-clay storativity of the Evangeline aquifer in the 

Houston Area Groundwater Model study area ....................................................................29

17. Map showing simulated inelastic-clay storativity of the Burkeville confining unit 

in the Houston Area Groundwater Model study area .........................................................30

18. Map showing simulated inelastic-clay storativity of the Jasper aquifer in the 

Houston Area Groundwater Model study area ....................................................................31

19. Map showing simulated and measured potentiometric surfaces of the Chicot 

aquifer, 2009, and location of monitoring wells in the Houston Area Groundwater 

Model study area .......................................................................................................................32

20. Map showing simulated and measured potentiometric surfaces of the Evangeline 

aquifer, 2009, and location of monitoring wells in the Houston Area Groundwater 

Model study area .......................................................................................................................33

21. Map showing simulated and measured potentiometric surfaces of the Jasper 

aquifer, 2009, and location of monitoring wells in the Houston Area Groundwater 

Model study area .......................................................................................................................34

22. Graph showing relation between simulated and measured heads for the Chicot, 

Evangeline, and Jasper aquifers, 2009, in the Houston Area Groundwater Model 

study area ...................................................................................................................................36

23. Map showing spatial distribution of water-level (head) residuals (measured minus 

simulated heads) for the Chicot aquifer, 2009, in the Houston Area Groundwater 

Model study area .......................................................................................................................37

24. Map showing spatial distribution of water-level (head) residuals (measured 

minus simulated heads) for the Evangeline aquifer, 2009, in the Houston Area 

Groundwater Model study area ..............................................................................................38
vii

25. Map showing spatial distribution of water-level (head) residuals (measured minus 

simulated heads) for the Jasper aquifer, 2009, in the Houston Area Groundwater 

Model study area .......................................................................................................................39

26. Graph showing hydrographs showing simulated and measured water levels in 

selected observation wells screened in the Chicot aquifer in A, Brazoria, 

B, Galveston, C, Harris, and D, Fort Bend Counties in the Houston Area 

Groundwater Model study area ..............................................................................................40

27. Graph showing hydrographs showing simulated and measured water levels in 

selected observation wells screened in the Evangeline aquifer in A, Brazoria, 

B, Fort Bend, and C, D, Harris Counties in the Houston Area Groundwater Model 

study area ...................................................................................................................................41

28. Graph showing hydrographs showing simulated and measured water levels in 

selected observation wells screened in the Jasper aquifer in A, B, C, Harris and 

D, Montgomery Counties in the Houston Area Groundwater Model study area ............42

29. Simulated 2009 water-budget components of the hydrogeologic units of the 

Houston Area Groundwater Model ........................................................................................43

30. Map showing simulated (1891–2000) and measured (1906–2000) land-surface 

subsidence in the Houston Area Groundwater Model study area ....................................45

31. Map showing simulated (1891–2009) and measured (1906–2000) land-surface 

subsidence in the Houston Area Groundwater Model study area ....................................46

32. Map showing predictions Relating Effective Stress to Subsidence (PRESS) 

model site locations and PRESS simulated land-surface subsidence, 1906–2000, 

and Houston Area Groundwater Model simulated land-surface subsidence 

(1891–2009) and measured land-surface subsidence (1906–2000) in the Houston 

Area Groundwater Model study area ....................................................................................47

33. Graph showing sensitivity of simulated water levels to changes in selected 

calibrated model input data of the Houston Area Groundwater Model ...........................48

34. Graph showing sensitivity of simulated land-surface subsidence to changes in 

selected calibrated model input data of the Houston Area Groundwater Model ..........49

Tables

1. Groundwater withdrawal (stress) periods used in the Houston Area Groundwater 

Model ...........................................................................................................................................19

2. Comparison of calibrated-parameter values used in the Groundwater Availability 

Model (2004) and the Houston Area Groundwater Model ..................................................20

3. Number of water-level (head) measurements, root-mean-square errors of 

simulated head, and range of total simulated head in the Chicot, Evangeline, and 

Jasper aquifers, 2009 ................................................................................................................35

4. Volumetric budget for the Houston Area Groundwater Model at the end of stress 

period 78, 2009 ............................................................................................................................44
viii

Conversion Factors and Datums

Inch/Pound to SI

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

square mile (mi2) 259.0 hectare (ha)

square mile (mi2) 2.590 square kilometer (km2) 

Volume

gallon (gal) 3.785 liter (L) 

gallon (gal) 0.003785 cubic meter (m3) 

million gallons (Mgal) 3,785 cubic meter (m3)

cubic foot (ft3) 0.02832 cubic meter (m3) 

cubic foot per second (ft3/s) 0.02832 cubic meter per second (m3/s)

cubic foot per day (ft3/d) 0.02832 cubic meter per day (m3/d)

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

Flow rate

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

Mass

pound, avoirdupois (lb) 0.4536 kilogram (kg) 

Pressure

pound per square foot (lb/ft2) 0.04788 kilopascal (kPa) 

Density

pound per cubic foot (lb/ft3) 16.02 kilogram per cubic meter (kg/m3)

pound per cubic foot (lb/ft3) 0.01602 gram per cubic centimeter (g/cm3)

Hydraulic conductivity

foot per day (ft/d) 0.3048 meter per day (m/d)

Hydraulic gradient

foot per mile (ft/mi) 0.1894 meter per kilometer (m/km)

Transmissivity*

foot squared per day (ft2/d) 0.09290 meter squared per day (m2/d) 
ix

SI to Inch/Pound 

Multiply By To obtain

Volume

liter (L) 33.82 ounce, fluid (fl. oz)

liter (L) 2.113 pint (pt)

liter (L) 1.057 quart (qt)

liter (L) 0.2642 gallon (gal)

Mass

gram (g) 0.03527 ounce, avoirdupois (oz)

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 North American Vertical Datum of 1988 

(NAVD 88).

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

Altitude, as used in this report, refers to distance above or below 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.

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

or micrograms per liter (μg/L).

Hydrogeology and Simulation of Groundwater Flow and 

Land-Surface Subsidence in the Northern Part of the 

Gulf Coast Aquifer System, Texas, 1891–2009

By Mark C. Kasmarek

Abstract

In cooperation with the Harris–Galveston Subsidence 

District, Fort Bend Subsidence District, and Lone Star 

Groundwater Conservation District, the U.S. Geological 

Survey developed and calibrated the Houston Area 

Groundwater Model (HAGM), which simulates groundwater 

flow and land-surface subsidence in the northern part of the 

Gulf Coast aquifer system in Texas from predevelopment 

(before 1891) through 2009. Withdrawal of groundwater 

since development of the aquifer system has resulted in 

potentiometric surface (hydraulic head, or head) declines in 

the Gulf Coast aquifer system and land-surface subsidence 

(primarily in the Houston area) from depressurization 

and compaction of clay layers interbedded in the aquifer 

sediments.

The MODFLOW-2000 groundwater flow model 

described in this report comprises four layers, one for each 

of the hydrogeologic units of the aquifer system except the 

Catahoula confining system, the assumed no-flow base of 

the system. The HAGM is composed of 137 rows and 245 

columns of 1-square-mile grid cells with lateral no-flow 

boundaries at the extent of each hydrogeologic unit to the 

northwest, at groundwater divides associated with large rivers 

to the southwest and northeast, and at the downdip limit of 

freshwater to the southeast. The model was calibrated within 

the specified criteria by using trial-and-error adjustment of 

selected model-input data in a series of transient simulations 

until the model output (potentiometric surfaces, land-surface 

subsidence, and selected water-budget components) acceptably 

reproduced field measured (or estimated) aquifer responses 

including water level and subsidence. The HAGM-simulated 

subsidence generally compared well to 26 Predictions Relating 

Effective Stress to Subsidence (PRESS) models in Harris, 

Galveston, and Fort Bend Counties. Simulated HAGM 

results indicate that as much as 10 feet (ft) of subsidence has 

occurred in southeastern Harris County. Measured subsidence 

and model results indicate that a larger geographic area 

encompassing this area of maximum subsidence and much 

of central to southeastern Harris County has subsided at 

least 6 ft. For the western part of the study area, the HAGM 

simulated as much as 3 ft of subsidence in Wharton, Jackson, 

and Matagorda Counties. For the eastern part of the study 

area, the HAGM simulated as much as 3 ft of subsidence at 

the boundary of Hardin and Jasper Counties. Additionally, in 

the southeastern part of the study area in Orange County, the 

HAGM simulated as much as 3 ft of subsidence. Measured 

subsidence for these areas in the western and eastern parts of 

the HAGM has not been documented.

Introduction

The availability of groundwater for municipal, industrial, 

and agricultural uses, as well as the potential subsidence 

associated with groundwater use, has been of concern in the 

Houston, Texas, area for decades (Lang and Winslow, 1950; 

Doyel and Winslow, 1954; Wood, 1956; Wood and others, 

1963; Wood and Gabrysch, 1965; Jorgenson, 1975; Gabrysch 

and Bonnett, 1975; Gabrysch, 1982). In 2004, in cooperation 

with Texas Water Development Board and Harris–Galveston 

Coastal Subsidence District (now known as the Harris–

Galveston Subsidence District), the U.S. Geological Survey 

(USGS) developed a groundwater flow model referred to as 

the “Northern Gulf Coast Groundwater Availability Model” 

(GAM) (Kasmarek and Robinson, 2004), which simulated 

the potentiometric surface (hydraulic head, or head) and 

clay compaction in the main water-bearing units of the Gulf 

Coast aquifer system from 1891 to 2000. Because areal 

distribution of groundwater withdrawals has changed in the 

study area (and subsequently, areas undergoing land-surface 

subsidence as a result) since 2000, a need was identified by 

water managers in the greater Houston area to update the 

GAM (Kasmarek and Robinson, 2004) to more accurately 

reflect recent (2009) conditions. Accordingly, the USGS, in 

cooperation with the Harris–Galveston Subsidence District 

(HGSD), the Fort Bend Subsidence District (FBSD), and 

the Lone Star Groundwater Conservation District (LSGCD), 

prepared a groundwater model of the Houston area, referred 

to hereinafter as the Houston Area Groundwater Model 

(HAGM). The objective of the HAGM is to accurately 

simulate and provide reliable, timely data on groundwater 
2 Hydrogeology and Simulation of Groundwater Flow and Land-Surface Subsidence in the Northern Part of the Gulf Coast Aquifer

availability and land-surface subsidence in the Houston area 

through 2009. Local and regional water managers can use 

the HAGM as a tool to simulate aquifer response (changes in 

water levels and clay compaction) to future estimated water 

demands. The previous model (GAM) simulated groundwater 

flow in the Chicot and Evangeline aquifers and in parts of 

the Burkeville confining unit and Jasper aquifer that contain 

freshwater (Kasmarek and Robinson, 2004, figs. 20 and 21) 

and simulated land-surface subsidence in the Chicot and 

Evangeline aquifers. Like the GAM, the HAGM simulates 

groundwater flow in the Chicot and Evangeline aquifers and 

parts of the Jasper aquifer and Burkeville confining unit, but 

unlike the GAM the HAGM also simulates subsidence in the 

Jasper aquifer and the Burkeville confining unit.

Purpose and Scope

The purpose of this report is to describe the 

hydrogeology and simulation of groundwater flow and landsurface subsidence in the northern part of the Gulf Coast 

aquifer system in the HAGM study area (fig. 1). Additionally, 

this report documents changes made to the previous model 

(GAM), the parent model of the HAGM. For this report, 

“predevelopment” refers to conditions prior to 1891, and 

“postdevelopment” refers to 1891–2009. The hydrogeologic 

units, hydraulic properties, flow conditions, and development 

(groundwater withdrawals) of the HAGM are based on 

available information and have been modified from the 

original GAM as necessary. The hydrogeologic units from 

land surface downward are the Chicot aquifer, Evangeline 

aquifer, Burkeville confining unit, Jasper aquifer, and 

Catahoula confining system. Little mention of the Catahoula 

confining system is included because it was not simulated in 

the model. Groundwater flow was simulated for parts of the 

hydrogeologic units that contain freshwater.

Previous Studies

The Gulf Coast aquifer system in the Houston region has 

been extensively studied. Nine previous groundwater-flowmodeling studies, including two that simulated land-surface 

subsidence, have been completed in all or parts of the HAGM 

study area. From the earliest to most recent, the models were 

authored by Wood and Gabrysch (1965); Jorgensen (1975); 

Meyer and Carr (1979); Trescott (1975); Espey, Huston and 

Associates, Inc. (1982); Carr and others (1985); LBG-Guyton 

and Associates (1997); Kasmarek and Strom (2002); and 

Kasmarek and Robinson (2004). LBG-Guyton and Associates 

(1997) were the first to use the USGS groundwater-flow 

model MODFLOW to simulate water levels (heads) in the 

Houston area (McDonald and Harbaugh, 1988; Harbaugh and 

McDonald, 1996).

The first model to simulate land-surface subsidence 

is known as the Predictions Relating Effective Stress to 

Subsidence (PRESS) model, which uses a modified version 

of the compaction (COMPAC) code developed by Helm 

(1975; 1976a, b; 1978). A model of land-surface subsidence 

(Fugro–McClelland [Southwest], Inc., 1997) was designed to 

be used with, but was not part of, the LBG-Guyton Associates 

(1997) groundwater-flow model. Similar to the model by 

Espey, Huston and Associates, Inc. (1982), the model by 

Fugro–McClelland (Southwest), Inc. (1997), used the PRESS 

code to simulate land-surface subsidence. The simulated 

water-level declines from the LBG-Guyton Associates (1997) 

groundwater-flow model were used as input data for PRESS 

models at 22 separate sites in the Houston area. Kasmarek 

and Strom (2002) and Kasmarek and Robinson (2004) used 

MODFLOW (Harbaugh and McDonald, 1996) to simulate 

groundwater flow in the Chicot and Evangeline aquifers of 

the Houston–Galveston region and the northern part of the 

Gulf Coast aquifer system, respectively, and the InterbedStorage (IBS) package (Leake and Prudic, 1991) was used 

to simulate clay compaction and storage in the aquifers. 

Additional summary information about the previous models 

described in this section is presented in Kasmarek and 

Robinson (2004).

Description of Study Area for the Houston Area 

Groundwater Model

The HAGM study area (fig. 1) includes all or parts 

of 38 counties in southeastern Texas. The HAGM area is a 

gently sloping coastal plain, and land-surface elevations are 

topographically highest along the northwestern boundary. The 

vegetation in the northern parts of the HAGM area generally 

is composed of hardwood and pine forests, but as land-surface 

altitude decreases toward the coast, the vegetation becomes 

increasingly dominated by shrubs and grasses. Numerous 

constructed lakes and reservoirs are in the HAGM area, but 

those surficial water bodies generally only influence the water 

table on a local scale. The Gulf of Mexico and Galveston 

Bay have a large effect on the downdip groundwater-flow 

system and climate of the area. Winters in the HAGM area are 

mild with few days of freezing temperatures. During winter, 

moisture-laden Pacific and Canadian air masses produce 

regionally extensive bands of moderate rainfall. Summers 

are hot with high relative humidity, and prevailing winds are 

from the south to southwest (Kasmarek and Robinson, 2004). 

During summer, atmospheric convective cells can produce 

rates of precipitation from light to extreme (0.01 inches [in.] 

per hour to 2.0 in. per hour or more) (Federal Aviation 

Agency, 2007). Infrequently, moisture-laden tropical air 

masses produce light to extreme rates of precipitation with a 

reported rate of 38.8 in. being recorded from June 5 to 

June 9, 2001, related to Tropical Storm Allison (National 

Oceanic and Atmospheric Administration, 2012a). The 

average annual rainfall for the greater Houston area is 

47.84 in., and the average annual temperature is about 

68.8 degrees Fahrenheit (National Oceanic and Atmospheric 

Administration, 2012).
Introduction 3

COLUMNS

ROWS

1

1

245

137

95°

94°

96°

97°

30°

29°

31°

93°

TEXAS

LOUISIANA

Houston Area

Groundwater Model

study area

West Fork San Jacinto River

Colorado River

 AddicksReservoir

 LakeHouston

Galveston

 Bay

Barker

 Reservoir

Trinity River

 LakeLivingston

 LakeConroe

Brazos

River

Neches River

aS i b en i R ev r

Sam Rayburn Reservoir

Toledo BendReservoir

B A Steinhagen Lake

Calcasieu Lake

Sabine Lake

Matagorda

 Bay

Somerville Lake

Lake Texana

Lavaca River

GULF OF MEXICO

West Fork San Jacinto River

Colorado River

 AddicksReservoir

 LakeHouston

Galveston

 Bay

Barker

 Reservoir

Trinity River

 LakeLivingston

 LakeConroe

Brazos

River

Neches River

aS i b en i R ev r

Sam Rayburn Reservoir

Toledo BendReservoir

B A Steinhagen Lake

Calcasieu Lake

Sabine Lake

Matagorda

 Bay

Somerville Lake

Lake Texana

Lavaca River

GULF OF MEXICO

POLK

SAN JACINTO

LIBERTY

GRIMES

MONTGOMERY

WALLER

HARRIS

HARDIN

JEFFERSON

CHAMBERS

AUSTIN

COLORADO

WHARTON

WASHINGTON

WALKER

TRINITY

MATAGORDA

BRAZORIA

FORT BEND

JASPER

TYLER

NEWTON

VERNON

BEAUREGARD

CAMERON

CALCASIEU

JEFF DAVIS

ALLEN

ORANGE

SABINE

NACOGDOCHES

SHELBY

 SAN

 AUGUSTINE

JACKSON

GONZALES

BURLESON

BRAZOS

BASTROP

LEE

CALDWELL

LAVACA

GALVESTON

ANGELINA

CALHOUN

FAYETTE

SABINE

TEXAS

LOUISIANA

Base modified from U.S. Geological Survey digital data

Scale 1:250,000

Albers equal-area projection 

North American Datum of 1983

Standard parallels 34°55’ and 27°25’, central meridian 100°

30 40 MILES

30 40 KILOMETERS

0 10 20

0 10 20

Huntsville

Houston

Beaumont

 Port Arthur

Baytown

Evadale

The finite-difference grid (shown here)consists of 137 rows and 245 columnswith the origin of the grid (row 137, column 1) located here.

Figure 1. Location of the Houston Area Groundwater Model study area and finite-difference grid, southeastern Texas and southwestern Louisiana.
4 Hydrogeology and Simulation of Groundwater Flow and Land-Surface Subsidence in the Northern Part of the Gulf Coast Aquifer

Hydrogeology

In a generalized conceptual model of the Gulf Coast 

aquifer system, the fraction of precipitation that does not 

evaporate, transpire through plants, or run off the land 

surface to streams enters the groundwater-flow system in 

topographically high updip outcrop areas of the hydrogeologic 

units in the northwestern part of the system. Most precipitation 

infiltrating into the saturated zone flows relatively short 

distances through shallow zones and then discharges to 

streams. The remainder of the water flows to intermediate and 

deep zones of the system southeastward of the outcrop areas 

where it is discharged by wells (in the developed system) and 

by upward leakage in topographically low areas near or along 

the coast (in both predevelopment and postdevelopment, but 

appreciably less in postdevelopment). Near the coast and at 

depth, saline water is present. The saline water causes lessdense freshwater that has not been captured and discharged 

by wells to be redirected upward as diffuse leakage to 

shallow zones of the aquifer system and ultimately to be 

discharged to coastal water bodies. Because groundwater 

flow was simulated in the HAGM only as far as the downdip 

limit of freshwater, only the parts of the hydrogeologic units 

containing freshwater are described in this report (Kasmarek 

and Robinson, 2004).

Hydrogeologic Units and Geologic Setting

The thicknesses of the four stratigraphic units used in the 

HAGM coincide with the GAM of Kasmarek and Robinson 

(2004) and originated from Strom and others (2003c). From 

land surface downward, the Chicot aquifer, the Evangeline 

aquifer, the Burkeville confining unit, the Jasper aquifer, and 

the Catahoula confining system are the hydrogeologic units of 

the Gulf Coast aquifer system (fig. 2), as described by Baker 

(1979, 1986) and by Ashworth and Hopkins (1995). In general, 

where the hydrogeologic units crop out, they do so parallel 

to the coast and thicken downdip to the southeast with the 

older units having a greater dip angle (fig. 2). The correlation 

of hydrogeologic units with stratigraphic units is shown in 

figure 3. The Chicot aquifer comprises (youngest to oldest) 

the alluvium, Beaumont Formation, Montgomery Formation, 

Bentley Formation, and Willis Formation. The Evangeline 

aquifer comprises (youngest to oldest) the Goliad Sand and the 

upper part of the Fleming Formation. The Burkeville confining 

unit consists entirely of the Fleming Formation. The Jasper 

aquifer comprises (youngest to oldest) the lower part of the 

Fleming Formation throughout its subsurface extent and the 

upper part of the Catahoula Sandstone in its outcrop and updip 

parts (fig. 3). The basal unit for this report is the Catahoula 

confining system, which comprises the Catahoula Sandstone 

and, downdip, the Anahuac and Frio Formations (Kasmarek 

and Robinson, 2004).

The updip limit of the Chicot aquifer is an undulating 

boundary approximately parallel to the coast and extending 

as far north as Lavaca, Colorado, Austin, Waller, Grimes, 

Montgomery, San Jacinto, Polk, Tyler, Jasper, and Newton 

Counties (fig. 4). To the southeast, the freshwater part of the 

aquifer extends beneath the Gulf of Mexico. The altitude 

of the top of the Chicot aquifer in the HAGM study area 

approximates the land-surface altitude and ranges from 

the North American Vertical Datum of 1988 (NAVD 88, 

hereinafter, datum) at the coast to as much as 445 feet (ft) 

above datum at its updip limit (Kasmarek and Robinson, 

2004, fig. 9). The altitude of the base of the Chicot aquifer in 

the HAGM study area (Kasmarek and Robinson, 2004, fig. 

10) ranges from more than 1,500 ft below Datum southeast 

of the coast to more than 420 ft above Datum in the outcrop 

area and varies locally because of numerous salt domes in 

the study area (Kasmarek and Robinson, 2004, fig. 27). The 

altitude of the base of the Chicot aquifer was constructed 

from hydrogeologic digital data of Strom and others (2003a). 

The original cumulative clay thickness of the Chicot aquifer 

(Kasmarek and Robinson, 2004, fig. 12) was subtracted from 

aquifer thickness to construct cumulative sand thickness 

(Kasmarek and Robinson, 2004, fig. 13).

The updip limit of the Evangeline aquifer is an 

undulating boundary approximately parallel to the coast and 

extending as far north as Lavaca, Fayette, Austin, Washington, 

Grimes, Montgomery, Walker, San Jacinto, Polk, Tyler, Jasper, 

and Newton Counties (fig. 5). The downdip limit of freshwater 

is approximately coincident with the coast. The altitude of the 

top of the Evangeline aquifer in the HAGM study area ranges 

from more than 1,440 ft below datum to as much as 469 ft 

above datum at its updip limit (Kasmarek and Robinson, 2004, 

fig. 15). The altitude of the base of the Evangeline aquifer in 

the HAGM study area (Kasmarek and Robinson, 2004, fig. 

16) ranges from more than 5,300 ft below datum at the coast 

to 430 ft above datum in the outcrop area and varies locally 

because of numerous salt domes (Kasmarek and Robinson, 

2004, fig. 27). The base of the Evangeline aquifer transgresses 

the stratigraphic boundary between the Goliad Sand and the 

Fleming Formation. (This transgression is not shown in the 

section depicted in figure 2, as only outcropping stratigraphic 

units are shown.) The altitude of the base of the Evangeline 

aquifer is presented in Strom and others (2003b). The 

original cumulative clay thickness of the Evangeline aquifer 

(Kasmarek and Robinson, 2004, fig. 18) is from Gabrysch 

(1982, fig. 37) and was subtracted from aquifer thickness to 

construct cumulative sand thickness (Kasmarek and Robinson, 

2004, fig. 19).

The updip limit of the Burkeville confining unit is an 

undulating boundary approximately parallel to the coast and 

extending as far north as Lavaca, Fayette, Austin, Washington, 

Grimes, Montgomery, Walker, San Jacinto, Polk, Tyler, Jasper, 

and Newton Counties (fig. 6). The Burkeville confining 

unit lies stratigraphically below the Evangeline aquifer and 

above the Jasper aquifer (fig. 2) and restricts flow between 

the Evangeline and Jasper aquifers because of its relatively 

large percentage of silt and clay compared to the percentages 

of the adjacent aquifers (Baker, 1979). Southeast of the 
Hydrogeology 5

Jasperoutcrop

Catahoula

 outcrop

Quaternary outcrop Tertiary outcrop

Evangeline

 outcrop

Outcrop ofthe Burkevilleconfining unit

A

A'

10

1

2

3 4 5

6

7

8 9

11

WALLER COUNTY

GRIMES COUNTY

GALVESTON COUNTY

HARRIS COUNTY

MONTGOMERY COUNTY

KURDICK

No. 1 Stoneham

LONE STAR

No. 1 Goforth

SUPERIOR

No. B-1 McWhorter

NEWMAN & GEOTEK

No. 1 Ziegenhain

SPARTA

No. 1 Suttles et. al.

TENNESSEE GAS

No. 1 Martin Unit

HUGHES

No. 1 Goar Est.

PHILLIPS

No. 5 Guaranty

SOHIO

No. 1 Kopperl

No. 1 Bradley

No. 1 Sealy

HUMBLE

EXETER

Well number on location map

A A'

NORTHWEST SOUTHEAST

MONTGOMERY

COUNTY HARRIS COUNTY GALVESTON COUNTY GRIMES COUNTY +400

-400

-800

-1,200

-1,600

-2,000

-2,400

-2,800

-3,200

-3,600

-4,000

-4,400

-4,800-5,200

-5,600

-6,000

-6,400

-6,800

-7,200

-7,600

NAVD 88

FEET

NAVD 88, North American Vertical Datum of 1988

FEET+400NAVD 88

-400

-800

-1,200

-1,600

-2,000

-2,400

-2,800

-3,200

-3,600

-4,000

-4,400

-4,800

-5,200

-5,600

-6,000

-6,400

-6,800-7,200

-7,600

3 4 5 6 7 8 9 10 11 1 2

VERTICAL SCALE GREATLY EXAGGERATED

16 MILES 12 8 4 0

16 KILOMETERS 12 8 4 0

Anahuac

Formation

Formation

Upper part of 

Frio

Catahoula

Sandstone

Base of 

Catahoula 

Sandstone 

Catahoula 

confining 

system (restricted) 

Jasper

aquifer

Base of 

Fleming 

Formation

Chicot aquifer

Evangeline aquifer

Burkeville

confining

unit

land surface Approximate

Figure 2. Hydrogeologic section of the Gulf Coast aquifer system in Harris County and adjacent counties, Texas (modified from Baker, 1979, fig. 4).

(Holocene- and Pleistocene-age sediments)

(Pliocene- and Miocene-age sediments)

(Miocene-age sediments)

(Miocene-age sediments)

(Miocene-age sediments)

Undifferentiated Pre-Miocene deposits

Figure 2. Hydrogeologic section of the Gulf Coast aquifer system in Harris County and adjacent counties, Texas.
6 Hydrogeology and Simulation of Groundwater Flow and Land-Surface Subsidence in the Northern Part of the Gulf Coast Aquifer

downdip limit of freshwater (fig. 6), this unit is considered 

(for HAGM simulation purposes) a no-flow unit that prevents 

diffuse upward leakage of saline water from the Jasper aquifer. 

In updip areas of the Burkeville confining unit (fig. 6), the 

sediments are slightly more transmissive and thus able to 

supply small quantities of water for domestic use. In the 

outcrop area, the altitude of the top of the Burkeville confining 

unit is equal to the land-surface altitude, and in the subcrop 

area, the top of the Burkeville confining unit is coincident with 

the base of the Evangeline aquifer. The altitude of the base of 

the Burkeville confining unit is coincident with the top of the 

Jasper aquifer and varies locally because of the numerous salt 

domes in the area (Kasmarek and Robinson, 2004, fig. 27).

The updip limit of the Jasper aquifer is an undulating 

boundary approximately parallel to the coast and extending 

as far north as Lavaca, Gonzales, Fayette, Washington, 

System Series Formation Aquifers and

confining units

Chicot

aquifer

Evangeline

aquifer

Alluvium

Beaumont

 Formation

Montgomery

 Formation

 Bentley

Formation

Willis Formation

Goliad Sand

 Fleming

Formation

Holocene

Pleistocene

Pliocene

Miocene

Quaternary

Tertiary

Burkeville

confining

unit

Jasper 

aquifer

 Oakville

Sandstone 

Catahoula

confining

system

Catahoula

Sandstone 

Anahuac

Formation1

Frio

Formation1

Hydrogeologic

units

Geologic (stratigraphic) units

Model

layer

1

2

3

4

1

Present only in subsurface.

Figure 3. Correlation of stratigraphic and hydrogeologic units in the Houston Area Groundwater Model study area.
Hydrogeology 

7

EXPLANATION

Outcrop

30 40 MILES

30 40 KILOMETERS

0 10 200 10 20

Base modified from U.S. Geological Survey digital data

Scale 1:24,000 (except Louisiana hydrography 1:100,000)

Albers equal-area projection 

North American Datum of 1983

Standard parallels 34°55’ and 27°25’, central meridian 100°

95°

94°

96° 97° 30°

29°

31°

93°

N

POLK

SAN

 JACINTO

LIBERTY

GRIMES

WALLER

HARRIS

HARDIN

JEFFERSON

CHAMBERS

AUSTIN

COLORADO

WHARTON

GALVESTON

WASHINGTON WALKER

TRINITY

MATAGORDA BRAZORIA

FORT BEND

FAYETTE

JASPER

TYLER

NEWTON

VERNON

BEAUREGARD

CAMERON

CALCASIEU

JEFF DAVIS

ALLEN

ORANGE

SABINE

ANGELINA

SHELBY

 SAN

 AUGUSTINE

JACKSON

GONZALES CALHOUN BURLESON BRAZOS BASTROP LEE CALDWELL LAVACA TEXAS LOUISIANA MONTGOMERY NACOGDOCHES

Estimated downdip limit of freshwaterin the Chicot aquifer

Updip limit of the Chicot aquifer

Figure 4. Extent and outcrop area of the Chicot aquifer in the Houston Area Groundwater Model study area.
8 Hydrogeology and Simulation of Groundwater Flow and Land-Surface Subsidence in the Northern Part of the Gulf Coast Aquifer Outcrop Subcrop EXPLANATION 30 40 MILES 30 40 KILOMETERS

0 10 200 10 20

Base modified from U.S. Geological Survey digital data

Scale 1:24,000 (except Louisiana hydrography 1:100,000)

Albers equal-area projection 

North American Datum of 1983

Standard parallels 34°55’ and 27°25’, central meridian 100°

95°

94°

96°

97°

30°

29°

31°

93°

N

POLK

SAN

 JACINTO

LIBERTY

GRIMES

WALLER

HARRIS

HARDIN

JEFFERSON

CHAMBERS

AUSTIN

COLORADO

WHARTON

GALVESTON

WASHINGTON WALKER

TRINITY

MATAGORDA BRAZORIA

FORT BEND

FAYETTE

JASPER

TYLER

NEWTON

VERNON

BEAUREGARD

CAMERON

CALCASIEU

JEFF

 DAVIS

ALLEN

ORANGE

SABINE

ANGELINA

SHELBY

 SAN

 AUGUSTINE

JACKSON

GONZALES CALHOUN BURLESON

BRAZOS BASTROP LEE CALDWELL

LAVACA

TEXAS

LOUISIANA

MONTGOMERY

NACOGDOCHES

Estimated downdip limit of freshwaterin the Evangeline aquifer

Updip limit of the Evangeline aquifer

Figure 5. Extent, outcrop area, and subcrop area of the Evangeline aquifer in the Houston Area Groundwater Model study area.
Hydrogeology 

9

EXPLANATION

OutcropSubcrop

30 40 MILES

30 40 KILOMETERS

0 10 200 10 20

Base modified from U.S. Geological Survey digital data

Scale 1:24,000 (except Louisiana hydrography 1:100,000)

Albers equal-area projection 

North American Datum of 1983

Standard parallels 34°55’ and 27°25’, central meridian 100°

95°

94°

96° 97° 30°

29°

31°

93°

N

POLK

SAN

 JACINTO

LIBERTY

GRIMES

WALLER

HARRIS

HARDIN

JEFFERSON

CHAMBERS

AUSTIN

COLORADO

WHARTON

GALVESTON

WASHINGTON WALKER

TRINITY

MATAGORDA BRAZORIA

FORT BEND

FAYETTE

JASPER

TYLER

NEWTON

VERNON

BEAUREGARD

CAMERON

CALCASIEU

JEFF DAVIS

ALLEN

ORANGE

SABINE

ANGELINA

SHELBY

 SAN

 AUGUSTINE

JACKSON

GONZALES CALHOUN BURLESON BRAZOS BASTROP LEE CALDWELL LAVACA TEXAS LOUISIANA MONTGOMERY NACOGDOCHES Updip limit of the Burkeville confining unit Estimated downdip limit of freshwater in the Burkeville confining unit

Figure 6. Extent, outcrop area, and subcrop area of the Burkeville confining unit in the Houston Area Groundwater Model study area.
10 Hydrogeology and Simulation of Groundwater Flow and Land-Surface Subsidence in the Northern Part of the Gulf Coast Aquifer

Brazos, Grimes, Walker, Trinity, Polk, Tyler, Angelina, 

Jasper, Newton, and Sabine Counties (fig. 7). Southeast of 

the downdip limit of freshwater, this unit is considered (for 

HAGM simulation purposes) a no-flow unit that prevents 

diffuse upward leakage of saline water. The altitude of the top 

of the Jasper aquifer in the HAGM study area ranges from 

less than 2,800 ft below datum to about 900 ft above datum at 

its updip limit (Kasmarek and Robinson, 2004, fig. 22). The 

altitude of the base of the freshwater part of the Jasper aquifer 

(Kasmarek and Robinson, 2004, fig. 23) ranges from about 

3,800 ft below datum near the downdip limit of freshwater 

to about 500 ft above datum in the outcrop area and varies 

locally because of numerous salt domes (Kasmarek and 

Robinson, 2004, fig. 27). The base of the Jasper aquifer in 

updip areas transgresses the stratigraphic boundary between 

the Fleming Formation and the Catahoula Sandstone (figs. 2 

and 3). Strom and others (2003c) estimated the altitudes of the 

top and base of the Jasper aquifer and evaluated the thickness 

of the aquifer (Kasmarek and Robinson, 2004, fig. 24). The 

original cumulative clay thickness of the Jasper aquifer 

(Kasmarek and Robinson, 2004, fig. 25) was subtracted from 

aquifer thickness to construct the cumulative sand thickness 

(Kasmarek and Robinson, 2004, fig. 26). The basal unit 

for the HAGM (fig. 2) is the Catahoula confining system, 

which comprises the Catahoula Sandstone and, downdip, the 

Anahuac and Frio Formations. The Jasper aquifer is underlain 

by the Catahoula confining system, which is composed mostly 

of clay or tuff. The Catahoula confining system impedes 

substantial exchange of water between the Jasper aquifer and 

underlying units (Baker, 1986).

The paleodepositional environment of the sediments that 

formed the Gulf Coast aquifer system was a fluvial-deltaic 

or shallow-marine environment that produced interlayered, 

discontinuous sequences of clay, silt, sand, and gravel 

(Kasmarek and Robinson, 2004). (In this report, the term 

“sand” refers to coarse-grained sand and gravel sediments, 

whereas “clay” refers to fine-grained sediments including 

clay and silt.) Changes in land-surface altitudes related to 

naturally occurring land-surface subsidence of the depositional 

basin and sea-level transgressions and regressions created 

cyclical sedimentation facies. During periods when the sea 

level declined, fluvial deltaic processes deposited continental 

sediments, but as the sea level rose, the deposited continental 

sediments were reworked, and marine sediments were 

deposited. Because of this complex depositional process, the 

facies alternate cyclically from the predominantly continental 

sediments that compose the aquifers to the predominantly 

marine sediments that compose the confining units and clay 

layers within aquifers; therefore, the Gulf Coast aquifer 

system has a high degree of heterogeneity in both lateral and 

vertical extents (Sellards and others, 1932).

Normal growth faults are common throughout the 

unconsolidated sediments of the HAGM study area, and traces 

of some of these faults have been mapped and named. Based 

on the study of well logs and seismic-line data, these faults 

have been delineated to depths of 3,000–12,000 ft below land 

surface (Verbeek and others, 1979). The presence of most of 

these faults is associated with natural geologic processes. The 

scale of fault movement is insufficient to completely offset 

entire hydrogeologic units; however, if an offset results in the 

juxtaposition of relatively more permeable sediments against 

relatively less permeable sediments, the rate and direction 

of groundwater flow could be affected. Although growth 

faults are common in the study area, the exact locations and 

frequency with which associated offsets appreciably affect 

groundwater flow is unknown. Because the distribution 

and magnitude of such occurrences in the study area are 

unknown, accounting for them in the HAGM was not possible. 

Numerous salt domes originating from the Jurassic-age 

Louann Salt have risen through the overlying strata (Halbouty, 

1967) and have been mapped in the HAGM area (Beckman 

and Williamson, 1990). In some areas, the salt domes have 

penetrated the aquifers. The upward intrusions of the salt 

domes decrease the thickness of the adjacent aquifer sediments 

and radially alter the prevailing hydraulic characteristics 

and flow paths in the adjacent aquifer sediments. These 

widely distributed salt domes increase the heterogeneity of 

the hydraulic characteristics of the aquifers (Kasmarek and 

Robinson, 2004).

Hydraulic Properties

Carr and others (1985) estimated transmissivity and 

storativity of the Chicot and Evangeline aquifers from 

simulation and are approximately the same as that used in 

the HAGM. Estimated transmissivity of the Chicot aquifer 

ranged from about 3,000 to about 50,000 square feet per 

day (ft2/d), and storativity ranged from about 0.0004 to 

0.1(dimensionless). Estimated transmissivity of the Evangeline 

aquifer ranges from about 3,000 to about 15,000 ft2/d, and 

storativity ranged from about 0.00005 to 0.1. For both 

aquifers, the simulations indicated that the larger storativities 

are in the updip outcrop areas that are under water-table 

conditions; the smaller storativities are in downdip areas 

that are under confined conditions. Baker (1986) estimated 

transmissivity of the Jasper aquifer from simulation for an area 

coincident with most of the Jasper aquifer in the HAGM area; 

the transmissivity of the Jasper aquifer simulated in that study 

ranged from less than 2,500 to about 35,000 ft2/d. Wesselman 

(1967) estimated transmissivity for all three aquifers and 

storativity for the Chicot and Evangeline aquifers from 

aquifer tests in Jasper, Newton, Orange, and Hardin Counties. 

Transmissivities of the Chicot aquifer ranged from 12,300 to 

68,000 ft2/d; the Evangeline aquifer, 2,130 to 14,800 ft2/d; and 

the Jasper aquifer, 1,070 to 14,000 ft2/d. Wesselman (1967) 

also estimated storativities of the Evangeline aquifer ranging 

from 0.00063 to 0.0015 and of the Jasper aquifer ranging 

from 0.000382 to 0.00119. Strom and others (2003c) reported 

storativities for the Jasper aquifer as large as 0.2. Several other 

previous studies (for example Jorgensen, 1975) estimated 

transmissivity in aquifers for parts of counties in the HAGM 

study area; those estimates generally are within the ranges 

listed above.
Hydrogeology 11

EXPLANATION

OutcropSubcrop

30 40 MILES

30 40 KILOMETERS

0 10 200 10 20

Base modified from U.S. Geological Survey digital data

Scale 1:24,000 (except Louisiana hydrography 1:100,000)

Albers equal-area projection 

North American Datum of 1983

Standard parallels 34°55’ and 27°25’, central meridian 100°

95°

94°

96° 97° 30°

29°

31°

93°

N

POLK

SAN

 JACINTO

LIBERTY

GRIMES

WALLER

HARRIS

HARDIN

JEFFERSON

CHAMBERS

AUSTIN

COLORADO

WHARTON

GALVESTON

WASHINGTON WALKER

TRINITY

MATAGORDA BRAZORIA

FORT BEND

FAYETTE

JASPER

TYLER

NEWTON

VERNON

BEAUREGARD

CAMERON

CALCASIEU

JEFF DAVIS

ALLEN

ORANGE

SABINE

ANGELINA

SHELBY

 SAN

 AUGUSTINE

JACKSON

GONZALES CALHOUN BURLESON BRAZOS BASTROP LEE CALDWELL LAVACA TEXAS LOUISIANA MONTGOMERY NACOGDOCHES Updip limit of the Jasper aquifer Estimated downdip limit of freshwater in the Jasper aquifer

Figure 7. Extent, outcrop area, and subcrop area of the Jasper aquifer in the Houston Area Groundwater Model study area.
12 Hydrogeology and Simulation of Groundwater Flow and Land-Surface Subsidence in the Northern Part of the Gulf Coast Aquifer

The transmissivity of an aquifer is equal to the hydraulic 

conductivity multiplied by the thickness of the aquifer (Freeze 

and Cherry, 1979, p. 59); “hydraulic conductivity” is used 

extensively in this report. Initial transmissivity distributions 

for the aquifers were constructed with data from Wesselman 

(1967), Carr and others (1985), Baker (1986), and Kasmarek 

and Strom (2002) by using geographic information system 

(GIS) applications. The initial transmissivity of the Burkeville 

confining unit was computed by multiplying values of 

hydraulic conductivity representative of a midrange between 

silty sand and marine clay (average of 0.01 foot per day 

[ft/d]) (Freeze and Cherry, 1979, table 2.2, p. 29) by the 

areally distributed thickness of the confining unit. In this 

report, hydraulic conductivity refers to horizontal hydraulic 

conductivity, unless otherwise noted.

Groundwater Flow Conditions, Recharge, and 

Discharge

The uppermost parts of the Gulf Coast aquifer system 

(shallow zones), which include outcrop areas, are under 

shallow, unconfined water-table conditions. As depth increases 

in the aquifer system and the cumulative thicknesses of the 

interbedded sand and clay increase, water-table conditions 

transition to confined potentiometric conditions. Thus, the 

lowermost parts of the aquifer system (deep zones) are 

under confined conditions. The middle parts of the aquifer 

system (intermediate zones) therefore are under semiconfined 

conditions. Because the transition from water table to confined 

conditions incrementally increases with depth, assigning 

specific depth horizons to shallow, intermediate, and deep 

zones is problematic (Kasmarek and Robinson, 2004). 

Assuming that groundwater flows downgradient and 

perpendicular to equipotential lines, simulated predevelopment 

potentiometric surfaces of the Chicot, Evangeline, and Jasper 

aquifers (Kasmarek and Robinson, 2004, figs. 62–64) confirm 

the generalized conceptual model of the natural groundwaterflow system. Recharge enters the system in topographically 

high updip outcrops of the hydrogeologic units in the 

northwestern parts of the HAGM study area and either flows 

relatively short distances discharging into topographically 

lower areas to features such as streams or flows longer 

distances southeastward through deeper zones, where it is 

discharged by diffuse-upward leakage in topographically low 

areas along coastal areas.

As first described by Tóth (1963) and summarized by 

Johnston (1999) relative to regional aquifer systems, natural 

(predevelopment) groundwater flow can be subdivided into 

local, intermediate, and regional flow systems. Local flow 

follows relatively short flow paths in shallow zones and is 

controlled mainly by topography. Recharge to local flow 

systems occurs in topographically high areas, and discharge 

occurs in nearby, topographically low areas. Intermediate flow 

moves along relatively deeper flow paths compared to local 

flow, with groundwater flowing from recharge areas through 

intermediate zones to downgradient discharge areas. Regional 

flow follows relatively long flow paths from regional recharge 

areas through deep zones to distal discharge areas such as the 

downgradient limits of an aquifer system. Referring to the 

local, intermediate, and deep flow systems of the aquifer is 

a basic way to explain the groundwater flow in the aquifer 

system, but the true nature of the flow system is more complex 

because of the paleodepositional environment and the stresses 

of groundwater withdrawals on the aquifer. Tóth (1963) noted 

that to assume an exact, one-to-one correspondence among 

local, intermediate, and regional flow systems would be an 

oversimplification.

If this concept of subdividing natural groundwater flow is 

applied to the Gulf Coast aquifer system, the implications are 

that an appreciable amount of the precipitation that infiltrates 

the subsurface (total recharge) in the relatively topographically 

high outcrop areas of the hydrogeologic units joins local 

flow systems. Thus, much of the total precipitation enters 

from and exits to the shallow subsurface by streams and in 

topographically low areas. A proportionally smaller amount 

of the total recharge joins intermediate flow systems, and 

an even smaller amount of the total recharge joins regional 

flow systems. Wood (1956, p. 30–33), in an early study of 

the availability of groundwater in the Gulf Coast region of 

Texas, stated that, “Within the rainfall belts of 40–50 inches 

per year, probably 1 inch or more of the water that enters the 

outcrop of the aquifers updip from the heavily pumped areas 

is discharged to the streams in the outcrop area as base flow or 

rejected recharge.”

The natural groundwater-flow system has been altered 

in places (the Houston area, for example) by decades of 

substantial and concentrated withdrawals in the Chicot and 

Evangeline aquifers. By 1977, water levels had declined 

to as much as 250 ft and 350 ft below datum in the Chicot 

and Evangeline aquifers, respectively (Gabrysch, 1979). 

Because the Chicot and Evangeline aquifers are hydraulically 

connected, in these areas, withdrawals have increased verticalhead gradients and have induced downward flow from local 

and intermediate flow systems into the regional flow system, 

thus capturing some flow that would have discharged naturally 

(Gabrysch, 1979).

Few studies that focus specifically on recharge to the 

system in the HAGM study area are available. For example, 

Baker (1986) and a study of potential recharge in the Houston 

area by the U.S. Geological Survey Robert K. Gabrysch 

[retired] and Fred Liscum [retired], U.S. Geological Survey, 

written commun., 1995) estimated that the recharge rate 

across the area ranged from 0.25 in. per year (in./yr) to 7 

in./yr. A few additional studies report recharge rates within 

this range (Tarver, 1968; Sandeen, 1972; Loskot and others, 

1982). An in-depth discussion of the results from previous 

recharge studies in the study area is available in Kasmarek and 

Robinson (2004).
Hydrogeology 13

Groundwater Development

Rates of recharge to and discharge from the Chicot, 

Evangeline, and Jasper aquifers are affected by groundwater 

withdrawals from those aquifers. “Predevelopment” relative 

to the HAGM refers to aquifer conditions before 1891 or 

before the aquifers were measurably stressed by groundwater 

withdrawals; “postdevelopment” refers to aquifer conditions 

after the stress of withdrawals became measurable. Initially, 

the principal areas of concentrated groundwater withdrawals 

from the aquifer system in the HAGM study area were located 

in Harris, Galveston, and Fort Bend Counties (the Houston 

area). Much of the early groundwater-use information for the 

area, as summarized here, is from Lang and Winslow (1950) 

and Wood and Gabrysch (1965).

In the area of Houston (founded in 1836), surface water 

was initially used to meet water-supply demands. In 1886, 

the first well was drilled to a depth of 140 ft and was reported 

as free flowing at more than 1,000 gallons per minute (gal/

min) (Lang and Winslow, 1950). By 1906, groundwater 

withdrawals had the capacity of as much as 19 million gallons 

per day (Mgal/d). By 1935, withdrawals averaged 24.5 Mgal/d 

and by 1941 had increased to 27.2 Mgal/d. From 1941 to 

1950, groundwater use more than doubled. In 1954, water 

released from the newly constructed Lake Houston began 

to be used to augment groundwater supplies. The additional 

surface-water supply from Lake Houston resulted in reduced 

groundwater withdrawals from 1954 to 1960. From the early 

1960s to the mid-1970s, however, groundwater withdrawals 

increased at rates comparable to pre-1954 rates (Lang and 

Winslow, 1950). In 1975, because of increasing groundwater 

withdrawals and subsequent land-surface subsidence in 

Harris and Galveston Counties, the Harris–Galveston Coastal 

Subsidence District (HGCSD) was created and began to 

control land-surface subsidence by regulating groundwater 

withdrawals. In late 1976, groundwater withdrawals began to 

decrease in eastern Harris County because part of the demand 

began to be supplied by water from Lake Livingston. The 

policies of the newly created HGCSD resulted in decreased 

groundwater withdrawals in the Baytown and southeastern 

Harris County areas. The groundwater withdrawal rate 

exceeded 450 Mgal/d in 1976 and decreased to about 390 

Mgal/d in the early 1980s, but the trend reversed, and by 

1990, withdrawals had increased to 493 Mgal/d. A downward 

trend began again in the 1990s when withdrawals were about 

463 Mgal/d by 1996. By 2000, withdrawals were about 895 

Mgal/d (Harris–Galveston Subsidence District, 2012).

Potentiometric Surfaces and Land-Surface 

Subsidence

In the updip outcrop area of the Chicot aquifer and 

the outcrop areas of the Evangeline and Jasper aquifers and 

Burkeville confining unit (figs. 4–7), water-table conditions 

generally exist. The water table is assumed to be a subdued 

replica of the topography (Williams and Williamson, 1989). 

In outcrops of the Chicot and Evangeline aquifers in parts 

of Harris and Montgomery Counties, a seismic refraction 

investigation indicated that the water table ranges from about 

10 to 30 ft below land surface (Noble and others, 1996). 

Hydrographs of water levels in wells screened in the water 

table of the Chicot and Evangeline aquifers indicate that the 

water levels were not influenced by increased groundwater 

withdrawal in the area and have remained fairly stable 

(Kasmarek and Robinson, 2004, fig. 28). The USGS annually 

has measured water levels in wells and constructed maps 

of potentiometric surfaces of the Chicot and Evangeline 

aquifers in the greater Houston area since 1977 (Gabrysch, 

1979) and of the Jasper aquifer since 2000. Related to 

groundwater withdrawal in the HAGM study area, the 2009 

report (Kasmarek, Houston, and Ramage, 2009) in this series 

indicates that water-level-altitude contours ranged from 

250 ft below datum (hereinafter, datum) in a small area in 

southwestern Harris County to 200 ft above datum in central 

to southwestern Montgomery County in the Chicot aquifer; 

from 300 ft below datum in south-central Montgomery 

County to 200 ft above datum at the intersecting borders of 

Waller, Montgomery, and Grimes Counties in the Evangeline 

aquifer; and from 175 ft below datum in south-central 

Montgomery County to 250 ft above datum in east-central 

Grimes County in the Jasper aquifer (Kasmarek, Houston, and 

Ramage, 2009).

In the 1830s, before groundwater withdrawals from 

the aquifer system occurred in the HAGM study area, the 

potentiometric surfaces in the confined parts of the aquifers 

were higher than land surface. This was demonstrated by 

a well in Houston that was drilled to 140 ft and flowed at 

more than 1,000 gal/min. Groundwater development has 

caused substantial declines of as much as 350 ft below datum 

(Gabrysch, 1979) of the potentiometric surfaces of the aquifers 

(and subsequent land-surface subsidence), primarily in Harris, 

Galveston, and Fort Bend Counties (Kasmarek and Robinson, 

2004, figs. 48 and 49). These potentiometric-surface declines 

in unconsolidated confined aquifers cause a decrease in 

hydraulic pressure that creates a load on the skeletal matrix of 

the aquifer (Galloway and others, 1999, p. 9). Because coarsegrained sediments (sand layers) are more transmissive and less 

compressible than are fine-grained sediments (clay layers), the 

depressurization of sand layers is relatively rapid compared 

to that of clay layers and causes only slight skeletal-matrix 

consolidation. The depressuring and subsequent dewatering 

of clay layers requires more time compared to that of the sand 

layers, however, and is dependent on the thickness of the 

clay layers, the hydraulic characteristics of the clay layers, 

and the vertical-stress load of the sediment overburden. The 

delayed drainage of the clay layers continues to occur until 

the residual excess (transient) pore pressure in the clay layers 

equals the pore pressure of the adjacent sand layers. Until 

pressure equilibrium is attained, dewatering of the clay layers 

continues to apply a load to the skeletal matrix of the clay 

layers. This loading process is similar to what occurs in the 
14 Hydrogeology and Simulation of Groundwater Flow and Land-Surface Subsidence in the Northern Part of the Gulf Coast Aquifer

sand layers, but additionally, the reorientation of the individual 

clay grains occurs, becoming perpendicular to the applied 

vertical load (Galloway and others, 1999, p. 9). Therefore, 

the dewatering caused by the depressurization of the clay 

layers combined with clay-grain realignment reduces the 

porosity and groundwater-storage capacity of the clay layers, 

which in turn allows them to inelastically and permanently 

compact. More than 10 ft of land-surface subsidence has 

been documented in the Baytown area in southwestern Harris 

County (Gabrysch and Neighbors, 2005; Kasmarek, Gabrysch, 

and Johnson, 2009). Because of the weight (sediment load) 

of the overburden and the inelastic compaction characteristics 

of the clay layers, about 90 percent of the compaction is 

permanent (Gabrysch and Bonnett, 1975). Thus, when 

potentiometric surfaces rise and repressure compacted clay 

layers, there is little, if any, rebound of the land surface 

(Gabrysch and Bonnett, 1975). Although the compaction of 

one clay layer generally will not cause a noticeable decrease 

in the land-surface altitude, if numerous stacked clay-layer 

sequences (which are characteristic of the Gulf Coast aquifer 

system) depressure and compact, then appreciable decreases 

in land-surface altitude can and do occur (Gabrysch and 

Bonnett, 1975). A substantial amount of the total water 

withdrawn is derived from dewatering of the numerous clay 

layers of the aquifer: model simulations indicated that as much 

as 19 and 10 percent of the total water budget of the Chicot 

and Evangeline aquifers, respectively, is derived from the 

dewatering of the clay layers of the aquifers (Kasmarek and 

Strom, 2002).

Simulation of Groundwater Flow and 

Land-Surface Subsidence

Model Description

The finite-difference computer code MODFLOW-2000 

(Harbaugh and others, 2000) was used to create and calibrate 

the HAGM to simulate groundwater flow and land-surface 

subsidence in the northern Gulf Coast aquifer system from 

predevelopment (1891) through 2009. The Subsidence and 

Aquifer-System Compaction (SUB) package designed for 

the MODFLOW-2000 model (Hoffman and others, 2003) 

was used to simulate clay compaction and storage, and thus 

land-surface subsidence, in the Chicot, Evangeline, and 

Jasper aquifers and the Burkeville confining unit. The Chicot, 

Evangeline, and Jasper aquifers and the Burkeville confining 

unit were simulated as four separate layers and discretized 

into two-dimensional finite-difference grids (fig. 1). By using 

GIS applications, model input data were georeferenced and 

assigned to model grid cells. 

Mathematical Representation

The MODFLOW-2000 model uses finite-difference 

methods to solve the partial differential equation for threedimensional movement of groundwater of constant density 

through heterogeneous, anisotropic porous materials. The 

equation can be written as follows:

h h h h Kxx Kyy Kzz W Ss x x y y z z t∂  ∂  ∂  ∂  ∂  ∂  ∂   +   +   − = ∂  ∂  ∂  ∂  ∂  ∂  ∂ (1)

where

Kxx, Kyy, and Kzz represent the hydraulic conductivity along 

the x, y, and z coordinate axes, which are 

assumed parallel to the major axes of 

hydraulic conductivity (Lt-1);

h is hydraulic head (Lt-1);

W is a volumetric flux per unit volume 

representing sources and/or sinks of 

water, with W < 0.0 for flow out of the 

groundwater system and W > 0.0 for flow 

in (Lt-1);

Ss is specific storage of the porous material (L-1);

L is length;

t is time; and

Lt-1 is length divided by time

(Harbaugh and McDonald, 1996). This equation, with 

specification of appropriate boundary and initial conditions, 

constitutes a mathematical representation of the groundwaterflow system. In this application, the aquifer system was 

assumed to be horizontally isotropic; thus, there was no 

preferred direction of hydraulic conductivity in the horizontal.

The storage coefficient (Ss) in equation 1 is particularly 

important in a confined and unlithified aquifer system like 

the Gulf Coast aquifer system. Because the aquifers do 

not have a rigid skeletal matrix, water is released not only 

from coarse-grained sediments like sand and gravel but also 

from fine-grained sediments like clay and silt. Therefore, 

the compressibility of water (Sw) is necessarily considered, 

computed as 

Sw = Ssw×b, (2)

where 

Ssw is specific storage due to compressibility of 

water (L);

Ssw is computed as Ssw = q×gw/Ew (L); and

b is thickness of the layer (L)

where

	 θ is porosity (dimensionless);

	 γw is unit weight of water (62.4 pounds [lb] per 

cubic foot [ft3]);

Ew is the bulk modulus of elasticity of water 

(4.5×107 lb/ft2); and

L is length (modified from Leake and Prudic, 

1991).
Simulation of Groundwater Flow and Land-Surface Subsidence 15

An additional important component of the aquifer 

system is the compressibility of the sediment skeleton, or Sk,

computed as 

Sk = Ssk×b, (3)

where 

Ssk is specific storage due to compressibility of 

water, and

b is thickness of sediments (L) (modified from 

Leake and Prudic, 1991).

As in equation 2, equation 3 is relevant to coarse- and 

fine-grained sediments, and thickness of the aquifer (b) is 

present. Thus, as the thickness of the aquifer increases, the 

storage coefficient from compressibility of water (Sw) and 

storage coefficient from compressibility of the sediment 

skeleton (Sk) correspondingly increase, providing a greater 

volume of water from storage in the downdip areas of 

the aquifers along the coast. In the Layer-Property Flow 

package of MODFLOW (LPF), a single combined specific 

storage value, Ss = Ssw + Ssk, is specified and multiplied by 

layer thickness for the case where head is above the top of 

a model layer (confined conditions). Where the aquifer is 

unconfined (head is below the top of the layer), LPF applies 

a value of specific yield in formulation of the equations for 

groundwater flow. Use of the confined storage coefficient, S

= Ss×b, is appropriate where compression and expansion of 

the aquifer skeleton and water are elastic; however, if inelastic 

(nonrecoverable) compaction of fine-grained sediments occurs 

and is important, an add-on package such as the SUB package 

(Hoffman and others, 2003) should be used with the no-delay 

interbeds option for the Gulf Coast aquifer system. For details 

on representing all storage properties in a model with aquifersystem compaction, see Leake and Prudic (1991).

Grid Design

The finite-difference grid (fig. 1) for the HAGM 

covers 33,565 square miles (mi2) in southeastern Texas and 

southwestern Louisiana. The model grid was rotated 37.6 

degrees clockwise so that the orientation of the model closely 

coincides with the natural groundwater divides, model 

boundaries, and predevelopment and postdevelopment flow 

paths. The four layers of the model together contain 134,260 

grid blocks. Each layer consists of 137 rows and 245 columns. 

Layer 1 represents the Chicot aquifer, layer 2 the Evangeline 

aquifer, layer 3 the Burkeville confining unit, and layer 4 the 

Jasper aquifer. The grid blocks are uniformly spaced with each 

model cell area equal to 1 mi2.

Boundaries

Model boundaries control where and how much water 

enters and exits the simulated aquifer system. The selection 

of model boundaries for the aquifers in this model was based 

on a conceptual interpretation of the flow system developed 

by using information reported by Meyer and Carr (1979), 

Carr and others (1985), Williamson and others (1990), and 

Strom and others (2003a, b, c). The northwestern boundaries 

of the three aquifers and the Burkeville confining unit are 

the northwestern extent of the updip outcrop sediments for 

each unit (Kasmarek and Robinson, 2004, figs. 8, 14, 20, 21). 

Northwest of these boundaries, the model grid blocks were 

assigned a hydraulic conductivity of zero to simulate no-flow 

boundaries. The downdip limit of freshwater (defined for this 

study as the location where the dissolved solids concentration 

is as much as 10,000 milligrams per liter [mg/L]) was chosen 

as the southeastern boundary of flow in each hydrogeologic 

unit. Southeast of these limits, the model grid blocks were 

assigned a hydraulic conductivity of zero to simulate no-flow 

boundaries. The location of the 10,000-mg/L line in each 

hydrogeologic unit was estimated from geophysical log data 

and from the coastward extent of freshwater withdrawals 

(Kasmarek and Robinson, 2004). A no-flow boundary at 

specified locations reflects an assumption of a stable downdip 

freshwater/saline-water interface. Along the coast in most of 

the HAGM study area, this assumption probably is valid: little 

or no human-induced stresses on the aquifer system in most of 

the coastal region likely have allowed long-term equilibrium 

to be established between the freshwater and the slightly more 

dense saline water that lies laterally adjacent to and beneath 

the freshwater. The southwestern and northeastern lateral 

boundaries for the Chicot, Evangeline, and Jasper aquifers and 

the Burkeville confining unit were selected to coincide with 

groundwater-flow divides associated with major rivers in the 

study area. The southwestern lateral boundary was located 

generally along the Lavaca River, and the northeastern lateral 

boundary was located in the general vicinity of the Sabine 

River (fig. 1). The assumption is that little lateral flow occurs 

across these boundaries, and thus they can reasonably be 

simulated as no-flow boundaries. The Catahoula confining 

system underlies the Jasper aquifer. The assumption is that 

the brackish water within the Catahoula confining system 

sufficiently impedes the exchange of water between the Jasper 

aquifer and deeper units, so the Catahoula confining system 

can reasonably be simulated as a no-flow base-of-system 

boundary.

Recharge and Discharge

The MODFLOW General-Head Boundary (GHB) 

package was used to simulate recharge and discharge in 

the outcrops of the Chicot, Evangeline, and Jasper aquifers 

and the Burkeville confining unit. This package allows the 

simulated water table of an aquifer system to function as a 

head-dependent flux (flow per unit area) boundary (Franke 

and others, 1987); that is, a condition in which the rate of 

flow between the water table and the adjacent deeper zone of 

the system is controlled by the difference between the water 

table (constant head) and the head in the adjacent deeper 

zone (which changes with model simulation time) and by the 

vertical hydraulic conductance between the water table and 

the immediately adjacent deeper zone. In interstream outcrop 
16 Hydrogeology and Simulation of Groundwater Flow and Land-Surface Subsidence in the Northern Part of the Gulf Coast Aquifer

areas, the head differences indicate general downward flow 

or areas of recharge, and in stream and downdip areas along 

the coast, the head differences generally indicate upward 

flow or areas of discharge. Simulating the water table as a 

constant-head source (or sink) of water to the system requires 

an assumption that no long-term trends in the water table are 

indicated, as shown in the example hydrographs in Kasmarek 

and Robinson (2004, fig. 28). These hydrographs indicate that 

the water table remains stable even during documented periods 

of drought that occurred during 1932–34, 1938–40, 1947–48, 

1950–57, and 1960–67 (State of Texas Drought Preparedness 

Council, 2006). Water-table-altitude data for the shallow 

zones of the hydrogeologic units from the model of Kasmarek 

and Robinson (2004) were used for HAGM model grid 

blocks in areas where the two models are coincident. These 

water-table-altitude data were originally created by using the 

method described by Williams and Williamson (1989) that 

used multiple linear regressions of depth-to-water data and 

topographic data to derive relations between depth to water 

and topography. This assumption is believed reasonable over 

most of the HAGM study area.

Flow between streams and the aquifer system (essentially 

discharge from aquifers to incised streams in outcrops) 

was not explicitly simulated in the model. The rationale for 

this approach is that the GHB package, assuming that the 

model is adequately calibrated, would account for stream 

discharge to the level of accuracy that such discharge is 

known. Additionally, few measured data are available on 

streamflow gains or losses for the major streams that flow 

across the outcrops of the Gulf Coast aquifer system. Because 

aquifer discharge to streams is not well known, such data are 

not particularly helpful for comparison with simulated data 

for purposes of calibration; there was little incentive to add 

more complexity to an already complex model by explicitly 

computing flow between streams to the aquifers. Although 

some additional recharge rates have recently been determined 

(Tarver, 1968; Sandeen, 1972; Loskot and others, 1982; Baker, 

1986; and Kasmarek and Robinson, 2004), the additional 

complexity of including that information specifically, by 

substituting the GHB package with the River or Stream 

package and the Recharge package, was determined to be 

beyond the scope of this report.

Initial Conditions

Initial conditions, including heads and spatial 

distributions of hydraulic conductivity, leakance, sand 

storativity, clay storativity, and general-head boundary 

conductance from Kasmarek and Robinson (2004), provided 

the initial data before model calibration began. The leakance 

parameter is equivalent to vertical hydraulic conductivity 

divided by the vertical distance between the centers of model 

layers. The spatial distributions of head in each hydrogeologic 

unit for the initial predevelopment steady-state simulation 

also were coincident with Kasmarek and Robinson (2004). 

Additionally, the simulated values of head from the stress 

period associated with the year 2000 in the GAM (Kasmarek 

and Robinson, 2004) were consistent with the initial heads of 

the HAGM in year 2001. For more detailed information on the 

initial development of these datasets, refer to Kasmarek and 

Robinson (2004).

Land-Surface Subsidence and Storage in Clays

Simulation of land-surface subsidence (actually, 

compaction of clays) and release of water from storage in 

the clays of the Chicot, Evangeline, and Jasper aquifers and 

the Burkeville confining unit was accomplished by using the 

SUB package designed for use with MODFLOW-2000 by 

Hoffman and others (2003). As explained in Leake and Prudic 

(1991), effective stress is defined as the difference between 

geostatic pressure (overburden load) and fluid pressure 

(head). Head decreases in a confined aquifer do not change 

geostatic pressure if, as assumed in this application, watertable heads remain constant. With constant geostatic pressure, 

effective stress thus will increase by the same amount that 

heads decrease. Previous studies (Riley, 1969; Helm, 1975) 

indicate that compaction (or expansion) of interbedded clays 

is proportional, or nearly so, to change in effective stress. 

For sediments in confined aquifers with constant geostatic 

pressure, compaction also is proportional, or nearly so, to 

change in head. The relation is

s b hS b Δ = Δ °

, (4)

where

Δb is the amount of compaction or expansion (L);

Δh is the change in head (L);

Ss is the skeletal (sand and clay) component of 

elastic or inelastic specific storage (L-1);

bo is the thickness of the interbed (L); and

L is length (modified from Leake and Prudic, 

1991).

For changes in hydraulic head in which head remains 

above preconsolidation head, an elastic response is 

computed. For changes in head in which head declines 

below preconsolidation head, an inelastic response is 

computed, permanent clay compaction is calculated, and the 

preconsolidation head is reset to the new head value. For the 

HAGM, an initial value of preconsolidation head of about 70 

ft below the starting head was used.

A preconsolidation head of about 70 ft was used by 

Meyer and Carr (1979), Carr and others (1985), Kasmarek 

and Strom (2002), and Kasmarek and Robinson (2004). For 

the Chicot and Evangeline aquifers in the HAGM study area, 

the initial values of elastic- and inelastic-clay storativity 

were coincident with the model of Kasmarek and Robinson 

(2004). The initial values of elastic-clay storativity used in 

the HAGM for the Burkeville confining unit and the Jasper 

aquifer were calculated by multiplying existing GAM values 

of clay thickness by 1.0×10-6. The initial values of inelasticclay storativity for the Burkeville confining unit and Jasper 

aquifer were derived by multiplying the values of elastic-clay 

storativity by 100.
Simulation of Groundwater Flow and Land-Surface Subsidence 17

Withdrawals

The primary sources of updated water-use data used in 

the HAGM are as follows: the Harris–Galveston Subsidence 

District (Harris and Galveston Counties); the Fort Bend 

Subsidence District (Fort Bend County); and the Lone 

Star Groundwater Conservation District, the Texas Water 

Development Board, and the San Jacinto River Authority 

(Montgomery County). HAGM simulations were made 

under transient conditions from 10,000 years before 1891 

through 2009 for 78 groundwater withdrawal (stress) periods 

of variable length (fig. 8 and table 1). Stress period 1 has 

a long duration without withdrawals, thereby enhancing 

model stability prior to actual withdrawals that began in 

stress period 2. For the years 1980, 1982, and 1988, monthly 

stress periods were applied. Substantially lower than average 

precipitation was recorded in the HAGM study area for those 

years. Monthly rather than annual stress periods allows the 

model to represent groundwater withdrawals on a monthly or 

seasonal basis if the model is used to simulate hypothetical 

drought scenarios in the future. Total groundwater withdrawals 

increased from an estimated 41 Mgal/d in 1891 to about 

1,130 Mgal/d in 1976, peaked at about 1,135 Mgal/d in 1980, 

and varied during the next 20 years but generally trended 

downward to about 895 Mgal/d in 2000. Water-use data from 

2001 to 2009 were compiled and provided by LBG-Guyton 

and Associates, and subsequently, the USGS joined these data 

with the GAM water-use dataset of Kasmarek and Robinson 

(2004). Evaluation of these data indicates that groundwater 

withdrawals varied from 799 Mgal/d in 2001 to 869 Mgal/d in 

2009. The lowest withdrawals, 747 Mgal/d, occurred in 2007, 

and the highest withdrawals, 876 Mgal/d, occurred in 2005. 

Additional water-use data (compiled by LBG-Guyton and 

Associates) was combined with the GAM water-use dataset 

for the Evangeline and Jasper aquifers in Montgomery County 

for the periods 1955–2000 and 1969–2000, respectively.

Model Calibration

Before calibration began, an initial predevelopment 

(no withdrawals) steady-state simulation was run to obtain 

starting heads for the hydrogeologic units for transient 

calibration simulations. Periodically during calibration, 

predevelopment steady-state simulations were run with 

the most current input data to obtain starting heads for 

successive transient calibration simulations. The input data 

that were adjusted from initial values on the basis of model 

output from successive transient simulations were hydraulic 

conductivity (transmissivity divided by aquifer thickness) 

of the aquifers, storativity of sands, vertical hydraulic 

conductance (leakance) between the water table and deeper 

zones of each hydrogeologic unit in outcrop areas, leakance 

between hydrogeologic units in subcrop areas, and inelasticclay storativity (actually, inelastic-clay-specific storage, which 

is multiplied by aquifer or confining unit thickness) in the 

Chicot, Evangeline, and Jasper aquifers and the Burkeville 

confining unit. Water-table heads, hydraulic conductivity, 

and storativity of the Burkeville confining unit, storativity of 

the Jasper aquifer, and temporal and spatial distributions of 

withdrawals were adjusted. Elastic-specific storage of clays 

in the Chicot and Evangeline aquifers were computed by 

multiplying inelastic-clay storativities by 0.01.

The HAGM was calibrated by an iterative trial-and-error 

adjustment of selected model input data (the aquifer properties 

that control water flow, recharge, discharge, and storage) 

in a series of transient simulations until the model output 

(simulated heads and land-surface subsidence and selected 

water-budget components) reasonably reproduced field 

measured (or estimated) aquifer responses and specified model 

calibration criteria. Transient model calibration comprised 

eight elements:

1. qualitative comparison of simulated and measured 

potentiometric surfaces of the Chicot, Evangeline, and 

Jasper aquifers for 2009 (Kasmarek, Houston, and 

Ramage, 2009);

2. quantitative comparison of simulated water levels and 

annually measured water levels of selected wells screened 

in the Chicot, Evangeline, and Jasper aquifers (calibration 

targets) by computing and evaluating the areal distribution 

of the root-mean-square error (RMSE) (square root of the 

sum of the squares of the differences between simulated 

and measured heads divided by the total number of 

calibration targets) of 497 sites for the three aquifers for 

2009;

3. qualitative comparison of hydrographs of simulated and 

measured water levels for each aquifer;

4. quantitative comparison of simulated and measured 

subsidence by computation and areal distribution of the 

RMSE for 474 calibration target sites was performed—

RMSE values were calculated by using standard GIS 

techniques, whereby a gridded surface of the 2000 

land-surface subsidence data (Gabrysch and Neighbors, 

2005) was intersected with the simulated subsidence data 

for model cells coinciding with the locations of the 474 

calibration targets, providing a spatial distribution of 

RMSE;

5. qualitative comparison of simulated subsidence from 

the 1890s through 2000 was compared to measured 

cumulative long-term land-surface subsidence from 1906 

to 2000 (Gabrysch and Neighbors, 2005);

6. qualitative comparison of simulated predevelopment 

potentiometric surfaces of the aquifers to conceptualized 

configurations of the predevelopment surfaces based 

on hydrogeologic knowledge of the Gulf Coast aquifer 

system;

7. quantitative comparison of simulated water-budget 

components—primarily recharge and withdrawal rates. 

The simulated recharge rate was compared to the range 
18 Hydrogeology and Simulation of Groundwater Flow and Land-Surface Subsidence in the Northern Part of the Gulf Coast Aquifer

1891–1900

1901–30

1931–40

1941–45

1946–53

1954–60

1961–62

1963–70

1971–73

1974–75 1976

1978

1980

1982

1984

1986

1988

1990

1992

1994

1996

1998

2000

2009

2002

2004

2006

Stress periods, in years (table 1)

0

900

800

700600

500

400

300

200

100

1,200

1,100

1,000

Groundwater withdrawal, in million gallons per day

Figure 8. Total groundwater withdrawals used during transient Houston Area Groundwater Model simulations, by stress periods, 1891–2009.
Simulation of Groundwater Flow and Land-Surface Subsidence 19

of rates from previous recharge studies (see “GroundWater-Flow Conditions, Recharge, and Discharge” 

section in Kasmarek and Robinson, 2004) to ensure 

that the value was reasonable. Similarly, simulated 

groundwater withdrawal rates were compared to the 

cumulative withdrawal rates published by HGSD, FBSD, 

and LSGCD for accuracy. Additionally, comparisons of 

simulated spatial distributions of recharge and discharge 

in the outcrops of aquifers to estimates of physically 

reasonable distributions based on knowledge of the 

hydrology of the Gulf Coast aquifer system also were 

used.

8. quantitative determination to ensure that the calibrated 

RMSE for each aquifer is 10 percent or less of the total 

range of calibrated simulated head.

Calibrated model parameters of the four layers of the 

GAM (Kasmarek and Robinson, 2004) and HAGM were 

compared to quantify the parameter differences (table 2). The 

additional water-use data (2001–9) used in the HAGM since 

the GAM was finalized required modification of the calibrated 

parameters, particularly in layer 4 (Jasper aquifer), to achieve 

recalibration.

The maximum value of simulated GHB conductance 

in layer 1 (Chicot aquifer) was decreased by more than two 

orders of magnitude, but the minimum value was increased by 

two orders of magnitude. All other maximum and minimum 

values of conductance in layer 2 (Evangeline aquifer), layer 3 

(Burkeville confining unit), and layer 4 (Jasper aquifer) were 

unchanged (table 2).

The maximum value of inelastic-clay storativity 

(inelastic storage coefficient) was increased by about one 

Table 1. Groundwater withdrawal (stress) periods used in the Houston Area Groundwater Model.

Stress

period

Length 

of time

(years)

Time

interval

Stress

period

Length 

of time

(years)

Time

interval

Stress

period

Length 

of time

(years)

Time

interval

1 Transient state1 10,000 years 27 0.085 Dec. 1980 53 0.085 Aug. 1988

2 10 1891–1900 28 1 1981 54 0.082 Sept. 1988

3 30 1901–30 29 0.085 Jan. 1982 55 0.085 Oct. 1988

4 10 1931–40 30 0.077 Feb. 1982 56 0.082 Nov. 1988

5 5 1941–45 31 0.085 Mar. 1982 57 0.085 Dec. 1988

6 8 1946–53 32 0.082 Apr. 1982 58 1 1989

7 7 1954–60 33 0.085 May 1982 59 1 1990

8 2 1961–62 34 0.082 June 1982 60 1 1991

9 8 1963–70 35 0.085 July 1982 61 1 1992

10 3 1971–73 36 0.085 Aug. 1982 62 1 1993

11 2 1974–75 37 0.082 Sept. 1982 63 1 1994

12 1 1976 38 0.085 Oct. 1982 64 1 1995

13 1 1977 39 0.082 Nov. 1982 65 1 1996

14 1 1978 40 0.085 Dec. 1982 66 1 1997

15 1 1979 41 1 1983 67 1 1998

16 0.085 Jan. 1980 42 1 1984 68 1 1999

17 0.077 Feb. 1980 43 1 1985 69 1 2000

18 0.085 Mar. 1980 44 1 1986 70 1 2001

19 0.082 Apr. 1980 45 1 1987 71 1 2002

20 0.085 May 1980 46 0.085 Jan. 1988 72 1 2003

21 0.082 June 1980 47 0.077 Feb. 1988 73 1 2004

22 0.085 July 1980 48 0.085 Mar. 1988 74 1 2005

23 0.085 Aug. 1980 49 0.082 Apr. 1988 75 1 2006

24 0.082 Sept. 1980 50 0.085 May 1988 76 1 2007

25 0.085 Oct. 1980 51 0.082 June 1988 77 1 2008

26 0.082 Nov. 1980 52 0.085 July 1988 78 1 2009

1

A 10,000-year transient period was used without withdrawals for model stability. 
20 Hydrogeology and Simulation of Groundwater Flow and Land-Surface Subsidence in the Northern Part of the Gulf Coast Aquifer

order of magnitude in layer 1 and was increased by about two 

orders of magnitude for layer 2. The minimum inelastic-clay 

storativity was increased by about one order of magnitude 

in layer 1 but decreased by about one order of magnitude in 

layer 2. A comparison of inelastic-clay storativity values for 

layers 3 and 4 was not possible because clay compaction was 

not simulated for these layers in the GAM.

The maximum value of simulated hydraulic conductivity 

(HC) value decreased about two orders of magnitude in 

layer 1, decreased slightly for layer 2, remained constant in 

layer 3, and decreased by about half in layer 4. The minimum 

HC was decreased by about two orders of magnitude for the 

layer 1, increased slightly for layer 2, remained the same for 

the layer 3, and increased by about three orders of magnitude 

for layer 4.

The maximum value of simulated storativity (sand 

storage) remained about constant for layers 1, 2, and 3 but 

increased by about one order of magnitude for layer 4. The 

minimum values of storativity for layers 1 and 3 remained 

constant, increased by about one order of magnitude for 

layer 2, and decreased by about one order of magnitude for 

layer 4.

Table 2. Comparison of calibrated-parameter values used in the Groundwater Availability Model (GAM) (2004) and the Houston Area 

Groundwater Model (HAGM).

[min, minimum; max, maximum; GHB, general head boundary; ICS, inelastic-clay storativity; HC, hydraulic conductivity; ft, feet; ft2/day, square feet per day; 

n/s, not simulated; <, less than; n/a, not applicable; ft3/day, cubic feet per day]

Simulated parameter GAM min GAM max HAGM min HAGM max

GHB conductance, in ft2/day

Chicot aquifer GHB 1.0×10-6 51,776 1.46×10-4 199

Evangeline aquifer GHB 1.202 69,700 1.202 69,700

Burkeville confining unit GHB 2.2×10-2 9.4×10-1 2.2×10-2 9.4×10-1

Jasper aquifer GHB 6.34 1,500 6.34 1,500

ICS (dimensionless)

Chicot aquifer ISC 2.06×10-7 5.18×10-3 5.3×10-6 1.49×10-2

Evangeline aquifer ISC 1.03×10-6 1.08×10-3 2.28×10-7 1.49×10-1

Burkeville confining unit ISC n/s n/s 2.05×10-6 9.24×10-5

Jasper aquifer ISC n/s n/s 1.0×10-6 9.47×10-4

HC, in ft2/day

Chicot aquifer HC 1.0×10-1 2,877 4.0×10-3 39.9

Evangline aquifer HC 2.0×10-1 49.5 3.9×10-1 30.8

Burkeville confining unit HC 9.0×10-6 2.1×10-2 9.0×10-6 2.1×10-2

Jasper aquifer HC 9.1×10-5 47.6 1.0×10-2 19.7

Storativity (dimensionless)

Chicot aquifer storativity 2.0×10-3 1.578×10-1 2.0×10-3 1.56×10-1

Evangeline aquifer storativity 2.0×10-4 1.8×10-1 1.0×10-3 1.82×10-1

Burkeville confining unit storativity 1.0×10-5 5.0×10-2 1.0×10-5 5.0×10-2

Jasper aquifer storativity 2.0×10-5 2.0×10-2 4.1×10-6 2.01×10-1

Leakance, in foot per day per foot

Chicot aquifer leakance 2.0.0×10-11 4.43×10-4 1.1×10-7 4.43×10-4

Evangeline aquifer leakance 5.0.0×10-11 5.0×10-3 9.0×10-8 5.0×10-3

Burkeville confining unit leakance 4.47.0×10-11 2.06×10-4 7.18.0×10-11 2.06×10-5

Jasper aquifer leakance n/a n/a n/a n/a

Total groundwater withdrawals for

 each aquifer Chicot aquifer Evangeline aquifer Burkeville confining unit Jasper aquifer

Total 2000 withdrawal, ft3/day 67,806,324 41,553,744 Negligible 9,380,838

Total 2009 withdrawal, ft3/day 51,383,890 55,668,241 Negligible 9,102,101

Change in withdrawls from 2000 to 2009 -16,422,434 14,114,497 -278,737
Simulation of Groundwater Flow and Land-Surface Subsidence 21

The maximum value of simulated leakance for layers 1, 

2, and 3 remained constant between the GAM and HAGM 

calibrated models. The minimum leakance in layer 1 was 

increased by about four orders of magnitude, was increased by 

about three orders of magnitude in layer 2, and remained about 

constant in layer 3. Additionally, a comparison of groundwater withdrawals for 2000 and 2009 for the four model layers 

indicates withdrawals decreased by 16,422,434 ft3/d for layer 

1, increased by 14,114,497 cubic feet per day (ft3/d) for layer 

2, and decreased by 278,737 ft3/day for layer 4. Water-use data 

for the Burkeville confining unit were unreported, therefore 

unknown, but are thought to be negligible.

Model Results

Simulated Hydraulic Properties Associated with 

Groundwater Flow and Subsidence

The calibrated spatial distributions of simulated 

hydraulic conductivity in the Chicot, Evangeline, and Jasper 

aquifers are shown in figures 9–11 and listed in table 2. 

Hydraulic conductivities of the Chicot aquifer ranged from 

4.0×10-3 to 39.91 ft/d, with the larger values located in Harris, 

Fort Bend, Liberty, Chambers, Galveston, Wharton, Colorado, 

Tyler, Jasper, and Newton Counties. Hydraulic conductivities 

of the Evangeline aquifer ranged from 3.9×10-1 to 30.79 ft/d, 

with largest values located in southeast Fort Bend County. 

Hydraulic conductivities of the Burkeville confining unit 

are coincident with values used in the GAM (Kasmarek and 

Robinson, 2004). Hydraulic conductivities of the Jasper 

aquifer ranged from 1.0×10-2 to 19.67 ft/d, with the larger 

values located in northern Harris and Montgomery Counties. 

Spatial distributions of hydraulic conductivity indicate that, 

generally, the largest values are coincident with areas of 

large withdrawals and are consistent with previous studies 

(Wesselman, 1972; Jorgensen, 1975; Carr and others, 1985; 

Baker, 1986; Kasmarek and Strom, 2002; Ryder and Ardis, 

2002; see “Initial Conditions,” Kasmarek and Robinson, 

2004).

Simulated sand storativities of the Chicot and Evangeline 

aquifers (2.0×10-3 to 1.56×10-1 and 1.0×10-3 to 1.82×10-1, 

figs. 12 and 13, respectively) reflect aquifer conditions from 

confined to semiconfined to water table. Sand storativities 

of the Chicot and Evangeline aquifers (figs. 12 and 13) 

generally are largest in the updip, outcrop areas, where 

water-table conditions prevail. Storativities of the Burkeville 

confining unit are coincident with values used in the GAM 

(Kasmarek and Robinson, 2004). Storativities of the Jasper 

aquifer (4.1×10-6 to 2.01×10-1) are generally largest in the 

updip, outcrop areas associated with water-table conditions 

(fig. 14).

The simulated calibrated spatial distributions of inelasticclay storativity for the Chicot aquifer, the Evangeline aquifer, 

the Burkeville confining unit, and the Jasper aquifer are 

shown in figures 15–18, respectively. Because a large area 

of land-surface subsidence has been documented (Gabrysch 

and Neighbors, 2005; Kasmarek, Gabrysch, and Johnson, 

2009) in Harris County and parts of Galveston, Fort Bend, 

Montgomery, Brazoria, Waller, Liberty, and Chambers 

Counties, only these areas of the model study area can 

be considered calibrated for elastic- and inelastic-clay 

storativity. Inelastic-clay storativities for the Chicot aquifer, 

the Evangeline aquifer, the Burkeville confining unit, and 

the Jasper aquifer range from 5.3×10-6 to 1.49×10-2, from 

2.28×10-7 to 1.49×10-1, from 2.05×10-6 to 9.24×10-5, and from 

1.0×10-6 to 9.47×10-4, respectively. A total of 474 calibrationtarget sites in Harris and surrounding counties were used 

to evaluate simulated subsidence compared to measured 

subsidence. After numerous iterative trial-and-error transient 

model simulations, the final RMSE was 0.37 ft.

The simulated potentiometric surfaces of the Chicot, 

Evangeline, and Jasper aquifers for 2009 (figs. 19–21; also 

shown are the selected wells used as calibration targets) 

indicate general agreement with measured potentiometric 

surfaces from Kasmarek, Houston, and Ramage (2009). The 

simulated 2009 potentiometric surfaces of the aquifers are 

shown in this report, but the simulated potentiometric surfaces 

for 1977, 1990, and 2000 compare favorably with coincident 

published water-level-altitude maps for 1977 (Gabrysch, 

1979); 1990 (Kasmarek, 1997); and 2000 (Coplin and Santos, 

2000: Chicot and Evangeline aquifer water-level altitudes; 

Kasmarek and Houston, 2007: 2000 Jasper aquifer water-level 

altitude). The RMSE of the simulated water levels for the three 

aquifers for 2009 were about 31.06 ft for the Chicot aquifer, 

about 33.73 ft for the Evangeline aquifer, and about 23.32 ft 

for the Jasper aquifer (table 3). The RMSE were calculated to 

be about 8, 6, and 6 percent, respectively, for the total range 

in simulated heads for the three aquifers, with a -0.03 percent 

water-budget difference between the total simulated inflow 

and the total simulated outflow.

Water levels were measured from December 2008 

through March 2009 in wells completed in the Chicot, 

Evangeline, and Jasper aquifers (Kasmarek, Houston, and 

Ramage, 2009). Simulated heads were compared to measured 

heads to evaluate the calibration validity of the groundwaterflow model. This comparison of simulated and measured 

heads of the Chicot aquifer, 2009 (fig. 22), indicates that the 

model is acceptable throughout the range of measured heads; 

however, simulated heads are lower than measured heads 

for values of measured head from about +60 ft to about -120 

ft. Similarly, for the simulated and measured heads of the 

Evangeline aquifer, 2009 (fig. 22), the model is acceptable 

throughout the range of heads, but simulated heads are lower 

than measured heads for values of measured head from 

about -105 ft to about -235 ft. Comparisons of simulated and 

measured heads for the Jasper aquifer, 2009 (fig. 22), indicate 

close correlation. These graphical comparisons between the 

simulated and measured heads correlate well with the RMSE 

shown in table 3.
22 Hydrogeology and Simulation of Groundwater Flow and Land-Surface Subsidence in the Northern Part of the Gulf Coast Aquifer EXPLANATION Hydraulic conductivity, in feet per day 0.004 to 4.99 5.00 to 9.99 10.00 to 14.99 15.00 to 19.99 20.00 to 24.99 25.00 to 29.99 30.00 to 34.99 35.00 to 39.91 30 40 MILES 30 40 KILOMETERS

0 10 200 10 20

Base modified from U.S. Geological Survey digital data

Scale 1:24,000 (except Louisiana hydrography 1:100,000)

Albers equal-area projection 

North American Datum of 1983

Standard parallels 34°55’ and 27°25’, central meridian 100°

95°

94°

96°

97°

30°

29°

31°

93°

N

POLK

SAN

 JACINTO

LIBERTY

GRIMES

WALLER

HARRIS

HARDIN

JEFFERSON

CHAMBERS

AUSTIN

COLORADO

WHARTON

GALVESTON

WASHINGTON WALKER

TRINITY

MATAGORDA BRAZORIA

FORT BEND

FAYETTE

JASPER

TYLER

NEWTON

VERNON

BEAUREGARD

CAMERON

CALCASIEU

JEFF

 DAVIS

ALLEN

ORANGE

SABINE

ANGELINA

SHELBY

 SAN

 AUGUSTINE

JACKSON

GONZALES CALHOUN BURLESON

BRAZOS BASTROP LEE CALDWELL

LAVACA

TEXAS

LOUISIANA

MONTGOMERY

NACOGDOCHES Updip limit of the Chicot aquifer

Estimated downdip limit of freshwater

in the Chicot aquifer

Figure 9. Simulated hydraulic conductivity of the Chicot aquifer in the Houston Area Groundwater Model study area.
Simulation of Groundwater Flow and Land-Surface Subsidence 23 EXPLANATION Hydraulic conductivity, in feet per day 0.39 to 4.19 4.20 to 7.99 8.00 to 11.79 11.80 to 15.59 15.60 to 19.39 19.40 to 23.19 23.20 to 26.99 27.00 to 30.79 30 40 MILES 30 40 KILOMETERS

0 10 200 10 20

Base modified from U.S. Geological Survey digital data

Scale 1:24,000 (except Louisiana hydrography 1:100,000)

Albers equal-area projection 

North American Datum of 1983

Standard parallels 34°55’ and 27°25’, central meridian 100°

95°

94°

96°

97°

30°

29°

31°

93°

N

POLK

SAN

 JACINTO

LIBERTY

GRIMES

WALLER

HARRIS

HARDIN

JEFFERSON

CHAMBERS

AUSTIN

COLORADO

WHARTON

GALVESTON

WASHINGTON WALKER

TRINITY

MATAGORDA BRAZORIA

FORT BEND

FAYETTE

JASPER

TYLER

NEWTON

VERNON

BEAUREGARD

CAMERON

CALCASIEU

JEFF

 DAVIS

ALLEN

ORANGE

SABINE

ANGELINA

SHELBY

 SAN

 AUGUSTINE

JACKSON

GONZALES CALHOUN BURLESON

BRAZOS BASTROP LEE CALDWELL

LAVACA

TEXAS

LOUISIANA

MONTGOMERY

NACOGDOCHES Updip limit of the Evangeline aquifer

Estimated downdip limit of freshwaterin the Evangeline aquifer

Figure 10. Simulated hydraulic conductivity of the Evangeline aquifer in Houston Area Groundwater Model study area.
24 Hydrogeology and Simulation of Groundwater Flow and Land-Surface Subsidence in the Northern Part of the Gulf Coast Aquifer EXPLANATION Hydraulic conductivity, in feet per day 0.01 to 1.49 1.50 to 2.80 2.81 to 4.82 4.83 to 6.83 6.84 to 8.61 8.62 to 10.31 10.32 to 12.01 12.02 to 13.79 13.80 to 16.27 16.28 to 19.67 30 40 MILES 30 40 KILOMETERS

0 10 200 10 20

Base modified from U.S. Geological Survey digital data

Scale 1:24,000 (except Louisiana hydrography 1:100,000)

Albers equal-area projection 

North American Datum of 1983

Standard parallels 34°55’ and 27°25’, central meridian 100°

95°

94°

96°

97°

30°

29°

31°

93°

N

POLK

SAN

 JACINTO

LIBERTY

GRIMES

WALLER

HARRIS

HARDIN

JEFFERSON

CHAMBERS

AUSTIN

COLORADO

WHARTON

GALVESTON

WASHINGTON WALKER

TRINITY

MATAGORDA BRAZORIA

FORT BEND

FAYETTE

JASPER

TYLER

NEWTON

VERNON

BEAUREGARD

CAMERON

CALCASIEU

JEFF

 DAVIS

ALLEN

ORANGE

SABINE

ANGELINA

SHELBY

 SAN

 AUGUSTINE

JACKSON

GONZALES CALHOUN BURLESON

BRAZOS BASTROP LEE CALDWELL

LAVACA

TEXAS

LOUISIANA

MONTGOMERY

NACOGDOCHES Updip limit of the Jasper aquifer

Estimated downdip limit of freshwaterin the Jasper aquifer

Figure 11. Simulated hydraulic conductivity of the Jasper aquifer in the Houston Area Groundwater Model study area.
Simulation of Groundwater Flow and Land-Surface Subsidence 25 EXPLANATION Storativity, dimensionless 0.002 to 0.019 0.020 to 0.038 0.039 to 0.058 0.059 to 0.077 0.078 to 0.097 0.098 to 0.116 0.117 to 0.136 0.137 to 0.156 30 40 MILES 30 40 KILOMETERS

0 10 200 10 20

Base modified from U.S. Geological Survey digital data

Scale 1:24,000 (except Louisiana hydrography 1:100,000)

Albers equal-area projection 

North American Datum of 1983

Standard parallels 34°55’ and 27°25’, central meridian 100°

95°

94°

96°

97°

30°

29°

31°

93°

N

POLK

SAN

 JACINTO

LIBERTY

GRIMES

WALLER

HARRIS

HARDIN

JEFFERSON

CHAMBERS

AUSTIN

COLORADO

WHARTON

GALVESTON

WASHINGTON WALKER

TRINITY

MATAGORDA BRAZORIA

FORT BEND

FAYETTE

JASPER

TYLER

NEWTON

VERNON

BEAUREGARD

CAMERON

CALCASIEU

JEFF

 DAVIS

ALLEN

ORANGE

SABINE

ANGELINA

SHELBY

 SAN

 AUGUSTINE

JACKSON

GONZALES CALHOUN BURLESON

BRAZOS BASTROP LEE CALDWELL

LAVACA

TEXAS

LOUISIANA

MONTGOMERY

NACOGDOCHES Updip limit of the Chicot aquifer

Estimated downdip limit of freshwaterin the Chicot aquifer

Figure 12. Simulated sand storativity of the Chicot aquifer in the Houston Area Groundwater Model study area.
26 Hydrogeology and Simulation of Groundwater Flow and Land-Surface Subsidence in the Northern Part of the Gulf Coast Aquifer EXPLANATION Storativity, dimensionless 0.001 to 0.023 0.024 to 0.045 0.046 to 0.068 0.069 to 0.091 0.092 to 0.114 0.115 to 0.136 0.137 to 0.159 0.160 to 0.182 30 40 MILES 30 40 KILOMETERS

0 10 200 10 20

Base modified from U.S. Geological Survey digital data

Scale 1:24,000 (except Louisiana hydrography 1:100,000)

Albers equal-area projection 

North American Datum of 1983

Standard parallels 34°55’ and 27°25’, central meridian 100°

95°

94°

96°

97°

30°

29°

31°

93°

N

POLK

SAN

 JACINTO

LIBERTY

GRIMES

WALLER

HARRIS

HARDIN

JEFFERSON

CHAMBERS

AUSTIN

COLORADO

WHARTON

GALVESTON

WASHINGTON WALKER

TRINITY

MATAGORDA BRAZORIA

FORT BEND

FAYETTE

JASPER

TYLER

NEWTON

VERNON

BEAUREGARD

CAMERON

CALCASIEU

JEFF

 DAVIS

ALLEN

ORANGE

SABINE

ANGELINA

SHELBY

 SAN

 AUGUSTINE

JACKSON

GONZALES CALHOUN BURLESON

BRAZOS BASTROP LEE CALDWELL

LAVACA

TEXAS

LOUISIANA

MONTGOMERY

NACOGDOCHES Updip limit of the Evangeline aquifer

Estimated downdip limit of freshwaterin the Evangeline aquifer

Figure 13. Simulated sand storativity of the Evangeline aquifer in the Houston Area Groundwater Model study area.
Simulation of Groundwater Flow and Land-Surface Subsidence 27 EXPLANATION Storativity, dimensionless 4.1 x 10-6 to 0.025 0.026 to 0.051 0.052 to 0.075 0.076 to 0.101 0.102 to 0.125 0.126 to 0.151 0.152 to 0.176 0.177 to 0.201 30 40 MILES 30 40 KILOMETERS

0 10 200 10 20

Base modified from U.S. Geological Survey digital data

Scale 1:24,000 (except Louisiana hydrography 1:100,000)

Albers equal-area projection 

North American Datum of 1983

Standard parallels 34°55’ and 27°25’, central meridian 100°

95°

94°

96°

97°

30°

29°

31°

93°

N

POLK

SAN

 JACINTO

LIBERTY

GRIMES

WALLER

HARRIS

HARDIN

JEFFERSON

CHAMBERS

AUSTIN

COLORADO

WHARTON

GALVESTON

WASHINGTON WALKER

TRINITY

MATAGORDA BRAZORIA

FORT BEND

FAYETTE

JASPER

TYLER

NEWTON

VERNON

BEAUREGARD

CAMERON

CALCASIEU

JEFF

 DAVIS

ALLEN

ORANGE

SABINE

ANGELINA

SHELBY

 SAN

 AUGUSTINE

JACKSON

GONZALES CALHOUN BURLESON

BRAZOS BASTROP LEE CALDWELL

LAVACA

TEXAS

LOUISIANA

MONTGOMERY

NACOGDOCHES Updip limit of the Jasper aquifer

Estimated downdip limit of freshwaterin the Jasper aquifer

Figure 14. Simulated sand storativity of the Jasper aquifer in the Houston Area Groundwater Model study area.
28 Hydrogeology and Simulation of Groundwater Flow and Land-Surface Subsidence in the Northern Part of the Gulf Coast Aquifer EXPLANATION Inelastic storativity, dimensionless 5.30 x 10-6 to 1.85 x 10-2 1.86 x 10-2 to 3.72 x 10-2 3.73 x 10-2 to 5.58 x 10-2 5.59 x 10-2 to 7.45 x 10-2 7.46 x 10-2 to 9.31 x 10-2 9.32 x 10-2 to 1.11 x 10-1 1.12 x 10-1 to 1.30 x 10-1 1.31 x 10-1 to 1.49 x 10-1 30 40 MILES 30 40 KILOMETERS

0 10 200 10 20

Base modified from U.S. Geological Survey digital data

Scale 1:24,000 (except Louisiana hydrography 1:100,000)

Albers equal-area projection 

North American Datum of 1983

Standard parallels 34°55’ and 27°25’, central meridian 100°

95°

94°

96°

97°

30°

29°

31°

93°

N

POLK

SAN

 JACINTO

LIBERTY

GRIMES

WALLER

HARRIS

HARDIN

JEFFERSON

CHAMBERS

AUSTIN

COLORADO

WHARTON

GALVESTON

WASHINGTON WALKER

TRINITY

MATAGORDA BRAZORIA

FORT BEND

FAYETTE

JASPER

TYLER

NEWTON

VERNON

BEAUREGARD

CAMERON

CALCASIEU

JEFF

 DAVIS

ALLEN

ORANGE

SABINE

ANGELINA

SHELBY

 SAN

 AUGUSTINE

JACKSON

GONZALES CALHOUN BURLESON

BRAZOS BASTROP LEE CALDWELL

LAVACA

TEXAS

LOUISIANA

MONTGOMERY

NACOGDOCHES Updip limit of the Chicot aquifer

Estimated downdip limit of freshwaterin the Chicot aquifer

Figure 15. Simulated inelastic-clay storativity of the Chicot aquifer in the Houston Area Groundwater Model study area.
Simulation of Groundwater Flow and Land-Surface Subsidence 29 EXPLANATION Inelastic storativity, dimensionless 2.28 x 10-7 to 1.86 x 10-2 1.87 x 10-2 to 3.73 x 10-2 3.74 x 10-2 to 5.59 x 10-2 5.60 x 10-2 to 7.45 x 10-2 7.46 x 10-2 to 9.32 x 10-2 9.33 x 10-2 to 1.11 x 10-1 1.12 x 10-1 to 1.30 x 10-1 1.31 x 10-1 to 1.49 x 10-1 30 40 MILES 30 40 KILOMETERS

0 10 200 10 20

Base modified from U.S. Geological Survey digital data

Scale 1:24,000 (except Louisiana hydrography 1:100,000)

Albers equal-area projection 

North American Datum of 1983

Standard parallels 34°55’ and 27°25’, central meridian 100°

95°

94°

96°

97°

30°

29°

31°

93°

N

POLK

SAN

 JACINTO

LIBERTY

GRIMES

WALLER

HARRIS

HARDIN

JEFFERSON

CHAMBERS

AUSTIN

COLORADO

WHARTON

GALVESTON

WASHINGTON WALKER

TRINITY

MATAGORDA BRAZORIA

FORT BEND

FAYETTE

JASPER

TYLER

NEWTON

VERNON

BEAUREGARD

CAMERON

CALCASIEU

JEFF

 DAVIS

ALLEN

ORANGE

SABINE

ANGELINA

SHELBY

 SAN

 AUGUSTINE

JACKSON

GONZALES CALHOUN BURLESON

BRAZOS BASTROP LEE CALDWELL

LAVACA

TEXAS

LOUISIANA

MONTGOMERY

NACOGDOCHES Updip limit of the Evangeline aquifer

Estimated downdip limit of freshwaterin the Evangeline aquifer

Figure 16. Simulated inelastic-clay storativity of the Evangeline aquifer in the Houston Area Groundwater Model study area.
30 Hydrogeology and Simulation of Groundwater Flow and Land-Surface Subsidence in the Northern Part of the Gulf Coast Aquifer EXPLANATION Inelastic storativity, dimensionless 2.05 x 10-6 to 1.33 x 10-5 1.34 x 10-5 to 2.46 x 10-5 2.47 x 10-5 to 3.59 x 10-5 3.60 x 10-5 to 4.72 x 10-5 4.73 x 10-5 to 5.85 x 10-5 5.86 x 10-5 to 6.98 x 10-5 6.99 x 10-5 to 8.11 x 10-5 8.12 x 10-5 to 9.24 x 10-5 30 40 MILES 30 40 KILOMETERS

0 10 200 10 20

Base modified from U.S. Geological Survey digital data

Scale 1:24,000 (except Louisiana hydrography 1:100,000)

Albers equal-area projection 

North American Datum of 1983

Standard parallels 34°55’ and 27°25’, central meridian 100°

95°

94°

96°

97°

30°

29°

31°

93°

N

POLK

SAN

 JACINTO

LIBERTY

GRIMES

WALLER

HARRIS

HARDIN

JEFFERSON

CHAMBERS

AUSTIN

COLORADO

WHARTON

GALVESTON

WASHINGTON WALKER

TRINITY

MATAGORDA BRAZORIA

FORT BEND

FAYETTE

JASPER

TYLER

NEWTON

VERNON

BEAUREGARD

CAMERON

CALCASIEU

JEFF

 DAVIS

ALLEN

ORANGE

SABINE

ANGELINA

SHELBY

 SAN

 AUGUSTINE

JACKSON

GONZALES CALHOUN BURLESON

BRAZOS BASTROP LEE CALDWELL

LAVACA

TEXAS

LOUISIANA

MONTGOMERY

NACOGDOCHES Updip limit of the Burkeville confining unit outcrop

Estimated downdip limit of freshwaterin the Burkeville confining unit

Figure 17. Simulated inelastic-clay storativity of the Burkeville confining unit in the Houston Area Groundwater Model study area.
Simulation of Groundwater Flow and Land-Surface Subsidence 31 EXPLANATION Inelastic storativity, dimensionless 1.00 x 10-6 to 1.34 x 10-4 1.35 x 10-4 to 2.20 x 10-4 2.21 x 10-4 to 3.20 x 10-4 3.21 x 10-4 to 4.31 x 10-4 4.32 x 10-4 to 5.46 x 10-4 5.47 x 10-4 to 6.69 x 10-4 6.70 x 10-4 to 7.92 x 10-4 7.93 x 10-4 to 9.47 x 10-4 30 40 MILES 30 40 KILOMETERS

0 10 200 10 20

Base modified from U.S. Geological Survey digital data

Scale 1:24,000 (except Louisiana hydrography 1:100,000)

Albers equal-area projection 

North American Datum of 1983

Standard parallels 34°55’ and 27°25’, central meridian 100°

95°

94°

96°

97°

30°

29°

31°

93°

N

POLK

SAN

 JACINTO

LIBERTY

GRIMES

WALLER

HARRIS

HARDIN

JEFFERSON

CHAMBERS

AUSTIN

COLORADO

WHARTON

GALVESTON

WASHINGTON WALKER

TRINITY

MATAGORDA BRAZORIA

FORT BEND

FAYETTE

JASPER

TYLER

NEWTON

VERNON

BEAUREGARD

CAMERON

CALCASIEU

JEFF

 DAVIS

ALLEN

ORANGE

SABINE

ANGELINA

SHELBY

 SAN

 AUGUSTINE

JACKSON

GONZALES CALHOUN BURLESON

BRAZOS BASTROP LEE CALDWELL

LAVACA

TEXAS

LOUISIANA

MONTGOMERY

NACOGDOCHES Updip limit of the Jasper aquifer

Estimated downdip limit of freshwaterin the Jasper aquifer

Figure 18. Simulated inelastic-clay storativity of the Jasper aquifer in the Houston Area Groundwater Model study area.
32 Hydrogeology and Simulation of Groundwater Flow and Land-Surface Subsidence in the Northern Part of the Gulf Coast Aquifer EXPLANATION -50 -50 Simulated potentiometric contour—Shows altitude at which water would have stood in tightly cased well. Interval 50 feet. Datum is NAVD 88 Measured potentiometric contour—Shows altitude at which water would have stood in tightly cased well. Interval 50 feet. Datum is NAVD 88 Data point—Well in which water-level measurement was made Data point and well number—Well in which water-level measurement was made and for which hydrograph is shown on figure 26

!

!

LJ–65–14–912

30 40 MILES

30 40 KILOMETERS

0 10 200 10 20

Base modified from U.S. Geological Survey digital data

Scale 1:24,000 (except Louisiana hydrography 1:100,000)

Albers equal-area projection 

North American Datum of 1983

Standard parallels 34°55’ and 27°25’, central meridian 100°

95°

94°

96°

97°

30°

29°

31°

93°

N

POLK

SAN

 JACINTO

LIBERTY

GRIMES

WALLER

HARRIS

HARDIN

JEFFERSON

CHAMBERS

AUSTIN

COLORADO

WHARTON

GALVESTON

WASHINGTON WALKER

TRINITY

MATAGORDA BRAZORIA

FORT BEND

FAYETTE

JASPER

TYLER

NEWTON

VERNON

BEAUREGARD

CAMERON

CALCASIEU

JEFF

 DAVIS

ALLEN

ORANGE

SABINE

ANGELINA

SHELBY

 SAN

 AUGUSTINE

JACKSON

GONZALES CALHOUN BURLESON

BRAZOS BASTROP LEE CALDWELL

LAVACA

TEXAS

LOUISIANA

MONTGOMERY

NACOGDOCHES

JY–65–43–101

BH–65–51–901

LJ–65–14–912

KH–64–33–918

!

!

!

!

Estimated downdip limit of freshwaterin the Chicot aquifer

Updip limit of the Chicot aquifer

-200

250

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!

!

!

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!

! !

! ! !

!

!

!

! !

! ! ! ! ! ! !

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!

!

!

!! !! !

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!

!

!

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!

!

!!

!

!

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!

!

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!

! ! ! ! !

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!

!! ! ! !! !

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!

!

! !

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!

!

!

! !! !

! !! !

!

! ! !! ! !

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! !!!! !

!

! !

! !

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!

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!

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!

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!

!

!

!

!

!!!

!

!

!

NAVD 88, North American Vertical Datum of 1988

Figure 19. Simulated and measured potentiometric surfaces of the Chicot aquifer, 2009, and location of monitoring wells in the Houston Area Groundwater Model study area.
Simulation of Groundwater Flow and Land-Surface Subsidence 33 EXPLANATION -50 -50 Simulated potentiometric contour—Shows altitude at which water would have stood in tightly cased well. Interval 50 feet. Datum is NAVD 88 Measured potentiometric contour—Shows altitude at which water would have stood in tightly cased well. Intervals 50, 100, and 250 feet. Datum is NAVD 88 Data point—Well in which water-level measurement was made Data point and well number—Well in which water-level measurement was made and for which hydrograph is shown on figure 27

!

!

LJ–65–14–909

Base modified from U.S. Geological Survey digital data

Scale 1:24,000 (except Louisiana hydrography 1:100,000)

Albers equal-area projection 

North American Datum of 1983

Standard parallels 34°55’ and 27°25’, central meridian 100°

95°

94°

96°

97°

30°

29°

31°

93°

N

POLK

SAN

 JACINTO

LIBERTY

GRIMES

WALLER

HARRIS

HARDIN

JEFFERSON

CHAMBERS

AUSTIN

COLORADO

WHARTON

GALVESTON

WASHINGTON WALKER

TRINITY

MATAGORDA BRAZORIA

FORT BEND

FAYETTE

JASPER

TYLER

NEWTON

VERNON

BEAUREGARD

CAMERON

CALCASIEU

JEFF

 DAVIS

ALLEN

ORANGE

SABINE

ANGELINA

SHELBY

 SAN

 AUGUSTINE

JACKSON

GONZALES CALHOUN BURLESON

BRAZOS BASTROP LEE CALDWELL

LAVACA

TEXAS

LOUISIANA

MONTGOMERY

NACOGDOCHES

Estimated downdip limit of freshwaterin the Evangeline aquifer

Updip limit of the Evangeline aquifer

0

50

-50 -100

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100

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50

-50

100

!

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!

!

!

!

!

!

!

!

!

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! !

! !

! ! ! !

!

! ! ! ! !! ! !!! !! !!! !!!

! ! !

!

! !!!

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!

! ! ! !! ! ! !

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! ! !! ! !

!

!

!

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!

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!

!

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!

!

!

!

!!

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!

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!

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!

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!!!

!

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!

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!!

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!

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!

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!

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!

!! ! ! ! ! ! !

!

!

!

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!!

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!

!!!! !

!

!

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!

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!

! !

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!

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!!

!

!

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!

!

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!

!

!

!

!

JY–65–33–210

BH–65–30–601

LJ–65–14–909

LJ–65–22–618

30 40 MILES

30 40 KILOMETERS

0 10 200 10 20

NAVD 88, North American Vertical Datum of 1988

Figure 20. Simulated and measured potentiometric surfaces of the Evangeline aquifer, 2009, and location of monitoring wells in the Houston Area Groundwater Model study area.
34 Hydrogeology and Simulation of Groundwater Flow and Land-Surface Subsidence in the Northern Part of the Gulf Coast Aquifer EXPLANATION -50 -50 Simulated potentiometric contour—Shows altitude at which water would have stood in tightly cased well. Interval 50 feet. Datum is NAVD 88 Measured potentiometric contour—Shows altitude at which water would have stood in tightly cased well. Interval 50 feet. Datum is NAVD 88 Data point—Well in which water-level measurement was made Data point and well number—Well in which water-level measurement was made and for which hydrograph is shown on figure 28

!

!

LJ–60–60–306

Base modified from U.S. Geological Survey digital data

Scale 1:24,000 (except Louisiana hydrography 1:100,000)

Albers equal-area projection 

North American Datum of 1983

Standard parallels 34°55’ and 27°25’, central meridian 100°

95°

94°

96°

97°

30°

29°

31°

93°

N

POLK

SAN

 JACINTO

LIBERTY

GRIMES

WALLER

HARRIS

HARDIN

JEFFERSON

CHAMBERS

AUSTIN

COLORADO

WHARTON

GALVESTON

WASHINGTON WALKER

TRINITY

MATAGORDA BRAZORIA

FORT BEND

FAYETTE

JASPER

TYLER

NEWTON

VERNON

BEAUREGARD

CAMERON

CALCASIEU

JEFF

 DAVIS

ALLEN

ORANGE

SABINE

ANGELINA

SHELBY

 SAN

 AUGUSTINE

JACKSON

GONZALES CALHOUN BURLESON

BRAZOS BASTROP LEE CALDWELL

LAVACA

TEXAS

LOUISIANA

MONTGOMERY

NACOGDOCHES Updip limit of the Jasper aquifer

Estimated downdip limit of freshwaterin the Jasper aquifer

!

!

!

!

!

!

! !

!!

!

!

!

! !!

! !

! ! ! !

! ! ! ! ! !!

!

! !!

! !

! ! ! !

! ! !

!

!

! ! ! ! !! ! ! ! ! ! !! !!!

!! ! !

! ! !

! !

! ! ! !

!

!

!

!

! !

!

!!

!!!

!

! ! !

!

!

!

TS–60–53–829

LJ–60–60–306 LJ–65–04–320

LJ–60–61–727

30 40 MILES

30 40 KILOMETERS

0 10 200 10 20

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NAVD 88, North American Vertical Datum of 1988

Figure 21. Simulated and measured potentiometric surfaces of the Jasper aquifer, 2009, and location of monitoring wells in the Houston Area Groundwater Model study area.
Simulation of Groundwater Flow and Land-Surface Subsidence 35

The spatial distribution of water-level residuals 

(measured values of head minus simulated values of head) 

for the Chicot aquifer (fig. 23) indicates that most residuals 

are positive in the area of the model that contains monitoring 

wells, which means that the model computes head below 

the measured value. In other areas of the Fort Bend, 

Brazoria, Galveston, southwest Harris, Chambers, Liberty 

and Montgomery Counties, areas of negative and positive 

residual values are prevalent, which means that the model 

computes head above the measured value in these areas. From 

a spatial distribution of water-level (head) residuals for the 

Evangeline aquifer (fig. 24), most of the residuals are positive, 

with isolated areas of negative residuals in southeast Harris, 

northern Galveston, western Chambers, northern Waller, and 

southeast Grimes Counties; an area of negative residuals 

also extends from northern Waller County into Montgomery 

County. The spatial distribution of water-level (head) residuals 

for the Jasper aquifer (fig. 25) indicates an almost even 

distribution between negative and positive residuals. These 

residual values are less than residual values of the Chicot and 

Evangeline aquifers (figs. 23 and 24).

Simulated and Measured Hydrographs

Hydrographs of simulated and measured water levels 

for observation wells in Brazoria, Galveston, Harris, and Fort 

Bend Counties in wells screened in the Chicot aquifer (fig. 26) 

indicate that simulated and measured water levels match 

closely. The hydrographs for Galveston and Harris Counties 

(fig. 26B and C) reflect generally declining heads through the 

mid- to late 1970s followed by rises associated with decreased 

withdrawals. The hydrographs of simulated and measured 

water levels in observation wells in Brazoria and Fort Bend 

Counties for the Evangeline aquifer (fig. 27A and B) also 

match closely. The two hydrographs from wells in Harris 

County (fig. 27C and D) indicate similar matches between 

simulated and measured water levels from about 1998 through 

2009, which spans the calibration period used for the HAGM. 

The hydrographs of simulated heads and measured heads in 

observation wells in Harris and Montgomery Counties for the 

Jasper aquifer (fig. 28) have similar water-level trends and 

become almost coincident in 2008 and 2009.

Simulated and Estimated Water-Budget 

Components

Simulated recharge and discharge in outcrops of the 

hydrogeologic units, vertical leakage between units, changes 

in storage, and withdrawals for 2009 are summarized in 

figure 29. The diagram indicates a net recharge (total recharge 

minus natural discharge) of 779.6 cubic feet per second 

(ft3/s) (about 0.56 in./yr) in the Chicot aquifer outcrop, 35.0 

ft3/s (about 0.68 in./yr) in the Evangeline aquifer outcrop, 

negligible net recharge in the Burkeville confining unit 

outcrop, and 16.8 ft3/s (about 0.26 in./yr) in the Jasper aquifer 

outcrop. For the entire system, the simulated total recharge 

for 2009 was 945.4 ft3/s (about 0.51 in./yr) in the outcrop 

areas. As a comparison, the simulated total recharge for the 

GAM in 2000 was 995 ft3/s (about 0.54 in./yr) (Kasmarek 

and Robinson, 2004, p. 90). In terms of a water balance 

(within 2.8 ft3/s because of rounding error) for the entire 

system in 2009, 945.4 ft3/s of total recharge plus 391.5 ft3/s 

from depletion of water in coarse-grained sediments (sands) 

and 102.5 ft3/s from inelastic compaction of clays is offset 

by 114.0 ft3/s of natural discharge and 1,328.2 ft3/s (about 

858.4 Mgal/d) of groundwater withdrawal. The net difference 

between total recharge (945.4 ft3/s) and withdrawal (1,328.2 

ft3/s) is 382.8 ft3/s (about 247.4 Mgal/d), and the volume of 

withdrawal from the Chicot, Evangeline, and Jasper aquifers 

was about 44, 48, and 8 percent, respectively. The volumetric 

budget (expressed in cubic feet per day) for the transient 

simulation for the HAGM in 2009, at the end of stress period 

78, is shown in table 4.

Simulated and Measured Land-Surface 

Subsidence

Simulated land-surface subsidence from 1891 

(predevelopment) to 2000 and measured land-surface 

subsidence from 1906 to 2000 is shown in figure 30. In Harris 

County and counties immediately adjacent, where the main 

area of subsidence has been measured, the simulated and 

measured values of subsidence match closely. As much as 

10 ft of measured subsidence has occurred in southeastern 

Harris County. A larger geographic area encompassing the 

maximum measured land-surface subsidence area and much 

of central to southeastern Harris County has subsided at least 

6 ft. In the western part of the HAGM study area, another area 

of simulated subsidence centered in Wharton County has as 

much as 3 ft of subsidence. In the eastern part of the HAGM 

study area, at the boundary of Hardin and Jasper Counties, 

an area of subsidence with as much as 3 ft of subsidence was 

simulated. An isolated area with as much as 3 ft of simulated 

Table 3. Number of water-level (head) measurements, root-meansquare errors of simulated head, and range of total simulated head 

in the Chicot, Evangeline, and Jasper aquifers, 2009.

Aquifer

Number of

water-level

measurements

Root-meansquare error 

of simulated 

water levels 

(feet)

Range of total 

simulated head 

(feet)

Chicot 165 31.06 366

Evangeline 251 33.73 541

Jasper 81 23.32 414
36 Hydrogeology and Simulation of Groundwater Flow and Land-Surface Subsidence in the Northern Part of the Gulf Coast Aquifer

subsidence is located in southeast Orange County. Measured 

subsidence has not been documented for these western and 

eastern areas of the HAGM study area. Measured compaction 

of subsurface sediments at 11 borehole extensometer sites in 

Harris and Galveston Counties has been continually recorded 

since as early as 1973 (Kasmarek and others, 2009).

Simulated land-surface subsidence (1891–2009) and 

measured land-surface subsidence (1906–2000) is shown in 

figure 31. For these periods in Harris County and counties 

immediately adjacent, where the main area of measured 

subsidence is present, the simulated and measured subsidence 

match closely, but not as closely as in figure 30. The most 

recent areas of simulated subsidence are generally in southern 

Montgomery, northwest Harris, and Fort Bend Counties, 

where water demand has increased and has resulted in 

sustained groundwater withdrawals during 2001–9. The two 

distal areas with as much as 3 ft of simulated subsidence in the 

eastern and western areas of the HAGM study area depicted 

in figure 31 are similar to the areal extent of simulated 

subsidence shown for 2000 in figure 30.

Figure 22. Relation between simulated and measured heads for the Chicot, Evangeline, and Jasper aquifers, 2009, in the Houston Area 

Groundwater Model study area.

-400 -300 -200 -100 0 100 200 300 400

Measured head, in feet above North American Vertical Datum of 1988

-400

-300

-200

-100

0

100

200

300

400

Simulated head, in feet above North American Vertical Datum of 1988

Head (water level),

 by aquifer

 Chicot

 Evangeline

 Jasper

EXPLANATION
Simulation of Groundwater Flow and Land-Surface Subsidence 37 EXPLANATION Data point—Well in which water-level measurement was made Residual, in feet -83.1 to -80.0 -79.9 to -60.0 -59.9 to -40.0 -39.9 to -20.0 -19.9 to 0.0 0.1 to 20.0

!!!!!!

! 20.1 to 40.0

 40.1 to 60.0 60.1 to 80.0 80.1 to 100.0 100.1 to 120.0 120.1 to 140.0

!!!!!

30 40 MILES

30 40 KILOMETERS

0 10 200 10 20

Base modified from U.S. Geological Survey digital data

Scale 1:24,000 (except Louisiana hydrography 1:100,000)

Albers equal-area projection 

North American Datum of 1983

Standard parallels 34°55’ and 27°25’, central meridian 100°

95°

94°

96°

97°

30°

29°

31°

93°

N

POLK

SAN

 JACINTO

LIBERTY

GRIMES

WALLER

HARRIS

HARDIN

JEFFERSON

CHAMBERS

AUSTIN

COLORADO

WHARTON

GALVESTON

WASHINGTON WALKER

TRINITY

MATAGORDA BRAZORIA

FORT BEND

FAYETTE

JASPER

TYLER

NEWTON

VERNON

BEAUREGARD

CAMERON

CALCASIEU

JEFF

 DAVIS

ALLEN

ORANGE

SABINE

ANGELINA

SHELBY

 SAN

 AUGUSTINE

JACKSON

GONZALES CALHOUN BURLESON

BRAZOS BASTROP LEE CALDWELL

LAVACA

TEXAS

LOUISIANA

MONTGOMERY

NACOGDOCHES

Estimated downdip limit of freshwaterin the Chicot aquifer

Updip limit of the Chicot aquifer

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!!

!

!

!

!

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!

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!!

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!

!

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!

!!

!

!

!

!

!!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

!

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!

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!!

!

!

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!

!

!

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!

!

!

!

!

!

!

!

Figure 23. Spatial distribution of water-level (head) residuals (measured minus simulated heads) for the Chicot aquifer, 2009, in the Houston Area Groundwater Model study area.
38 Hydrogeology and Simulation of Groundwater Flow and Land-Surface Subsidence in the Northern Part of the Gulf Coast Aquifer EXPLANATION Data point—Well in which water-level measurement was made Residual, in feet -136.1 to -120.0 -119.9 to -100.0 -99.9 to -80.0 -79.9 to -60.0 -59.9 to -40.0 -39.9 to -20.0

!!!!!!

!

!!!!!

 -19.9 to 0.0 0.1 to 20.0 20.1 to 40.0 40.1 to 60.0 60.1 to 80.0 80.1 to 95.2

Base modified from U.S. Geological Survey digital data

Scale 1:24,000 (except Louisiana hydrography 1:100,000)

Albers equal-area projection 

North American Datum of 1983

Standard parallels 34°55’ and 27°25’, central meridian 100°

95°

94°

96°

97°

30°

29°

31°

93°

N

POLK

SAN

 JACINTO

LIBERTY

GRIMES

WALLER

HARRIS

HARDIN

JEFFERSON

CHAMBERS

AUSTIN

COLORADO

WHARTON

GALVESTON

WASHINGTON WALKER

TRINITY

MATAGORDA BRAZORIA

FORT BEND

FAYETTE

JASPER

TYLER

NEWTON

VERNON

BEAUREGARD

CAMERON

CALCASIEU

JEFF

 DAVIS

ALLEN

ORANGE

SABINE

ANGELINA

SHELBY

 SAN

 AUGUSTINE

JACKSON

GONZALES CALHOUN BURLESON

BRAZOS BASTROP LEE CALDWELL

LAVACA

TEXAS

LOUISIANA

MONTGOMERY

NACOGDOCHES

Estimated downdip limit of freshwaterin the Evangeline aquifer

Updip limit of the Evangeline aquifer

!

!

!

!

!

!

!!

!

!

!

! !

!

! !

!

!

!

!

!

!!

!!!

!

!!!

!!

!

!!

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30 40 MILES

30 40 KILOMETERS

0 10 200 10 20

Figure 24. Spatial distribution of water-level (head) residuals (measured minus simulated heads) for the Evangeline aquifer, 2009, in the Houston Area Groundwater Model study area.
Simulation of Groundwater Flow and Land-Surface Subsidence 39 EXPLANATION Data point—Well in which water-level measurement was made Residual, in feet -85.53 to -68.03 -68.02 to -50.53 -50.52 to -33.04 -33.03 to -15.54 -15.53 to 1.96

!!!!!

1.97 to 19.4619.47 to 36.9636.97 to 54.4654.47 to 71.9571.96 to 89.45

!!!!!

Base modified from U.S. Geological Survey digital data

Scale 1:24,000 (except Louisiana hydrography 1:100,000)

Albers equal-area projection 

North American Datum of 1983

Standard parallels 34°55’ and 27°25’, central meridian 100°

95°

94°

96°

97°

30°

29°

31°

93°

N

POLK

SAN

 JACINTO

LIBERTY

GRIMES

WALLER

HARRIS

HARDIN

JEFFERSON

CHAMBERS

AUSTIN

COLORADO

WHARTON

GALVESTON

WASHINGTON WALKER

TRINITY

MATAGORDA BRAZORIA

FORT BEND

FAYETTE

JASPER

TYLER

NEWTON

VERNON

BEAUREGARD

CAMERON

CALCASIEU

JEFF

 DAVIS

ALLEN

ORANGE

SABINE

ANGELINA

SHELBY

 SAN

 AUGUSTINE

JACKSON

GONZALES CALHOUN BURLESON

BRAZOS BASTROP LEE CALDWELL

LAVACA

TEXAS

LOUISIANA

MONTGOMERY

NACOGDOCHES Updip limit of the Jasper aquifer

Estimated downdip limit of freshwaterin the Jasper aquifer

!

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30 40 MILES

30 40 KILOMETERS

0 10 200 10 20

Figure 25. Spatial distribution of water-level (head) residuals (measured minus simulated heads) for the Jasper aquifer, 2009, in the Houston Area Groundwater Model study area.
40 Hydrogeology and Simulation of Groundwater Flow and Land-Surface Subsidence in the Northern Part of the Gulf Coast Aquifer

HARRIS COUNTY

Well LJ–65–14–912

BRAZORIA COUNTY

Well BH–65–51–901

FORT BEND COUNTY

Well JY–65–43–101

GALVESTON COUNTY

Well KH–64–33–918

A

B

C

D 

Year

1891 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2009

1891 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2009

1891 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2009

1891 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2009

Water-level altitude, in feet above North American Vertical Datum of 1988

120

100

80

60

40

20

0

-80

-60

-40

-20

-120

-100

-160

-140

-180

-200

120

100

80

60

40

20

0

-80

-60

-40

-20

-120

-100

-160

-140

-180

-200

80

60

40

20

0

-20

-40

-120

-100

-80

-60

-160

-140

-200

-180

-220

-240

-260

-280

120

100

80

60

40

20

0

-80

-60

-40

-20

-120

-100

-160

-140

-180

-200

Simulated water level

Measured water level

EXPLANATION

Simulated water level

Measured water level

EXPLANATION

Simulated water level

Measured water level

EXPLANATION

Simulated water level

Measured water level

EXPLANATION

Figure 26. Hydrographs showing simulated and measured water levels in selected observation wells screened in the Chicot aquifer in 

A, Brazoria, B, Galveston, C, Harris, and D, Fort Bend Counties in the Houston Area Groundwater Model study area.
Simulation of Groundwater Flow and Land-Surface Subsidence 41

HARRIS COUNTY

Well LJ–65–14–909

HARRIS COUNTY

Well LJ–65–22–618

BRAZORIA COUNTY

Well BH–65–30–601

FORT BEND COUNTY

Well JY–65–33–210

A

B

C

D 

Year

1891 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2009

1891 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2009

1891 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2009

1891 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2009

Water-level altitude, in feet above North American Vertical Datum of 1988

150

100

50

0

-200

-150

-100

-50

-300

-250

-350

-400

150

100

50

0

-200

-150

-100

-50

-300

-250

-350

-400

150

100

50

0

-200

-150

-100

-50

-300

-250

-350

-400

150

100

50

0

-200

-150

-100

-50

-300

-250

-350

-400

Simulated water level

Measured water level

EXPLANATION

Simulated water level

Measured water level

EXPLANATION

Simulated water level

Measured water level

EXPLANATION

Simulated water level

Measured water level

EXPLANATION

Figure 27. Hydrographs showing simulated and measured water levels in selected observation wells screened in the Evangeline 

aquifer in A, Brazoria, B, Fort Bend, and C, D, Harris Counties in the Houston Area Groundwater Model study area.
42 Hydrogeology and Simulation of Groundwater Flow and Land-Surface Subsidence in the Northern Part of the Gulf Coast Aquifer

HARRIS COUNTY

Well LJ–65–04–320

HARRIS COUNTY

Well LJ–60–60–306

MONTGOMERY COUNTY

Well TS–60–53–829

HARRIS COUNTY

Well LJ–60–61–727

A

B

C

D 

Year

1891 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2009

1891 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2009

1891 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2009

1891 1900 1910 1920 1930 1940 1950 1960 1970 1980 1990 2000 2009

Water-level altitude, in feet above North American Vertical Datum of 1988

300

250

200

150

100

50

0

-50

-100

-150

-200

300

250

200

150

100

50

0

-50

-100

-150

-200

300

250

200

150

100

50

0

-50

-100

-150

-200

300

250

200

150

100

50

0

-50

-100

-150

-200

Simulated water level

Measured water level

EXPLANATION

Simulated water level

Measured water level

EXPLANATION

Simulated water level

Measured water level

EXPLANATION

Simulated water level

Measured water level

EXPLANATION

Figure 28. Hydrographs showing simulated and measured water levels in selected observation wells screened in the Jasper aquifer in 

A, B, C, Harris and D, Montgomery Counties in the Houston Area Groundwater Model study area.
Simulation of Groundwater Flow and Land-Surface Subsidence 43 EXPLANATION Recharge or discharge in outcrop area—Number is flow rate in cubic feet per second. Net equals recharge (total recharge in the aquifer outcrop) minus natural discharge Leakage through top or bottom of vertically adjacent hydrogeologic units—Number is flow rate in cubic feet per second. Net equals downward minus upward SSSC GW Net rate of water depleted from coarse-grained sediments, in cubic feet per second Net rate of water released from clay storage (clay compaction), in cubic feet per second Groundwater withdrawal rate, in cubic feet per second DOWNWARD UPWARD NET RECHARGE NATURAL DISCHARGE NET 785.3 5.7 (779.6) 582.5 34.5 (548.0) SS SC 945.4 + 391.5 + 102.5 = 114.0 + 1,328.2 1,439.4 = 1,442.2 WATER BALANCE1 RECHARGE + + = NATURAL DISCHARGE + GW 1 2.8 cubic feet per second difference because of numeric rounding.

OUTCROP AREA,

Chicot aquifer

OUTCROP AREAS,

Evangeline aquifer, Burkeville 

confining unit, and Jasper aquifer

No-flow boundary

(Catahoula confining system)

Jasper aquifer

Burkeville confining unit

Chicot aquifer

Evangeline aquifer

Constant-head

water table

5.8 2.9 (2.9) 58.0 41.2 (16.8)

NEGLIGIBLE

582.5 34.5 (548.0) 102.1 67.1 (35.0) 

6.0 2.8 (3.2)

785.3 5.7 (779.6)

SS = 77.3SC = 7.4

GW = 104.6

SS = 6.0SC = 0.1

GW = 0.0

SS = 11.0SC = 45.2

GW = 643.8

SS = 297.2SC = 49.9

GW = 579.8

(Conceptualization layer only)

(Model layer 1)

(Model layer 2)

(Model layer 3)

(Model layer 4)

Figure 29. Simulated 2009 water-budget components of the hydrogeologic units of the Houston Area Groundwater Model.
44 Hydrogeology and Simulation of Groundwater Flow and Land-Surface Subsidence in the Northern Part of the Gulf Coast Aquifer

An additional approach of simulating subsidence in 

Harris, Galveston, and Fort Bend Counties was the use of 

PRESS models developed by Helm (1975; 1976a, b; 1978). 

This model solves the Terzaghi equations of consolidation 

based on constant, one-dimensional total stress and transient 

changes of pore pressure at specific sites (Kasmarek and 

Strom, 2002). PRESS models were developed for 26 sites 

(fig. 32) by Freese and Nichols Inc. (Mike Reedy, Freese and 

Nichols Inc., written commun., 2011). For each PRESS site, 

a hydrograph was created by using coincident model cells 

of the simulated water-level data of the HAGM, and a value 

of subsidence was determined. A good correlation exists 

between the PRESS and HAGM simulated subsidence values. 

For example, the Pasadena site (fig. 32) indicates a PRESS 

determined subsidence value of 10.523 ft, and immediately 

adjacent to that site is a HAGM-simulated isolated 10-ft 

contour. Because the PRESS site locations (shown as polygons 

on fig. 32) encompass numerous model cells and may or may 

not extend across individual subsidence contours, a direct 

cell-by-cell or contour comparison is not a feasible evaluation. 

Instead, a more general areal comparison is appropriate.

Sensitivity Analysis

The sensitivity of calibrated model responses to 

changes in input data (the aquifer properties that control flow, 

recharge [general head boundary in the HAGM], discharge, 

subsidence, and storage, plus withdrawals) was evaluated. 

The values of selected model input data were iteratively and 

individually varied over ranges that may reflect plausible 

uncertainty (potential lack of accuracy of estimated or 

simulated values) in a series of simulations to present the 

effects of the uncertainty on simulated heads and subsidence. 

The effects of those changes on simulated 2009 water levels 

and land-surface subsidence were measured in terms of 

increases in RMSE (figs. 33 and 34, respectively). The plots 

depicting sensitivity of simulated water levels to changes in 

selected calibrated model input data (fig. 33) indicate that 

the model is more sensitive to groundwater withdrawals 

than to inelastic-clay storativity. In contrast, the plots 

depicting sensitivity of simulated land-surface subsidence 

to changes in selected calibrated model input data (fig. 34) 

indicate that the model is more sensitive to both groundwater 

withdrawals and sand storativity than to leakance. This 

analysis has implications if the HAGM is used for prediction 

of aquifer responses to future stresses. For example, the 

plots on figures 33 and 34 indicate that accurate estimates 

of withdrawals are more important to reliable predictions of 

heads and subsidence compared to accurate estimates of sand 

storativity.

Model Limitations

Several factors limit, or detract from, the ability of the 

HAGM to reliably simulate aquifer responses to groundwater withdrawals. The HAGM, like any nonlinear numeric 

model, is a simplification of the actual, complex aquifer 

system it simulates. As Brooks and others (1994) explain, 

simplification not only is necessary to make the problem 

tractable but also is necessary because the structure, 

properties, modeled boundaries, and stresses on the aquifer 

system can never be fully known. Simplifications involve 

assumptions about the actual system and the way it functions. 

Knowledge (or lack of knowledge) of the system is reflected 

in the quality and quantity of input data. The scale of the 

model, which is associated with the necessity to discretize a 

continuous system in space, also affects the ability of a model 

to produce reliable results.

Table 4. Volumetric budget for the Houston Area Groundwater Model at the end of stress period 78, 2009.

[ft3/day, cubic feet per day; E, exponent]

Cumulative

volumetric budget

Sand 

storage

Groundwater

withdrawal

Recharge and 

natural discharge

Clay

storage

Total

volume

Volume inflow (ft3/day) 1.016E+12 0.000E+00 7.701E+13 4.416E+11 7.846E+13

Volume outflow (ft3/day) 1.496E+11 2.580E+12 7.563E+13 5.290E+09 7.836E+13

Cumulative volumetric percent error 0.13

2009 volumetric budget

Volume inflow (ft3/day) 3.474E+07 0.000E+00 8.168E+07 9.112E+06 1.255E+08

Volume outflow (ft3/day) 9.169E+05 1.148E+08 9.850E+06 4.234E+04 1.256E+08

2009 volumetric percent error -0.03
Simulation of Groundwater Flow and Land-Surface Subsidence 45 EXPLANATION Land-surface subsidence, in feet 1891–2000 Simulated contour—Interval 1 foot 1906–2000 Measured contour—Interval 1 foot (from Gabrysch and Neighbors, 2005)

5

5

Base modified from U.S. Geological Survey digital data

Scale 1:24,000 (except Louisiana hydrography 1:100,000)

Albers equal-area projection 

North American Datum of 1983

Standard parallels 34°55’ and 27°25’, central meridian 100°

95°

94°

96°

97°

30°

29°

31°

93°

N

POLK

SAN

 JACINTO

LIBERTY

GRIMES

WALLER

HARRIS

HARDIN

JEFFERSON

CHAMBERS

AUSTIN

COLORADO

WHARTON

GALVESTON

WASHINGTON WALKER

TRINITY

MATAGORDA BRAZORIA

FORT BEND

FAYETTE

JASPER

TYLER

NEWTON

VERNON

BEAUREGARD

CAMERON

CALCASIEU

JEFF

 DAVIS

ALLEN

ORANGE

SABINE

ANGELINA

SHELBY

 SAN

 AUGUSTINE

JACKSON

GONZALES CALHOUN BURLESON

BRAZOS BASTROP LEE CALDWELL

LAVACA

TEXAS

LOUISIANA

MONTGOMERY

NACOGDOCHES

30 40 MILES

30 40 KILOMETERS

0 10 200 10 20

7

10 6

1

2

3

4

5

6

7

9 8 8

9

9

8

5

4

1

8

2

5

7

10 6

1

2

3

4

5

6

7

9 8 8

9

9

8

5

4

1

6

8

2

5

1

1

3

7

6

6

1 2

3

4

5

6

7

9 8

4 5

2

1

1

1

1 1

0

1

1

2

2

8

9

9

1

0

2

1

0

2

3

8

10

3

1

1

1

3

7

6

6

1 2

3

4

5

6

7

9 8

4 5

3 2

1

1

1

1 1

0

1

1

2

2

8

9 8

9

1

0

2

1

0

2

3

8

10

1

3

1

1

Figure 30. Simulated (1891–2000) and measured (1906–2000) land-surface subsidence in the Houston Area Groundwater Model study area.
46 Hydrogeology and Simulation of Groundwater Flow and Land-Surface Subsidence in the Northern Part of the Gulf Coast Aquifer EXPLANATION Land-surface subsidence, in feet 1891–2009 Simulated contour—Interval 1 foot 1906–2000 Measured contour—Interval 1 foot (from Gabrysch and Neighbors, 2005)

5

5

Base modified from U.S. Geological Survey digital data

Scale 1:24,000 (except Louisiana hydrography 1:100,000)

Albers equal-area projection 

North American Datum of 1983

Standard parallels 34°55’ and 27°25’, central meridian 100°

95°

94°

96°

97°

30°

29°

31°

93°

N

POLK

SAN

 JACINTO

LIBERTY

GRIMES

WALLER

HARRIS

HARDIN

JEFFERSON

CHAMBERS

AUSTIN

COLORADO

WHARTON

GALVESTON

WASHINGTON WALKER

TRINITY

MATAGORDA BRAZORIA

FORT BEND

FAYETTE

JASPER

TYLER

NEWTON

VERNON

BEAUREGARD

CAMERON

CALCASIEU

JEFF

 DAVIS

ALLEN

ORANGE

SABINE

ANGELINA

SHELBY

 SAN

 AUGUSTINE

JACKSON

GONZALES CALHOUN BURLESON

BRAZOS BASTROP LEE CALDWELL

LAVACA

TEXAS

LOUISIANA

MONTGOMERY

NACOGDOCHES

30 40 MILES

30 40 KILOMETERS

0 10 200 10 20

7

10 6

2

3

5

6

7

9 8 8

9

9

8

5

4

8

2

4

8

9

3

7

6 5

7

10 6

1

2

3

4

5

6

7

9 8

8

9

9

8

5

1

6

8

2

5 4

4

1

1

3

7

6

6

4 6

7

9 8

4 5

3

2

1

1

1

1 1

0

1

1

2

2

9

8

9

1

0

2

1

0

2

3

2

1

3 3

2

2

4 5

5

8

1

3

4

2

3

2

2 3

4

2

3

1

10

1

3 2

4

1

2

2

0

1 1

2

8

5

1

1 2

4 1

3

5

Figure 31. Simulated (1891–2009) and measured (1906–2000) land-surface subsidence in the Houston Area Groundwater Model study area.
Simulation of Groundwater Flow and Land-Surface Subsidence 47

GULF OF MEXICO

MONTGOMERY

WALLER

GRIMES

LIBERTY

HARRIS

CHAMBERS

BRAZORIA GALVESTON

FORT BEND

SAN

 JACINTO

WHARTON

MATAGORDA

NASA

AUSTIN

POLK

Base modified from U.S. Geological Survey digital data

Scale 1:24,000 (except Louisiana hydrography 1:100,000)

Albers equal-area projection

North American Datum of 1983

Standard parallels 34°55’ and 27°25’, central meridian 100°

7

7

7

5

3

4

8

7

7

7

1

2

5

3

4

6

8

6

6

4

9

6

9 8

8

5

5

7

3

1

4

5

6

1

9

7

2

1

1

1

2

1

1

6

7

4

5

6

9 8

8

1

1

1

1

5

1

2

3

1

2

5

1

1

9

10

9

8

8

8

9

4

5

10

9

8

8

8

10

9

96°00' 95°30' 95°00'

30°00'

29°30'

29°00'

Cypress Creek

Langham 

Creek

Katy

Needville

Richmond-Rosenberg

Smithers

 Lake

Arcola

Long Point

North Houston

Humble

Sheldon

Crosby

Galveston County

Bellaire

 West

Downtown

Genoa

Pasadena

Harrisburg

Galena Park Bellaire

Baytown

Eagle Point

Hobby

FM 1960

NASA

La Porte

6.704

5.154

0.667

6.634

4.992

1.522

6.365

6.535

2.192

4.820

6.138

6.774

1.439

4.026

10.523

6.283

1.061 

2.733

2.382

6.629 9.139

2.183

7.428

4.161

5.961

3.829

10

45

105

59

610

321 336

290

105

229900

59

90

10

45

10

105

45

90

59

90

PRESS Site location

 polygon—Value shown 

 covers period from 1906–

 2000, derived from PRESS

 models using HAGM

 simulated water-level

 data, in feet

HAGM Simulated land-

 surface subsidence, 

 1891–2000, in feet

Measured land-surface

 subsidence, 1906–2000 

 (Gabrysch and Neighbors,

 2005), in feet

2.192

EXPLANATION

5

5

PRESS, Predictions relating effective stress to subsidence model

HAGM, Houston Area Groundwater Model

30 MILES

30 KILOMETERS

0 10 20

0 10 20

Area enlarged

TEXAS

Figure 32. Predictions Relating Effective Stress to Subsidence (PRESS) model site locations and PRESS simulated land-surface 

subsidence, 1906–2000 (Mike Reedy, Freese and Nichols Inc., written commun., 2011), and Houston Area Groundwater Model simulated 

land-surface subsidence (1891–2009) and measured land-surface subsidence (1906–2000) in the Houston Area Groundwater Model 

study area.
48 Hydrogeology and Simulation of Groundwater Flow and Land-Surface Subsidence in the Northern Part of the Gulf Coast Aquifer

Assumption

A basic assumption is that the hydrogeologic units of 

the Gulf Coast aquifer system can be adequately represented 

by four discrete layers. This simplification is made because 

in the actual aquifer system the change from one aquifer to 

another with depth likely is transitional rather than abrupt. 

Other assumptions pertain to the boundary conditions. 

The conceptualization of the downdip boundaries of each 

hydrogeologic unit as the downdip limit of freshwater flow 

probably is realistic—salinity increases and flow becomes 

increasingly sluggish with distance downdip in each unit; 

however, the simplifying assumption that the downdip limit of 

freshwater flow in each unit is a sharp interface across which 

no flow occurs, the position of which is known and static 

over time, is more tenuous, as was discussed in the section 

“Hydrogeologic Units and Geologic Setting.” The assumption 

of the southwestern and northeastern aquifer-system 

boundaries as no-flow, coincident with the Lavaca and Sabine 

Rivers, respectively, is not entirely realistic. Although those 

rivers likely represent effective groundwater-flow divides in 

the shallow subsurface, the vertical extent of their influence 

on groundwater flow is unknown. Those lateral boundaries 

are far enough from areas of major withdrawals, however, 

so that they likely have negligible influence on the simulated 

response of the aquifer to withdrawals. The base of the Jasper 

aquifer is assumed to be a no-flow boundary, although in 

the actual aquifer system, a relatively small amount of water 

probably flows between the Jasper aquifer and the underlying 

Catahoula confining system. Another assumption is that in 

areas of large withdrawals and substantial declines in the 

potentiometric surface of an aquifer, the overlying water table 

has not declined in response to increased downward gradients; 

water-table heads are held constant during simulations. If 

this assumption is not valid, then more recharge than actually 

occurs in the actual system could be simulated in such areas, 

which also could result in simulated heads higher than actual 

heads. Although the validity of this assumption has not been 

studied, that annual rainfall is likely sufficient to keep any 

actual long-term water-table declines to a minimum. As noted 

in the section on “Land-Surface Subsidence and Storage in 

Clays,” assuming a constant-head water table also means 

constant geostatic pressure, which in turn makes changes 

in effective stress a function only of changes in head. If the 

Figure 33. Sensitivity of simulated water levels to changes in selected calibrated model input data of the Houston Area Groundwater 

Model.

0.05 0.1 0.2 0.5 1 2 5 10 15

Factor by which calibration value was multiplied

Root-mean-square error, in feet

25

50

75

100

125

150

175

200

225

250

Groundwater withdrawal

 (pumpage)

General head boundary

 (conductance)

Hydraulic conductivity

Sand storativity

Leakance

Inelastic-clay storativity

EXPLANATION
Simulation of Groundwater Flow and Land-Surface Subsidence 49

assumption of a constant water table was not valid and the 

water table in the actual system was to decline appreciably, 

then the model could overestimate effective stress and thus 

overestimate compaction (subsidence). Also pertaining to 

the simulation of land-surface subsidence, the assumption 

was made that head changes within a model time step in the 

aquifer sands are the same as those in the interbedded clays; 

in other words, head changes in the clays do not lag those in 

the sands. If simulated time steps are too short to allow for 

dissipation of all excess-residual-pore pressure in the clays 

of the actual system, then the amount of water released by 

the clays in the simulated system will be unrealistically large 

for the time step. Leake and Prudic (1991, p. 7) provide an 

equation for the upper limit on the time required for excessresidual-pore pressure in the actual system to dissipate on the 

basis of interbedded clay properties, which can be compared 

to the length of model time steps. Computations for the 

interbedded clays in the aquifer system indicate that excessresidual-pore pressure will dissipate in about 300 days. Thus 

the 1-year model time steps that were applied for all of the 

transient period except for 1980, 1982, and 1988 appear to 

be adequate, but the 1-month model time steps during those 

3 years probably are not, which implies that the simulated 

amount of water released by the clays for each of those 3 years 

probably is greater than the actual amount. 

Input Data

Associated with each of the input datasets is a level 

of uncertainty and a degree of bias, neither of which is 

quantitatively known. The uncertainty arises from the fact 

that point measurements or estimates of the input data 

represent regions around the points. The bias originates from 

the facts that some properties are better known than others 

are and individual properties are better known in some areas 

than in others (data points commonly are concentrated in 

some areas and are sparse in others). The result is that the 

optimum (but non-unique) spatial distributions of input 

data arrived at through calibration, or history matching, are 

distributions of effective properties, not actual properties; that 

is, the set of property distributions for the calibrated model 

is one of potentially many plausible sets that would allow 

simulated heads, subsidence, and water-budget components 

to reasonably match those of the actual system under selected 

Figure 34. Sensitivity of simulated land-surface subsidence to changes in selected calibrated model input data of the Houston Area 

Groundwater Model.

0.05 0.1 0.2 0.5 1 2 5 10 15

Factor by which calibration value was multiplied

Root-mean-square error, in feet

0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

10.0

Groundwater withdrawal

 (pumpage)

General head boundary

 (conductance)

Hydraulic conductivity

Sand storativity

Leakance

Inelastic-clay storativity

EXPLANATION
50 Hydrogeology and Simulation of Groundwater Flow and Land-Surface Subsidence in the Northern Part of the Gulf Coast Aquifer

conditions. In all likelihood, the property distributions reflect 

the order of magnitude of the actual-system properties but 

not the true distributions of the actual-system properties. 

For example, the simulated spatial distributions of hydraulic 

conductivity of the Chicot, Evangeline, and Jasper 

aquifers (figs. 9–11), while generally of the correct orders 

of magnitude, indicate larger values and generally more 

“definition” in areas coincident with large withdrawals. 

The distributions reflect the availability of more historical 

information for those areas and thus more attention to those 

areas during calibration. It is likely that if comparable 

groundwater development, subsurface information, head data, 

and calibration attention were focused on the system in other 

parts of the HAGM study area, the distributions of hydraulic 

conductivity in those areas would reflect that situation and be 

different from the distributions of figures 9, 10, and 11. What 

can be said about the spatial distributions of aquifer-system 

properties after calibration is that, collectively, they are one set 

of probably multiple sets of input data that allows the model 

to reasonably reproduce selected historical heads, land-surface 

subsidence, and groundwater flow. The possibility of multiple 

sets of input data implies that the reliability of the model for 

predictive simulation is uncertain.

Scale of Application

The HAGM is a regional-scale model, and as such, it is 

intended for regional-scale rather than local-scale analyses. 

Discretization of the HAGM area into 1-mi2 grid blocks in 

which aquifer properties and conditions are assumed to be 

averages over the area of each grid block precludes sitespecific analyses. For example, the simulated head in a grid 

block encompassing one or more pumping wells will represent 

an average head in the actual grid-block area rather than 

the head at or near the pumping well, which is much lower. 

An implication of simulated areal average heads is that, for 

calibration, comparison of simulated heads to measured heads 

might not always be comparable. Although explicit care is 

taken to ensure that static (nonpumping) water-level data are 

collected, undoubtedly some measured heads are influenced 

by nearby pumping or by antecedent pumping conditions or 

for other reasons are not representative of an average head in 

the grid-block area. Another scale-related issue—the “scale 

problem” as defined by Johnston (1999)—was described in 

the “Groundwater-Flow Conditions, Recharge, and Discharge” 

section. Because flow that enters and exits the actual system 

within the area encompassed by a single grid block cannot be 

simulated except by superposition of sources or sinks, which 

would be impractical over a regional area, the model does 

not simulate total recharge (and thus total [actual-system] 

groundwater flow). The fraction of total flow simulated is 

unknown, but the fraction of total flow simulated decreases 

as the grid-block size increases. This unknown flow fraction 

implies that any simulated components of flow not explicitly 

specified (for example, natural recharge and discharge) will 

be less than their actual-system counterparts. Explicitly 

specified components (for example, withdrawals) are based on 

measured or estimated actual-system data and therefore will 

more closely approximate actual-system magnitudes.

Summary

The availability of groundwater for municipal, industrial, 

and agricultural uses, as well as the potential subsidence 

associated with groundwater use, has been a concern in the 

Houston, Texas, area for decades. In cooperation with the 

Harris–Galveston Subsidence District, Fort Bend Subsidence 

District, and Lone Star Groundwater Conservation District, the 

U.S. Geological Survey developed and calibrated the Houston 

Area Groundwater Model (HAGM). Groundwater flow and 

land-surface subsidence in the northern part of the Gulf Coast 

aquifer system in Texas from predevelopment (before 1891) 

through 2009 were simulated; the objective of the HAGM is 

to accurately simulate and provide reliable, timely data on 

groundwater availability and land-surface subsidence in the 

Houston area through 2009. Results from the HAGM can be 

used to simulate aquifer response (changes in water levels and 

clay compaction) to future estimated water demands.

In a generalized conceptual model of the Gulf Coast 

aquifer system, the fraction of precipitation that does not 

evaporate, transpire through plants, or run off the land 

surface to streams enters the groundwater-flow system in 

topographically high updip outcrop areas of the hydrogeologic 

units in the northwestern part of the system. Most precipitation 

infiltrating into the saturated zone flows relatively short 

distances through shallow zones and then discharges to 

streams. The remainder of the water flows to intermediate and 

deep zones of the system southeastward of the outcrop areas 

where it is discharged by wells (in the developed system) and 

by upward leakage in topographically low areas near or along 

the coast. Because groundwater flow was simulated in the 

HAGM only as far as the downdip limit of freshwater, only 

the parts of the hydrogeologic units containing freshwater are 

described in this report.

The HAGM was developed to simulate groundwater 

flow and land-surface subsidence in the northern Gulf Coast 

aquifer system (Chicot aquifer, Evangeline aquifer, Burkeville 

confining unit, and Jasper aquifer) from predevelopment 

(1891) through 2009. The finite-difference computer code 

MODFLOW-2000 was used in this application. The finitedifference grid for the numerical model covers 33,565 square 

miles in southeastern Texas and southwestern Louisiana. The 

model grid was rotated 37.6 degrees clockwise so that the 

orientation of the model closely coincides with the natural 

groundwater divides, model boundaries, and predevelopment 

and postdevelopment flow paths. The four layers of the model 

together contain 134,260 grid blocks. Each layer consists of 

137 rows and 245 columns. Layer 1 represents the Chicot 

aquifer, layer 2 the Evangeline aquifer, layer 3 the Burkeville 

confining unit, and layer 4 the Jasper aquifer. The grid blocks 

are uniformly spaced with each model cell area equal to 
Summary 51

1 square mile. The MODFLOW General-Head Boundary 

package was used to simulate recharge and discharge in the 

outcrops of the Chicot, Evangeline, and Jasper aquifers and 

the Burkeville confining unit. This package allows the water 

table of an aquifer system to function as a head-dependent 

flux. Initial conditions, including heads and hydraulic 

properties, provided a starting point for the model simulation. 

The initial conditions for head and hydraulic properties 

were coincident with the calibrated groundwater flow model 

previously created (2004) for the northern Gulf Coast by the 

USGS and cooperators.

Simulation of land-surface subsidence (actually, 

compaction of clays) and release of water from storage in 

the clays of the Chicot, Evangeline, and Jasper aquifers and 

the Burkeville confining unit was accomplished by using 

the Subsidence and Aquifer-System Compaction package 

designed for use with MODFLOW-2000. Simulations were 

made under transient conditions from 1891 through 2009 

for 78 withdrawal (stress) periods of variable length. Total 

groundwater withdrawals increased from an estimated 41 

million gallons per day in 1891 to about 869 million gallons 

per day in 2009.

The HAGM was calibrated by an iterative trial-and-error 

adjustment of selected model input data (the aquifer properties 

that control water flow, recharge, discharge, and storage) 

in a series of transient simulations until the model output 

(simulated heads, land-surface subsidence, selected waterbudget components) reasonably reproduced field measured 

aquifer responses. 

Calibrated model parameters from each layer within the 

GAM and HAGM were compared to identify any differences 

in values. Generally, the additional data available in the model 

area since the development of the GAM required substantial 

modification of GAM parameters, particularly in the Jasper 

aquifer, for a complete calibration. Maximum general-head 

boundary conductance in the Chicot aquifer was reduced 

by more than two orders of magnitude, whereas generalhead boundary conductance values in the other model layers 

remained unchanged. Inelastic-clay storativity maximum and 

minimum values varied slightly between the two models in 

the Chicot and Evangeline aquifers but were of a consistent 

magnitude. Minimum hydraulic conductivity values decreased 

about two orders of magnitude in the Chicot aquifer, increased 

less than an order of magnitude in the Evangeline aquifer, 

and increased about three orders of magnitude in the Jasper 

aquifer. Maximum hydraulic conductivity values decreased 

nearly two orders of magnitude in the Chicot and less than 

one order of magnitude in the Evangeline and Jasper aquifers. 

Spatial distributions of simulated parameters of specific 

storage and leakance were similar between the GAM and 

HAGM calibrated models.

Hydraulic conductivities of the Chicot aquifer ranged 

from 4.0×10-3 to 39.91 feet per day (ft/d), with the larger 

values located in Harris, Fort Bend, Liberty, Chambers, 

Galveston, Wharton, Colorado Tyler, Jasper, and Newton 

Counties. Hydraulic conductivities of the Evangeline aquifer 

ranged from 3.9×10-1 to 30.79 ft/d, with largest values located 

in northeast Fort Bend County. Hydraulic conductivities of 

the Burkeville confining unit are coincident with values used 

in the GAM. Hydraulic conductivities of the Jasper aquifer 

ranged from 1.0×10-2 to 19.67 ft/d, with the larger values 

located in northern Harris and Montgomery Counties.

Simulated sand storativities of the Chicot and Evangeline 

aquifers (2×10-3 to 1.56×10-1 and 1×10-3 to 1.82×10-1, 

respectively) reflect aquifer conditions from confined 

to semiconfined to water table. Sand storativities of the 

Chicot and Evangeline aquifers generally are largest in the 

updip, outcrop areas where water-table conditions prevail. 

Storativities of the Burkeville confining unit are coincident 

with values used in the GAM. Storativities of the Jasper 

aquifer (4.1×10-6 to 2.01×10-1) are generally largest in the 

updip, outcrop areas associated with water-table conditions.

Because a large area of land-surface subsidence has been 

documented in Harris County and parts of Galveston, Fort 

Bend, Montgomery, Brazoria, Waller, Liberty, and Chambers 

Counties, only these areas of the HAGM can be considered 

calibrated for elastic- and inelastic-clay storativity. Inelasticclay storativities for the Chicot aquifer, the Evangeline 

aquifer, the Burkeville confining unit, and the Jasper 

aquifer range from 5.3×10-6 to 1.49×10-2, from 2.28×10-7 to 

1.49×10-1, from 2.05×10-6 to 9.24×10-5, and from 1.0×10-6 to 

9.47×10-4, respectively. A total of 474 sites located in Harris 

and surrounding counties were used to evaluate simulated 

subsidence compared to measured subsidence. After numerous 

iterative trial-and-error transient model simulations, the final 

land-surface subsidence RMSE was 0.37 ft.

The simulated potentiometric surfaces of the Chicot, 

Evangeline, and Jasper aquifers for 2009 indicate general 

agreement with the measured potentiometric surfaces. The 

RMSE of the three aquifer potentiometric surfaces for 

2009 were 31.06 ft for the Chicot aquifer, 33.73 ft for the 

Evangeline aquifer, and 23.32 ft for the Jasper aquifer. The 

RMSE were about 8, 6, and 6 percent, respectively, for the 

total range in simulated heads for the three aquifers, with 

a -0.03 percent water-budget difference between the total 

simulated inflow and the total simulated outflow.

Hydrographs were used to compare simulated and 

measured water levels; selected water wells with screened 

intervals in the Chicot, Evangeline, and Jasper aquifers match 

closely relative to the ranges of water-level change. Simulated 

water budget components for 2009 indicate that a net recharge 

(total recharge minus natural discharge) of 779.6 cubic feet per 

second (ft3/s) (about 0.56 inches per year [in./yr]) in the Chicot 

aquifer outcrop, 35.0 ft3/s (about 0.68 in./yr) in the Evangeline 

aquifer outcrop, negligible net recharge in the Burkeville 

confining unit outcrop, and 16.8 ft3/s (about 0.26 in./yr) in the 

Jasper aquifer outcrop. For the entire system, the simulated 

total recharge for 2009 was 945.4 ft3/s (about 0.51 in./yr).

In Harris County and counties immediately adjacent, 

where the main area of subsidence has been measured, the 

1891–2000 simulated subsidence matches closely with 

the 1906–2000 measured subsidence. As much as 10 ft of 
52 Hydrogeology and Simulation of Groundwater Flow and Land-Surface Subsidence in the Northern Part of the Gulf Coast Aquifer

subsidence has occurred in southeastern Harris County near 

the northern end of Galveston Bay. A larger geographic 

area encompassing the maximum land-surface subsidence 

area and much of central to southeastern Harris County has 

subsided at least 6 ft. Again, in Harris County and counties 

immediately adjacent, where the main area of subsidence is 

present, the 1891–2009 simulated subsidence matches closely 

with the 1906–2000 measured subsidence, but not as closely 

as the simulated subsidence for 1891–2000. The most recent 

areas of subsidence are approximately located in southern 

Montgomery, northwest Harris, and Fort Bend Counties, 

where development has occurred and required sustained 

groundwater withdrawals during 2001–9.

An additional approach of simulating and predicting 

subsidence in Harris, Galveston, and Fort Bend Counties was 

the use of Predictions Relating Effective Stress to Subsidence 

(PRESS) model. For each PRESS site, a hydrograph was 

created by using coincident model cells of the simulated 

water-level data of the HAGM, and a value of subsidence was 

determined. A good correlation exists between the PRESS 

and HAGM simulated subsidence values. For example, at the 

Pasadena PRESS site, the simulated value is 10.523 ft and the 

site is located immediately adjacent to a HAGM-simulated 

isolated 10 ft contour.

The sensitivity of calibrated-model responses to changes 

in input data (the aquifer properties that control flow, recharge, 

discharge, subsidence, and storage, plus withdrawals) was 

evaluated. The HAGM sensitivity results indicate that accurate 

estimates of hydraulic conductivity and withdrawals are more 

important to reliable predictions of heads and subsidence 

compared to accurate estimates of sand storativity. 

Several factors limit, or detract from, the ability of 

the HAGM to reliably predict aquifer responses to future 

conditions. The HAGM, like any nonlinear numeric model, 

is a simplification of the actual, complex aquifer system it 

simulates. Additionally, the HAGM is a regional-scale model, 

and as such, it is intended for regional-scale rather than localscale analyses. Discretization of the HAGM study area into 

1-square-mile grid blocks in which aquifer properties and 

conditions are assumed to be averages over the area of each 

grid block precludes site-specific analyses.

Associated with each of the input datasets are a level 

of uncertainty and a degree of bias, neither of which is 

quantitatively known. The uncertainty arises from the fact 

that point measurements or estimates of the input data 

represent regions around the points. The bias originates from 

the facts that some properties are better known than others 

are and individual properties are better known in some areas 

than in others (data points commonly are concentrated in 

some areas and are sparse in others). The result is that the 

optimum (but non-unique) spatial distributions of input 

data arrived at through calibration, or history matching, are 

distributions of effective properties, not actual properties; that 

is, the set of property distributions for the calibrated model 

is one of potentially many plausible sets that would allow 

simulated heads, subsidence, and water-budget components 

to reasonably match those of the actual system under selected 

conditions.

A basic assumption is that the hydrogeologic units of 

the Gulf Coast aquifer system can be adequately represented 

by four discrete layers, a simplification because, in the 

actual system, the change from one aquifer to another with 

depth likely is transitional rather than abrupt. Downdip 

salinity changes and lateral boundary conditions also are not 

absolutely known.

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Kasmarek—Hydrogeology and Simulation of Groundwater Flow and Land-Surface Subsidence in the Northern Part of the Gulf Coast Aquifer System—SIR 2012–5154

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