Water-level change 2010–2040 for Scenario 3

Water-level decline

Water-level rise

Leon-Belding area

PECOS COUNTY

BREWSTER COUNTY

REEVES COUNTY

TERRELL COUNTY

CROCKETT

COUNTY

JEFF DAVIS COUNTY

UPTON COUNTY

WARD

COUNTY

CRANE COUNTY

Belding Fort Stockton

Pecos River

Oblique view of the Pecos County region with selected wells extending into the simulated water table

of the Edwards-Trinity aquifer (vertically exaggerated)

U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Report 2013–5228 

Version 1.1, August 2014

Prepared in cooperation with the Middle Pecos Groundwater Conservation District, Pecos 

County, City of Fort Stockton, Brewster County, and Pecos County Water Control and 

Improvement District No. 1

Simulation of Groundwater Flow in the Edwards-Trinity and 

Related Aquifers in the Pecos County Region, Texas
 
Simulation of Groundwater Flow in the 

Edwards-Trinity and Related Aquifers in the 

Pecos County Region, Texas

By Brian R. Clark, Johnathan R. Bumgarner, Natalie A. Houston, and 

Adam L. Foster

Prepared in cooperation with the Middle Pecos Groundwater Conservation 

District, Pecos County, City of Fort Stockton, Brewster County, and Pecos County 

Water Control and Improvement District No. 1

Scientific Investigations Report 2013–5228

Version 1.1, August 2014

U.S. Department of the Interior

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

SALLY JEWELL, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2014

First release: 2014, online 

Revised: August 2014 (ver.1.1), online 

Suggested citation:

Clark, B.R., Bumgarner, J.R., Houston, N.A., and Foster, A.L., 2014, Simulation of groundwater flow in the EdwardsTrinity and related aquifers in the Pecos County region, Texas (ver.1.1, August 2014): U.S. Geological Survey Scientific 

Investigations Report 2013–5228, 56 p., http://dx.doi.org/10.3133/sir20135228. 

For more information on the USGS—the Federal source for science about the Earth, its natural and living 

resources, natural hazards, and the environment, visit http://www.usgs.gov or call 1–888–ASK–USGS.

For an overview of USGS information products, including maps, imagery, and publications, 

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

U.S. Government.

Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials 

as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner.

ISSN 2328-031X (print)

ISSN 2328-0328 (online)

ISBN 978 1 4113 0
iii

Acknowledgments

Jonathan Thomas, Andrew Teeple, and Jason Payne were instrumental in development of the 

model layers and water level information. Paul Weatherby of the Middle Pecos Groundwater 

Conservation District provided insightful, detailed knowledge of the area and water-use 

information. Greg Stanton and George Ozuna provided oversight and conceptual knowledge of 

the system. Geoff Delin, Randall Hanson, and Mark Kasmarek provided technical reviews of 

this report.

v

Contents

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

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

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

Purpose and Scope .............................................................................................................................3

Previous Investigations .......................................................................................................................3

Description of the Pecos County Region Model Area ..................................................... 3

Geologic and Hydrogeological Setting ............................................................................................4

Hydrogeologic Framework ........................................................................................................4

Geochemistry ...............................................................................................................................7

Groundwater-Flow System ........................................................................................................7

Simulation of Groundwater Flow ................................................................................................................7

Groundwater-Flow Model Construction ..........................................................................................7

Spatial Discretization and Layering ..................................................................................................7

Temporal Discretization ......................................................................................................................7

Hydrologic Boundaries .......................................................................................................................8

Recharge ......................................................................................................................................8

Discharge .....................................................................................................................................8

Springs .................................................................................................................................8

Groundwater Pumping ......................................................................................................8

Specified Head Boundaries ...........................................................................................10

No-Flow Boundaries ................................................................................................................12

Structure ..............................................................................................................................................12

Initial Conditions ................................................................................................................................12

Hydraulic Properties ..........................................................................................................................13

Hydraulic Conductivity .............................................................................................................13

Vertical Hydraulic Conductivity and Storage .......................................................................18

Model Calibration ........................................................................................................................................28

Optimal Parameter Estimates ..........................................................................................................28

Model Fit and Model Error ......................................................................................................28

Calibration Targets ....................................................................................................................28

Hydraulic Head Observations and Errors ....................................................................28

Spring Flow .......................................................................................................................29

Hydrographs .....................................................................................................................32

Potentiometric Surfaces .................................................................................................32

Sensitivity Analysis ...................................................................................................................32

Normality of Residuals and Goodness of Fit ........................................................................35

Geochemical Modeling .....................................................................................................................37

Groundwater-Flow Budget ...............................................................................................................39

Model Limitations ...............................................................................................................................39

Development of Groundwater Pumping Scenarios ..............................................................................41

Scenario 1 ...........................................................................................................................................41

Scenario 2 ...........................................................................................................................................43

Scenario 3 ...........................................................................................................................................43
vi

Summary .......................................................................................................................................................46

References Cited .........................................................................................................................................47

Appendixes

1. Wells completed in the Edwards-Trinity aquifer providing observed hydraulic 

head data in the Pecos County region model area, Texas, 1940–2010 .............................53

2. Amount of pumping in 2008 by county, category, and aquifer in the Pecos 

County region, Texas .................................................................................................................54

3. Data-collection sites providing data for the geochemical analysis in the Pecos 

County region, Texas .................................................................................................................55

Figures

1. Map showing location and generalized geological structural features of the 

model area in the Pecos County region of Texas ...................................................................2

2. Map showing major aquifers and minor aquifers in the Pecos County region 

model area, Texas ........................................................................................................................6

3. Map showing simulated net recharge in the Pecos County region model area, 

Texas ..............................................................................................................................................9

4. Map showing specified head and horizontal flow (fault) boundaries used in the 

model simulation (excluding Constant Head Boundary of the Rustler aquifer) and 

locations of geochemical data collection in the Pecos County region model area, 

Texas ............................................................................................................................................11

5. Map showing distribution of pilot points throughout the Pecos County region 

model area, Texas ......................................................................................................................14

6. Maps showing calibrated hydraulic conductivity distribution of the Pecos Valley 

aquifer (alluvial layer) in the Pecos County region model area, Texas ............................15

7. Maps showing calibrated vertical hydraulic conductivity distribution of the Pecos 

Valley aquifer (alluvial layer) in the Pecos County region model area, Texas .................19

8. Maps showing calibrated specific storage distribution of the Pecos Valley 

aquifer (alluvial layer) in the Pecos County region model area, Texas ............................24

9. Map showing calibrated specific yield distribution of the Pecos Valley aquifer 

(alluvial layer) in the Pecos County region model area, Texas ..........................................27

10. Graph showing comparison of simulated and observed spring flow at Comanche 

Springs in the Pecos County region model area, Texas ......................................................31

11. Hydrographs showing simulated and observed hydrographs of hydraulic head 

in selected wells in the Edwards-Trinity aquifer in the Pecos County region model 

area, Texas ..................................................................................................................................33

12. Map showing potentiometric surface (1980–2010) and simulated groundwaterlevel altitudes for the Edwards-Trinity aquifer in the Pecos County region model 

area, Texas, 2010 ........................................................................................................................34

13. Histogram showing composite sensitivities of all parameter groups of the 

groundwater-flow model of the Edwards-Trinity and related aquifers in the 

Pecos County region model area, Texas ...............................................................................35

14. Histogram of residuals for the Pecos County region model, Texas ..................................36

15. Graph showing relation between simulated and observed hydraulic heads for the 

Pecos County region model, Texas .........................................................................................36

16. Map showing spatial distribution of hydraulic-head residuals after 1940 in the 

Edwards-Trinity aquifer in the Pecos County region model area, Texas ..........................38
vii

17. Graph showing net simulated groundwater flow from 1940 to 2010 in the Pecos 

Valley and Edwards-Trinity aquifers in the Pecos County region model area, 

Texas ............................................................................................................................................40

18. Map showing water-level difference in the Trinity part of the Edwards-Trinity 

aquifer (Trinity layer) in the Pecos County region model area, Texas, between 

2010 and 2040 for scenario 1 ....................................................................................................42

19. Map showing water-level difference in the Trinity part of the Edwards-Trinity 

aquifer (Trinity layer) in the Pecos County region model area, Texas, between 

2010 and 2040 for scenario 2 ....................................................................................................44

20. Map showing water-level difference in the Trinity part of the Edwards-Trinity 

aquifer (Trinity layer) in the Pecos County region model area, Texas, between 

2010 and 2040 for scenario 3 ....................................................................................................45

Tables

1. Hydrogeologic section in the Pecos County regional model area, Texas ..........................5

2. Final calibrated hydraulic parameter values for the Pecos County region model 

area, Texas ..................................................................................................................................29

3. Summary of hydraulic-head residual statistics for the Edwards-Trinity aquifers 

in the Pecos County region model area, Texas ....................................................................30

4. Summary of PHREEQC inverse modeling results in the Leon-Belding and Fort 

Stockton areas in the Pecos County region model area, Texas ........................................39
viii

Conversion Factors

Inch/Pound to SI

Multiply By To obtain

Length

inch (in.) 25.4 millimeter (mm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

square mile (mi2

) 2.590 square kilometer (km2) 

Flow rate

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

cubic foot per second (ft3

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

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

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

/d)

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

/s)

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

Hydraulic conductivity

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

foot squared per day (ft2

/d)

gallon per day per foot of drawdown 

(gal/d/ft)

 0.09290

0.01242

meter squared per day (m2/d) 

meter squared per day (m2

/d)

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

°F=(1.8×°C)+32

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

°C=(°F-32)/1.8

Vertical coordinate information is referenced to the North American Vertical Datum of 1988 

(NAVD88).

Horizontal coordinate information is referenced to the North American Datum of 1983 (NAD83).

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

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

foot of aquifer thickness [(ft3

/d)/ft2]ft. In this report, gallon per day per foot of drawdown (gal/d/ft) 

is used for convenience.
Simulation of Groundwater Flow in the Edwards-Trinity 

and Related Aquifers in the Pecos County Region, Texas

By Brian R. Clark, Johnathan R. Bumgarner, Natalie A. Houston, and Adam L. Foster

Abstract 

The Edwards-Trinity aquifer is a vital groundwater 

resource for agricultural, industrial, and public supply uses 

in the Pecos County region of western Texas. The U.S. 

Geological Survey completed a comprehensive, integrated 

analysis of available hydrogeologic data to develop a numerical 

groundwater-flow model of the Edwards-Trinity and related 

aquifers in the study area in parts of Brewster, Jeff Davis, 

Pecos, and Reeves Counties. The active model area covers 

about 3,400 square miles of the Pecos County region of Texas 

west of the Pecos River, and its boundaries were defined to 

include the saturated areas of the Edwards-Trinity aquifer. 

The model is a five-layer representation of the Pecos Valley, 

Edwards-Trinity, Dockum, and Rustler aquifers. The Pecos 

Valley aquifer is referred to as the alluvial layer, and the 

Edwards-Trinity aquifer is divided into layers representing 

the Edwards part of the Edwards-Trinity aquifer and the 

Trinity part of the Edwards-Trinity aquifer, respectively. The 

calibration period of the simulation extends from 1940 to 2010. 

Simulated hydraulic heads generally were in good agreement 

with observed values; 1,684 out of 2,860 (59 percent) of the 

simulated values were within 25 feet of the observed value. 

The average root mean square error value of hydraulic head 

for the Edwards-Trinity aquifer was 34.2 feet, which was 

approximately 4 percent of the average total observed change 

in groundwater-level altitude (groundwater level). Simulated 

spring flow representing Comanche Springs exhibits a pattern 

similar to observed spring flow. Independent geochemical 

modeling corroborates results of simulated groundwater flow 

that indicates groundwater in the Edwards-Trinity aquifer in the 

Leon-Belding and Fort Stockton areas is a mixture of recharge 

from the Barilla and Davis Mountains and groundwater that has 

upwelled from the Rustler aquifer. 

The model was used to simulate groundwater-level 

altitudes resulting from prolonged pumping to evaluate 

sustainability of current and projected water-use demands. 

Each of three scenarios utilized a continuation of the calibrated 

model. Scenario 1 extended recent (2008) irrigation and 

nonirrigation pumping values for a 30-year period from 2010 

to 2040. Projected groundwater-level changes in and around 

the Fort Stockton area under scenario 1 change little from 

current conditions, indicating that the groundwater system is 

near equilibrium with respect to recent (2008) pumping stress. 

Projected groundwater-level declines in the eastern part of the 

model area ranging from 5.0 to 15.0 feet are likely the result of 

nonequilibrium conditions associated with recent increases in 

pumping after a prolonged water-level recovery period of little 

or no pumping. Projected groundwater-level declines (from 

15.0 to 31.0 feet) occurred in localized areas by the end of 

scenario 1 in the Leon-Belding area. Scenario 2 evaluated the 

effects of extended recent (2008) pumping rates as assigned in 

scenario 1 with year-round maximum permitted pumping rates 

in the Belding area. Results of scenario 2 are similar in waterlevel decline and extent as those of scenario 1. The extent of 

the projected groundwater-level decline in the range from 5.0 to 

15.0 feet in the Leon-Belding irrigation area expanded slightly 

(about a 2-percent increase) from that of scenario 1. Maximum 

projected groundwater-level declines in the Leon-Belding 

irrigation area were approximately 31.3 feet in small isolated 

areas. Scenario 3 evaluated the effects of periodic increases 

in pumping rates over the 30-year extended period. Results 

of scenario 3 are similar to those of scenario 2 in terms of the 

areas of groundwater-level decline; however, the maximum 

projected groundwater-level decline increased to approximately 

34.5 feet in the Leon-Belding area, and the extent of the 

decline was larger in area (about a 17-percent increase) 

than that of scenario 2. Additionally, the area of projected 

groundwater-level declines in the eastern part of the model 

area increased from that of scenario 2—two individual areas 

of decline coalesced into one larger area. The localized nature 

of the projected groundwater-level declines is a reflection of 

the high degree of fractured control on storage and hydraulic 

conductivity in the Edwards-Trinity aquifer. Additionally, the 

finding that simulated spring flow is highly dependent on the 

transient nature of hydraulic heads in the underlying aquifer 

indicates the importance of adequately understanding and 

characterizing the entire groundwater system.

Introduction

The Edwards-Trinity aquifer is a vital groundwater 

resource for agricultural, industrial, and public supply uses in 

the Pecos County region of western Texas (fig. 1) (Barker and 

Ardis, 1992; Freese and Nichols, Inc. and LBG-Guyton, Inc., 

2010). Resource managers would like to understand the future 

availability of water in the Edwards-Trinity aquifer in and near 

Pecos County, Tex., and the effects of the possible increase or 

temporal redistribution of groundwater withdrawals. To provide 
Crane Pecos Alpine Fort Stockton 285 10 20 Leon-Belding area Pecos River

WARD COUNTY

CRANE COUNTY

CULBERSON COUNTY

JEFF DAVIS COUNTY BREWSTER COUNTY

TERREL COUNTY PRESIDIO COUNTY

PECOS COUNTY

REEVES COUNTY DELAWA RE BASI N

DIA BL O PLATF ORM CEN TRA L BA SIN PL ATFO RM

MIDLAND BA

VA L VER

MONUMENT DRAW TROUGH AND BELDING-COYANOSA TROUGH PECOS TROUGH Grandfalls

Balmorhea

Toyahvale

Belding

Leon

Springs

Fort Davis

San

Solomon

Spring

Santa

Rosa

Spring

Comanche

Springs

Diamond

Y Spring

TEXAS

PERMIAN

BASIN

LOCATION MAP

Model area

104°30' 104°00’ 103°30' 103°00’ 102°30' 102°00’ 

31°30'

31°00’

30°30'

EXPLANATION

Groundwater-flow model boundaryConceptual model study area boundaryPermian geologic structure boundaryPermian to Cenozoic geologic structure boundary

Belding Fault SystemDiamond Y Fault SystemFault zone—Represents numerous faultsSpring

GLASS

MOUNTAINS

BARILLA

MOUNTAINS

DAVIS

MOUNTAINS

Belding Fau

Diam

Belding-Coyanosa Trough modified from Boghici (1997), Baumgardner and 

Permian geologic structure boundaries 

Cenozoic geologic structure boundaries 

Base modified from U.S. Geological Survey, 1:2,000,000-scale digital dataAlbers Equal Area Projection, Texas State Mapping SystemNorth American Datum of 1983

Figure 1. Location and generalized geological structural features of the model area in the Pecos County region of Texas.
Introduction 3

resource managers with that information, the U.S. Geological 

Survey (USGS), in cooperation with the Middle Pecos 

Groundwater Conservation District (MPGCD), Pecos County, 

City of Fort Stockton, Brewster County, and Pecos County 

Water Control and Improvement District No. 1, completed a 

comprehensive, integrated analysis of available hydrogeologic 

data to develop a conceptual model of the Edwards-Trinity and 

related aquifers in parts of Brewster, Jeff Davis, Pecos, and 

Reeves Counties (fig. 1) (Bumgarner and others, 2012). The 

Edwards-Trinity and related aquifers (hereinafter referred to 

as the groundwater system) include the Pecos Valley, Igneous, 

Edwards-Trinity, Dockum, Rustler, and Capitan Reef aquifers. 

The Edwards-Trinity aquifer was the focus of the investigation 

presented in this report, and the related aquifers (with the 

exception of the Capitan Reef aquifer) were evaluated in terms 

of how they potentially interact with and affect the EdwardsTrinity aquifer. The Capitan Reef aquifer was not evaluated 

directly because the Rustler aquifer underlying the EdwardsTrinity aquifer was simulated as a constant head source, which 

would negate the effects of other underlying hydrogeologic 

units.

Development of the groundwater-flow model described 

in this report is the last phase of a three-phase groundwateravailability study being conducted in the Pecos County region 

by the USGS and its cooperators. The first phase of the study 

was to collect groundwater, surface-water, geochemical, 

geophysical, and geologic data in the Pecos County region 

and develop a geodatabase of historical and collected data 

(Pearson and others, 2012). The data compiled in the first 

phase of the study were used in the second phase to develop 

the conceptual model of the hydrogeologic framework, 

geochemistry, and groundwater-flow system of the area 

(Bumgarner and others, 2012).

A groundwater-flow model for the active model area 

(about 3,400 square-miles [mi2

] of the Pecos County region 

study area) was developed to simulate groundwater flow 

under varying pumping conditions. The model incorporates 

conceptual information provided by Bumgarner and others 

(2012) and data collected and compiled by Pearson and others 

(2012) in order to evaluate the sustainability of recent (2008) 

and projected water-use demands on groundwater resources in 

the study area. The hydrostratigraphy and structural features 

of the groundwater system were used to develop hydrologic 

boundaries and the numerical framework; aquifer-test data 

were used to estimate aquifer hydraulic properties. Knowledge 

of groundwater-flow paths inferred from groundwater-quality 

data (geochemical data) was used to guide model development 

and calibration. Groundwater-level altitude (groundwater 

level) and spring-flow data were used as calibration targets to 

evaluate the goodness of fit of the model simulation.

Purpose and Scope

This report documents the development of a numerical 

model describing groundwater flow of the Edwards-Trinity 

and related aquifers in the Pecos County region, Tex., and 

summarizes potential future pumping scenarios simulated with 

the model. The sustainability of recent (2008) and projected 

water-use demands on groundwater resources in the Pecos 

County region study area were evaluated through the year 

2040. The model was developed using data collected by the 

USGS during 2009–11 as well as historical data from 1930 

to 2011 collected by various State and local agencies and 

compiled by the USGS (Bumgarner and others, 2012; Pearson 

and others, 2012). The various components of the numerical 

model are described including the spatial and temporal 

discretization, initial conditions, hydraulic properties, model 

calibration, model evaluation, and simulations of groundwaterlevel changes as a result of changes in groundwater pumping. 

Inverse geochemical modeling (PHREEQC; Parkhurst and 

Appelo, 1999) was used to evaluate some of the groundwaterflow model results. The limitations of the model are also 

described.

Previous Investigations

Several scientific investigations have been done to 

develop conceptual and numerical groundwater-flow models 

of the Edwards-Trinity aquifer in the Pecos County region. 

Thornhill Group, Inc. (2008) and Daniel B. Stephens and 

Associates (2010) developed conceptual and numerical 

groundwater-flow models of the Edwards-Trinity aquifer 

that focused on a 100 mi2 area near Belding, Tex., referred 

to hereinafter as the Leon-Belding area (fig. 1). The LeonBelding area is an agricultural area about 7 miles southwest 

of Fort Stockton that includes about 30 mi2 of cultivated 

land. Water primarily from the Edwards-Trinity aquifer is 

used for irrigation purposes in the Leon-Belding area. Using 

the conceptual and numerical groundwater-flow models, 

simulations to project future aquifer conditions in the LeonBelding area based on various groundwater-withdrawal 

scenarios were published (Thornhill Group, Inc., 2008; 

Daniel B. Stephens and Associates, 2010). Anaya and Jones 

(2009) developed a 44,000 mi2 regional groundwater-flow 

model of the Pecos Valley and Edwards-Trinity aquifers 

in central and west Texas to determine the availability of 

groundwater based on projected water demand. Ewing and 

others (2012) developed a groundwater availability model for 

the Rustler aquifer to provide a tool to help refine estimates 

of groundwater availability for the Texas Water Development 

Board.

Description of the Pecos County Region Model 

Area1

The active model area covers about 3,400 mi2 of the 

Pecos County region of Texas west of the Pecos River. 

The boundaries of the model were defined to include the 

saturated areas of the Edwards-Trinity aquifer. The model 

1

 This section modified from Bumgarner and others (2012).
4 Simulation of Groundwater Flow in the Edwards-Trinity and Related Aquifers in the Pecos County Region, Texas

area is reduced in size from the area described by Bumgarner 

and others (2012) to focus on groundwater flow within 

the Edwards-Trinity aquifer. Therefore, areas where the 

Edwards-Trinity aquifer is absent or where the saturated 

thickness precludes the use as a viable water source were 

excluded from the model area. The southwestern and southern 

boundaries of the model area are rimmed by the Barilla and 

Davis Mountains, located primarily in northeastern Jeff 

Davis County as well as southwestern Reeves County, and 

the Glass Mountains located in northeastern Brewster County 

and southern Pecos County. The northeastern boundary of 

the model area is the Pecos River. The western part of the 

MPGCD management area (Pecos County) is in the model 

area. Altitude ranges from approximately 4,600 feet (ft) in the 

southern part of the model area in Pecos County to 2,150 ft 

in the northeastern part of the model area near the Pecos 

River in Pecos County. Climate in the model area is arid with 

average annual rainfall during 1970–2000 at Fort Stockton of 

approximately 14 inches (National Oceanic and Atmospheric 

Administration, 2013). Rainfall at Fort Stockton is annually 

quite variable with 2004 being the wettest year with an annual 

rainfall of about 26 inches and 2011 being the driest year 

with an annual rainfall of about 3 inches (National Oceanic 

and Atmospheric Administration, 2013). Potential annual 

evaporation of as much as 109 inches has been estimated 

(Boghici, 1997). Temperatures during 1970–2000 ranged from 

an average low of about 32 degrees Fahrenheit (oF) in January 

to an average high of about 96 oF in July (National Oceanic 

and Atmospheric Administration, 2013). 

Geologic and Hydrogeological Setting2

There were several periods of seawater inundation, 

deposition, uplift, and erosion during the geologic history of 

the Pecos County region of west Texas. Sedimentary rocks 

of Pennsylvanian, Permian, Triassic, and Cretaceous age; 

Tertiary-age igneous rocks; and Cenozoic-age alluvium are 

present in the subsurface. Many of the bedrock formations 

are exposed at the surface in the model area (Texas Water 

Development Board, 1972). This investigation focuses 

on subsurface rocks deposited from the Permian to the 

Quaternary Period (table 1). Dissolution of Permian-age 

evaporite deposits that began at the time of deposition and 

continued through the Cretaceous Period caused the Permian 

beds to collapse and form the north-south Belding-Coyanosa 

depositional trough (fig. 1) (Armstrong and McMillion, 1961; 

Boghici, 1997). During the Cenozoic Era, two depositional 

troughs that roughly trend north-south formed in the central 

and western parts of the model area because of the continued 

dissolution of the Permian-age evaporite deposits and 

subsequent collapse of the overlying sediments (Armstrong 

and McMillion, 1961). These troughs subsequently filled with 

Cenozoic-age alluvium and are known as the Monument Draw 

(central) and Pecos (western) troughs (fig. 1). For simplicity, 

2 This section modified from Bumgarner and others (2012).

hereinafter, the name ‘Monument Draw trough’ will be used 

to represent both the Cenozoic-age Monument Draw and 

Permian to Cretaceous-age Belding-Coyanosa troughs because 

the spatial extents and separation of these structural features 

are not well defined. Additional detail about the geologic 

and hydrogeologic setting of the model area can be found in 

Bumgarner and others (2012).

Hydrogeologic Framework

The geologic setting contributed to formation of two 

major and four minor aquifers in the model area. The Pecos 

Valley aquifer is a major aquifer composed of Cenozoic-age 

alluvium consisting of unconsolidated silt, sand, gravel and 

clay (fig. 2) (Small and Ozuna, 1993). In the northern part 

of the model area, the Pecos Valley aquifer unconformably 

overlies the Edwards-Trinity aquifer, the other major (and 

primary) aquifer in the model area (fig. 2). Minor aquifers 

include the Igneous, Dockum, Rustler, and Capitan Reef 

aquifers (table 1). The Edwards-Trinity aquifer is composed of 

lower Cretaceous-age limestone, marl, and clay of the Washita 

Group; limestone of the Fredericksburg Group; and sand, 

limestone, and shale of the Trinity Group (fig. 2, table 1). The 

Edwards part of the aquifer is composed of upper Cretaceous 

rocks of the Washita and Fredericksburg Groups, which locally 

are referred to as the Edwards and Sixshooter Groups (Brand 

and DeFord, 1958; Small and Ozuna, 1993; Smith and others, 

2000). The Trinity Group comprises the Trinity part of the 

aquifer and is composed of the Maxon Sand, the Glen Rose 

Formation, and the Basal Cretaceous Sand (Anaya and Jones, 

2009). The individual formations in the Trinity Group are not 

separated for the purposes of this report. Locally the Trinity 

Group is known as the Trinity Sands (Rees and Buckner, 1980; 

Small and Ozuna, 1993).

The Dockum aquifer is a minor aquifer and is composed 

of Triassic-age shale, sand, sandstone, and conglomerate of 

the Dockum Group (Bradley and Kalaswad, 2003). In Pecos 

County, a sand unit within the Dockum aquifer is recognizable 

in some geophysical logs, but the individual formations of 

the Dockum Group are not separated for the purposes of 

this report. Because of the shale content within the Dockum 

aquifer and contrast in aquifer properties in relation to the 

Edwards-Trinity aquifer, it is treated as a confining unit over 

much of the model area. Locally, the Dockum aquifer is also 

known as the Santa Rosa aquifer (Small and Ozuna, 1993).

The Rustler aquifer is composed of Permian-age rocks 

(table 1) consisting of mostly dolomite, anhydrite, and some 

limestone of the Rustler Formation. A basal unit of the Rustler 

aquifer consists of sand, conglomerate, and some shale (Small 

and Ozuna, 1993; LBG-Guyton, Inc., 2003).

The Capitan Reef aquifer is composed of Permian-age 

rocks consisting of reef, fore-reef, and back-reef facies of 

dolomite and limestone of the older Capitan Limestone. 

Additional detail about the hydrogeologic framework is 

available in Bumgarner and others (2012).
Introduction 5

Era Period Series

or group 

Stratigraphic

unit 

Major

and minor

aquifers

Cenozoic

Quaternary

and Tertiary Alluvium Pecos valley

Tertiary Volcanic Rocks, undivided Igneous

Mesozoic

Cretaceous

Gulfian

Series

Terlingua 

Group Boquillas Formation

Comanchean Series

Washita Group

Western Pecos County Eastern Pecos County

Alluvial layer

Sixshooter Group

Buda Limestone

Boracho 

Formation

Edwards Group

Fort Lancaster 

Formation

EdwardsTrinityEdwards layer

Trinity layer

University Mesa 

Marl

Burt Ranch 

Member

Fredericksburg 

Group Finlay Formation Fort Terrett Formation

Trinity Group Trinity Sands

Maxon Sand

Glen Rose Formation

Basal Cretaceous Sand

Triassic Dockum Group Middle

Dockum

Lower

Paleozoic

Permian

Ochoan Series

Southern 

Pecos County Northern Pecos County

Tessey 

Limestone

Rustler Formation Rustler

Dockum layer

Salado Formation

Rustler layer

Castile Formation

Guadalupian 

Series

Whitehorse 

Group

Gilliam 

Limestone

Capitan 

Limestone

Guadalupian 

Formations; 

undivided

Capitan

Reef

Lower Guadalupian Formations; undivided

Lower Permian Formations; undivided

Pennsylvanian Pennsylvanian Formations; undivided

Model

layer

Table 1. Hydrogeologic section in the Pecos County regional model area, Texas (modified from Bumgarner and others, 2012).
Figure 2. Major aquifers and minor aquifers in the Pecos County region model area, Texas.

Belding

Fort Stockton

PECOS COUNTY PRESIDIO BREWSTER COUNTY COUNTY

REEVES COUNTY

TERRELL COUNTY

CROCKETT 

JEFF DAVIS COUNTY

UPTON C

WARD COUNTY

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EXPLANATION

Major aquifer systems (George and others, 2011)

 Pecos Valley Edwards-Trinity (outcrop) Edwards-Trinity (subcrop)

Minor aquifer systems (George and others, 2011) 

 Igneous  Dockum Rustler Capitan Reef Complex

Groundwater-flow mo boundaryConceptual model stud area boundaryFault zone—Represen numerous faults
Simulation of Groundwater Flow 7

Geochemistry

The second phase of this study by Bumgarner and others 

(2012) indicated that the Edwards-Trinity aquifer in the model 

area was dominated by mineralized, regional groundwater that 

most likely recharged during the Pleistocene with variable 

contributions of recent, local recharge. Four end members 

were identified to use as part of the qualitative groundwaterflow and mixing analysis. The end members represented: 

(1) mineralized groundwater that likely recharged northwest 

of the model area during the Pleistocene; (2) dilute, recent 

recharge from the Barilla and Davis Mountains; (3) dilute, 

recent recharge from the Glass Mountains; and (4) mineralized 

water that is likely a mixture of recharge under recent and 

Pleistocene climate conditions. Bumgarner also noted that 

groundwater from the Dockum and Rustler aquifers likely 

mixes with groundwater compositions of the Edwards-Trinity 

aquifer. Additional detail about the geochemistry of the 

groundwater in the model area is available in Bumgarner and 

others (2012). 

Groundwater-Flow System

Groundwater generally flows northward into the downdip 

extent of the Edwards-Trinity aquifer (corresponding to 

the northern extent of the Edwards-Trinity subcrop area) or 

eastward out of the model area (fig. 2). Regional groundwater 

flow entering the model area from the northwest naturally 

discharges from springs or flows northward into the Pecos 

trough where it discharges into the Pecos Valley or Dockum 

aquifers at the downdip extent of the Edwards-Trinity 

aquifer. Recharge from the Barilla and Davis Mountains 

also predominantly flows toward the Pecos trough and 

most likely discharges to other aquifers of the groundwater 

system. Groundwater flow in the Edwards-Trinity aquifer 

in the Monument Draw trough originated as recharge in the 

Barilla, Davis, and Glass Mountains, agricultural return flow, 

or upwelling groundwater from lower units. Edwards-Trinity 

aquifer water generally flows north and northeast in the 

Monument Draw trough and naturally discharges from springs 

or to other aquifers of the groundwater system at the downdip 

extent. Groundwater in the eastern part of the model area 

likely originated in the Glass Mountains and generally flows 

northeast and out of the model area. Some groundwater in the 

eastern part of the model area also naturally discharges from 

springs to other aquifers of the groundwater system at the 

downdip extent or to the Pecos River. Additional detail about 

conceptual groundwater flow in the model area is available in 

Bumgarner and others (2012).

Simulation of Groundwater Flow

The modular finite-difference code, USGS 

MODFLOW-2005 (Harbaugh, 2005), was used to approximate 

the solution of the equations governing three-dimensional 

groundwater flow. The preconditioned conjugate gradient 

solver (Hill, 1990) was used for the numerical solution 

technique. 

Groundwater-Flow Model Construction

The groundwater-flow system is represented by a set 

of grid cells, and within each individual cell the hydraulic 

properties are the same. Three finite-difference equations 

describe flow through each cell, which can be solved for 

steady-state or transient conditions, to simulate groundwaterlevel changes within the flow system resulting from 

pumping stress over discrete periods of time. The model was 

constructed using much of the compiled and field-collected 

data gathered from Pearson and others (2012). The transient 

model simulates 70 years (1940–2010) of system response to 

stress by using 144 stress periods.

Spatial Discretization and Layering

The active model area includes 13,680 active model cells 

representing the Trinity part of the Edwards-Trinity aquifer. 

The active model was discretized into a finite-difference 

grid of 156 rows and 174 columns with uniform cells 0.25 

mi2 in area, a refinement from previous regional modeling 

and coincident with previous local-scale models. The grid is 

aligned approximately perpendicular to regional groundwater 

flow and coincides with the Groundwater Availability Model 

(GAM) of Anaya and Jones (2009) with the y-coordinate axis 

at an azimuth of approximately N.48° W.

The layers of the numerical model were developed 

from the hydrostratigraphy of Bumgarner and others (2012) 

and Ewing and others (2012). The model has five layers that 

represent the Pecos Valley aquifer (referred to as the alluvial 

layer), the Edwards part of the Edwards-Trinity aquifer 

(referred to as the Edwards layer), the Trinity part of the 

Edwards-Trinity aquifer (referred to as the Trinity layer), the 

Dockum aquifer (referred to as the Dockum layer), and the 

Rustler aquifer (referred to as the Rustler layer) (table 1). In 

areas that a hydrogeologic unit is thin or nonexistent, such as 

the Dockum aquifer, the hydraulic properties of the layer are 

similar to those of the underlying unit.

Temporal Discretization

The model is discretized temporally into 144 stress 

periods that represent intervals of time with constant stresses 

(such as pumping) applied throughout. Stress period 1 

represents a steady-state period of time prior to 1940 in 

which water levels were less likely affected by widespread 

groundwater pumping. Stress period 2 begins the transient 

period of the simulation starting January 1, 1940. Between 

stress period 2 (January 1, 1940, to March 31, 1940) and stress 

period 144 (October 1, 2010, to December 31, 2010), each 

intervening stress period is 6 months in length to represent 

irrigation (April through September) and nonirrigation seasons 

(October through March).
8 Simulation of Groundwater Flow in the Edwards-Trinity and Related Aquifers in the Pecos County Region, Texas

Hydrologic Boundaries

The hydrologic boundaries of the model include areal 

recharge (Recharge Package), rivers (River Package), no-flow, 

constant heads (Time-Variant Constant Head Package), 

and general-head boundaries (General Head Boundary 

Package). Each boundary was included to represent a specific 

aspect of the groundwater-flow system and is discussed 

briefly in the following sections. A complete description 

of MODFLOW-2005 hydrologic boundaries and model 

specifications is available in Harbaugh (2005).

Recharge

Because precipitation is generally relatively low and 

evapotranspiration is relatively high (Anaya and Jones, 

2009), net recharge was expected to be low to nonexistent 

over much of the model area. Therefore, net areal recharge 

in the model primarily occurs in front of the Barilla, Davis, 

and Glass Mountains along the western edge of the model 

domain (fig. 3) (Anaya and Jones, 2009). This mountain-front 

recharge was simulated using the MODFLOW-2005 Recharge 

Package in an assumed width of approximately 5 miles along 

the base of the mountains at an initial rate of 2.0 inches per 

year (in/yr) based on higher estimates of recharge for the 

Edwards-Trinity aquifer (Long, 1958; Rees and Buckner, 

1980). The recharge rate was further adjusted through model 

calibration. Additional recharge may be introduced through 

irrigation return flow (Bumgarner and others, 2012). Return 

flow recharge was estimated based on streamflow increases 

and pumping data and varied widely from 0.15 to 0.60 in/

yr (Kuniansky and Holligan, 1994). Recharge was specified 

as 0.2 in/yr in irrigated areas to represent irrigation return 

flow based on low but detectable concentrations of nutrients 

and pesticides (Bumgarner and others, 2012). The specified 

recharge representing irrigation return flow is approximately 

equal to 2 percent of recent (2008) pumping in the LeonBelding irrigation area. Because the purpose of the model is an 

evaluation of projected water use through the year 2040 (longterm demands) rather than seasonal changes in water use and 

water levels, the above rates of recharge for both mountainfront and irrigation return flow areas were held constant 

throughout the model simulation.

Discharge

Multiple hydrologic boundaries representing 

groundwater discharge were included in the model. Springs 

and groundwater pumping represented in the model serve 

exclusively as net discharge from the groundwater system. 

Specified heads representing flow horizontally, vertically, 

and from the Pecos River may serve as both recharge and 

discharge at various times and locations within the model. 

Because the majority of the total flow from specified heads 

is out of the model (discharge), they are included here in the 

“Discharge” section of the report.

Springs

Multiple springs exist within the model area, though 

few discharge records are available to aid model calibration. 

Until the 1950s, Comanche Springs were the largest in the 

Pecos County region and sixth largest in the State (Sharp, 

2001). Comanche Springs first went dry in 1955, and 

perennial flow ceased in 1961 (Texas Parks and Wildlife 

Department, 2012). According to Freese and Nichols, Inc. and 

LBG-Guyton, Inc. (2010), Comanche Springs have flowed 

occasionally since 1987. The springs are in faulted areas, and 

the presence of faults likely contributed to the formation of the 

springs (Anaya and Jones, 2009; Armstrong and McMillion, 

1961; Baumgarner and others, 1982; Boghici, 1997; Sharp 

and others, 1999; Small and Ozuna, 1993; Texas Water 

Development Board, 2005; Veni, 1991). 

Comanche Springs had usable discharge data to aid 

in setting boundary conditions and to calibrate the model. 

Monthly discharge measurements from Comanche Springs 

corresponding to each 6-month stress period were averaged 

to represent observed flow. The location and elevation 

of Comanche Springs were implemented by using the 

Streamflow Routing Package (SFR) (Niswonger and Prudic, 

2005). SFR was designed to simulate the interaction between 

surface-water features and aquifers as well as to route the 

flow of water along a linear, downgradient path with the flow 

system. While SFR is typically used to represent streams, it 

is analogous to the simulation of springs that have a fracture 

controlled contributing area. The preferential flow paths that 

provide flow to Comanche Springs surveyed by Veni (1991) 

lie along N.60° to N.65° W trending joints. In addition to 

allowing the user to locate the spring discharge points in the 

model domain, SFR allows the user to delineate the fracture 

network that provides flow to the spring discharge point. In 

this way, the springs can be more accurately represented in 

model simulations.

Groundwater Pumping

Groundwater-pumping estimates were compiled from 

multiple sources to develop a pumping record for 1940 to 

2010 (Armstrong and McMillion, 1961; Audsley, 1956; Dante 

1947; Ogilbee and others, 1962; Paul Weatherby, Middle 

Pecos Groundwater Conservation District, written commun., 

2012; Rees and Buckner, 1980; Small and Ozuna, 1993; 

Texas Water Development Board, 1986, 2001, 2012a). 

Site-specific pumping was used when available, though 

much of the record for irrigation pumping contained only 

aggregated amounts of withdrawals by county, aquifer, and 

year. Public supply, manufacturing, mining, and powergeneration (industrial) pumping represented actual monthly 

or annually reported amounts by these water users. All 

pumping totals were aggregated to annual amounts and 

assigned to the appropriate stress period of the model. 

Nonirrigation wells were assigned annual pumping amounts 

for growing and nongrowing seasons for the corresponding 

year. 
Figure 3. Simulated net recharge in the Pecos County region model area, Texas. 

PECOS COUNTY BREWSTER COUNTY

REEVES COUNTY

TERRELL COUNTY

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JEFF DAVIS COUNTY

UPTON C

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MOUNTAINS

D – 4557603

E – 4656201

B – 4656301

G – 5302102

A – 5302708

F – 5309105 C – 5309301

Belding

Fort Stockton

Leon-Belding area

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Base modified from U.S. Geological Survey, 1:2,000,000-scale digital dataAlbers Equal Area Projection, Texas State Mapping SystemNorth American Datum of 1983 EXPLANATION

G – 5302102

Recharge, in inches per year

 0.01 to 0.50 0.51 to 1.00 1.01 to 1.50 1.51 to 2.00 2.01 to 2.80

Groundwater-flow model boundaryConceptual model study area boundaryFault zone—Represents numerous faultsObservation well completed in Edwards-Trinity aquifer and Texas Water Development Board Well number—Map identifier represents hydrograph of groundwater-level altitudes in figure 11 and appendix 1
10 Simulation of Groundwater Flow in the Edwards-Trinity and Related Aquifers in the Pecos County Region, Texas

Irrigation wells were assigned total pumping amounts 

for the growing season months of April through October only, 

with a pumping amount of zero assigned during nongrowing 

season months. Nonirrigation pumping data were available 

for 1955 through 2005. Historical estimates of groundwater 

pumping from 1955 to 1999 were based on the water-usesurvey database used in the Texas Water Development Board’s 

(TWDB) GAM program (Cindy Ridgeway, Texas Water 

Development Board, written commun., 2001). Groundwaterpumping data from 2000 to 2005 also were obtained from 

the TWDB water-use-survey database (Kevin Kluge, Texas 

Water Development Board, written commun., 2012). Pumping 

amounts then were assigned to wells based on the TWDB 

groundwater database. The water user in the water-use-survey 

database was linked to wells in the groundwater database 

by a unique identifier. If more than one well was referenced 

for a given water user, the groundwater-pumping amounts 

were divided evenly among those wells. Wells then were 

intersected using a Geographic Information System (GIS) 

(Environmental Systems Research Institute, 2013) with the 

spatial data representing the tops and bottoms of the aquifers 

in the model area and based on the latitude, longitude, and 

well-screen interval. Wells were assigned a model layer if 

the screen interval of the well was contained within the top 

and bottom of a given model layer. If a well was screened in 

multiple aquifers, the well was assigned to multiple model 

layers; pumping amounts were distributed to each layer using 

the Multi-Node Well (MNW1) Package (described in the 

latter part of this section). Pumping amounts for 2006 through 

2008 for all categories were obtained from the Middle Pecos 

Groundwater Conservation District.

Irrigation amounts were obtained using data from the 

TWDB’s historical water-use-information database (Texas 

Water Development Board, 2012b). Data for 1955, 1958, 

1964, 1969, 1974, and 1979 were obtained from the TWDB’s 

Survey of Irrigation studies (Texas Water Development Board, 

1986, 2001). Data were obtained for 1980 and 1984 through 

2005 from the TWDB historical water-use-information 

database (Texas Water Development Board, 2012b). Data also 

were obtained from published reports (Hood and Knowles, 

1952; Ogilbee and others, 1962). Based on the assumption 

that pumping amounts increased every year from the 1940s to 

the peak reported value in 1964, temporal gaps in the wateruse dataset were estimated by linear interpolation between 

reported pumping values. Irrigation pumping estimates 

were applied to wells that had available latitude, longitude, 

documented period of record, and well construction (well 

depth, well screen) information (Armstrong and McMillion, 

1961; Audsley, 1956; Dante, 1947; Ogilbee and others, 1962; 

Rees and Buckner, 1980; Small and Ozuna, 1993; Texas Water 

Development Board, 2001, 2012a). Rather than distribute the 

pumping amounts equally to all irrigation wells, the pumping 

amount distributed to each irrigation well was based on the 

percentage of total pumping that each well represented for 

years that the individual amounts were known or estimated. 

Groundwater pumping for domestic and livestock uses 

typically is small compared to public supply, industrial, and 

irrigation and was not compiled or used in the simulation 

for 1940 through 2005 (Rees and Buckner, 1980; Small and 

Ozuna, 1993; Texas Water Development Board 2012a), though 

some information for domestic and livestock was available 

after 2005 (appendix 2).

All pumping wells were represented with the MNW1 

Package (Halford and Hanson, 2002). The MNW1 allows the 

simulation of groundwater flow within the well bore and the 

extraction of groundwater from multiple hydrogeologic units 

at a single well to better represent pumping from wells open 

to more than one hydrogeologic unit. Flow into or out of the 

well bore can be affected by the difference in transmissivity 

between the formation and the disrupted radius around the 

well bore, noted by a skin friction coefficient. The skin friction 

coefficient is derived from the linear well-loss coefficient, 

which defines head loss from flow through formation damaged 

during well drilling, the gravel pack, and the well screen. The 

linear well-loss coefficient can be used directly to define head 

loss or can be recast in terms of a dimensionless skin friction 

coefficient (Cooley and Cunningham, 1979; Earlougher, 

1977). For all withdrawal wells, a final skin friction coefficient 

of 4 was used, which results in a contrast of the transmissivity 

of the formation (T) to transmissivity of the disrupted radius 

(Tskin) value of 6.77 (T/Tskin). The contrast of T/Tskin allows 

variation in flow into and out of hydrogeologic units based 

on the different hydraulic properties of each unit. MNW1 

also can be used to limit pumping on the basis of changes 

to the drawdown constraint; simulated pumping is reduced 

(possibly to the point of zero pumping) when the water level 

near a well reaches a specified altitude within the well bore. 

Approximately 1 percent of the aquifer thickness of the model 

cell at a given well location was used to specify the altitude of 

drawdown constraints on all pumping wells.

Specified Head Boundaries

There are three types of MODFLOW-2005 specified 

head boundary packages represented in the model: General 

Head, Time-Variant Constant Head, and River (Harbaugh, 

2005). The General Head Boundary Package (GHB) was 

used to represent horizontal flow into or out of the model area 

(fig. 4). GHBs were placed along the western, northwestern, 

north, and southeastern perimeters of the model area in layer 3 

(Trinity layer). This placement allows for flow between the 

alluvial, Edwards, and Trinity layers; whereas, the vertical 

hydraulic conductivity was sufficient to allow groundwater 

movement vertically and horizontally in the model. The 

groundwater levels assigned to the GHBs were interpolated 

using minimum curvature spline that includes input for 

barriers (with the assumption that features such as faults create 

a barrier to, or reduction in, horizontal flow) from observation 

data that represented 1940, 1963, and 2010. The locations of 

the observation data coincide with most of the locations also 

used as pilot points described later in the section “Hydraulic 

Properties.” The interpolation of water levels included major 

faults to act as hydraulic barriers because of large changes in 
Figure 4. Specified head and horizontal flow (fault) boundaries used in the model simulation (excluding Constant Head Boundary of the Rustler aquif

geochemical data collection in the Pecos County region model area, Texas.

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Base modified from U.S. Geological Survey, 1:2,000,000-scale digital dataAlbers Equal Area Projection, Texas State Mapping SystemNorth American Datum of 1983

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General head boundaryRiver boundaryGeochemistry site and site identifier 

(appendix 3)

Groundwater-flow model boundaryConceptual model study area boundaryHorizontal flow barrierFault zone—Represents numerous faults

EXPLANATION
12 Simulation of Groundwater Flow in the Edwards-Trinity and Related Aquifers in the Pecos County Region, Texas

hydraulic gradient in the area of the faults. These dates were 

chosen to depict water levels at the beginning of the model 

simulation, at the low point of groundwater-level decline, and 

at the end of the model simulation. The water level at each 

GHB was linearly interpolated temporally between these 

three years to allow a smooth transition of groundwater-level 

decline and recovery during transient model simulation. 

Geochemical data from Bumgarner and others (2012) 

indicate that water from the Rustler aquifer is likely upwelling 

in some areas of the model domain. Therefore, Time-Variant 

Constant Heads were used in the model to represent water 

levels in the Rustler aquifer. Simulated water levels from 

Ewing and others (2012) were digitized for the 1959, 1970, 

and 2000 time periods and applied as groundwater-level 

altitude of the constant heads in the Rustler aquifer. The three 

time periods were used as endpoints to interpolate constanthead water levels in the transient simulation from 1940 to 

1970 (by using the 1959 and 1970 digitized surface) and from 

1970 to 2010 (by using the 1970 and 2000 digitized surface). 

The 1970 and the 2000 digitized surfaces were reduced by 

30 ft after model simulations indicated the surfaces were too 

high relative to water levels for the Rustler aquifer in the 

Belding and Fort Stockton areas. Groundwater in the Rustler 

aquifer may be of higher salinity than overlying hydrogeologic 

units, though salinity differences are probably not great 

enough to result in variable density-driven flow; thus, the 

assumption of a constant density simulation is thought to be 

adequate.

The MODFLOW-2005 River Package was used to 

represent the Pecos River on the northeastern side of the 

model area (fig. 4). Estimates of river bottom elevation were 

calculated as the minimum elevation within each river cell 

based on a 10-meter digital elevation model. The river stage 

was set as 5 ft above the river bottom, and the streambed 

conductance was assigned a value of 8,001 ft2

/d after test 

simulation indicated values higher or lower than the assigned 

value would adversely affect the ability to simulate observed 

conditions.

No-Flow Boundaries

The southwestern perimeter of the model area (fig. 4) 

and the base of the Rustler aquifer (table 1) are represented as 

no-flow boundaries. The southwestern perimeter represents 

an area where the hydrogeologic units do not exist, or flow 

into or out of the model area is assumed to be negligible. 

The base of the Rustler aquifer is represented as a no-flow 

boundary because, while it is underlain by the Capitan Reef 

Complex aquifer, the hydraulic connection between the two 

units is unknown. Additionally, Time Variant Constant Head 

boundaries were applied to the Rustler aquifer as discussed in 

“Specified Head Boundaries,” which could reduce or remove 

the effects of water levels from underlying units. 

Structure

Faulting in the area is widespread and can have a 

substantial effect on groundwater flow, particularly in areas 

of large displacement between fault blocks, which can serve 

as horizontal barriers to groundwater flow if the faulting 

results in offsets that hydraulically isolate different parts of 

the same hydrogeologic unit (Bumgarner and others, 2012). 

The maximum displacement interpreted by Bumgarner and 

others (2012) was about 1,025 ft in the western part of the 

model area, which can hydraulically isolate one part of the 

aquifer from another. While faulting is recognized as typically 

occurring in wide zones of numerous faults, the delineation 

of a fault zone was represented as a horizontal flow barrier 

in the groundwater-flow model (fig. 4) using the Horizontal 

Flow Barrier (HFB) Package of MODFLOW-2005, which 

simulates reduced conductance between individual pairs 

of cells. This particular zone, while not explicitly mapped, 

appeared to have substantial effects on water levels in the 

western part of the model area and thus was explicitly 

included in the simulation.

Initial Conditions 

Initial conditions were simulated by using a steady-state 

stress period (representing conditions prior to January 1, 1940) 

at the beginning of the simulation. Specified head boundaries 

of GHB and constant heads for this steady-state stress period 

were developed using the observation data and previous model 

simulations (described in the “Specified Head Boundaries” 

section). Specified heads representing the Pecos River were 

held at constant altitudes throughout the steady-state and 

transient simulation periods.
Simulation of Groundwater Flow 13

Hydraulic Properties

Hydraulic properties in the model were assigned by 

using discrete zones (large areas possessing the same property 

value) and pilot points (Doherty, 2003). Pilot points allow 

for greater flexibility in the spatial assignment of aquifer 

properties. Each point at a specified location can be assigned 

a value of a hydraulic property, which can change throughout 

the calibration process within specified limits. A hydraulic 

property value for each model cell is interpolated based on 

the values of surrounding pilot points, which can serve to 

spatially vary the properties in a gradational manner, rather 

than as fixed discrete zones of hydraulic properties. Doherty 

(2011) provides additional information on pilot points and the 

geostatistical methods associated with their use. During initial 

model development, pilot points were distributed uniformly 

across the model domain at a spacing of approximately 2 

miles, but this distribution did not adequately represent 

hydraulic property heterogeneity in the groundwater system. 

As a result, the pilot points were reconfigured to locations of 

existing groundwater-level observations (fig. 5) (288 pilot 

points in the alluvial layer, 535 in each of the Edwards and 

Trinity layers, and 480 in the Dockum layer), which allowed 

for better representation of hydraulic property heterogeneity 

within the model domain. Pilot points representing recharge 

remained at the original uniform spacing of 2 miles.

Hydraulic Conductivity

Horizontal hydraulic conductivity values for each 

hydrogeologic unit were based on available aquifer test 

information compiled by Bumgarner and others (2012) and 

Christian and Wuerch (2012), and aquifer properties cited in 

Anaya and Jones (2009). Information from each data source 

was used as initial estimates of hydraulic conductivity for 

the alluvial, Edwards, Trinity, and Rustler layers. Pilot points 

were used to interpolate the hydraulic conductivity field of 

the alluvial (fig. 6A), Edwards (fig. 6B), and Trinity (fig. 6C) 

layers by using ordinary Kriging and a spherical variogram 

(Doherty, 2011). The hydraulic conductivities for the Dockum 

and Rustler layers were assigned uniform values of 1.5 and 

100 feet per day (ft/d), respectively, to approximate typical 

values of similar lithology; pilot points were not used to 

represent hydraulic conductivity in these units.
Figure 5. Distribution of pilot points throughout the Pecos County region model area, Texas.

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Groundwater-flow model boundaryConceptual model study area boundaryFault zone—Represents numerous faultsLocation of pilot points based on well locations  Dockum layer Edwards and Trinity layers Alluvial layer
Figure 6A. Calibrated hydraulic conductivity distribution of the Pecos Valley aquifer (alluvial layer) in the Pecos County region model area, Texas. 

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Base modified from U.S. Geological Survey, 1:2,000,000-scale digital dataAlbers Equal Area Projection, Texas State Mapping SystemNorth American Datum of 1983

A

Hydraulic conductivity, in feet per day 0.5 to 5.0 5.1 to 10.0 10.1 to 15.0 15.1 to 20.0 20.1 to 25.0

25.1 to 35.0

 35.1 to 40.0 40.1 to 50.0 50.1 to 60.0

EXPLANATION

Groundwater-flow model boundaryConceptual model study area boundaryFault zone—Represents numerous faults
Figure 6B. Calibrated hydraulic conductivity distribution of the Edwards part of the Edwards-Trinity aquifer (Edwards layer) in the Pecos County regi

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Leon-Belding area

PECOS COUNTY BREWSTER COUNTY

REEVES COUNTY

TERRELL COUNTY

CROCKETT 

JEFF DAVIS COUNTY

UPTON C

WARD COUNTY

CRANE COUNTY

Belding

Fort Stockton

Pecos River

104°00’ 103°30' 103°00’ 102°30' 

31°00’

30°30'

0 5 10 1

0 5 10 15 20 KIL

Base modified from U.S. Geological Survey, 1:2,000,000-scale digital dataAlbers Equal Area Projection, Texas State Mapping SystemNorth American Datum of 1983 EXPLANATION

Hydraulic conductivity, in feet per day

 0.1 to 2.0 2.1 to 5.0 5.1 to 10.0 10.1 to 25.0

 25.1 to 50.0 50.1 to 100.0 100.1 to 150.0 150.1 to 229.5

Groundwater-flow model boundaryConceptual model study area boundaryFault zone—Represents numerous faults

B
Figure 6C. Calibrated hydraulic conductivity distribution of the Trinity part of the Edwards-Trinity aquifer (Trinity layer) in the Pecos County region mo

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PECOS COUNTY BREWSTER COUNTY

REEVES COUNTY

TERRELL COUNTY

CROCKETT 

JEFF DAVIS COUNTY

UPTON C

WARD COUNTY

CRANE COUNTY

Belding

Fort Stockton

EXPLANATION

Hydraulic conductivity, in feet per day 0.0 to 25.0 25.1 to 75.0 75.1 to 150.0 150.1 to 250.0

 250.1 to 500.0 500.1 to 700.0 700.1 to 900.0 900.1 to 1,200.0

Groundwater-flow model boundaryConceptual model study area boundaryFault zone—Represents numerous faults

104°00’ 103°30' 103°00’ 102°30' 

31°00’

30°30'

0 5 10 1

0 5 10 15 20 KIL

Base modified from U.S. Geological Survey, 1:2,000,000-scale digital dataAlbers Equal Area Projection, Texas State Mapping SystemNorth American Datum of 1983

C
18 Simulation of Groundwater Flow in the Edwards-Trinity and Related Aquifers in the Pecos County Region, Texas

Vertical Hydraulic Conductivity and Storage

Vertical hydraulic conductivity (VK) and storage play 

a large role in groundwater flow because of the faulted and 

fractured nature of the geology in the model area. Faults and 

fractures can act as conduits or barriers to groundwater flow 

depending on the hydraulic characteristics of the structural 

feature. For example, conductive faults and fractures that 

penetrate multiple hydrogeologic units in a groundwater 

system might allow vertical movement of groundwater 

between the units, even if a confining unit is present. 

Conversely, faults that are only marginally conductive or result 

in an offset that disconnects a unit from itself horizontally 

can act as barriers to flow in the vertical, horizontal, or both 

directions (Maclay and Small, 1983). Scant high-resolution 

information pertaining to VK and storage in the model area 

was available; VK typically is determined by using laboratory 

techniques on cores obtained from well bore holes (Dagan, 

1986), and the calculation of storage typically requires 

lengthy pumping tests with multiple observation wells (Heath, 

1983). As a result, literature values (Ewing and others, 2008; 

Anaya and Jones, 2009) were used to assign VK values to 

pilot points for a given model layer (fig. 5) and modified 

during calibration. The VK of the alluvial (fig. 7A), Trinity 

(fig. 7B), Edwards (fig. 7C), and Dockum (fig. 7D) layers was 

represented using pilot points, while the VK of the Rustler 

layer was assigned a uniform value of 0.49 ft/d to approximate 

typical values of similar lithology.
Figure 7A. Calibrated vertical hydraulic conductivity distribution of the Pecos Valley aquifer (alluvial layer) in the Pecos County region model area, T

Leon-Belding area

PECOS COUNTY BREWSTER COUNTY

REEVES COUNTY

TERRELL COUNTY

CROCKETT 

JEFF DAVIS COUNTY

UPTON C

WARD COUNTY

CRANE COUNTY

Belding

Fort Stockton

Pecos River

A

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104°00’ 103°30' 103°00’ 102°30' 

31°00’

30°30'

0 5 10 1

0 5 10 15 20 KIL

Base modified from U.S. Geological Survey, 1:2,000,000-scale digital dataAlbers Equal Area Projection, Texas State Mapping SystemNorth American Datum of 1983

EXPLANATION

Vertical hydraulic conductivity, in feet per day

 0.50 to 0.75

 0.76 to 1.00 1.01 to 1.25

 1.26 to 1.50 1.51 to 1.75 1.76 to 2.00

Groundwater-flow model boundaryConceptual model study area boundaryFault zone—Represents numerous faults
Figure 7B. Calibrated vertical hydraulic conductivity distribution of the Edwards part of the Edwards-Trinity aquifer (Edwards layer) in the Pecos Cou

Texas.

Leon-Belding area

PECOS COUNTY BREWSTER COUNTY

REEVES COUNTY

TERRELL COUNTY

CROCKETT 

JEFF DAVIS COUNTY

UPTON C

WARD COUNTY

CRANE COUNTY

Belding

Fort Stockton

Pecos River

B

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104°00’ 103°30' 103°00’ 102°30' 

31°00’

30°30'

0 5 10 1

0 5 10 15 20 KIL

Base modified from U.S. Geological Survey, 1:2,000,000-scale digital dataAlbers Equal Area Projection, Texas State Mapping SystemNorth American Datum of 1983 EXPLANATION

Vertical hydraulic conductivity, in feet per day 0.0 to 1.0 1.1 to 3.0 3.1 to 6.0 6.1 to 9.0

 9.1 to 12.0 12.1 to 15.0 15.1 to 18.0 18.1 to 27.3

Groundwater-flow model boundaryConceptual model study area boundaryFault zone—Represents numerous faults
Figure 7C. Calibrated vertical hydraulic conductivity distribution of the Trinity part of the Edwards-Trinity aquifer (Trinity layer) in the Pecos County re

Leon-Belding area

PECOS COUNTY BREWSTER COUNTY

REEVES COUNTY

TERRELL COUNTY

CROCKETT 

JEFF DAVIS COUNTY

UPTON C

WARD COUNTY

CRANE COUNTY

Belding

Fort Stockton

Pecos River

C

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104°00’ 103°30' 103°00’ 102°30' 

31°00’

30°30'

0 5 10 1

0 5 10 15 20 KIL

Base modified from U.S. Geological Survey, 1:2,000,000-scale digital dataAlbers Equal Area Projection, Texas State Mapping SystemNorth American Datum of 1983 EXPLANATION

Vertical hydraulic conductivity, in feet per day 0.0 to 1.0 1.1 to 3.0 3.1 to 6.0 6.1 to 9.0

 9.1 to 12.0 12.1 to 15.0 15.1 to 18.0 18.1 to 27.3

Groundwater-flow model boundaryConceptual model study area boundaryFault zone—Represents numerous faults
Figure 7D. Calibrated vertical hydraulic conductivity distribution of the Dockum layers in the Pecos County region model area, Texas.

Leon-Belding area

PECOS COUNTY BREWSTER COUNTY

REEVES COUNTY

TERRELL COUNTY

CROCKETT 

JEFF DAVIS COUNTY

UPTON C

WARD COUNTY

CRANE COUNTY

Belding

Fort Stockton

Pecos River £¤285

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D

104°00’ 103°30' 103°00’ 102°30' 

31°00’

30°30'

0 5 10 1

0 5 10 15 20 KIL

Base modified from U.S. Geological Survey, 1:2,000,000-scale digital dataAlbers Equal Area Projection, Texas State Mapping SystemNorth American Datum of 1983

EXPLANATION

Vertical hydraulic conductivity, in feet per day

 0.00 to 0.10

 0.11 to 0.30 0.31 to 0.60

 0.61 to 0.90 0.91 to 1.20 1.21 to 2.67

Groundwater-flow model boundaryConceptual model study area boundaryFault zone—Represents numerous faults
Simulation of Groundwater Flow 23

Specific storage (confined aquifers) and specific yield 

(unconfined aquifers) can have a large effect on water levels, 

particularly in areas of potentially unconfined conditions 

(Freeze and Cherry, 1979). Specific storage represents the 

change in storage associated with compressibility of water 

and the geologic material and when multiplied by thickness 

provides the storage coefficient for a confined aquifer. Specific 

yield represents the volume of water that can be drained from 

a volume of aquifer per unit decline in water-table altitude. 

Specific storage values were assigned to pilot points within 

the alluvial (fig. 8A) layer and initially to pilot points within 

the Edwards and Trinity layers. The distribution of points was 

identical to those used for horizontal hydraulic conductivity in 

each unit (fig. 5). As noted in the “Model Limitations” section 

of this report, changing the specific storage of the Edwards 

and Trinity layers was found to have relatively little effect 

in many areas of the model. Specific storage values within 

these layers were then removed from the calibration and 

were instead calculated by using literature values (Domenico 

and Mifflin, 1964) of storage coefficient (product of the 

layer thickness and the specific storage) or specific yield for 

unconfined conditions and the spatially varying thickness of 

the layers. The resultant specific storage of the Edwards layer 

(fig. 8B) and the Trinity layer (fig. 8C) reflects approximate 

areas of confined and unconfined conditions in the model 

area. During the calibration procedure, specific storage of 

the Dockum and Rustler layers was uniformly calibrated 

as 8.8×10-6 and 5.0×10-6 1/ft, respectively. The alluvial and 

Edwards layers were represented as convertible layers in the 

model; meaning that each layer can convert from confined to 

unconfined in areas where water levels decline below the top 

of the unit. Because the Trinity layer is specified as a confined 

layer in the model, the specific storage value may be higher 

than expected for the lithology because of the representation 

of potentially unconfined conditions. During the calibration 

procedure, specific yield of the alluvial layer (fig. 9) was 

assigned by using pilot points (fig. 5), and the specific yield 

assigned to the Edwards layer was uniformly 5.0×10-2 1/ft 

based on model calibration. 
Figure 8A. Calibrated specific storage distribution of the Pecos Valley aquifer (alluvial layer) in the Pecos County region model area, Texas.

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REEVES COUNTY

TERRELL COUNTY

CROCKETT 

JEFF DAVIS COUNTY

UPTON CO

WARD COUNTY

CRANE COUNTY

Belding

Fort Stockton

Pecos River

104°00’ 103°30' 103°00’ 102°30' 

31°00’

30°30'

0 5 10 15 

0 5 10 15 20 KILO

Base modified from U.S. Geological Survey, 1:2,000,000-scale digital dataAlbers Equal Area Projection, Texas State Mapping SystemNorth American Datum of 1983 EXPLANATION

Specific storage, in 1/foot 0.0013 to 0.0020 0.0021 to 0.0030 0.0031 to 0.0040

 0.0041 to 0.0050 0.0051 to 0.0060

Groundwater-flow model boundaryConceptual model study area boundaryFault zone—Represents numerous faults

A
Figure 8B. Calibrated specific storage distribution of the Pecos Valley aquifer (alluvial layer) in the Pecos County region model area, Texas.

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MOUNTAINS

PECOS COUNTY BREWSTER COUNTY

REEVES COUNTY

TERRELL COUNTY

CROCKETT 

JEFF DAVIS COUNTY

UPTON CO

WARD COUNTY

CRANE COUNTY

Leon-Belding area

Belding

Fort Stockton

Pecos River

104°00’ 103°30' 103°00’ 102°30' 

31°00’

30°30'

0 5 10 15 

0 5 10 15 20 KIL

Base modified from U.S. Geological Survey, 1:2,000,000-scale digital dataAlbers Equal Area Projection, Texas State Mapping SystemNorth American Datum of 1983

EXPLANATION

Specific storage, in 1/foot 0.000002 to 0.000010 0.000011 to 0.000050 0.000051 to 0.000100

 0.000101 to 0.00050 0.00051 to 0.010000

Groundwater-flow model boundaConceptual model study area bouFault zone—Represents numerou

Layer less than 5 feet thickConfined conditions based  on Bumgarner and others (2012)

B
Figure 8C. Calibrated specific storage distribution of the Pecos Valley aquifer (alluvial layer) in the Pecos County region model area, Texas.

Leon-Belding area

PECOS COUNTY BREWSTER COUNTY

REEVES COUNTY

TERRELL COUNTY

CROCKETT 

JEFF DAVIS COUNTY

UPTON C

WARD COUNTY

CRANE COUNTY Pecos River

Belding

Fort Stockton

C

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104°00’ 103°30' 103°00’ 102°30' 

31°00’

30°30'

0 5 10 15 

0 5 10 15 20 KIL

Base modified from U.S. Geological Survey, 1:2,000,000-scale digital dataAlbers Equal Area Projection, Texas State Mapping SystemNorth American Datum of 1983

Specific storage, in 1/foot 0.000002 to 0.000010 0.000011 to 0.000050 0.000051 to 0.000100

 0.000101 to 0.000500 0.000501 to 0.000603

Groundwater-flow model boundaryConceptual model study area boundaryFault zone—Represents numerous fault

Confined conditions based  on Bumgarner and others (2012)

EXPLANATION
Figure 9. Calibrated specific yield distribution of the Pecos Valley aquifer (alluvial layer) in the Pecos County region model area, Texas.

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PECOS COUNTY BREWSTER COUNTY

REEVES COUNTY

TERRELL COUNTY

CROCKETT 

JEFF DAVIS COUNTY

UPTON CO

WARD COUNTY

CRANE COUNTY

Belding

Fort Stockton

104°00’ 103°30' 103°00’ 102°30' 

31°00’

30°30'

Base modified from U.S. Geological Survey, 1:2,000,000-scale digital dataAlbers Equal Area Projection, Texas State Mapping SystemNorth American Datum of 1983

0 5 10 15 0 5 10 15 20 KIL

EXPLANATION

Specific yield, dimensionless 0.006 to 0.010 0.011 to 0.020 0.021 to 0.030 0.031 to 0.040

 0.041 to 0.050 0.051 to 0.060 0.061 to 0.070

Groundwater-flow model boundaryConceptual model study area boundaryFault zone—Represents numerous faults
28 Simulation of Groundwater Flow in the Edwards-Trinity and Related Aquifers in the Pecos County Region, Texas

Model Calibration

The ability of the model to simulate observed conditions 

was accomplished by a combination of manual changes to 

input values and automated calibration methods. The value 

of each pilot point, as well as other discrete zone parameters, 

was manually and automatically adjusted through the use of 

the Parameter ESTimation code PEST (Doherty, 2005) and 

a series of model simulations. The PEST code is an opensource, public domain software suite that allows modelindependent parameter estimation analysis. PEST, along 

with extensive documentation, can be downloaded from 

http://www.pesthomepage.org. After each model simulation, 

simulated hydraulic-head (simulated groundwater-level 

altitude) and spring-flow values were compared to observed 

values. The iterative simulations continued until a best fit 

(minimizing the numeric difference) between simulated 

hydraulic head and spring flow and observed hydraulic head 

(measured groundwater-level altitude) and spring flow was 

attained. The calibration approach that was used differs 

from traditional nonlinear regression parameter estimation 

by taking advantage of Tikhonov regularization (Doherty, 

2003; Fienen and others, 2009; Tikhonov, 1963) and hybrid 

singular value decomposition (Hunt and others, 2007; Tonkin 

and Doherty, 2005), also referred to as SVD-Assist (Doherty, 

2005). The model was evaluated through a comparison of 

residuals, or the difference between simulated and observed 

values of groundwater levels and spring flow at Comanche 

Springs, as well as the feasibility of estimated parameter 

values compared to literature values within the model area 

and of similar lithology (Bumgarner and others, 2012; Freeze 

and Cherry, 1979; Kunianksy and Holligan, 1994).

Optimal Parameter Estimates

The final calibrated parameter values of the model 

(table 2) are within the range of reported values and 

considered reasonable for the type of material and conditions 

found in the groundwater system (Domineco and Mifflin, 

1965; Freeze and Cherry, 1979). The range of horizontal 

hydraulic conductivity values applied to the Edwards-Trinity 

aquifer (Edwards and Trinity layers) from 1.0×10-3 to 1.2×103

ft/d was within the range of hydraulic conductivities for 

a fractured rock and karst limestone (Freeze and Cherry, 

1979). Specific yield values throughout the model range 

from 6.0×10-3 to 7.0×10-2. Specific storage values range from 

2.0×10-6 to 1.0×10-2 1/ft. Net recharge values range from 

1.0×10-8 to 6.2×10-4 ft/d (effectively from 0 to 2.7 in/yr) along 

the western model boundary. Comparatively, investigations 

by Long (1958), Iglehart (1967), Reeves (1969), and Rees and 

Buckner (1980) estimated average recharge values ranging 

from 0.3 to 1.4 in/yr over various parts of the Edwards-Trinity 

aquifer. The higher end-range value of simulated net recharge 

likely is a result of finer-scale discretization resulting in 

greater variability represented in the model, as compared with 

reported estimated values that are averaged across wide areas, 

for example countywide averages (Iglehart, 1967; Reeves, 

1969). 

Model Fit and Model Error

Differences between simulated and observed values 

of water level and spring flow are referred to as residuals. 

Residuals for groundwater levels and spring flow were 

evaluated based on the mean, minimum, maximum, absolute 

mean, and root mean square error (RMSE). RMSE is 

determined using the equation:

2 (( ) / ) RMSE h h n = √ − ∑ s o

where

hs is simulated value,

h0 is observed values, and

n is number of observations.

The mean of residuals indicates model bias depending on 

the magnitude and direction of the mean from zero. A mean 

value close to zero indicates a balance between positive and 

negative residuals, or less model bias. A large positive mean 

indicates that the model primarily overpredicts (simulated 

values greater than observed), and a negative mean indicates 

underprediction (simulated values less than observed). Low 

values of RMSE indicate better model fit to observed values 

(Anderson and Woessner, 1992; Reilly and Harbaugh, 2004).

Calibration Targets

Measurements of hydraulic head and spring flow were 

used as the primary calibration targets to evaluate model fit. 

Hydraulic head and spring flow were used quantitatively 

as part of manual and automatic calibration. Qualitative 

comparisons of hydrographs at selected wells in the model area 

and a comparison of a simulated potentiometric surface to that 

produced by Bumgarner and others (2012) are provided in the 

“Hydrographs” and “Potentiometric Surfaces” sections of the 

report.

Hydraulic Head Observations and Errors

Simulated hydraulic heads were compared to 

observed hydraulic heads (2,860 groundwater-level altitude 

measurements made at 288 wells) in the Edwards-Trinity 

aquifer (fig. 5). Simulated hydraulic heads generally were 

in good agreement with observed hydraulic head values, 

with 1,684 (59 percent) simulated values within 25 ft of the 

observed value. The RMSE values of most years were within 

5 percent of the total observed groundwater-level altitude 

range in the Edwards-Trinity aquifer (maximum total range of 

1,420.5 ft), which was deemed acceptable. The average RMSE 

value of hydraulic head for the Edwards-Trinity aquifer was 

34.2 ft, which was approximately 4 percent of the average total 

observed groundwater-level change. The RMSE of hydraulic 

heads ranged from 19.3 ft in 2002 to 55.4 ft in 1987 (table 3). 
Model Calibration 29

The mean residual approached zero with an absolute value 

less than 5 ft for 20 of the 63 years for which residuals were 

available. Out of the 2,860 observations in the EdwardsTrinity aquifer, 1,536 residuals were greater than or equal to 

zero (overprediction), and 1,324 residuals were less than zero 

(underprediction). The maximum and minimum residuals were 

149 ft and -286 ft, respectively.

Spring Flow

Simulated spring flow representing Comanche Springs 

indicates a similar pattern as compared to the observed spring 

flow (fig. 10). During the calibration process, it was noted that 

the overall magnitude of spring flow could be controlled by 

different aquifer properties, such as hydraulic conductivity of 

the Edwards or Trinity layers. However, each change in this 

aquifer property did not allow the gradual decline and eventual 

cessation of simulated spring flow. Only after the addition of 

the time-variant constant heads in the Rustler layer (described 

in “Specified Head Boundaries” by Ewing and others [2012]) 

did the simulated spring flow begin to follow the pattern of 

the observed data. These findings are corroborated by the 

geochemical analysis by Bumgarner and others (2012) that 

indicate upwelling of groundwater from the Rustler aquifer 

in localized areas. Droughts were common in Texas during 

the 1940s and 1950s (Thomas and others, 1963), and the 

preliminary analysis of cumulative below-normal departure 

from normal precipitation during model calibration followed 

an almost identical decline as that of spring flow from 1940 

to the late 1950s (Sharp, 2001). It is likely that this dry period 

resulted in the increased pumping to account for the lack 

of rainfall, resulting in groundwater-level and spring-flow 

declines; however, specified pumping from the Rustler aquifer 

would not substantially affect simulated hydraulic heads in the 

Rustler aquifer because of the time-variant constant heads in 

the Rustler layer.

Table 2. Final calibrated hydraulic parameter values for the Pecos County region model area, Texas.

Parameter group Parameter description Parameter name Final value or range Units

Hydraulic conductivity 

in horizontal direction

Pecos Valley aquifer (alluvial layer) hk_alvm* 0.5 to 60 feet/day

Edwards part of the Edwards-Trinity aquifer 

(Edwards layer)

hk_ed* 0.1 to 230 feet/day

Trinity part of the Edwards-Trinity aquifer 

(Trinity layer)

hk_trin* 1.0×10-3 to 1.2×103 feet/day

Dockum aquifer (Dockum layer) hk_dock 1.5 feet/day

Rustler aquifer (Rustler layer) hk_rust 100 feet/day

Vertical hydraulic 

conductivity

Pecos Valley aquifer (alluvial layer) vk_alvm* 0.5 to 1.8 feet/day

Edwards part of the Edwards-Trinity aquifer 

(Edwards layer)

vk_ed* 1.2×10-4 to 27 feet/day

Trinity part of the Edwards-Trinity aquifer 

(Trinity layer)

vk_trin* 0.02 to 22 feet/day

Dockum aquifer (Dockum layer) vk_dock* 6.2×10-6 to 2.7 feet/day

Rustler aquifer (Rustler layer) vk_rust 0.49 feet/day

Specific storage Pecos Valley aquifer (alluvial layer) ss_alvm* 1.3×10-3 to 5.0×10-3 1/foot

Edwards part of the Edwards-Trinity aquifer 

(Edwards layer)

ss_ed* 2.0×10-6 to 1.0×10-2 1/foot

Trinity part of the Edwards-Trinity aquifer 

(Trinity layer)

ss_trin* 2.0×10-6 to 6.0×10-4 1/foot

Dockum aquifer (Dockum layer) ss_dock 8.8×10-6 1/foot

Rustler aquifer (Rustler layer) ss_rust 5.0×10-6 1/foot

Specific yield Pecos Valley aquifer (alluvial layer) sy_alvm* 6.0×10-3 to 0.07 Dimensionless

Edwards part of the Edwards-Trinity aquifer 

(Edwards layer)

sy_ed 5.0×10-2 Dimensionless

Recharge Recharge prch* 1.0×10-8 to 6.2×10-4 feet/day

Horizontal flow barrier North/south trending fault within 

Pecos Trough

westpcstrgh 1.0×10-6 feet/day

*

 Represented by pilot points, see figure 5.
30 Simulation of Groundwater Flow in the Edwards-Trinity and Related Aquifers in the Pecos County Region, Texas

Table 3. Summary of hydraulic-head residual statistics for the Edwards-Trinity aquifers in the Pecos County region model area, 

Texas.

[–, too few observations to compute root mean square error (RMSE)]

Year

Mean 

residual

(feet)

Minimum 

residual

(feet)

Maximum 

residual

(feet)

RMSE

(feet)

Mean 

absolute error 

(feet)

Number of 

observations

Range 

(feet)

Ratio 

of RMSE 

to range

1940 -22.1 -61.8 11.2 34.3 27.1 9 501.9 0.07

1941 -66.4 -66.4 -66.4 -- 66.4 1 -- --

1942 23.3 -8.06 54.7 -- 31.4 2 -- --

1946 9.54 -28.3 42.0 22.6 17.0 14 762.0 0.03

1947 -0.394 -97.3 102 54.1 40.5 21 1,031.5 0.05

1948 -4.57 -86.6 90.1 46.3 37.0 20 870.0 0.05

1949 -15.5 -87.4 28.3 38.1 27.3 18 712.0 0.05

1950 -13.6 -88.1 25.1 36.9 28.5 36 750.8 0.05

1951 -14.9 -85.2 23.0 33.8 27.7 21 751.7 0.04

1952 -4.18 -85.0 52.3 28.6 23.0 51 777.4 0.04

1953 -8.84 -84.0 82.7 38.4 31.2 31 779.2 0.05

1954 -10.6 -77.7 41.1 30.2 25.2 35 781.5 0.04

1955 -12.6 -64.8 61.9 26.8 22.5 61 893.1 0.03

1956 -11.7 -102 80.9 39.8 32.6 44 1,155.3 0.03

1957 1.84 -231 129 32.7 23.0 232 1,063.1 0.03

1958 -9.38 -276 119 41.4 28.3 231 1,199.2 0.03

1959 -7.93 -188 131 50.3 38.2 98 1,080.1 0.05

1960 5.85 -98.0 59.2 46.6 38.1 14 363.4 0.13

1961 -15.0 -286 70.1 51.3 34.6 71 1,420.5 0.04

1962 1.14 -90.6 81.7 36.1 28.6 57 1,132.5 0.03

1963 5.09 -88.5 131 45.4 37.0 61 1,135.9 0.04

1964 14.0 -86.3 112 43.4 34.6 69 1,131.0 0.04

1965 11.8 -83.3 138 40.5 28.8 75 1,134.2 0.04

1966 6.14 -79.1 112 35.4 27.3 75 1,135.4 0.03

1967 10.6 -75.6 115 36.4 26.5 68 1,132.2 0.03

1968 7.27 -75.2 118 35.6 26.2 67 1,128.5 0.03

1969 3.94 -106 110 37.7 26.4 81 1,374.0 0.03

1970 7.16 -84.4 148 39.0 26.2 111 1,124.8 0.03

1971 6.93 -153 115 37.4 26.6 104 1,204.3 0.03

1972 10.9 -70.3 139 41.7 29.6 46 1,119.2 0.04

1973 8.42 -126 145 45.6 31.2 32 961.3 0.05

1974 14.1 -54.5 109 37.9 26.4 24 697.2 0.05

1975 -2.25 -71.8 72.9 35.4 27.9 40 1,102.4 0.03

1976 7.21 -66.3 97.6 39.0 30.9 35 1,102.8 0.04

1977 -2.80 -65.9 66.4 30.8 24.6 25 1,102.5 0.03

1978 1.46 -74.7 75.2 35.1 29.0 23 1,109.3 0.03

1979 0.609 -66.8 53.0 28.1 22.9 24 1,097.5 0.03

1980 6.87 -39.1 66.8 29.0 23.2 18 770.9 0.04

1981 16.1 -38.3 70.8 33.2 25.7 10 733.7 0.05

1982 -11.0 -81.6 13.3 34.0 19.1 6 451.1 0.08

1983 21.5 -39.4 86.3 38.8 28.9 14 738.0 0.05

1984 2.88 -69.4 59.6 33.7 26.8 20 1,111.5 0.03

1985 52.1 -5.51 110 -- 57.6 2 -- --

1986 -6.27 -76.4 73.7 36.1 26.6 12 781.6 0.05

1987 -3.76 -140 149 55.4 41.9 51 1,095.6 0.05

1988 7.30 -85.4 63.2 32.0 25.1 52 1,083.7 0.03

1989 7.83 -39.9 54.7 25.5 20.4 31 766.0 0.03

1990 7.39 -67.4 55.9 28.7 22.5 28 1,076.7 0.03

1991 2.39 -68.1 57.7 27.8 21.6 27 1,084.6 0.03

1992 -1.44 -68.9 47.4 27.5 22.1 26 1,087.6 0.03
Model Calibration 31

Year

Mean 

residual

(feet)

Minimum 

residual

(feet)

Maximum 

residual

(feet)

RMSE

(feet)

Mean 

absolute error 

(feet)

Number of 

observations

Range 

(feet)

Ratio 

of RMSE 

to range

1993 13.2 -40.3 115 40.2 27.5 39 724.3 0.06

1994 -5.77 -90.7 35.6 28.0 21.4 27 809.2 0.03

1995 -2.86 -39.8 34.9 19.9 16.7 26 744.4 0.03

1996 2.98 -39.3 79.7 26.1 20.2 23 742.3 0.04

1997 -14.9 -164 35.1 41.3 25.5 26 1,342.7 0.03

1998 -2.74 -42.7 40.1 22.3 19.3 23 762.7 0.03

1999 -1.86 -41.1 56.0 23.1 18.7 26 758.5 0.03

2000 -4.21 -40.9 41.7 21.9 18.3 19 760.5 0.03

2001 -5.53 -154 40.6 34.6 21.8 30 1,332.1 0.03

2002 0.683 -41.2 42.3 19.3 15.6 39 924.1 0.02

2003 -5.42 -34.5 18.5 19.5 17.1 8 118.5 0.16

2004 -11.9 -45.7 39.1 23.6 19.7 24 923.4 0.03

2005 -18.1 -80.4 36.8 31.1 25.8 19 935.4 0.03

2006 -14.5 -54.0 34.4 26.6 23.4 20 931.7 0.03

2007 -8.57 -54.3 33.8 22.2 17.5 48 933.0 0.02

2008 -11.3 -58.0 27.7 19.8 16.2 139 933.0 0.02

Table 3. Summary of hydraulic-head residual statistics for the Edwards-Trinity aquifers in the Pecos County region model area, 

Texas.—Continued

[–, too few observations to compute root mean square error (RMSE)]

Figure 10. Comparison of simulated and observed spring flow at Comanche Springs in the Pecos County region model area, Texas.

Simulated

Observed

EXPLANATION

50

Spring flow, in cubic feet per second

40

30

20

10

0

1931

Year

1944 1958 1972 1985 1999 2013 2027
32 Simulation of Groundwater Flow in the Edwards-Trinity and Related Aquifers in the Pecos County Region, Texas

Hydrographs

Hydrographs of simulated and observed hydraulic heads 

within the Edwards-Trinity aquifer were used to evaluate 

calibration of the model at selected wells in the model area 

(fig. 11). The hydrographs generally show good agreement 

between simulated and observed hydraulic heads for most 

locations with relatively long periods of record. Simulated 

hydraulic heads were appreciably larger than observed 

hydraulic heads at two wells (hydrographs 11B and 11E) 

during the midyears of the simulation. Differences between 

simulated and observed hydraulic heads are caused by 

uncertainty in hydraulic property values and the placement and 

timing of pumping wells in the model, which are dependent on 

the accuracy of the available pumping data.

Potentiometric Surfaces

The observed potentiometric surface for 1980 to 2010 

and the simulated potentiometric surface for 2010 of the 

Edwards-Trinity aquifer generally are in agreement (fig. 12). 

Although the observed potentiometric surface (fig. 12) was 

constructed by using the average winter groundwater-level 

altitude measurements from 1980 to 2010 (Bumgarner and 

others 2012), the comparison is useful to evaluate regional 

groundwater-levels and flow directions. The observed 

and simulated potentiometric surfaces indicate regional 

groundwater flow from south and southwest to north and 

northeast in the model area. The simulated surface also 

coincides roughly with the two groundwater divides in 

the model area described by Bumgarner and others (2012) 

(fig. 12).

Sensitivity Analysis

Sensitivity of simulated hydraulic heads to adjustments 

made to the various model parameters and parameter groups 

was evaluated using PEST (Doherty, 2005). Sensitivity 

analysis aids in determining if there is adequate information 

in the calibration data to estimate a particular parameter. 

Sensitivity analysis results can also be useful in evaluating 

where additional data need to be collected to enhance 

model accuracy. Composite sensitivities (related to the 

sensitivity of each parameter with respect to all observations 

[Doherty, 2005]) were calculated for groups of model 

parameters for each model layer, recharge, and horizontalflow barrier conductance that serves as the control for the 

amount of horizontal flow passing through a fault zone 

(fig. 13). Two parameters, hydraulic conductivity of the 

Trinity layer (hk_trin) and vertical hydraulic conductivity 

of the Dockum layer (vk_dock) have the largest composite 

sensitivity, as might be expected because most groundwaterlevel observations used in the analysis are within the Trinity 

layer. Additionally, according to the conceptual model 

of the groundwater-flow system (Bumgarner and others, 

2012), upwelling from the Rustler hydrogeologic unit that 

underlies the Edwards-Trinity aquifer is a critical component 

of flow in the Monument Draw trough area of the aquifer. 

The conceptual model also is corroborated by the most 

sensitive parameters in the analysis in that simulated 

groundwater flow is predominately controlled by hydraulic 

conductivity through the Edwards-Trinity aquifer and vertical 

hydraulic conductivity that controls upwelling from lower 

units.
Model Calibration 33

Figure 11. Simulated and observed hydrographs of hydraulic head in selected wells in the Edwards-Trinity aquifer in the Pecos County 

region model area, Texas.

1940 1950 1960 1970 1980 1990 2000 2010 

1940 1950 1960 1970 1980 1990 2000 2010 

1940 1950 1960 1970 1980 1990 2000 2010 

1940 1950 1960 1970 1980 1990 2000 2010 

3,000 

2,950 

2,900 

2,850 

2,800 

3,000 

2,950 

2,900 

2,850 

2,800 

2,400 

2,350 

2,300 

2,250 

3,000 

2,950 

2,900 

2,850 

3,050 

2,750 

2,700 

2,650 

2,600 

2,800 

2,350 

2,300 

2,250 

2,200 

2,400 

1940 1950 1960 1970 1980 1990 2000 2010 

1940 1950 1960 1970 1980 1990 2000 2010 

1940 1950 1960 1970 1980 1990 2000 2010 

E. TWDB well 4656201 F. TWDB well 5309105

C. TWDB well 5309301 D. TWDB well 4557603

A. TWDB well 5302708 B. TWDB well 4656301

G. TWDB well 5302102, layer 3

2,850 

2,800 

2,750 

2,700 

2,900 

Year Year

Year Year

Year Year

Year

EXPLANATION

Texas Water Development Board

Observed and simulated water-level 

altitude for wells completed in the 

Edwards-Trinity aquifer. Datum is 

North American Vertical Datum of 1988

 Simulated

 Observed

Altitude, in feet above North American Vertical Datum of 1988

TWDB
Figure 12. Potentiometric surface (1980–2010) and simulated groundwater-level altitudes for the Edwards-Trinity aquifer in the Pecos County region 

3,100 3,000

2,900

3,1003,000

2,800

3,500

3,400

3,300

3,200

2,700

2,600

3,600

2,700

2,600

2,500

2,400 2,600

2,500

2,600

2,300

£¤285

£¤285

¦§ ̈20

¦§ ̈10

GLASS

MOUNTAINS

BARILLA

MOUNTAINS

DAVIS

MOUNTAINS

Leon-Belding area

Pecos River

PECOS COUNTY BREWSTER COUNTY

REEVES COUNTY

TERRELL COUNTY

CROCKETT 

JEFF DAVIS COUNTY

UPTON C

WARD COUNTY

CRANE COUNTY

Belding

Fort Stockton

0 5 10 15 0 5 10 15 20 KILO

Base modified from U.S. Geological Survey, 1:2,000,000-scale digital dataAlbers Equal Area Projection, Texas State Mapping SystemNorth American Datum of 1983.

Potentiometric surface is from Bumgarner and other

104°00’ 103°30' 103°00’ 102°30' 

31°00’

30°30'

EXPLANATION

Simulated water level, in feet above North American 

 Vertical Datum of 1988 (NAVD 88)

 2,181.1 to 2,200.0 2,200.1 to 2,400.0 2,400.1 to 2,600.0 2,600.1 to 2,800.0

 2,800.1 to 3,000.0 3,000.1 to 3,200.0 3,200.1 to 3,400.0 3,400.1 to 3,600.0

Measured water level 1980 to 2010 (Bumgarner and 

 others, 2012)—Dashed where approximately  located. Interval 25 feet. Datum is NAVD 88

Minor contour

2,600 Major contour

Groundwater divideGroundwater-flow model bouConceptual model study area Fault zone—Represents nume
Model Calibration 35

Normality of Residuals and Goodness of Fit

Normality of residuals is a prerequisite for a 

valid regression. If the model accurately represents the 

groundwater-flow system, the residuals are expected to be 

random, independent, and normally distributed (Hill, 1998). 

The independence and normality of the residuals can be 

assessed through use of (1) the summary statistic RN 

2

, the 

correlation coefficient between the weighted residuals ordered 

from smallest to largest and the order statistics derived from 

the normal probability distribution function (Brockwell and 

Davis, 1989; Hill and others, 1998) and (2) a histogram of 

the residuals. The residuals are thought to be independent 

and normally distributed if the computed value of RN 

2

 for a 

calibration is greater than the tabulated critical value (Hill 

and others, 1998; Hill and others, 2000). Hill (1992, p. 64) 

provides a table of critical values for 35 to 200 observations. 

The critical value of RN 

2

 is 0.987 for a set of 200 observations. 

The value of RN 

2

 for hydraulic heads in the model calibration 

is 0.99, which is slightly greater than the critical value. A 

histogram (fig. 14) of the 2,860 Edwards-Trinity residuals 

shows an approximately normal distribution with the 

maximum value occurring within the interquartile range (25th 

to 75th percentile) of -25 to 25 ft.

Graphical analyses of the residuals facilitate assessment 

of model bias or error and of model fit to the calibration data. 

These analyses include plots of the observed and simulated 

values and of the spatial and temporal distribution of the 

hydraulic-head residuals.

The plot of observed and simulated equivalents for a 

reasonable calibration effort should approximate a fit to a 1:1 

line extending through the data. The model has a reasonable fit 

to the 1:1 line with a coefficient of determination (R2

) of 0.98 

(fig. 15). In the upper range of groundwater-level altitudes 

of 3,200 ft and larger where data are sparse, the simulated 

hydraulic heads are appreciably less than the observed 

hydraulic heads.

Parameter name (table 2)

Composite sensitivity 

hk_alvm 

h k_ed 

hk_trin 

hk_dock 

hk_rust 

vk_alvm 

vk_ed 

vk_trin 

vk_dock 

vk_rust 

ss_alvm 

ss_trin 

ss_ed 

ss_dock 

ss_rust 

sy_alvm 

sy_ed 

prch 

westpcstrgh 

0 

0.1 

0.2 

0.3 

0.4 

0.5 

0.6 

Figure 13. Composite sensitivities of all parameter groups of the groundwater-flow model of the Edwards-Trinity and related aquifers 

in the Pecos County region model area, Texas.
36 Simulation of Groundwater Flow in the Edwards-Trinity and Related Aquifers in the Pecos County Region, Texas

Figure 14. Residuals for the Pecos County region model, Texas.

Figure 15. Relation between simulated and observed hydraulic heads for the Pecos County region model, Texas.

300

Head residual, in feet

Number of residuals

-300 -275 -250 -225 -200 -175 -150 -125 -100 -75 -50 -25 0 25 50 75 100 125 150 175 200 225 250 275

1,000

800

600

400

200

0

2,000 2,200 2,400 2,600 2,800 3,000 3,200 3,400 3,600 3,800

3,800

3,600

3,400

3,200

3,000

2,800

2,600

2,400

2,200

2,000

Coefficient of determination (R2) = 0.98

Observed, in feet

Simulated, in feet
Model Calibration 37

Additional assessments of model error were 

accomplished through analysis of the spatial and temporal 

distribution of weighted residuals of the Edwards-Trinity 

aquifer for years after 1940 (fig. 16). Different ranges in 

residuals represented by a variety of geometric symbols in 

figure 16 allow for a visual analysis of model bias. Positive 

residuals, shown in blue, indicate simulated hydraulic heads 

that are higher than observed, while negative residuals indicate 

simulated hydraulic heads that are lower than observed.

Ideally, negative and positive weighted residuals should 

be small and randomly distributed in space. Clustering of 

residuals with similar magnitudes and signs is indicative of 

model bias (Reilly and Harbaugh, 2004). Overall, residuals 

(fig. 16) appear to be evenly distributed in both magnitude and 

positive or negative sign. 

Geochemical Modeling

Geochemical modeling was used independently to 

evaluate some of the groundwater-flow model results. Results 

of the geochemical modeling were not used for quantitative 

comparison during the groundwater-flow model calibration. 

Because of this, the mixing proportions and source areas of the 

various types of groundwater may differ slightly between the 

geochemical and flow models; however, the basic assumptions 

of the conceptual model of the system were substantiated by 

the geochemical and flow modeling results. 

Bumgarner and others (2012) identified four principal 

sources of recharge to the Edwards-Trinity aquifer: 

(1) regional groundwater flow in the Edwards-Trinity 

aquifer that originated as recharge northwest of the study 

area and enters the study area near the western corner; 

(2) runoff from the Barilla, Davis, and Glass Mountains that 

percolates through underlying rocks and into gravels along 

the slopes of the mountains; (3) return flow from irrigation; 

and (4) upwelling from deeper aquifers. Higher specific 

conductance, chloride, and sulfate values were measured in 

water-quality samples collected from the Edwards-Trinity 

aquifer in the Leon-Belding (sites Q16, Q18, Q22, and Q24, 

fig. 4) and Fort Stockton areas (sites Q23 and Q26, fig. 4) 

compared to the specific conductance, chloride, and sulfate 

values measured in water-quality samples collected from the 

Edwards-Trinity aquifer in other areas. These data indicate 

that water from the Rustler aquifer is likely upwelling in the 

Leon-Belding and Fort Stockton areas (Bumgarner and others, 

2012). 

Inverse geochemical modeling (PHREEQC; Parkhurst 

and Appelo, 1999) was used to mix upgradient EdwardsTrinity groundwater recharged in the Barilla, Davis, and 

Glass Mountains (fig. 1) with Rustler groundwater to 

approximate the composition of groundwater from sites 

Q16, Q18, Q22, Q23, Q24, and Q26 (fig. 4). The inverse 

models were constrained by the concentrations of alkalinity, 

calcium, carbon, chloride, dissolved oxygen, magnesium, 

sodium, strontium, and sulfur. All models included phases 

for anhydrite, calcite, celestite, dolomite, halite, carbon 

dioxide, and dissolved oxygen; anhydrite and halite phases 

were only allowed to dissolve (no precipitation). The inverse 

modeling results are valid within the constraints of available 

thermodynamic, chemical, and mineralogical data. The 

composition of the sample collected from site Q15 (fig. 4) was 

used to represent upgradient Edwards-Trinity groundwater 

recharged in the Barilla and Davis Mountains. Water from site 

Q15 is a dilute (specific conductance 331 microsiemens per 

centimeter at 25 degrees Celsius [μS/cm]) calcium-sodiumbicarbonate (Ca-Na-HCO3) type water with a composition 

indicative of interaction with igneous rocks. The composition 

of the sample collected from site Q8 (fig. 4) was used to 

represent upgradient Edwards-Trinity groundwater recharged 

in the Glass Mountains. Water from site Q8 is a dilute (specific 

conductance 587 μS/cm) sodium bicarbonate (Na-HCO3) 

type water with a composition indicative of interaction with 

carbonate rocks. The composition of the sample collected from 

site Q19 (located within the Leon-Belding area, fig. 4) was 

used to represent Rustler groundwater in the Leon-Belding 

area, and the composition of the sample collected from site 

Q29 (located within the Fort Stockton area, fig. 4) was used to 

represent Rustler groundwater in the Fort Stockton area. Sites 

Q19 and Q29 are hypothesized to be representative of Rustler 

groundwater in the Leon-Belding and Fort Stockton areas, 

respectively. 

Geochemical model results indicate that Edwards Trinity 

groundwater in the Leon-Belding and Fort Stockton areas is 

a mixture of groundwater recharged in the Barilla and Davis 

Mountains and groundwater that has upwelled from the Rustler 

aquifer (table 4). In the Leon-Belding area, the proportion of 

Rustler groundwater in the Edwards-Trinity aquifer increased 

downgradient from south (site Q16; 0 to 48.8 percent) to north 

(site Q24; 87.1 to 100 percent) (fig. 4, table 4). Likewise, 

geochemical modeling indicates that as much as 40.7 percent 

of the water in the Edwards-Trinity aquifer in the area of 

Comanche Springs is from the Rustler aquifer. This is similar 

to groundwater-flow model results that indicate approximately 

55 percent of the water in the simulated spring flow in the early 

part of the simulation period is from the Rustler aquifer, which 

was determined from the amount of water moving upward 

through the Dockum layer into model cells containing the SFR.

Bumgarner and others (2012) observed a groundwater 

divide that originates in the southern part of the Leon-Belding 

area and extends northward along the center of Monument 

Draw Trough (figs. 1 and 12) and hypothesized that the divide 

was related to upwelling of water from deeper aquifers. 

Results of the geochemical modeling support that hypothesis. 

Geochemical modeling results also indicate that no proportion 

of groundwater from the six sites in the Leon-Belding and Fort 

Stockton areas originated as recharge in the Glass Mountains 

(table 4). These six sites (fig. 4) are located on or to the west 

of the groundwater divide (fig. 12); thus, groundwater in the 

Edwards-Trinity aquifer that originated as recharge in the Glass 

Mountains flows to the east of this groundwater divide. This 

geochemical result differs from that of the groundwater-flow 

model, which simulates that part of the recharge may have 

originated near the Glass Mountains about halfway between 

sites Q16 and Q8. The groundwater-flow and geochemical 
Figure 16. Spatial distribution of hydraulic-head residuals after 1940 in the Edwards-Trinity aquifer in the Pecos County region model area, Texas.

£¤285

£¤285

¦§ ̈20

¦§ ̈10

GLASS

MOUNTAINS

BARILLA

MOUNTAINS

DAVIS

MOUNTAINS

Leon-Belding area

Pecos River

PECOS COUNTY BREWSTER COUNTY

REEVES COUNTY

TERRELL COUNTY

CROCKETT 

JEFF DAVIS COUNTY

UPTON CO

WARD COUNTY

CRANE COUNTY

Belding

Fort Stockton

104°00’ 103°30' 103°00’ 102°30' 

31°00’

30°30'

Base modified from U.S. Geological Survey, 1:2,000,000-scale digital dataAlbers Equal Area Projection, Texas State Mapping SystemNorth American Datum of 1983

0 5 10 15 0 5 10 15 20 KIL

EXPLANATION

Hydraulic head residual, in feet (simulated 

 minus observed)

 -299.9 to -200.0 -199.9 to -100.0 -99.9 to -50.0 -49.9 to -20.0

 -19.9 to 0.0  0.1 to 20.0 20.1 to 50.0 50.1 to 100.0 100.1 to 200.0

Groundwater-flow model boundaryConceptual model study area boundaryFault zone—Represents numerous faults
Model Calibration 39

models also required similar recharge amounts from the 

underlying hydrogeologic units. Attempts to exclude this 

source of recharge from the models resulted in an unsuccessful 

fit to the observed data.

Groundwater-Flow Budget

The groundwater-flow budget of the Pecos Valley and 

Edwards-Trinity aquifers indicates changes in flow into 

(inflows) and out of (outflows) the model area from 1940 

to 2010 (fig . 17). Negative rates indicate outflows from the 

groundwater system, and positive rates indicate inflows to the 

groundwater system. Total flow (sum of inflows or outflows) 

through the Pecos Valley and Edwards-Trinity aquifers in the 

model area ranged from about 613 Mgal/d in predevelopment 

to over 1,100 Mgal/d during the highest pumping of the 

1950s. This increase in simulated flow through the simulated 

period reflects increases in pumping and inflow compared 

to the predevelopment condition. Total inflow consists of 

three primary net inflows to the model listed from smallest to 

largest: net recharge, storage, and upwelling from the Dockum 

and Rustler aquifers. Total outflow consists of two primary net 

discharges or outflows: head dependent boundaries (horizontal 

flow at the edge of the model perimeter) and pumping. The 

pumping from wells represents the largest outflow component 

with a net rate of approximately 600 Mgal/d in the 1950s

Model Limitations

An understanding of model limitations is essential 

to effectively use groundwater-flow and hydraulic-head 

simulation results (Reilly and Harbaugh, 2004). The accuracy 

of a groundwater model is limited by simplification of 

complexities within the groundwater-flow system (conceptual 

model), space and time discretization effects, assumptions 

made in the formulation of the governing flow equations, and 

simplifications of representations of boundary conditions and 

discretization and representation of climate. Model accuracy

also is affected by cell size, number of layers, accuracy of 

boundary conditions, accuracy and availability of hydraulic

property data, accuracy of withdrawal and areal recharge

estimates, historical data for calibration, parameter sensitivity,

and the interpolations and extrapolations that are inherent 

in using data in a model (Reilly and Harbaugh, 2004). The 

model provides a relatively good fit to groundwater levels in 

the Leon-Belding and Fort Stockton areas and spring flow 

from Comanche Springs. However, the finding that simulated 

spring flow is highly contingent on the transient nature of 

the underlying hydraulic heads indicates the importance 

of adequately understanding and characterizing the entire 

groundwater system.

Although a model might be calibrated, the parameter 

values likely are not unique in yielding acceptable 

distributions of hydraulic head. Results of the model must 

be evaluated while taking into account the resolution of 

these limitations. The placement and timing of pumping 

wells in the model, which are dependent on the accuracy of 

pumping data, play a crucial role in the simulated hydraulic 

head and flow values. Additionally, data pertaining to wells 

that are completed through multiple hydrogeologic units 

are sometimes incomplete. Though the model is capable of 

simulating wells open to multiple aquifers, assumptions were 

made regarding skin friction coefficients and the number of 

model layers through which wells are screened, which may not 

be accurate. Additionally, wells are simulated as being at the 

node (center) of a model cell, which likely is not accurate.

Data regarding predevelopment conditions for hydraulic

head are sparse to nonexistent, therefore model calibration 

to predevelopment conditions is not well constrained. With 

the exception of the pre-1940s steady-state period and 

initial and final 3-month stress periods in 1940 and 2010, 

respectively, temporal discretization of the model is based on 

a 6-month irrigation/nonirrigation period. Each stress period 

incorporates average input values for pumping and boundary 

heads for the given time interval. Groundwater flow from 

underlying or adjacent aquifer systems is not well defined, 

though geochemical data provide evidence for interaction 

from underlying units. The model framework, which includes 

the altitude and thickness of hydrogeologic units, is based on 

available geophysical information that varies spatially and 

vertically throughout the model area.

Areas of sparse geophysical information might affect 

model results through assumptions in the altitude and 

thickness of these hydrogeologic units and the lack of 

definition of structural controls such as faults that might 

affect groundwater movement. The assumptions of a no-flow 

boundary beneath the Rustler aquifer and constant density of 

water may not be entirely valid. However, the assumption of 

constant density is thought to be adequate because salinity 

differences were probably not great enough to result in 

variable density-driven flow. 

Table 4. Summary of PHREEQC inverse modeling (Parkhurst and 

Appelo, 1999) results in the Leon-Belding and Fort Stockton areas 

in the Pecos County region model area, Texas.

[Well locations are shown on figure 4 and described in appendix 3 (modified 

from Bumgarner and others, 2012)]

Target well Model results indicating the proportion of 

groundwater that originated as recharge (percent)

Site

identifier

Site Q15

(Barilla/Davis 

Mountains)

Site Q8

(Glass

Mountains)

Site Q19

(Rustler)

Q16 51.2–100.0 0.0 0.0–48.8

Q18 14.5 0.0 85.5

Q22 10.2 0.0 89.8

Q24 0.0–12.9 0.0 87.1–100.0

Site Q15 

(Barilla/Davis 

Mountains)

Site Q8 

(Glass

Mountains)

Site Q29

(Rustler)

Q23 67.3–100.0 0.0 0.0–32.7

Q26 59.3–100.0 0.0 0.0–40.7
40 Simulation of Groundwater Flow in the Edwards-Trinity and Related Aquifers in the Pecos County Region, Texas

Because of numerical instability, sparse observation data, 

and little information regarding the connection between the 

Rustler and Trinity layers, the Trinity layer type was specified 

as confined in the simulation. This means that regardless of 

the altitude of the simulated groundwater level in the Trinity 

layer, it would not become dewatered. Specifying this layer 

type provided for a more stable numerical simulation because 

transmissivity was calculated as the product of hydraulic 

conductivity and the total thickness of the layer for any given 

model cell rather than a variable transmissivity based on a 

moving water table. Additionally, the storage term remained 

fixed for each model cell rather than converting from specific 

storage to a specific yield value when the groundwater 

level declined below the top of the layer (unconfined 

conditions). Because of sparse observation data, unsaturated 

areas within the Trinity layer were not well known. Some 

indication of unsaturated conditions may be inferred from 

the potentiometric map developed by Bumgarner and others 

(2012), taking into account the map’s limitations (contour 

lines are dashed in most areas and missing in some because of 

a lack of available data). The degree of hydraulic connection 

between the Rustler and Trinity layers also was not well 

known. During test simulations late in the calibration, 

groundwater levels in the Rustler appeared to have a large 

effect on groundwater levels in the Trinity layer, particularly 

in areas where Trinity groundwater levels declined below 

the bottom of the layer. These test simulations also indicated 

a relative insensitivity to adjustments in model parameters 

such as hydraulic conductivity and storage in areas where 

water-level altitudes are below the base of the Trinity layer. 

This insensitivity indicated that additional characteristics of 

the system are not well known or represented in the model 

such as complex geologic structure or upwelling from lower 

hydrogeologic units. The geologic structure or upwelling from 

lower hydrogeologic units may have a greater influence on 

groundwater levels than hydraulic conductivity and storage. 

Figure 17. Net simulated groundwater flow from 1940 to 2010 in the Pecos Valley and Edwards-Trinity aquifers in the Pecos County 

region model area, Texas.

1940 1950 1960 1970 1980 1990 2000 2010

Year

-800

-600

-400

-200

0

200

400

600

Million gallons per day

Inflow

Outflow

EXPLANATION

River leakage

Net flow at the edge

of model perimeter

Net recharge

Spring discharge

Net upwelling from

lower units

Storage

Net pumping
Development of Groundwater Pumping Scenarios 41

Most of these insensitive areas are in the southern part of 

the model area along the base of the Glass Mountains and 

an area near the Reeves-Pecos County line. Because the 

specified groundwater levels in the Rustler layer were derived 

from another groundwater simulation (Ewing and others, 

2012), few data were available to constrain the groundwater 

levels and improve the calibration. Additionally, even though 

groundwater levels may decline below the base of the Trinity 

layer in some areas, groundwater continues to flow through 

these model cells. This is important to provide a continuous 

hydraulic connection that would not be allowed if the layer 

was specified as convertible (Harbaugh, 2005).

Development of Groundwater Pumping 

Scenarios

The model can be used to simulate groundwater levels 

resulting from hypothetical prolonged pumping to evaluate the 

sustainability of recent (2008) and projected water use. The 

following scenarios utilized a continuation of the calibrated 

model to simulate a 30-year period from 2010 to 2040. This 

30-year simulation includes a continuation of input conditions 

of river stage, net recharge, general-head boundaries (GHBs), 

constant head boundaries, and the appropriate pumping 

condition for the scenario under evaluation. For each scenario, 

the change in groundwater level from 2010 to 2040 was 

extracted from the model for comparison with regard to 

effects of changes in pumping. For simplicity, each scenario is 

summarized as:

• Scenario 1 – Simulation of seasonal 2008 pumping 

with return flow extended from 2010 to 2040;

• Scenario 2 – Simulation of 2008 pumping extended 

from 2010 to 2040 with the addition of year-round permitted values in the Leon-Belding area with no return 

flow; and

• Scenario 3 – Identical pumping as in scenario 2, with 

increases of 5 percent pumping every 10 years between 

2010 and 2040 for every well with no return flow.

Simulations of prolonged pumping provide insight to 

system response using an extension of recent (2008) pumping 

(scenario 1), recent pumping with year-round permitted 

pumping near Leon-Belding (scenario 2), and effects of 

5 percent increases in pumping in 10 year increments 

(scenario 3). While the results of these scenarios are useful 

in gaining an understanding of the groundwater system as 

a whole, caution should be used in interpreting the results 

considering the model limitations described earlier in the 

“Model Limitations” section.

Scenario 1

Scenario 1 extends recent (2008) irrigation and 

nonirrigation pumping rates for a 30-year period from 2010 to 

2040 represented by 61 stress periods that are each 6 months 

in length (with the exception of stress period 1). As in the 

calibration period of the model, the 6-month stress periods 

continue to represent irrigation (April through September) and 

nonirrigation (October through March). While recent pumping 

might not be expected to continue unchanged for the 30-year 

period, scenario 1 provides a baseline from which to compare 

additional scenarios where pumping increases in specific areas 

or increases linearly through time. Return-flow recharge from 

irrigation is included in scenario 1. Projected groundwaterlevel changes in and around the Fort Stockton area (fig. 18) 

indicate little if any change from current conditions indicating 

that the groundwater system is near equilibrium with respect 

to recent (2008) pumping stress. Projected groundwaterlevel declines (from 5.0 to 15.0 ft) in the eastern part of the 

model area are likely the result of nonequilibrium conditions 

associated with recent increases in pumping after a prolonged 

water-level recovery period of little or no pumping. Projected 

groundwater-level declines (from 15.0 to 31.0 ft) occurred in 

localized areas by the end of the scenario in the Leon-Belding 

area. Results of scenario 1 indicate relatively stable water 

levels ranging from -5.0 to 5.0 ft throughout most of the model 

area during the 30-year simulation using pumping amounts as 

specified for the year 2008.
Figure 18. Water-level difference in the Trinity part of the Edwards-Trinity aquifer (Trinity layer) in the Pecos County region model area, Texas, betwe

scenario 1.

£¤285

£¤285

¦§ ̈20

¦§ ̈10

GLASS

MOUNTAINS

BARILLA

MOUNTAINS

DAVIS

MOUNTAINS

Pecos River

Leon-Belding area

PECOS COUNTY BREWSTER COUNTY

REEVES COUNTY

TERRELL COUNTY

CROCKETT 

JEFF DAVIS COUNTY

UPTON CO

WARD COUNTY

CRANE COUNTY

Belding

Fort Stockton

104°00’ 103°30' 103°00’ 102°30' 

31°00’

30°30'

Base modified from U.S. Geological Survey, 1:2,000,000-scale digital dataAlbers Equal Area Projection, Texas State Mapping SystemNorth American Datum of 1983

0 5 10 10 5 10 15 20 KIL

Water-level change from 2010 to 2040, in feet 

 (2010 minus 2040)

 -5.1 to -10.7  4.9 to -5.0 14.9 to 5.0

 24.9 to 15.0 32.0 to 25.0

EXPLANATION

Groundwater-flow model boundaryConceptual model study area boundaryFault zone—Represents numerous faults
Development of Groundwater Pumping Scenarios 43

Scenario 2

Scenario 2 evaluates the effects of extended recent (2008) 

pumping rates as assigned in scenario 1, in addition to yearround maximum permitted pumping rates (about 42 Mgal/d) 

in the Leon-Belding area. The extended 2008 irrigation and 

nonirrigation pumping rates are represented as they were 

in the calibration period reflecting 6-month irrigation and 

nonirrigation periods, while the recently permitted values 

were assigned as constant annual pumping for the 30-year 

period. Return flow recharge from irrigation is not included in 

scenario 2. Results of scenario 2 (fig. 19) are similar in waterlevel decline and extent from those of scenario 1 (fig. 18). 

The extent of the projected groundwater-level decline in the 

range of 5.0 to 15.0 ft in the Leon-Belding irrigation area 

expanded slightly (about a 2-percent increase) from that of 

scenario 1. Maximum projected groundwater-level declines in 

the Leon-Belding irrigation area were approximately 31.3 ft in 

small isolated areas, which are depicted as water-level changes 

ranging from 25.0 to 32.0 in figure 19. The remaining area and 

magnitude of groundwater-level decline are almost identical to 

that of scenario 1.

Scenario 3

Scenario 3 evaluates the effects of periodic increases 

in pumping rates over the 30-year extended period using 

the same 6-month irrigation and nonirrigation stress 

periods in scenarios 1 and 2. Groundwater use is predicted 

to increase approximately 16 percent by 2040 given the 

projected population growth for Pecos County (Texas Water 

Development Board, 2013). Based on this projected increase, 

simulated pumping for all wells was increased by 5 percent 

every 10 years to account for about a 15-percent increase 

by the end of the 30-year simulation period. Return-flow 

recharge from irrigation is not included in scenario 3. Results 

of scenario 3 are similar to those of scenario 2 in terms of the 

areas of groundwater-level decline (fig. 20). The maximum 

projected groundwater-level decline in the Leon-Belding area 

was greater than scenario 1 or 2 at approximately 34.5 ft, and 

the extent of the decline is larger in area (about a 17-percent 

increase) than that of scenario 2. Additionally, the area of 

projected groundwater-level declines in the eastern part of 

the model area increased as compared to scenario 2, with two 

discrete areas of decline coalescing into one larger area. The 

lack of differences in the remaining areas associated with the 

results of scenarios 2 and 3 might be attributed to the low 

magnitude of pumping in 2008 (fig. 17) and the relatively 

small total increase in water use of about 15 percent over 

the 30-year period, which together produces small increases 

in pumping amounts. For example, in 2008, the average 

pumping rate for wells completed solely in the Trinity 

layer was approximately 370 gallons per minute (gal/min). 

After three decades of 5 percent increases, the new average 

pumping value at the end of the simulation was estimated 

to be approximately 425 gal/min. This represents an overall 

increase of 55 gal/min (0.08 million gallons per day), which 

is a relatively small increase in relation to the total water 

budget in the simulation of the groundwater system (fig. 17). 

While projected groundwater-level declines are evident in all 

scenarios throughout the model area (figs. 18–20), the largest 

projected groundwater-level declines occurred in localized 

areas of more intense pumping. The localized nature of the 

projected groundwater-level declines reflects the secondary 

porosity of the Edwards-Trinity aquifer. In such a system, 

groundwater levels that differ by tens or hundreds of feet 

over relatively short horizontal distances of a few miles 

are common because of the discontinuity of flow paths and 

barriers such as faults and fractures. An added complexity is 

upwelling of groundwater from underlying units, which model 

results indicate will increase in response to increased pumping 

in the overlying aquifer units.
Figure 19. Water-level difference in the Trinity part of the Edwards-Trinity aquifer (Trinity layer) in the Pecos County region model area, Texas, betwe

scenario 2.

Leon-Belding area

£¤285

£¤285

¦§ ̈20

¦§ ̈10

GLASS

MOUNTAINS

BARILLA

MOUNTAINS

DAVIS

MOUNTAINS

PECOS COUNTY BREWSTER COUNTY

REEVES COUNTY

TERRELL COUNTY

CROCKETT 

JEFF DAVIS COUNTY

UPTON CO

WARD COUNTY

CRANE COUNTY Pecos River

Belding

Fort Stockton

0 5 10 15 0 5 10 15 20 KIL

Base modified from U.S. Geological Survey, 1:2,000,000-scale digital dataAlbers Equal Area Projection, Texas State Mapping SystemNorth American Datum of 1983

104°00’ 103°30' 103°00’ 102°30' 

31°00’

30°30'

EXPLANATION

Water-level change from 2010 to 2040, in feet 

 (2010 minus 2040)

 -5.1 to -10.7  4.9 to -5.0 14.9 to 5.0

 24.9 to 15.0 32.0 to 25.0

Groundwater-flow model boundaryConceptual model study area boundaryFault zone—Represents numerous faults
Figure 20. Water-level difference in the Trinity part of the Edwards-Trinity aquifer (Trinity layer) in the Pecos County region model area, Texas, betwe

scenario 3.

£¤285

£¤285

¦§ ̈20

¦§ ̈10

GLASS

MOUNTAINS

BARILLA

MOUNTAINS

DAVIS

MOUNTAINS

Leon-Belding area

PECOS COUNTY BREWSTER COUNTY

REEVES COUNTY

TERRELL COUNTY

CROCKETT 

JEFF DAVIS COUNTY

UPTON CO

WARD COUNTY

CRANE COUNTY Pecos River

Belding

Fort Stockton

104°00’ 103°30' 103°00’ 102°30' 

31°00’

30°30'

Base modified from U.S. Geological Survey, 1:2,000,000-scale digital dataAlbers Equal Area Projection, Texas State Mapping SystemNorth American Datum of 1983

0 5 10 15 0 5 10 15 20 KIL

Water-level change from 2010 to 2040, in feet 

 (2010 minus 2040)

 -5.1 to -13.5  4.9 to -5.0 14.9 to 5.0

 24.9 to 15.0 34.5 to 25.0

EXPLANATION

Groundwater-flow model boundaryConceptual model study area boundaryFault zone—Represents numerous faults
46 Simulation of Groundwater Flow in the Edwards-Trinity and Related Aquifers in the Pecos County Region, Texas

Summary

The Edwards-Trinity aquifer is a vital groundwater 

resource for agricultural, industrial, and public supply uses in 

the Pecos County region of west Texas. Resource managers 

would like to know more about the future availability of water 

in the aquifer in Pecos County, Texas, and the effects of the 

possible increase or temporal redistribution of groundwater 

withdrawals. To provide resource managers with that 

knowledge, the U.S. Geological Survey, in cooperation with 

the Middle Pecos Groundwater Conservation District, Pecos 

County, City of Fort Stockton, Brewster County, and Pecos 

County Water Control and Improvement District No. 1, 

completed a comprehensive, integrated analysis of available 

hydrogeologic data to develop a numerical groundwater-flow 

model of the Edwards-Trinity and related aquifers in the 

study area in parts of Brewster, Jeff Davis, Pecos, and Reeves 

Counties.

The active model area covers about 3,400 square miles 

of the Pecos County region of Texas west of the Pecos River, 

and its boundaries were defined to include the saturated areas 

of the Edwards-Trinity aquifer. The southwestern and southern 

boundaries of the model area are rimmed by the Barilla and 

Davis Mountains in northeastern Jeff Davis County and 

southwestern Reeves County and the Glass Mountains in 

northeastern Brewster County and southern Pecos County. The 

northeastern boundary of the model area is the Pecos River.

The modular finite-difference code U.S. Geological 

Survey MODFLOW-2005 was used to approximate the 

solution of the equations governing three-dimensional 

groundwater flow. The model is a five-layer representation of 

the Pecos Valley aquifer (alluvial layer), the Edwards part of 

the Edwards-Trinity aquifer (Edwards layer), the Trinity part 

of the Edwards-Trinity aquifer (Trinity layer), the Dockum 

aquifer (Dockum layer), and the Rustler aquifer (Rustler 

layer). The calibration period of the simulation extends from 

1940 to 2010 for a total of 70 years and 144 stress periods, 

each one approximately 6 months in length. The hydrologic 

boundaries of the model include areal recharge, rivers, 

no-flow, constant heads, and general-head boundaries. Each 

boundary was included to represent a specific aspect of the 

groundwater-flow system. Groundwater pumping in the model 

represents public supply, manufacturing, mining, and power 

generation (industrial), and irrigation, with irrigation being the 

largest water-use category.

Groundwater pumping was compiled from multiple 

sources to develop a pumping record for the period from 1940 

to 2010. Site-specific pumping was used when available, 

though much of the record for irrigation pumping contained 

only aggregated amounts of withdrawals by county, aquifer, 

and year.

Hydraulic properties in the model were assigned through 

discrete zones (large areas possessing the same property value) 

and pilot points. The value of each pilot point, as well as other 

discrete zone parameters, was adjusted through manual and 

automated methods to achieve a best fit of observed values of 

hydraulic head and spring flow.

Simulated hydraulic heads were compared to 2,860 

measurements from 288 wells in the Edwards-Trinity aquifer 

within the model area. Simulated hydraulic heads were 

generally in good agreement with observed hydraulic head 

values with 1,684 (59 percent) simulated values within 25 feet 

(ft) of the observed value. The average root mean square 

error value of hydraulic head for the Edwards-Trinity aquifer 

for all time periods was 34.2 ft, which was approximately 

4 percent of the average total observed changes in groundwaterlevel altitudes (groundwater levels). Simulated spring flow 

representing Comanche Springs indicates a similar pattern 

as compared to the observed spring flow. Independent 

geochemical modeling corroborates results of simulated 

groundwater flow that indicates groundwater in the EdwardsTrinity aquifer in the Leon-Belding and Fort Stockton areas is a 

mixture of recharge from the Barilla and Davis Mountains and 

groundwater that has upwelled from the Rustler aquifer. 

The model was used to simulate groundwater levels 

resulting from prolonged pumping to evaluate sustainability 

of current and projected water-use demands. Each of three 

scenarios utilized a continuation of the calibrated model to 

simulate a 30-year period from 2010 to 2040.

Scenario 1 extended recent (2008) irrigation and 

nonirrigation pumping rates for a 30-year period from 

2010 to 2040 represented by 61 stress periods that are each 

approximately 6 months in length. Projected groundwater-level 

declines (from 5.0 to 15.0 ft) in the eastern part of the model 

area are likely the result of recent increases in pumping after a 

prolonged water-level recovery period of little or no pumping. 

Projected groundwater-level declines (from 15.0 to 31.0 ft) 

occurred in localized areas by the end of the scenario in the 

Leon-Belding area. Results of scenario 1 indicate relatively 

stable water levels ranging from -5.0 to 5.0 ft throughout most 

of the model area during the 30-year simulation using pumping 

amounts as specified for the year 2008. Scenario 2 evaluated 

the effects of extended recent (2008) pumping rates as 

assigned in scenario 1, with year-round maximum permitted 

pumping rates in the Leon-Belding area. Results of scenario 

2 are similar in water-level decline and extent as those of 

scenario 1. The extent of the projected groundwater-level 

decline in the range of 5.0 to 15.0 ft in the Leon-Belding 

irrigation area expanded slightly from that of scenario 1. 

Maximum projected groundwater-level declines in the LeonBelding irrigation area were approximately 31.3 ft in small 

isolated areas. Scenario 3 evaluated the effects of periodic 

increases in pumping rates over the 30-year extended period. 

Results of scenario 3 are similar to scenario 2 in terms of the 

areas of groundwater-level decline. The maximum projected 

groundwater-level decline in the Leon-Belding area was 

greater than scenario 1 or 2 at approximately 34.5 ft, and the 

extent of the decline is larger in area than that of scenario 2. 

Additionally, the area of projected groundwater-level declines 

in the eastern part of the model area increased as compared 

to scenario 2, with two discrete areas of decline coalescing 
References Cited 47

into one larger area. While groundwater-level declines are 

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projected groundwater-level declines occur in localized areas 

of more intense pumping. The localized nature of the projected 

groundwater-level declines is a reflection of the highly 

fractured nature of the Edwards-Trinity aquifer. Additionally, 

the finding that simulated spring flow is highly dependent on 

the transient nature of the underlying hydraulic heads indicates 

the importance of adequately understanding and characterizing 

the entire groundwater system. 

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Appendixes 1–3

Appendix 1 53

Appendix 1

Appendix 1. Wells completed in the Edwards-Trinity aquifer providing observed hydraulic head 

data in the Pecos County region model area, Texas, 1940–2010.

[TWDB, Texas Water Development Board]

Map

identifier

(fig. 3)

TWDB

well

number

Source 

station

number1

Latitude

(decimal

degrees)

Longitude

(decimal

degrees)

Period of data 

collected used for 

model calibration

A 5302708 305234102504301 30.87618 102.84521 1950–2008

B 5309301 305110102533401 30.85286 102.89278 1956–2008

C 4656301 4656301 31.21583 103.03389 1961–2008

D 4557603 4557603 31.05667 102.90722 1973–2009

E 4656201 311340103040101 31.22792 103.06766 1958–2008

F 5309105 5309105 30.84528 102.99917 1957–2008

G 5302102 5301102 30.99222 102.96583 1947–1997

1

Bumgarner and others (2012, table 9).
54 Simulation of Groundwater Flow in the Edwards-Trinity and Related Aquifers in the Pecos County Region, Texas

Appendix 2

Appendix 2. Amount of pumping in 2008 by county, category, and aquifer in the Pecos 

County region, Texas.

County Category Aquifer

Amount 

(million 

gallons 

per day)

Number 

of wells

Pecos Domestic/livestock1 Edwards-Trinity 11.28 34

Pecos Domestic/livestock1 Pecos Valley 4.99 9

Pecos Domestic/livestock1 Rustler 1.08 2

Pecos Irrigation Edwards-Trinity 46.66 101

Pecos Irrigation Pecos Valley 21.18 106

Pecos Irrigation Rustler 12.22 18

Pecos Municipal, industrial, mining Edwards-Trinity 0.97 31

Pecos Municipal, industrial, mining Pecos Valley 0.01 1

Pecos Municipal, industrial, mining Rustler 0.18 11

Pecos Unknown Edwards-Trinity 20.93 38

Pecos Unknown Pecos Valley 4.82 11

Pecos Unknown Rustler 0.22 4

Reeves Irrigation Edwards-Trinity 0.93 12

Reeves Irrigation Pecos Valley 6.62 7

1

Discrepancy in records indicates values could be largely attributed to irrigation.
Appendix 3 55

Appendix 3

Appendix 3. Data-collection sites providing data for the geochemical analysis in the Pecos County region, Texas.

[Modified from Bumgarner and others (2012, table 5). USGS, U.S. Geological Survey; –, not applicable]

Site 

identifier 

(fig. 4)

USGS 

station name

State 

well number

USGS 

station number

Latitude 

(decimal 

degrees)

Longitude 

(decimal 

degrees)

Site 

type

Contributing 

aquifer

Q1 San Solomon Spring – 08427500 30.94292 103.78824 Spring –

Q2 Santa Rosa Spring – 08437000 31.26743 102.95828 Spring –

Q4 Comanche Springs – 08444500 30.88628 102.87495 Spring –

Q6 Diamond Y Spring – 08446600 31.00190 102.92358 Spring –

Q27 PS-52-02-404 PS-52-02-404 305502103504101 30.91737 103.84518 Well Pecos Valley

Q36 WD-46-62-201 WD-46-62-201 310625103175201 31.10685 103.29777 Well Pecos Valley

Q42 US-45-43-807 US-45-43-807 311602102400601 31.26942 102.67609 Well Pecos Valley

Q43 US-45-43-8xx (PA 1)1 US-45-43-8xx 311602102400901 31.26934 102.68214 Well Pecos Valley

Q15 PS-52-11-702 PS-52-11-702 304605103444601 30.77100 103.74800 Well Igneous

Q7 PS-52-34-303 – 302955103451101 30.49860 103.75300 Well Igneous

Q11 US-52-22-8xx (Farm Well 3)1 US-52-22-8xx 303941103175001 30.66139 103.29720 Well Edwards-Trinity

Q12 PS-52-20-601 PS-52-20-601 304006103315601 30.66827 103.53216 Well Edwards-Trinity

Q14 US-53-17-501 US-53-17-501 304117102560101 30.68806 102.93361 Well Edwards-Trinity

Q16 US-52-16-910 US-52-16-910 304646103013401 30.77931 103.02615 Well Edwards-Trinity

Q17 US-52-13-801 US-52-13-801 304715103263501 30.78740 103.44343 Well Edwards-Trinity

Q18 US-52-16-611 US-52-16-611 304802103003901 30.80088 103.01110 Well Edwards-Trinity

Q22 US-52-16-3xx (S-21)1 US-52-16-3xx 305132103015701 30.85899 103.03244 Well Edwards-Trinity

Q23 US-53-09-306 US-53-09-306 305140102521101 30.87393 102.88229 Well Edwards-Trinity

Q24 US-52-08-909 US-52-08-909 305331103020501 30.89210 103.03516 Well Edwards-Trinity

Q25 US-53-03-9xx US-53-03-9xx 305354102373501 30.89825 102.62647 Well Edwards-Trinity

Q26 US-53-01-907 US-53-01-907 305419102545301 30.90560 102.91610 Well Edwards-Trinity

Q28 PS-52-02-4xx (Balmorhea)1 PS-52-02-4xx 305509103510101 30.91911 103.85027 Well Edwards-Trinity

Q30 WD-52-02-507 WD-52-02-507 305531103474201 30.92539 103.79511 Well Edwards-Trinity

Q32 US-53-07-105 US-53-07-105 305836102131701 30.97667 102.22139 Well Edwards-Trinity

Q33 US-53-01-210 US-53-01-210 305859102571001 30.98293 102.95271 Well Edwards-Trinity

Q35 US-45-60-903 US-45-60-903 310136102311601 31.02670 102.52102 Well Edwards-Trinity

Q37 US-45-58-2xx US-45-58-2xx 310718102484801 31.12162 102.81354 Well Edwards-Trinity

Q40 US-46-56-309 US-46-56-309 311235103000901 31.20974 103.00262 Well Edwards-Trinity

Q8 BK-52-29-8xx (Brewster 

County Edwards-Trinity Well)1

BK-52-29-8xx 303222103263701 30.53950 103.44346 Well Edwards-Trinity

Q9 US-52-07-502 US-52-07-502 303342103064001 30.93779 103.18711 Well Edwards-Trinity

Q21 US-53-13-208 US-53-13-208 305112102265901 30.85341 102.44965 Well Dockum

Q31 US-52-06-603 US-52-06-603 305559103154101 30.93305 103.26194 Well Dockum

Q34 US-53-01-208 US-53-01-208 305949102552301 30.99718 102.92291 Well Dockum

Q39 US-46-55-9xx (Weatherby 

Ranch)1

US-46-55-9xx 310949103090401 31.16341 103.15103 Well Dockum

Q44 US-46-48-701 US-46-48-701 311610103050901 31.26959 103.08683 Well Dockum

Q10 US-53-19-7xx (PC QW)1 US-53-19-7xx 303852102432902 30.64799 102.72470 Well Rustler

Q13 US-52-24-501 US-52-24-501 304020103025202 30.67295 103.05601 Well Rustler

Q19 US-52-16-609 US-52-16-609 304805103013301 30.80129 103.02618 Well Rustler

Q29 US-53-01-5xx (Apache 3)1 US-53-01-5xx 305529102560601 30.92470 102.93490 Well Rustler

Q38 WD-46-54-901 WD-46-54-901 310806103171901 31.13502 103.28796 Well Rustler

Q20 US-52-16-504 US-52-16-504 304807103025301 30.80241 103.04844 Well Capitan Reef

Q41 US-45-49-203 US-45-49-203 311422102555101 31.23974 102.93097 Well Capitan Reef

1

Local well name.
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Clark and others—Simulation of Groundwater Flow in the Edwards-Trinity and Related Aquifers in the Pecos County Region, Texas—SIR 2013–5228 ver. 1.1

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