0 10 20 MILES

0 10 20 KILOMETERS

104°30' 104°00’ 103°30' 103°00’ 102°30' 102°00’ 101°30'

31°30'

31°00’

30°30'

285

10

20

UPTON CO

U

NTY

REA

G

AN CO

U

NTY

CRA

NE

CO

U

NTY

WA

RD

CO

U

NTY

CROCKETT COUNTY

CUL

BERSO

N

C

O

UNTY

JEFF DAVIS COUNTY

BREWSTER COUNTY

TERREL CO

U

NTY

PRESIDIO COUNTY

PECOS CO

U

NTY

REEVES C

O

UNTY

DELAWARE BASIN

CENTRAL BASIN PLATFORM

DIABLO PLATFORM

MIDLAND BASIN

VAL VERDE BASIN

MONUMENT DRAW TROUGH

AND

BELDING-COYANOSA TROUGH

PECOS

TROUGH

Grandfalls

Balmorhea

Toyahvale

Belding

Fort Davis

Fort 

Stockton

Crane

Pecos

Alpine

San Solomon

Spring

Santa Rosa Spring

Comanche

Springs

Diamond Y Spring

Leon

Springs

Leon-Belding area

TEXAS

PERMIAN

BASIN

Model area

EXPLANATION

Belding Fault System

Diamond Y Fault System

Fault zone—Represents numerous faults

Spring

Groundwater-flow model boundary

Conceptual model study area boundary

Permian geologic structure boundary

Permian to Cenozoic geologic structure boundary

Base modified from U.S. Geological Survey digital data, 1:2,000,000

Albers Equal-Area Conic projection, Texas State Mapping System

North American Datum of 1983

Belding Fault System from Small and Ozuna (1993);

Diamond Y Fault System from Boghici (1997);

Belding-Coyanosa Trough modified from Boghici (1997);

Baumgardner and others (1982); Meyer and others (2011);

Permian geologic structure boundaries modified from Small and Ozuna (1993);

Cenozoic geologic structure boundaries modified from Meyer and others (2011)

GLASS

MOUNTAINS

BARILLA

MOUNTAINS

DAVIS

MOUNTAINS

Pecos River

U.S. Department of the Interior

U.S. Geological Survey

Fact Sheet 2014–3029

July 2014

Prepared in cooperation with Middle Pecos Groundwater Conservation District, Pecos County, City of Fort Stockton, 

Brewster County, and Pecos County Water Control and Improvement District No. 1

Introduction

The Edwards-Trinity aquifer, a major aquifer in the Pecos County region of western Texas, is a vital groundwater resource for 

agricultural, industrial, and public supply uses (Barker and Ardis, 1992; Freese and Nichols, Inc., and LBG-Guyton, Inc., written 

commun., 2010). Resource managers would like to better understand the future availability of water in the Edwards-Trinity aquifer in 

the Pecos County region and the effects of the possible increase or temporal redistribution of groundwater withdrawals. To that end, 

the U.S. Geological Survey (USGS), 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 groundwater-flow model of the Edwards-Trinity and related aquifers 

in parts of Brewster, Jeff Davis, Pecos, and Reeves Counties (fig. 1). The model incorporates conceptual information provided by 

Bumgarner and others (2012) and data collected and compiled by Pearson and others (2012). Following calibration, the model was 

used to evaluate the sustainability of recent (2008) and projected water-use demands on groundwater resources in the study area.

Figure 1. Location and general geological structural features of the model area in the Pecos County region, Texas 

(modified from Clark and others, 2014, fig. 1).

Simulation of Groundwater Flow

in the Edwards-Trinity and Related Aquifers in the Pecos County Region, Texas

Printed on recycled paper
Recharge

Because precipitation is relatively low and evapotranspiration is relatively high (Anaya and Jones, 2009), very little net recharge 

was expected over much of the model area. 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. Net areal recharge 

in the model primarily occurred as mountain-front recharge at the Barilla, Davis, and Glass Mountains. The width of the mountainfront recharge zone was assumed to be approximately 5 miles along the base of the mountains at an initial rate of 2.0 inches per 

year (in/yr) on the basis of higher estimates of recharge for the Edwards-Trinity aquifer (Long, 1958; Rees and Buckner, 1980) and 

was further adjusted through model calibration. Additional recharge may be introduced through irrigation return flow (Bumgarner 

and others, 2012). Recharge was specified as 0.2 in/yr in irrigated areas to represent irrigation return flow on the basis of low but 

detectable concentrations of nutrients and pesticides (Bumgarner and others, 2012). 

Discharge

Multiple springs exist within the model area, though few discharge records are available to aid model calibration. Sufficient 

discharge data were available for Comanche Springs, however, and these data were used to specify boundary conditions, as well as 

to calibrate the model. Monthly discharge measurements from Comanche Springs within each 6-month stress period were averaged 

Description of the Model Area

The model area includes the saturated areas of the Edwards-Trinity aquifer and covers about 3,400 square miles of the Pecos 

County region of Texas. 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.

Hydrogeologic Framework

The geologic setting contributed to the formation of two major and four minor aquifers in the model area. In addition to the 

Edwards-Trinity aquifer, the other major aquifer in the Pecos County region is the Pecos Valley aquifer. In the northern part of 

the model area, the Pecos Valley aquifer unconformably overlies the Edwards-Trinity aquifer. Minor aquifers include the Igneous, 

Dockum, Rustler, and Capitan Reef aquifers. Additional details about the geologic and hydrogeologic setting, as well as the 

hydrogeologic framework of the model area, can be found in Bumgarner and others (2012).

Groundwater-Flow System

The Edwards-Trinity aquifer in the model area is dominated by mineralized, regional groundwater that most likely recharged 

during the Pleistocene with variable contributions of more recent, local recharge. Groundwater generally flows northward into the 

downdip extent of the Edwards-Trinity aquifer or eastward out of the model area. 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. Additional details about the groundwater 

geochemistry and the conceptual groundwater flow in the model area are available in Bumgarner and others (2012). 

Groundwater-Flow Model Construction

The groundwater-flow system is represented in the model by 

a set of grid cells, and within each individual cell the hydraulic 

properties are the same. The transient model simulates 70 years 

(1940–2010) of system response to stress by using 144 stress periods. 

Stress period 1 is a steady-state stress period, used to establish 

equilibrium conditions, while the remaining stress periods are each 

6 months in length, representing irrigation (April through September) 

and non-irrigation (October through March) intra-annual variability. 

The model has five layers representing 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 hydrologic boundaries of the model 

include specified flux (areal recharge, pumping, and no-flow), 

specified groundwater levels (also referred to as “hydraulic heads,” 

or simply “heads”) for the Rustler aquifer, and head-dependent flux 

(general-head and river boundaries). Each boundary was included to 

represent a specific aspect of the groundwater-flow system.

Water-quality sampling by the U.S. Geological Survey, 

San Solomon Spring, Texas.
300

Head residual, in feet

Number of residuals

-300 -250 -200 -150 -100 -50 0 50 100 150 200 250

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

to represent measured flow. Groundwater-pumping estimates were compiled from multiple sources to develop a pumping record for 

1940–2010. Site-specific pumping data were used when available, though much of the record for irrigation pumping contained only 

aggregated amounts of withdrawals by county, aquifer, and year. All pumping totals were aggregated to annual amounts and assigned 

to the appropriate stress period of the model. 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, 

and pumping amounts were distributed to each layer.

Boundaries

General-head boundaries, which represent horizontal flow into or out of the model area, were placed along the western, 

northwestern, north, and southeastern perimeters of the model area in layer 3 (Trinity layer). To address the likely upwelling from 

the Rustler aquifer in some areas of the model area, time-variant constant heads were specified in the active cells of model layer 5 to 

represent groundwater levels in the Rustler aquifer as represented in the Groundwater Availability Model (GAM) of Anaya and Jones 

(2009). The MODFLOW-2005 River Package (Harbaugh, 2005) was used to represent the Pecos River on the northeastern side of the 

model area. The southwestern perimeter of the model area and the base of the Rustler aquifer are represented as no-flow boundaries. 

While faulting is recognized as typically occurring in fault zones, delineated fault zones were represented as horizontal flow barriers in 

the groundwater-flow model by using the Horizontal Flow Barrier Package of MODFLOW-2005 (Harbaugh, 2005), which simulates 

reduced conductance between individual pairs of cells.

Model Calibration Results

Simulated hydraulic heads were compared to 

observed hydraulic heads (2,860 groundwater-level altitude 

measurements made at 288 wells) in the Edwards-Trinity 

aquifer. Simulated hydraulic heads were generally in good 

agreement with observed hydraulic head values, with 1,684 (59 

percent) simulated values within 25 ft of the observed value. 

The average root mean square error value of hydraulic head 

for the Edwards-Trinity aquifer for all stress periods was 34.2 

ft, which was approximately 4 percent of the average total 

measured changes in groundwater-level altitudes (groundwater 

levels). Simulated spring flow representing Comanche Springs 

generally reproduces the measured spring flow. Results from 

the calibrated model are in agreement with the geochemical 

modeling analyses, which indicate that 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. Graphs depicting residuals for the Pecos County 

region model and the relation between simulated and measured 

hydraulic heads for the Pecos County region model are provided 

in figures 2 and 3, respectively.

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. The Pecos County region model 

provides a relatively good fit to measured groundwater levels 

in the Leon-Belding and Fort Stockton areas and to spring 

flow from Comanche Springs. The Pecos County region model 

provides a relatively good fit to measured groundwater levels 

in the Leon-Belding and Fort Stockton areas and to spring flow 

from Comanche Springs (fig. 4). One of the findings, however, 

was that simulated spring flow is highly contingent on the 

Figure 2. Residuals for the Pecos County region model, 

Texas (modified from Clark and others, 2014, fig. 14).

Figure 3. Relation between simulated and measured 

hydraulic heads for the Pecos County region model, Texas 

(modified from Clark and others, 2014, fig. 15).
PECOS

COUNTY 

BREWSTER

COUNTY 

PRESIDIO

COUNTY 

REEVES

COUNTY 

TERRELL COUNTY 

CROCKETT

 COUNTY 

JEFF DAVIS

COUNTY 

UPTON COUNTY 

WARD

COUNTY CRANE

COUNTY 

Pecos River

Leon-Belding area

Belding

Fort Stockton

£¤285

£¤285

¦§ ̈10

¦§ ̈20

¦§ ̈10

Glass Mountains

Davis

Mountains

Barilla

Mountains

3,100

3,000

2,900

3,100

3,000

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

0 10 20 MILES

0 10 20 KILOMETERS

Base modified from U.S. Geological Survey digital data, 1:2,000,000

Albers Equal-Area Conic projection, Texas State Mapping System

North American Datum of 1983

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 divide

Groundwater-flow model 

boundary

Conceptual model study 

area boundary

Fault zone—Represents 

numerous faults

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

31°00’

30°30'

Figure 4. Measured potentiometric surface (1980–2010) and simulated groundwater-level 

altitudes for the Edwards-Trinity aquifer in the model area of the Pecos County region, 

Texas, 2010 (modified from Clark and others, 2014, fig. 12).

transient nature of the underlying specified head boundary 

conditions in model layer 5 (Rustler layer), which indicates the 

importance of adequately understanding and characterizing the 

entire groundwater system.

Development of Groundwater-Pumping 

Scenarios

The model was used to simulate groundwater levels 

resulting from prolonged pumping to evaluate the sustainability 

of recent (2008) and projected water use. Each of three 

scenarios is a continuation of the 70-year calibration period 

and simulates a 30-year period from 2010 to 2040. 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 (fig. 5). The 30-year period 

is discretized into sixty 6-month stress periods generally 

representing seasonality of irrigation water demand. 

Pecos County region, Texas.
£¤285

£¤285

§ ̈¦10

¦§ ̈20

¦§ ̈10

GLASS

MOUNTAINS

M

BARILLA

OUNTAINS

DAVIS

MOUNTAINS

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

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

31 ̊00’

30 ̊30'

Base modified from U.S. Geological Survey, 1:2,000,000-scale digital data

Albers Equal Area Projection, Texas State Mapping System

North American Datum of 1983

0 5 10 15 20 MILES

0 5 10 15 20 KILOMETERS

EXPLANATION

Groundwater-flow model boundary

Conceptual model study area boundary

Fault zone—Represents numerous faults

 -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

Water-level change from 2010 to 2040,

in feet (2010 minus 2040)

Pecos River

Scenario 1

Scenario 1 extends recent (2008) irrigation and non-irrigation pumping rates for each year of the 30-year simulation period from 

2010 to 2040. Return flow recharge from irrigation is included in scenario 1. Scenario 1 provides a measure of the sustainability to the 

2008 water usage and also provides a baseline scenario for comparison against additional scenarios. Projected groundwater-level changes 

in and around the Fort Stockton area indicate little if any change from current conditions, indicating that the groundwater system is near 

equilibrium with respect to recent (2008) pumping stress. 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

Scenario 2 evaluates the effects of extended recent (2008) pumping rates as assigned in scenario 1, in addition to year-round 

maximum permitted groundwater-pumping rates (about 42 Mgal/d) in the Leon-Belding area for the 30-year simulation period. Return 

flow recharge from irrigation is not included in scenario 2. Results of scenario 2 are similar in water-level decline and extent from those 

of scenario 1. The extent of the projected groundwater-level decline in the range of 5–15 ft in the Leon-Belding irrigation area expanded 

slightly (about 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. 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 simulation period by using the same 6-month 

irrigation and non-irrigation stress periods as in scenarios 1 and 2; however, for this scenario, groundwater use is predicted to increase 

approximately 16 percent by 2040 because of the projected population growth for Pecos County (Texas Water Development Board, 2013). 

Based on this projected groundwater-use increase, simulated pumping for all wells was increased by 5 percent every 10 years to account 

for an approximate 15-percent increase by the end of the 30-year simulation period. Return flow recharge from irrigation is not included 

in scenario 3. Projected groundwater-level declines resulting from scenario 3 are depicted in figure 5 (results from all three scenarios 

are depicted in Clark and others, 2014). Results of scenario 3 are similar to those of scenario 2 in terms of the areas of groundwaterlevel decline. The maximum projected groundwater-level decline in the Leon-Belding area was greater than either scenario 1 or 2 at 

approximately 34.5 ft, and the extent of the decline is larger in area (about 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. 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 and the relatively small total increase in water use of about 15 percent over the 30-year period, which together produce small 

increases in pumping amounts. 

Figure 5. Simulated groundwater-level difference for the Edwards-Trinity aquifer in the model area of the Pecos County region, 

Texas, from 2010 to 2040 for groundwater-pumping scenario 3 (modified from Clark and others, 2014, fig. 20).
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

Revised July 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

This fact sheet is based 

on the following USGS 

report:

Clark, B.R., Bumgarner, J.R., 

Houston, N.A., and Foster, 

A.L., 2014, Simulation of 

groundwater flow in the 

Edwards-Trinity and related 

aquifers in the Pecos County 

region, Texas: U.S. Geological 

Survey Scientific Investigations 

Report 2013–5228, 56 p., http://

dx.doi.org/10.3133/sir20135228. 

(Revised July 2014).

References Cited

Anaya, Roberto, and Jones, Ian, 2009, Groundwater Availability 

Model for the Edwards-Trinity (Plateau) and Pecos Valley 

aquifers of Texas: Texas Water Development Board Report 373, 

103 p.

Barker, R.A., and Ardis, A.F., 1992, Configuration of the base of 

the Edwards-Trinity aquifer and hydrogeology of the underlying 

pre-Cretaceous rocks, west-central Texas: U.S. Geological 

Survey Water-Resources Investigations Report 91–4071, 25 p., 

1 pl.

Baumgardner, R. J., Jr., Hoadley, A. D., and Goldstein, A.G., 1982, 

Formation of the Wink Sink, a salt dissolution and collapse 

feature, Winkler County, Texas: The University of Texas at 

Austin, Bureau of Economic Geology Report of Investigations 

No. 114, 38 p.

Boghici, Radu, 1997, Hydrogeological investigations at Diamond 

Y Springs and surrounding area, Pecos County, Texas: Austin, 

University of Texas, master’s thesis, 132 p.

Bumgarner, J.R., Stanton, G.P., Teeple, A.P., Thomas, J.V., 

Houston, N.A., Payne, J.D., and Musgrove, MaryLynn, 

2012, A conceptual model of the hydrogeologic framework, 

geochemistry, and groundwater-flow system of the EdwardsTrinity and related aquifers in the Pecos County region, Texas: 

U.S. Geological Survey Scientific Investigations Report 

2012–5124, 74 p.

Long, A.T., 1958, Ground-water geology of Edwards County, 

Texas: Texas Water Commission Bulletin 6208, 128 p.

Meyer, J.E., Wise, M.R., and Kalaswad, Sanjeev, 2011, 

Pecos Valley aquifer, west Texas—Structure and brackish 

groundwater: Texas Water Development Board Brackish 

Resources Aquifer Characterization System (BRACS), 92 p.

ISSN 2327-6916 (print)

ISSN 2327-6932 (online)

http://dx.doi.org/10.3133/fs20143029

For additional information, contact

Director, Texas Water Science Center 

U.S. Geological Survey 

1505 Ferguson Lane 

Austin, Texas 78754–4501 

http://tx.usgs.gov/

Publishing support provided by

Lafayette and Rolla Publishing Service Centers

Pearson, D.K., Bumgarner, J.R., Houston, N.A., Stanton, G.P., 

Teeple, A.P., and Thomas, J.V., 2012, Data collection and 

compilation for a geodatabase of groundwater, surface-water, 

water-quality, geophysical, and geologic data, Pecos County 

region, Texas, 1930–2011: U.S. Geological Survey Data 

Series 678, 67 p.

Rees, Rhys, and Buckner, A.W., 1980, Occurrence and quality 

of ground water in the Edwards-Trinity (Plateau) aquifer in 

the Trans-Pecos Region of Texas: Texas Department of Water 

Resources Report 255, 41 p.

Reilly, T.E., and Harbaugh, A.W., 2004, Guidelines for evaluating 

ground-water flow models: U.S. Geological Survey Scientific 

Investigations Report 2004–5038, 30 p.

Small, T.A., and Ozuna, G.B., 1993, Ground-water conditions 

in Pecos County, Texas, 1987: U.S. Geological Survey WaterResources Investigations Report 92–4190, 68 p.

Texas Water Development Board, 2013, 2011 regional water 

plan: Accessed June 6, 2013, at http://www.twdb.state.

tx.us/waterplanning/data/projections/2012/doc/Population/

PopulationByRWPG/4PopulationF.pdf.

Pecos County region, Texas.

— Jonathan V. Thomas