In cooperation with the U.S. Department   of Defense and Edwards Aquifer Authority

**Conceptualization and Simulation   of the Edwards Aquifer, San Antonio Region, Texas**

By R.J. Lindgren1, A.R.Dutton2,   S.D. Hovorka3, S.R.H. Worthington4, and Scott Painter5

1  U.S. Geological Survey.  2  The University of Texas at San Antonio.  3  The University of Texas at Austin, Bureau of Economic Geology.  4  Worthington Groundwater, Dundas, Ontario, Canada.  5  Southwest Research Institute, San Antonio, Texas.

U.S. Geological Survey  Scientific Investigations Report 2004–5277

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report in pdf (7.64 MB)--(Revised May 2011)plates in pdf (14.5 MB)

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**Contents**

Abstract 

Introduction 

Purpose and Scope 

Physiography and Climate 

Previous Investigations and Research 

Acknowledgments 

Conceptualization of the Edwards Aquifer 

Geologic Setting 

Hydrogeologic Setting 

Hydrostratigraphy 

Structural Features 

Karst Hydrogeology 

Karst Development 

Karst Features 

Conduit Distribution and Characterization 

Hydraulic Properties 

Permeability 

Hydraulic Conductivity and Transmissivity 

Distributions Within Edwards Aquifer 

Distributions Developed For Edwards Aquifer Model 

Storativity 

Ground-Water-Flow System 

Regional Ground-Water Flow 

Recharge 

Discharge 

Withdrawals by Wells 

Springflow 

Water-Level Fluctuations and Storage 

Simulation of Ground-Water Flow 

Numerical Model Description 

Mathematical Representation 

Model Grid 

Model Boundaries 

Northern Model Boundary 

Eastern Model Boundary 

Western Model Boundary 

Southern Model Boundary 

Model Aquifer Structure 

Aquifer Top and Bottom Altitudes 

Simulation of Faults 

Simulation of Conduits 

Model Hydraulic Properties 

Horizontal Hydraulic Conductivity 

Storativity 

Recharge 

Discharge 

Withdrawals by Wells 

Springflow 

Numerical Model Calibration 

Steady-State Simulation 

Calibration Targets 

Changes From Initial Values 

Goodness of Fit 

Transient Simulation 

Calibration Targets 

Changes From Initial Values 

Goodness of Fit 

Numerical Model Results 

Ground-Water Flow 

Water Budget 

Steady-State Simulation 

Transient Simulation 

Numerical Model Sensitivity 

Hydraulic Heads 

Springflow 

Comparison With GWSIM Model 

Model Limitations 

Assumptions For Conceptual and Numerical Models 

Limitations of Input Data 

Scale of Application 

Needed Data and Future Work 

Summary 

References Cited 

**Plates**

| 1–7.   | Maps showing: |
|-------------------|---------------|
|              | 1.       | Locations of Edwards aquifer wells with water levels (hydraulic       heads) used to calibrate Edwards aquifer model (calibration target wells),       San Antonio region, Texas |
|              | 2.       | Measured water-level altitude in Edwards aquifer, 1939–46, and simulated       altitude of potentiometric surface, steady-state simulation, Edwards aquifer       model, San Antonio region, Texas |
|              | 3.       | Measured water-level altitude in Edwards aquifer and simulated altitude       of potentiometric surface for drought conditions, August 1956 (stress period       117), transient simulation, Edwards aquifer model, San Antonio region, Texas |
|              | 4.       | Measured water-level altitude in Edwards aquifer and simulated altitude       of potentiometric surface for above-normal rainfall conditions, February       1975 (stress period 339), transient simulation, Edwards aquifer model, San       Antonio region, Texas |
|              | 5.       | Simulated flow directions for steady-state simulation (1939–46),       Edwards aquifer model, San Antonio region, Texas |
|              | 6.       | Simulated flow directions for drought conditions, August 1956 (stress       period 117), transient simulation, Edwards aquifer model, San Antonio region,       Texas |
|              | 7.       | Simulated flow directions for above-normal rainfall conditions, February       1975 (stress period 339), transient simulation, Edwards aquifer model, San       Antonio region, Texas |

**Figures**

| 1.    | Map showing location of ground-water-flow model area, Edwards aquifer       segments, and physiographic regions, San Antonio region, Texas                                                                                                                                                                                                                                                                                     |
|------------|---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| 2.    | Map showing hydrogeologic zones and catchment area (upper parts of stream       basins that contribute recharge) of the Edwards aquifer, San Antonio region,       Texas                                                                                                                                                                                                                                                      |
| 3.    | Map showing paleogeographic and structural features in Texas associated       with the Edwards aquifer model area                                                                                                                                                                                                                                                                                                              |
| 4.    | Chart showing correlation of Cretaceous stratigraphic units and hydrogeologic       units, and relative permeabilities, in the Edwards aquifer model area, San       Antonio region, Texas                                                                                                                                                                                                                                    |
| 5.    | Map showing altitude of the top of the Edwards aquifer (in the subcrop)       and fault locations, San Antonio region, Texas                                                                                                                                                                                                                                                                                                   |
| 6.    | Map showing thickness of the Edwards aquifer, San Antonio region, Texas                                                                                                                                                                                                                                                                                                                                                         |
| 7.    | Map showing inferred locations of conduits in the Edwards aquifer, San       Antonio region, Texas                                                                                                                                                                                                                                                                                                                             |
| 8.    | Map showing distribution of horizontal hydraulic conductivity for the       Edwards aquifer, San Antonio region, Texas                                                                                                                                                                                                                                                                                                         |
| 9.    | Diagrammatic north-northwest-to-south-southeast section showing hydrogeologic       framework and generalized ground-water-flow directions, Edwards Plateau       to Gulf Coastal Plain, San Antonio region, Texas                                                                                                                                                                                                            |
| 10.   | Map showing potentiometric surface and inferred regional ground-water-flow       pattern in the Edwards aquifer, October 27–November 2, 2001, San Antonio       region, Texas                                                                                                                                                                                                                                           |
| 11.   | Graphs showing estimated annual recharge (from seepage losses from streams       and infiltration of rainfall) to the San Antonio segment of the Edwards       aquifer, San Antonio region, Texas, by recharge basin or contributing drainage       area, 1934–2003                                                                                                                                                    |
| 12.   | Graphs showing (a) annual ground-water discharge, springflow, and withdrawals,       (excluding Travis County); (b) annual withdrawals by water-use category       (excluding Travis County); and (c) annual withdrawals by county, in the       active model area of the Edwards aquifer, San Antonio region, Texas, 1939–2000                                                                                        |
| 13.   | Maps showing distributions of annual (a) water use by category and (b)       total withdrawals for the Edwards aquifer, San Antonio region, Texas, 1996                                                                                                                                                                                                                                                                        |
| 14.   | Hydrograph showing annual measured discharge for major springs in the       San Antonio segment of the Edwards aquifer, San Antonio region, Texas, 1939–2000                                                                                                                                                                                                                                                             |
| 15.   | Hydrographs showing measured and simulated water levels (hydraulic heads)       for Edwards aquifer wells in (a) Uvalde County and (b) Medina County, San       Antonio region, Texas                                                                                                                                                                                                                                         |
| 16.   | Hydrographs showing measured and simulated water levels (hydraulic heads)       for Edwards aquifer wells in Bexar County, San Antonio region, Texas                                                                                                                                                                                                                                                                           |
| 17.   | Hydrographs showing measured and simulated water levels (hydraulic heads)       for Edwards aquifer wells in (a) Comal County and (b) Hays County, San Antonio       region, Texas                                                                                                                                                                                                                                            |
| 18.   | Map showing boundary conditions and finite-difference grid for the Edwards       aquifer model, San Antonio region, Texas                                                                                                                                                                                                                                                                                                      |
| 19.   | Map showing thickness of underlying Trinity aquifer simulated in the Edwards       aquifer recharge zone, Edwards aquifer model, San Antonio region, Texas                                                                                                                                                                                                                                                                     |
| 20.   | Map showing simulated subzones of the recharge zone of the Edwards aquifer,       San Antonio region, Texas                                                                                                                                                                                                                                                                                                                    |
| 21.   | Map showing distribution of withdrawals during 1939–46 (steady-state       simulation) and 2000, and additional withdrawals for 2000, Edwards aquifer       model, San Antonio region, Texas                                                                                                                                                                                                                            |
| 22.   | Graph showing variation in monthly percentages of total annual withdrawals       for municipal and irrigation withdrawals simulated in the Edwards aquifer       model, San Antonio region, Texas                                                                                                                                                                                                                             |
| 23.   | Graph showing simulated relative to measured hydraulic heads, steady-state       simulation, Edwards aquifer model, San Antonio region, Texas                                                                                                                                                                                                                                                                                  |
| 24.   | Map showing simulated storativity zones for calibrated Edwards aquifer       model, San Antonio region, Texas                                                                                                                                                                                                                                                                                                                  |
| 25.   | Map showing simulated distribution of horizontal hydraulic conductivity       for calibrated Edwards aquifer model, San Antonio region, Texas                                                                                                                                                                                                                                                                                  |
| 26.   | Map showing simulated distribution of transmissivity for calibrated Edwards       aquifer model, San Antonio region, Texas                                                                                                                                                                                                                                                                                                     |
| 27.   | Graphs showing simulated relative to measured hydraulic heads for (a)       August 1956 (stress period 117) and (b) February 1975 (stress period 339),       transient simulation, Edwards aquifer model, San Antonio region, Texas                                                                                                                                                                                           |
| 28.   | Hydrographs showing measured and simulated springflows for (a) Comal Springs       and (b) San Marcos Springs, 1947–2000, Edwards aquifer model, San       Antonio region, Texas                                                                                                                                                                                                                                        |
| 29.   | Hydrographs showing measured and simulated springflows for (a) Leona Springs,       (b) San Antonio Springs, and (c) San Pedro Springs, 1947–2000, Edwards       aquifer model, San Antonio region, Texas                                                                                                                                                                                                               |
| 30.   | Schematic diagram showing simulated water-budget components for (a) steady-state       simulation and (b) transient simulation, Edwards aquifer model, San Antonio       region, Texas                                                                                                                                                                                                                                        |
| 31.   | Graphs showing simulated water-budget components, as percentages of total       sources to and discharges from the Edwards aquifer for (a) drought conditions,       1956, and (b) above-normal rainfall and recharge conditions, 1975, transient       simulation, Edwards aquifer model, San Antonio region, Texas                                                                                                         |
| 32.   | Graphs showing sensitivity of simulated hydraulic heads to changes in       model parameters at calibration wells for (a) steady-state simulation, (b)       drought conditions, August 1956 (stress period 117), transient simulation,       and (c) above-normal rainfall and recharge conditions, February 1975 (stress       period 339), transient simulation, Edwards aquifer model, San Antonio region,       Texas |
| 33.   | Hydrographs showing sensitivity of simulated hydraulic heads in (a) Bexar       County index well (J–17, 6837203) and (b) Uvalde County index well       (J–27, 6950302) to changes in location of the southern model boundary       and spring-orifice altitude, Edwards aquifer model, San Antonio region,       Texas                                                                                        |
| 34.   | Graphs showing sensitivity of simulated springflow to changes in model       parameters at simulated springs for (a) steady-state simulation, (b) drought       conditions, August 1956 (stress period 117), transient simulation, and (c)       above-normal rainfall and recharge conditions, February 1975 (stress period       339), transient simulation, Edwards aquifer model, San Antonio region, Texas             |
| 35.   | Graph showing sensitivity of simulated springflow to changes in hydraulic       conductivity at simulated springs for steady-state simulation, Edwards aquifer       model, San Antonio region, Texas                                                                                                                                                                                                                         |
| 36.   | Hydrographs showing sensitivity of simulated springflow from (a) Comal       and (b) Leona Springs to changes in location of the southern model boundary       and spring-orifice altitude, Edwards aquifer model, San Antonio region,       Texas                                                                                                                                                                           |
| 37.   | Hydrographs showing measured hydraulic heads and simulated hydraulic heads       by the Edwards aquifer model, San Antonio region, Texas, and the GWSIM model       for (a) BexarCounty index well (J–17, 6837203) and (b) Uvalde County       index well (J–27, 6950302), 1978–89                                                                                                                         |
| 38.   | Hydrographs showing measured springflows and simulated springflows by       the Edwards aquifer model, San Antonio region, Texas, and the GWSIM model       for Comal Springs for (a) 1947–59 and (b) 1978–89                                                                                                                                                                                                     |
| 39.   | Hydrographs showing measured springflows and simulated springflows by       the Edwards aquifer model, San Antonio region, Texas, and the GWSIM model       for San Marcos Springs for (a) 1947–59 and (b) 1978–89                                                                                                                                                                                           |
| 40.   | Hydrographs showing measured hydraulic heads and simulated hydraulic heads       by the Edwards aquifer model, San Antonio region, Texas, and the GWSIM model       for Hays County wells (a) 5857902, (b) 5858101, and (c) Uvalde County       well 6937402, 1978–89                                                                                                                                             |

**Tables**

| 1.    | Ground-Water-Model Advisory Panel (GWMAP) members and staff                                                                                                                                                                                                                                                                                                                                          |
|------------|------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------|
| 2.    | Fault displacement and simulated hydraulic characteristic for faults,       Edwards aquifer model, San Antonio region, Texas                                                                                                                                                                                                                                                                        |
| 3.    | Estimated recharge rates, by subzone of the recharge zone, Edwards aquifer       model, SanAntonio region, Texas, 1939–2000                                                                                                                                                                                                                                                                   |
| 4.    | Ground-water withdrawals, by county, Edwards aquifer model, San Antonio       region, Texas1939–2000                                                                                                                                                                                                                                                                                          |
| 5.    | Steady-state and transient simulation target wells, Edwards aquifer model,       San Antonio region, Texas                                                                                                                                                                                                                                                                                          |
| 6.    | Summary of changes from initial values for steady-state calibration, Edwards       aquifer model, San Antonio region, Texas                                                                                                                                                                                                                                                                         |
| 7.    | Estimated and simulated recharge rates, by subzone of the recharge zone,       steady-state calibration, Edwards aquifer model, San Antonio region, Texas                                                                                                                                                                                                                                           |
| 8.    | Steady-state calibration target wells and residuals, Edwards aquifer model,       San Antonio region, Texas                                                                                                                                                                                                                                                                                         |
| 9.    | Measured and simulated springflows for steady-state calibration, Edwards       aquifer model, San Antonio region, Texas                                                                                                                                                                                                                                                                             |
| 10.   | Estimated and simulated recharge rates and multiplication factors used       for years with greatly above-normal rainfall and recharge, Edwards aquifer       model, San Antonio region, Texas                                                                                                                                                                                                     |
| 11.   | Initial and final calibrated drain parameters for springs, Edwards aquifer       model, San Antonio region, Texas                                                                                                                                                                                                                                                                                   |
| 12.   | Summary of changes in parameter values (1) from initial calibrated steady-state       simulation, (2) from USGS-estimated recharge rates, and (3) from the       initial uniform storativity distribution for the transient simulation; and       changes from the calibrated transient simulation for transient simulation       testing, Edwards aquifer model, San Antonio region, Texas |
| 13.   | Transient simulation target wells and residuals for drought conditions,       Edwards aquifer model, San Antonio region, Texas                                                                                                                                                                                                                                                                      |
| 14.   | Transient simulation target wells and residuals for above-normal rainfall       and recharge conditions, Edwards aquifer model, San Antonio region, Texas                                                                                                                                                                                                                                           |
| 15.   | Simulated water budget for the steady-state simulation and for the transient       simulation by monthly stress period for 1956 and 1975, Edwards aquifer model,       San Antonio region, Texas                                                                                                                                                                                                   |
| 16.   | Comparison of the residuals for hydraulic heads and springflows for selected       observation wells and springs for the Edwards aquifer model and the GWSIM       model, San Antonio region, Texas                                                                                                                                                                                                |

**Vertical Datum**

Vertical coordinate information is referenced to the National Geodetic Vertical   Datum of 1929 (NGVD 29). 

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**Abstract**

A new numerical ground-water-flow model (Edwards aquifer model) that incorporates   important components of the latest information and plausible conceptualization   of the Edwards aquifer was developed. The model includes both the San Antonio   and Barton Springs segments of the Edwards aquifer in the San Antonio region,   Texas, and was calibrated for steady-state (1939–46) and transient (1947–2000)   conditions, excluding Travis County. Transient simulations were conducted using   monthly recharge and pumpage (withdrawal) data. The model incorporates conduits   simulated as continuously connected (other than being separated in eastern Uvalde   and southwestern Medina Counties), one-cell-wide (1,320 feet) zones with very   large hydraulic-conductivity values (as much as 300,000 feet per day). The locations   of the conduits were based on a number of factors, including major potentiometric-surface   troughs in the aquifer, the presence of sinking streams, geochemical information,   and geologic structures (for example, faults and grabens).

The simulated directions of flow in the Edwards aquifer model are most strongly   influenced by the presence of simulated conduits and barrier faults. The simulated   flow in the Edwards aquifer is influenced by the locations of the simulated   conduits, which tend to facilitate flow. The simulated subregional flow directions   generally are toward the nearest conduit and subsequently along the conduits   from the recharge zone into the confined zone and toward the major springs.   Structures simulated in the Edwards aquifer model influencing ground-water flow   that tend to restrict flow are barrier faults. The influence of simulated barrier   faults on flow directions is most evident in northern Medina County. 

A water budget is an accounting of inflow to, outflow from, and storage change   in the aquifer. For the Edwards aquifer model steady-state simulation, recharge   (from seepage losses from streams and infiltration of rainfall) accounts for   93.5 percent of the sources of water to the Edwards aquifer, and inflow through   the northern and northwestern model boundaries contributes 6.5 percent. The   largest discharges are springflow (73.7 percent) and ground-water withdrawals   by wells (25.7 percent). 

The principal source of water to the Edwards aquifer for the Edwards aquifer   model transient simulation was recharge, constituting about 60 percent of the   sources of water (excluding change in storage) to the Edwards aquifer during   1956, a drought period, and about 97 percent of the sources (excluding change   in storage) during 1975, a period of above-normal rainfall and recharge. The   principal discharges from the Edwards aquifer for the transient simulation were   springflow and withdrawals by wells. During 1956, representing drought conditions,   the change in storage (net water released from storage) was much greater than   recharge, accounting for 75.9 percent of the total flow compared to 14.5 percent   for recharge. Conversely, during 1975, representing above-normal rainfall and   recharge conditions, recharge constituted 79.9 percent of the total flow, compared   to 7.1 percent for the change in storage (net water added to storage). 

A series of sensitivity tests was made to ascertain how the model results were   affected by variations greater than and less than the calibrated values of input   data. Simulated hydraulic heads in the Edwards aquifer model were most sensitive   to recharge, withdrawals, hydraulic conductivity of the conduit segments, and   specific yield and were comparatively insensitive to spring-orifice conductance,   northern boundary inflow, and specific storage. Simulated springflow in the   Edwards aquifer model was most sensitive to recharge, withdrawals, hydraulic   conductivity of the conduit segments, specific yield, and increases in northern   boundary inflow and was comparatively insensitive to spring-orifice conductance   and specific storage.

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