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)
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).
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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