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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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