In cooperation with the U.S. Department of Defense and Edwards Aquifer Authority Conceptualization and Simulation of the Edwards Aquifer, San Antonio Region, Texas Scientific Investigations Report 2004–5277 U.S. Department of the Interior U.S. Geological Survey Conceptualization and Simulation of the Edwards Aquifer, San Antonio Region, Texas By R.J. Lindgren, A.R. Dutton, S.D. Hovorka, S.R.H. Worthington, and Scott Painter In cooperation with the U.S. Department of Defense and Edwards Aquifer Authority Scientific Investigations Report 2004–5277 U.S. Department of the Interior U.S. Geological Survey U.S. Department of the Interior Gale A. Norton, Secretary U.S. Geological Survey Charles G. Groat, Director U.S. Geological Survey, Reston, Virginia: 2004 For sale by U.S. Geological Survey, Information Services Box 25286, Denver Federal Center Denver, CO 80225 For more information about the USGS and its products: Telephone: 1-888-ASK-USGS World Wide Web: http://www.usgs.gov/ Any use of trade, product, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government. Although this report is in the public domain, permission must be secured from the individual copyright owners to reproduce any copyrighted materials contained within this report. Suggested citation: Lindgren, R.J., Dutton, A.R., Hovorka, S.D., Worthington, S.R.H., and Painter, Scott, 2004, Conceptualization and simulation of the Edwards aquifer, San Antonio region, Texas: U.S. Geological Survey Scientific Investigations Report 2004–5277, 143 p. iii Contents Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 Purpose and Scope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Physiography and Climate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Previous Investigations and Research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Conceptualization of the Edwards Aquifer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Geologic Setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Hydrogeologic Setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Hydrostratigraphy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10 Structural Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 Karst Hydrogeology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Karst Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Karst Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 Conduit Distribution and Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Hydraulic Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Permeability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Hydraulic Conductivity and Transmissivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Distributions Within Edwards Aquifer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Distributions Developed For Edwards Aquifer Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 Storativity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 Ground-Water-Flow System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 Regional Ground-Water Flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27 Recharge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 Discharge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 Withdrawals by Wells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 Springflow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 Water-Level Fluctuations and Storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 Simulation of Ground-Water Flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 Numerical Model Description . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 Mathematical Representation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47 Model Grid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 Model Boundaries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 Northern Model Boundary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48 Eastern Model Boundary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 Western Model Boundary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 Southern Model Boundary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 Model Aquifer Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 Aquifer Top and Bottom Altitudes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 Simulation of Faults . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51 Simulation of Conduits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 iv Model Hydraulic Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 Horizontal Hydraulic Conductivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 Storativity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 Recharge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54 Discharge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56 Withdrawals by Wells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56 Springflow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 Numerical Model Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60 Steady-State Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 Calibration Targets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 Changes From Initial Values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 Goodness of Fit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62 Transient Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 Calibration Targets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 Changes From Initial Values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64 Goodness of Fit . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69 Numerical Model Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 Ground-Water Flow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73 Water Budget . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74 Steady-State Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74 Transient Simulation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74 Numerical Model Sensitivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77 Hydraulic Heads . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77 Springflow . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81 Comparison With GWSIM Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 Model Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92 Assumptions For Conceptual and Numerical Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 Limitations of Input Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94 Scale of Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 Needed Data and Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 96 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 References Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101 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 v 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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Map showing hydrogeologic zones and catchment area (upper parts of stream basins that contribute recharge) of the Edwards aquifer, San Antonio region, Texas . . . . . . . . . . . . . . . . . . . . . . 5 3. Map showing paleogeographic and structural features in Texas associated with the Edwards aquifer model area . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 4. Chart showing correlation of Cretaceous stratigraphic units and hydrogeologic units, and relative permeabilities, in the Edwards aquifer model area, San Antonio region, Texas . . . . . . . . . . . . . 9 5. Map showing altitude of the top of the Edwards aquifer (in the subcrop) and fault locations, San Antonio region, Texas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 6. Map showing thickness of the Edwards aquifer, San Antonio region, Texas . . . . . . . . . . . . . . . . . . . . . . 14 7. Map showing inferred locations of conduits in the Edwards aquifer, San Antonio region, Texas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 8. Map showing distribution of horizontal hydraulic conductivity for the Edwards aquifer, San Antonio region, Texas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26 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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 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 . . . . . . . . . . . . . . . . . 29 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 . . . . . . . . . . . . . . . . . . . . . . . 33 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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 13. Maps showing distributions of annual (a) water use by category and (b) total withdrawals for the Edwards aquifer, San Antonio region, Texas, 1996 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37 14. Hydrograph showing annual measured discharge for major springs in the San Antonio segment of the Edwards aquifer, San Antonio region, Texas, 1939–2000 . . . . . . . . . . . . . . . . . . . . . . . . . . 39 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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42 16. Hydrographs showing measured and simulated water levels (hydraulic heads) for Edwards aquifer wells in Bexar County, San Antonio region, Texas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44 vi 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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 18. Map showing boundary conditions and finite-difference grid for the Edwards aquifer model, San Antonio region, Texas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 19. Map showing thickness of underlying Trinity aquifer simulated in the Edwards aquifer recharge zone, Edwards aquifer model, San Antonio region, Texas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 20. Map showing simulated subzones of the recharge zone of the Edwards aquifer, San Antonio region, Texas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 23. Graph showing simulated relative to measured hydraulic heads, steady-state simulation, Edwards aquifer model, San Antonio region, Texas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 24. Map showing simulated storativity zones for calibrated Edwards aquifer model, San Antonio region, Texas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 25. Map showing simulated distribution of horizontal hydraulic conductivity for calibrated Edwards aquifer model, San Antonio region, Texas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67 26. Map showing simulated distribution of transmissivity for calibrated Edwards aquifer model, San Antonio region, Texas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68 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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70 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 . . . . . . . . . 71 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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72 30. Schematic diagram showing simulated water-budget components for (a) steady-state simulation and (b) transient simulation, Edwards aquifer model, San Antonio region, Texas . . . . . . . 75 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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76 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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78 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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82 vii 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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83 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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86 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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 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) Bexar County index well (J–17, 6837203) and (b) Uvalde County index well (J–27, 6950302), 1978–89 . . . . . . 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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90 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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91 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 . . . . . . . . . . . . . . . 92 Tables 1. Ground-Water-Model Advisory Panel (GWMAP) members and staff . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Fault displacement and simulated hydraulic characteristic for faults, Edwards aquifer model, San Antonio region, Texas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 3. Estimated recharge rates, by subzone of the recharge zone, Edwards aquifer model, San Antonio region, Texas, 1939–2000 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109 4. Ground-water withdrawals, by county, Edwards aquifer model, San Antonio region, Texas 1939–2000 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121 5. Steady-state and transient simulation target wells, Edwards aquifer model, San Antonio region, Texas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 122 6. Summary of changes from initial values for steady-state calibration, Edwards aquifer model, San Antonio region, Texas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126 7. Estimated and simulated recharge rates, by subzone of the recharge zone, steady-state calibration, Edwards aquifer model, San Antonio region, Texas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126 8. Steady-state calibration target wells and residuals, Edwards aquifer model, San Antonio region, Texas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127 9. Measured and simulated springflows for steady-state calibration, Edwards aquifer model, San Antonio region, Texas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130 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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130 11. Initial and final calibrated drain parameters for springs, Edwards aquifer model, San Antonio region, Texas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131 viii 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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131 13. Transient simulation target wells and residuals for drought conditions, Edwards aquifer model, San Antonio region, Texas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132 14. Transient simulation target wells and residuals for above-normal rainfall and recharge conditions, Edwards aquifer model, San Antonio region, Texas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 136 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 . . . . . 140 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 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142 Vertical Datum Vertical coordinate information is referenced to the National Geodetic Vertical Datum of 1929 (NGVD 29). 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 Abstract 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 A new numerical ground-water-flow model (Edwards losses from streams and infiltration of rainfall) accounts for aquifer model) that incorporates important components of the 93.5 percent of the sources of water to the Edwards aquifer, latest information and plausible conceptualization of the and inflow through the northern and northwestern model Edwards aquifer was developed. The model includes both the boundaries contributes 6.5 percent. The largest discharges are San Antonio and Barton Springs segments of the Edwards aqui- fer in the San Antonio region, Texas, and was calibrated for springflow (73.7 percent) and ground-water withdrawals by steady-state (1939–46) and transient (1947–2000) conditions, wells (25.7 percent). excluding Travis County. Transient simulations were con- The principal source of water to the Edwards aquifer for ducted using monthly recharge and pumpage (withdrawal) data. the Edwards aquifer model transient simulation was recharge, The model incorporates conduits simulated as continuously constituting about 60 percent of the sources of water (excluding connected (other than being separated in eastern Uvalde and change in storage) to the Edwards aquifer during 1956, a southwestern Medina Counties), one-cell-wide (1,320 feet) drought period, and about 97 percent of the sources (excluding zones with very large hydraulic-conductivity values (as much change in storage) during 1975, a period of above-normal as 300,000 feet per day). The locations of the conduits were rainfall and recharge. The principal discharges from the based on a number of factors, including major potentiometric- Edwards aquifer for the transient simulation were springflow surface troughs in the aquifer, the presence of sinking streams, and withdrawals by wells. During 1956, representing drought geochemical information, and geologic structures (for example, conditions, the change in storage (net water released from stor- faults and grabens). age) was much greater than recharge, accounting for 75.9 per- The simulated directions of flow in the Edwards aquifer cent of the total flow compared to 14.5 percent for recharge. model are most strongly influenced by the presence of simu- Conversely, during 1975, representing above-normal rainfall lated conduits and barrier faults. The simulated flow in the and recharge conditions, recharge constituted 79.9 percent of Edwards aquifer is influenced by the locations of the simulated the total flow, compared to 7.1 percent for the change in storage conduits, which tend to facilitate flow. The simulated subre- gional flow directions generally are toward the nearest conduit (net water added to storage). and subsequently along the conduits from the recharge zone A series of sensitivity tests was made to ascertain how the into the confined zone and toward the major springs. Structures model results were affected by variations greater than and less simulated in the Edwards aquifer model influencing ground- than the calibrated values of input data. Simulated hydraulic water flow that tend to restrict flow are barrier faults. The influ- heads in the Edwards aquifer model were most sensitive to ence of simulated barrier faults on flow directions is most evi- recharge, withdrawals, hydraulic conductivity of the conduit dent in northern Medina County. segments, and specific yield and were comparatively insensi- 1 U.S. Geological Survey. tive to spring-orifice conductance, northern boundary inflow, 2 The University of Texas at San Antonio. and specific storage. Simulated springflow in the Edwards aqui- fer model was most sensitive to recharge, withdrawals, hydrau- 3 The University of Texas at Austin, Bureau of Economic Geology. lic conductivity of the conduit segments, specific yield, and 4 Worthington Groundwater, Dundas, Ontario, Canada. increases in northern boundary inflow and was comparatively 5 Southwest Research Institute, San Antonio, Texas. insensitive to spring-orifice conductance and specific storage.