Groundwater Availability in the Carrizo-Wilcox Aquifer in Central Texas: Numerical Simulations

RI0256. Groundwater Availability in the Carrizo-Wilcox Aquifer in Central Texas: Numerical Simulations of 2000 through 2050 Withdrawal Projections, by A. R. Dutton. 53 p., 23 figs., 5 tables, 2 appendices, 1999.  Print.

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ABSTRACT
Between 1951 and 1996, groundwater pumpage from the Carrizo-Wilcox aquifer, one of Texas' major aquifer systems, increased in the area between the Colorado and Brazos Rivers from approximately 10,600 to 37,900 acre-ft/yr, primarily as a result of mining needs. Continued (and possibly greatly accelerated) growth in groundwater demand for a variety of uses is expected through the year 2050. To assess the general availability of groundwater in the Carrizo- Wilcox aquifer between the Colorado and Brazos Rivers, five groundwater-development scenarios were simulated according to a finite-difference numerical model developed for this study. Simulated water-level change was related to the amount of groundwater withdrawal, its concentration in an area, hydrogeologic properties, and model characteristics. Actual locations and future rates of pumping of water wells, needless to say, might differ from what were simulated. Model calibration by means of historical water-level data had a mean absolute error of 32 ft.
 

On the basis of the calibrated model, groundwater in the Carrizo-Wilcox aquifer in the study area is predicted to remain available to meet specified withdrawal scenarios through the year 2050 and additional demands after 2050. Except for near the centers of simulated pumping areas, the aquifer units are forecast to remain fully saturated, and simulated water-level decline reflects mainly a change in artesian or pressure head. Simulated rate of decline of hydraulic head, however, is constant through the year 2050, and continued drawdown should be expected as long as pumping remains well above historical rates. Availability of groundwater is also determined by pumping lift, drilling depth, transportation to point of use, and property access, as well as other criteria.
 

Keywords: aquifer, groundwater, hydrogeologic properties, numerical model

CONTENTS

Abstract

Introduction

     Purpose and Objectives

Conceptual Hydrogeologic Model

     Data Availability

     Hydrostratigraphy

     Flow Paths and Flow Rates

     Recharge and Discharge

Model Design and Approach

     Model Architecture

     Aquifer Geometry

     Aquifer Parameters

     Boundary Conditions

     Modeling Sequence and Calibration

     Pumping Rates

     Future Groundwater-Withdrawal Scenarios

Results

     Baseline Historical Simulation (1951 through 1999)

     Projected Groundwater-Withdrawal Scenarios (2000 through 2050)

Recommendations

Summary and Conclusions

Acknowledgments

References

Appendix A

     Observation wells used for model calibration

Appendix B

     Maps showing location of pumping areas used in scenarios, as well as projected 1996 through 2050 drawdown in the Simsboro for scenarios 3 and 4, the difference between scenarios 3 and 2 in simulated hydraulic head for the Simsboro in 2050, and projected 1996 through 2050 drawdown in the Carrizo for scenario 4

Figures
1. Study area of the Carrizo-Wilcox aquifer in Central Texas

2. Conceptual hydrogeologic model used for constructing the numerical model of groundwater flow

3. Model grid consisting of 74 columns and 42 rows

4. Histograms of hydraulic conductivity measured in well tests in the different formations

5. Histograms of hydraulic conductivity estimated from geophysical well logs in the different formations

6. Hydraulic-conductivity distribution used to represent the Simsboro Formation in cells of the model

7. Hydraulic-conductivity distribution used to represent the Carrizo Formation in cells of the model

8. Change in total pumping rate from the Carrizo-Wilcox aquifer in Bastrop, Burleson, Fayette, Lee, Milam, and Robertson Counties in study area between 1980 and 1996

9. Location of wells to which pumping rates were specified in historical and baseline future stress periods

10. Simulated potentiometric surface representing groundwater in the Simsboro Formation for the year 1996

11. Southeast-northwest vertical hydrologic dip section A-A' along column 28 and southwest-northeast vertical hydrologic strike section B-B' along row 31 of the model

12. Simulated historical drawdown of hydraulic head in the Simsboro Formation

13. Comparison of measured and simulated hydraulic head

14. Projected 1996 through 2050 drawdown in the Simsboro for scenario 1, given pumping rates of the TWDB State Water Plan

15. Projected 1996 through 2050 drawdown in the Simsboro for scenario 2, superposing drawdown associated with pumping areas B and C on that for scenario 1

16. Difference between scenarios 2 and 1 in simulated hydraulic head for the Simsboro Formation in the year 2050, showing incremental effects of pumping areas B and C

17. Projected 1996 through 2050 drawdown in the Simsboro for scenario 5, including cumulative effects of all pumping areas superposed on the pumping rates of the TWDB State Water Plan

18. Projected 1996 through 2050 drawdown in the Carrizo for scenario 5, including cumulative effects of all pumping areas superposed on the pumping rates of the TWDB State Water Plan

19. Difference between scenarios 5 and 1 in simulated hydraulic head for the Simsboro Formation in the year 2050, showing the combined incremental effects of all pumping areas

20. Difference between scenarios 5 and 1 in simulated hydraulic head for the Carrizo Formation in the year 2050, showing the combined incremental effects of simulated withdrawal of Carrizo water mainly from area D but also from area E

21. Incremental drawdown of hydraulic head in the Simsboro, associated with pumping areas A and F in the year 2050

22. Difference between scenarios 5 and 4 in simulated hydraulic head for the Simsboro Formation in the year 2050, showing incremental effects of pumping area E

23. Difference between scenarios 5 and 3 in simulated hydraulic head for the Simsboro Formation in the year 2050, showing incremental effects of pumping area D

Appendix B Figures

B1. Location of pumping areas used in scenarios modeled in this study

B2. Projected 1996 through 2050 drawdown in the Simsboro for scenario 3, including cumulative effects of pumping areas A, B, C, E, and F superposed on the pumping rates of the TWDB State Water Plan

B3. Difference between scenarios 3 and 2 in simulated hydraulic head for the Simsboro Formation in the year 2050, showing the combined incremental effects of pumping areas A, E, and F

B4. Projected 1996 through 2050 drawdown in the Simsboro for scenario 4, including cumulative effects of pumping areas A, B, C, D, and F superposed on the pumping rates of the TWDB State Water Plan

B5. Projected 1996 through 2050 drawdown in the Carrizo for scenario 4, including cumulative effects of pumping areas A, B, C, D, and F superposed on the pumping rates of the TWDB State Water Plan

Tables

1. Groundwater-development scenarios modeled in this study

2. Horizontal and vertical values of hydraulic conductivity assigned to property zones of Carrizo Wilcox model by means of Visual MODFLOW

3. Projected rates of groundwater withdrawal by pumping areas shown in figures 1 and B1

4. Comparison of water-level differences between scenarios

5. Summary of budget for groundwater in Carrizo and Simsboro Formations




Citation
Dutton, A. R., 1999, Groundwater Availability in the Carrizo-Wilcox Aquifer in Central Texas: Numerical Simulations of 2000 through 2050 Withdrawal Projections: The University of Texas at Austin, Bureau of Economic Geology, Report of Investigations No. 256, 53 p.