1. Texas Groundwater 2004 Towards Sustainability
The Queen City and Sparta GAMs Strengths & Limitations for Groundwater Management
Texas Groundwater 2004
Towards Sustainability
Van Kelley, Neil Deeds & Dennis Fryar INTERA Inc. November 19, 2004

2. Acknowledgements TWDB Bureau of Economic Geology RJ Brandes Company
S. Wade, C. Ridgeway, R. Mace
Bureau of Economic Geology
J.P. Nicot and A. Dutton
RJ Brandes Company
J. Machin and G. Bruehl

3. Outline of Presentation
Introduction to Queen City & Sparta GAMs
Improvements to Carrizo-Wilcox GAMs
GAMs and Groundwater Management
GAM Strengths
Illustrative Example
Limitations of Applicability
Conclusions

4. GAM Objectives
Develop realistic and scientifically accurate GW flow models representing the physical characteristics of the aquifer and incorporating the relevant processes
GAMs are designed to be tools to help GWCDs, RWPGs, and individuals assess groundwater availability through 2050 based upon current data

5. Model Stratigraphy 5 6 7 8

6. Queen City and Sparta aquifers
1997 Use
QC – 14,000 AFY
Sparta – 6,800 AFY
Availability
QC – 680,000 AFY
SP – 160,000 AFY

7. Model Domains – Same as C/W GAMs
Northern
Model Area
Gonzales
Nixon
20,000 acres represents
Approximately 5 grid blocks
Southern
Model Area
Central
Model Area
Grid square mile each
Same Grid as Carrizo-Wilcox GAMs

8. Queen City and Sparta GAM Status
Queen City and Sparta GAMs have been completed and submitted to TWDB for approval:
Calibrated to specifications:
pre-development
transient conditions ( )
verified from ( )
Developed a consistent recharge distribution across CZWX and QCSP in Texas
Developed consistent parameterization between GAMs in the overlap

9. Consistent Properties - Statewide
617 TCEQ Mace for QC
Recharge
Hydraulic Conductivity

10. Regions of Applicability
Recommendation by the model developers
If modeling the Simsboro, always use the Central GAM

11. Groundwater Management Strengths & Limitations of GAMs

12. Management Concepts
Sustainability is a “value laden concept” Allay and Leake (2004)
Sustainable yield is an idea which has evolved and currently is a socioeconomic concept which may include consideration of:
Aquifer dynamics
Human and ecological impacts, and
Economic impacts
Bredehoeft has defined sustainable development from an aquifer dynamics perspective to be a condition when pumping can be balanced by natural aquifer discharge.

13. After Alley et al, (1999) and Bredehoeft (2002)
Aquifer Dynamics
Pre-development
recharge
discharge
Dynamic equilibrium:
Aquifer recharge is balanced
by aquifer discharge
aquifer
Post-development
pumping
Development is balanced by:
Decrease in storage
Reduction in discharge (capture)
Stream gains
Spring flows
Groundwater ET
Cross-formational flow
Increase in recharge (generally small in comparison to discharge reduction)
Dynamic equilibrium:
Pumping is balanced by a
Reduction in discharge and in some cases an increase in recharge – sometimes termed “capture”
recharge
discharge
aquifer
After Alley et al, (1999) and Bredehoeft (2002)

14. Sources of Groundwater Availability
from Alley et al, (1999)

15. GAM - Strengths
Provide a means to perform regional groundwater management
Provides a physical description of impacts of development describing
Sources of recharge
Sources of capture
Timing of capture
Groundwater storage
Provides key inputs to the definition of availability or sustainability
Without management framework, cannot determine availability or sustainability

16. GAM Groundwater Management Example
Develop ~5,000 AFY in the outcrop of the Queen City near a major river
Seven wells on one mile spacing
How can the GAM provide understanding for this development scenario?

17. County Flow Balance (AFY) – Year 0
ET 919
Recharge 4,649
Sparta
Pumping 184
Cross Flow
1,373
Unconfined
Queen City
Grid Cell
Confined
Queen City
Grid Cell
Baseflow 1,407
Downdip 2,060
Storage = 82
Carrizo
Cross Flow
1,387

18. County Flow Balance (AFY) – Year 100
ET 629
Recharge 4,649
Pumping 4,470
Sparta
Cross Flow
1,958
Unconfined
Queen City
Grid Cell
Confined
Queen City
Grid Cell
Baseflow 1,146
Storage = 533
Carrizo
Cross Flow
1,172

19. County Transient Flow Balance (AFY)
Storage
Streams
Pumping
One well goes dry

20. Sources of Capture (AFY)
Streams 2,552 (3.52 cfs)
Sparta 585
ET 302
60%
14 %
7 %
Pumped additional
4,285 AFY
11 %
4 %
Storage 451
Downdip
180
5 %
Carrizo 215

21. What did we learn?
Production is not physically sustainable (ie. available capture exists and water levels would stabilize) within 50 years
It would take > 50 years to see the ultimate impact of the development
It would be a good idea to refine the GAM grid, and possibly properties if available, in the vicinity of the development and stream

22. GAM Strengths for Groundwater Management
The GAM is well suited for studying institutional water resource issues
The GAM provides regional water balance information which can be used to assess:
Regional aquifer dynamics
Regional aquifer capture
Physical inputs to the determination of sustainability
The GAM allows regional consideration of interference between resource strategies
The GAM is well suited for refinement to address local-scale water resource questions.

23. Limitations & Applicability of the GAM
The GAM is a tool limited to groundwater assessments at the regional scale
The model is not well suited for studying operational water resource issues
The model would require refinement to study operational issues for a specific project
Our example is a case-in-point where refinement of the model in the vicinity of the well field and river would be warranted
GAMs have not been developed to assess water quality issues

24. Grid Impacts on Drawdown Predictions
Most GAMs developed to date have used a grid size of one square mile
As a result, the GAM model will not predict head drawdown at a specific pumped well
Re = x
K = 15 ft/day
b = 600 ft
S =

25. Error and Uncertainty
The calibration of GAMs, and most models, is assessed through the model misfit to observations.
In the GAMs we use the Root Mean Square Error (RMSE) as one of our calibration measures
These typically range from 20 to 35 feet
The GAMs are only as accurate at reproducing heads, on average, as the magnitude of the RMSE
The RMSE only describes the average misfit in the calibration or validation periods
The RMSE only provides a measure of the potential predictive error if future conditions are unchanged

26. Conclusions
GAMs are tools that can be used for regional institutional groundwater assessments
Issues of sustainability or availability require a management framework
GAMs only describe the physical constraints for aquifer management within any given management framework

27. Conclusions
A primary value of a model, regardless of the predictive accuracy, is it allows for a disciplined format for the improvement of the understanding of an aquifer (Konikow, 1995)
All models, including GAMs, are uncertain and require updating based upon revised conceptual models and new data