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The Queen City and Sparta GAMs Strengths & Limitations for Groundwater Management

The Queen City and Sparta GAMs Strengths & Limitations for Groundwater Management. Van Kelley, Neil Deeds & Dennis Fryar INTERA Inc. November 19, 2004. Texas Groundwater 2004 Towards Sustainability. Acknowledgements. TWDB S. Wade, C. Ridgeway, R. Mace Bureau of Economic Geology

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The Queen City and Sparta GAMs Strengths & Limitations for Groundwater Management

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

  2. Acknowledgements • TWDB • 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 Central Model Area Southern Model Area Grid - 1 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 (1980-1989) • verified from (1990-1999) • Developed a consistent recharge distribution across CZWX and QCSP in Texas • Developed consistent parameterization between GAMs in the overlap

  9. Consistent Properties - Statewide Recharge Hydraulic Conductivity

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

  11. Groundwater ManagementStrengths & 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. Aquifer Dynamics Pre-development recharge discharge Dynamic equilibrium: Aquifer recharge is balanced by aquifer discharge aquifer Post-development pumping 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 C B 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 Recharge 4,649 ET 919 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 Cross Flow 1,387 Carrizo

  18. County Flow Balance (AFY) – Year 100 Recharge 4,649 ET 629 Sparta Pumping 4,470 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 Carrizo 215 5 %

  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 = 0.198 x K = 15 ft/day b = 600 ft S = 0.0018

  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

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