The Queen City and Sparta GAMsStrengths & Limitations for Groundwater ManagementTexas Groundwater 2004Towards Sustainability

Van Kelley, Neil Deeds & Dennis FryarINTERA Inc.

November 19, 2004

Texas Groundwater 2004 November 18-19, 2004 *

AcknowledgementsTWDBS. Wade, C. Ridgeway, R. Mace

Bureau of Economic GeologyJ.P. Nicot and A. Dutton

RJ Brandes CompanyJ. Machin and G. Bruehl

Texas Groundwater 2004 November 18-19, 2004 *

Outline of PresentationIntroduction to Queen City & Sparta GAMsImprovements to Carrizo-Wilcox GAMsGAMs and Groundwater ManagementGAM StrengthsIllustrative Example Limitations of Applicability Conclusions

Texas Groundwater 2004 November 18-19, 2004 *

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

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Model Stratigraphy 8675

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Queen City and Sparta aquifers1997 UseQC 14,000 AFYSparta 6,800 AFYAvailabilityQC 680,000 AFYSP 160,000 AFY

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Model Domains Same as C/W GAMsNorthernModel AreaCentralModel AreaSouthernModel AreaGrid - 1 square mile eachSame Grid as Carrizo-Wilcox GAMs20,000 acres representsApproximately 5 grid blocksNixonGonzales

Texas Groundwater 2004 November 18-19, 2004 *

Queen City and Sparta GAM StatusQueen 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 TexasDeveloped consistent parameterization between GAMs in the overlap

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Consistent Properties - StatewideRechargeHydraulic Conductivity

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Regions of ApplicabilityRecommendation by the model developers If modeling the Simsboro, always use the Central GAM

Groundwater ManagementStrengths & Limitations of GAMs

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Management ConceptsSustainability 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 dynamicsHuman and ecological impacts, andEconomic impactsBredehoeft has defined sustainable development from an aquifer dynamics perspective to be a condition when pumping can be balanced by natural aquifer discharge.

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Aquifer DynamicsaquiferrechargedischargeaquiferrechargedischargepumpingAfter Alley et al, (1999) and Bredehoeft (2002)Dynamic equilibrium:Aquifer recharge is balancedby aquifer dischargePre-developmentPost-developmentDynamic equilibrium:Pumping is balanced by aReduction in discharge and in some cases an increase in recharge sometimes termed capture

Texas Groundwater 2004 November 18-19, 2004 *

Sources of Groundwater Availabilityfrom Alley et al, (1999)BC

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GAM - StrengthsProvide a means to perform regional groundwater managementProvides a physical description of impacts of development describingSources of rechargeSources of captureTiming of captureGroundwater storageProvides key inputs to the definition of availability or sustainabilityWithout management framework, cannot determine availability or sustainability

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GAM Groundwater Management ExampleDevelop ~5,000 AFY in the outcrop of the Queen City near a major riverSeven wells on one mile spacingHow can the GAM provide understanding for this development scenario?

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County Flow Balance (AFY) Year 0

UnconfinedQueen CityGrid CellConfinedQueen CityGrid Cell

SpartaCarrizoRecharge 4,649ET 919 Baseflow 1,407Cross Flow1,387Cross Flow1,373Pumping 184Storage = 82Downdip 2,060

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County Flow Balance (AFY) Year 100

UnconfinedQueen CityGrid CellConfinedQueen CityGrid Cell

SpartaCarrizoRecharge 4,649ET 629 Baseflow 1,146Cross Flow1,172Cross Flow1,958Pumping 4,470Storage = 533

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County Transient Flow Balance (AFY)PumpingStorageStreamsOne well goes dry

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Sources of Capture (AFY)Pumped additional4,285 AFYStreams 2,552 (3.52 cfs)60%5 %Carrizo 215Sparta 585ET 30214 %7 %Storage 45111 %4 %Downdip180

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

Texas Groundwater 2004 November 18-19, 2004 *

GAM Strengths for Groundwater ManagementThe GAM is well suited for studying institutional water resource issuesThe GAM provides regional water balance information which can be used to assess:Regional aquifer dynamicsRegional aquifer capturePhysical inputs to the determination of sustainabilityThe GAM allows regional consideration of interference between resource strategiesThe GAM is well suited for refinement to address local-scale water resource questions.

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Limitations & Applicability of the GAMThe GAM is a tool limited to groundwater assessments at the regional scaleThe model is not well suited for studying operational water resource issuesThe model would require refinement to study operational issues for a specific projectOur example is a case-in-point where refinement of the model in the vicinity of the well field and river would be warrantedGAMs have not been developed to assess water quality issues

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Grid Impacts on Drawdown PredictionsMost GAMs developed to date have used a grid size of one square mileAs a result, the GAM model will not predict head drawdown at a specific pumped well

Re = 0.198 x

K = 15 ft/dayb = 600 ftS = 0.0018

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Error and UncertaintyThe 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 measuresThese typically range from 20 to 35 feetThe GAMs are only as accurate at reproducing heads, on average, as the magnitude of the RMSEThe RMSE only describes the average misfit in the calibration or validation periodsThe RMSE only provides a measure of the potential predictive error if future conditions are unchanged

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ConclusionsGAMs are tools that can be used for regional institutional groundwater assessmentsIssues of sustainability or availability require a management frameworkGAMs only describe the physical constraints for aquifer management within any given management framework

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ConclusionsA 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

617 TCEQ + 412 Mace for QC

Development is balanced by:Decrease in storageReduction in discharge (capture)Stream gainsSpring flowsGroundwater ETCross-formational flowIncrease in recharge (generally small in comparison to discharge reduction)