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Issues and Strategies for Integrated Model Calibration

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Issues and Strategies for Integrated Model Calibration

  1. 1. 1 Issues and Strategies for Integrated Model Calibration MODFLOW and More 2015 Dirk Kassenaar, E.J. Wexler P.J. Thompson, M.G.S. Takeda Earthfx Inc.
  2. 2. 2 Integrated Modeling ► Integrated modelling can provide significant insights into the overall system behavior and response to complex stresses ► Numerous technical and non- technical issues: ► Rainfall runoff models are plagued by numerical daemons  Mary Hill, June 1, 2015 ► Without the non-linear pressure saturation relationship of variably saturated flow the terrestrial system would simply not work  Stephan Kollet, June 1, 2015 After USGS
  3. 3. 3 Presentation Objectives ► Issues and Strategies for Integrated Modelling  Is integrated modelling different?  Technical Issues: ► Complex non-linear processes, compensating errors, long run times…  Non-Technical Issues: ► Knowledge limitations, different conceptual models, biases, terminology… ► Strategies for addressing these issues:  We present a general strategy and flow chart for model development, with some examples
  4. 4. 4 Background ► Integrated Stratigraphic/Groundwater modelling  Some GW modellers have only a limited background in geology ► Geology is a “knowledge boundary”  Re-conceptualization of the stratigraphic model is rarely undertaken once the GW model calibration process has begun. ► Geologic refinements and issues usually addressed with K zones or parameter estimation ► Integrated SW/GW modelling  Similar knowledge boundaries, limitations and modelling issues  “Compensating errors” (adjustment of GW model parameters to account for SW processes, and vice versa) is a bigger issue
  5. 5. 5 Presentation Outline ► Technical Issues and Challenges  Discussion of issues, with examples of soil zone response and dynamic GW feedback to illustrate challenges ► Strategies for integrated model calibration  Presentation of an integrated model development “flow chart”  Other guidelines and recommendations ► Non-technical issues  Data management, blind spots, “Renaissance Hydrogeology”
  6. 6. 6 Technical Issues ► Historic simplifications  GW: Baseflow separation, too many constant heads  SW: Lumped parameter catchment models, deep groundwater reservoirs, hydrology/hydraulics ► Calibration approaches  GW: Emphasis on matching heads and spatial patterns ► Less emphasis on regional flux calibration; recharge guesstimates  SW: Emphasis on matching streamflow peaks ► Limited emphasis on spatial and low-flow calibration ► Both surface water and groundwater modellers have “blind spots” and convenient simplifications that must be addressed early in the integrated model development process
  7. 7. 7 Technical Issues ► The shallow subsurface, where the integration happens, is highly transient and complex ► Significant fluctuation in system feedback  GW Feedback is highly variable – wet year/dry year, seasonal  Empirical baseflow separation is only a first guess ► Strong seasonality means the average conditions never exist  Steady state calibration can be very limited in the upper system ► In summary, dynamic feedback is reality – get on with it  Recognizing the dynamic nature is essential to the calibration process
  8. 8. 8 Integrated Model Development Flowchart: Step 1 ► Identify areas and scale of integration ► Pre-identify areas of strong transient interaction  Shallow depth to water – Dunnian rejected recharge ► Enhanced ET in areas with shallow depth to water table  Dynamic wetlands – storage  Riparian zones and “contributing areas”  Reaches with significant river pickup and loss ► Headwaters, springs, intermittent streams ► Seepage areas ► Identify, but avoid, these areas during initial model construction!
  9. 9. 9 GW Feedback Zones ► Dunnian rejected recharge may likely occurs in areas with:  Depth to water table less than 2 m  Areas with flowing wells, springs and headwater seeps
  10. 10. 10 Time-varying GW Feedback ► The “contributing area” that generates true runoff depends on the time-varying position of the water table ► Example: Dunnian process area varies seasonally between 5 and 25% of the study area ► Runoff occurs, but it is a groundwater dependent process!
  11. 11. 11 GW Discharge to the Soil Zone (Daily) Click for Animation Daily GW discharge to soil zone
  12. 12. 12 Step 2: Data and Model Tool Integration ► Integrated relational database  You need an integrated database to build an integrated model  Reduce barriers to integrated understanding and calibration  Need ability to assess cross-system response, trends, etc. ► Integrated modelling tools  Spatial visualization of SW processes – look beyond the gauge  Temporal visualization of shallow GW dynamics  Encourage both the SW and GW team to “visit the other domain”
  13. 13. 13 Step 3: Integration Conceptualization ► Address the shallow conceptual model  Discuss soil zone properties, thickness, storage, drainage, interflow  Develop compatible groundwater layer 1 geometry and properties ► Avoid the temptation to over-simplify the shallow system.  Resist “old habits” previously used to avoid dry GW cells ► MODFLOW NWT – stable representation of shallow complexity  Beware of SW “discharge to deep groundwater”
  14. 14. 14 SW vs GW Conceptualization ► SW Conceptual Model  Macropores  Preferential flow  Throughflow  Interflow  Subsurface stormflow  Infiltration/percolation/ drainage/recharge  Event mobilized GW  Soil/rock contact zone interface flow  Seepage faces ► GW Conceptual Model  1-D or 3-D Richard’s equation from Lin, 2010
  15. 15. 15 Storage and 3D movement of water in the Soil Zone ► Soil zone moisture content Beach Deposits Till Upland - Till uplands drain both vertically and downslope - Lateral drainage to the beach deposits from the till uplands enhances recharge - Soil zone storage helps supply rate limited GW recharge to the lower layers Click for Animation
  16. 16. 16 Soil Zone Drainage (GW Recharge) ► When moisture is available (winter months) there is a near constant, but rate limited, drainage from the soil zone ► Click for Animation Beach DepositsTill Upland
  17. 17. 17 Step 4: Sub-model Development ► Focus on:  SW and GW model construction and parameter preparation  Data review, assessment and pattern identification  Understanding of general sensitivity ► GW: Focus on the deeper GW flow system ► SW: Pre-calibrate to a gauged sub-catchment with relatively modest GW/SW interaction  Assume parsimony (consistency) when later extrapolating parameters to adjacent catchments.
  18. 18. 18 Step 5: First Integration Simulation ► Get the models and the team working together ► Re-conceptualize as necessary ► Write a draft report to formulate your understanding and impress your boss/client with your progress
  19. 19. 19 Time Step ► The timing of the SW and GW processes is very different, and a major source of contention ► Daily time step in GSFLOW:  Too fine for GW modelers  Too coarse for SW modelers Click for Animation
  20. 20. 20 Step 6: Sub-model Refinement ► Uncoupled model refinement  Update the conceptual model as necessary  Refine model parameters  Focus on the timing of the interaction ► GW: Focus on transient shallow system response  Ensure that surface discharge and groundwater discharge to streams matches observed wetland patterns and surface stream flows ► SW: Focus on the split between interflow and recharge ► In this final uncoupled simulation phase, the modellers must recognize that model response will not reflect interaction
  21. 21. 21 Step 7: Final Integrated Calibration ► Lots of re-thinking and even re-conceptualization  System response timing and lag is sensitive ► Two key benefits of the final integrated calibration process  Model Input: Measured total precipitation  Calibrate to: Measured total streamflow ► Baseflow separation is only good for the preliminary stages ► Focus on matching low flows, and not just the peaks  Balanced calibration to heads (GW) and flux (streamflow)
  22. 22. 2222 Aquifer Head vs. Stream Stage • GW/SW discharge reverses during each storm event • Baseflow separation does not account for reversals • GSFLOW Simulated Hydrograph at Oro-Hawkstone stream gauge Storm Event Reversal: Stream level higher than aquifer Dry period: Aquifer level higher than stream = GW discharge
  23. 23. 23 VL-GSFLOW GW Recharge ► GSFLOW provides ground water recharge estimates on a daily basis Click for Animation
  24. 24. 24 Non-Technical Issues and Strategies ► Expect to do a lot of education: clients and peer reviewers  Include a plenty of simplified details about model integration in your reports (no one wants to read the manuals) ► Don’t get too attached to preliminary results  Integrated conceptual models frequently require change,  Watch for “blind spots” ► Management: Identify a someone who knows a little about everything to oversee integration  A polymath or renaissance hydrogeologist is needed for mediation, and “compromise”
  25. 25. 25 Conclusions ► Integrated Modelling is different; It requires:  Integrated calibration strategies ► Don’t become attached to your initial uncoupled calibration estimates! ► Consider re-conceptualization, even late in the integrated process  Integrated data management ► Data silos and barriers will only hide the relationships and response lag between the systems ► Integrated modelling and calibration tools  An integrated and balanced modelling team ► The skill, multi-disciplinary knowledge, and ability of the SW and GW experts to address their “blind spots” is very important ► Our experience after building 9 fully-integrated GSFLOW models: It’s hard, but it’s worth it.

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