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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 IAH Waterloo 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 ► Integrated Modelling: Modelling with dynamic feedback between the SW and GW systems ► NOT integrated: Conjunctive or loosely coupled models that are manually coupled or explicitly solved After USGS
  3. 3. 3 Presentation Objectives ► 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
  4. 4. 4 7dps (2 years in 1.7 minutes) Click for Animation
  5. 5. 5 Integrated Hydrostratigraphic Modelling ► We are already doing integrated modelling… ► Integrated Stratigraphic/Groundwater modelling  Some GW modellers have only a limited background in geology ► Geology is a “knowledge boundary” ► Desire to independently and completely develop the stratigraphic model  Re-conceptualization of the stratigraphic model is rarely undertaken once the GW model calibration process has begun. ► Geologic refinements often addressed with K zones or parameter estimation (compensating errors) ► Integrated SW/GW modelling  Similar “knowledge boundaries”, but integration is essential  Greater chance that “compensating errors” enter into the process
  6. 6. 6 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 SW/GW model calibration  Presentation of an integrated model development “flow chart”  Other guidelines and recommendations ► Non-technical issues  Data management, blind spots, “Renaissance Hydrogeology”
  7. 7. 7 Customs, Conventions and Blind Spots ► Historic simplifications  GW: Too many constant heads, assumption of stasis (steady state)  SW: Lumped parameter models, groundwater as a “decay reservoir”, separation of hydrology and hydraulics, empirical baseflow separation ► Different in calibration approach and emphasis  GW: Emphasis on matching heads and spatial patterns (head, xy) ► Less emphasis on regional flux calibration; poor recharge guesstimates  SW: Emphasis on matching streamflow peaks (flux, time) ► Limited emphasis on spatial and low-flow calibration ► Both SW and GW modellers have “blind spots”  Conventions and simplifications must be addressed during the integrated model development process (Watch out for a defensive response!)
  8. 8. 8 Integrated Model Development Flowchart 8 1. DATA COMPILATION AND SYNTHESIS 2.DEVELOPMENT OF CONCEPTUAL MODELS -STRATIGRAPHY -HYDROSTRAT -SOIL ZONE -WETLANDS -RIVER SYSTEMS 3. SUB-MODEL CONSTRUCTION, PRE-CALIBRATION AND PRE-PEST 4.INTEGRATION APPLICATION AND INSIGHT 5. CALIBRATION PRECIPITATION RUNOFF SUB-MODEL GROUNDWATER FLOW SUB-MODEL ONGOING DATA EVALUATION
  9. 9. 9 Model Construction Flowchart ► Model construction includes iterative data evaluation, conceptualization, sub-model pre-calibration and integration  Sub-models cannot be independently calibrated, but some components of the sub-model (e.g. snow pack sub-model) can be more or less developed ► Stages 1-5 are not actually “completed” in a linear manner  The most important conceptual insights are usually only identified once the model integration and final calibration is complete.
  10. 10. 10 1. DATA COMPILATION AND SYNTHESIS Stage 1: Data Compilation and Synthesis ► Data availability, quality and gaps  Unevenly distributed, but shallow subsurface data is common  Watch for an unbalanced calibration: Too much focus on the data at hand, and not the overall problem ► Integrated relational database  Reduce barriers to integrated understanding and cross-system response and calibration assessment  Integrated data analysis and modelling tools (xyzt) ► Data collection and synthesis end when the report is done!
  11. 11. 11 Stage 2: Preliminary Conceptualization ► Multiple conceptual models need to be generally formulated  Stratigraphy, hydrostratigraphy, soil zone, land cover, vegetation, snow pack, land use (pervious-impervious), climate ► Pre-identify areas of strong transient SW/GW 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!
  12. 12. 12 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
  13. 13. 13 Stage 3: Sub-Model Development ► Focus on:  SW and GW model construction and parameter preparation  Data review, assessment and pattern identification  Understanding of general sensitivity (Pre-PEST) ► GW: Focus on the deeper GW flow system (steady state) ► SW: Pre-calibrate to a gauged sub-catchment with modest GW/SW interaction  Assume parsimony (consistency) when later extrapolating parameters from the test or “donor” catchment to adjacent catchments.
  14. 14. 14 Compensating Errors ► Compensating errors arise when the parameters of one sub-model are adjusted incorrectly to offset processes and feedback from another sub-model ► Examples:  GW feedback (Dunnian rejected recharge) can be incorrectly represented as low permeability soil  Wetlands storage (and release) can mask GW discharge  Cross-watershed GW inflows can be missed as stream gauge underflow  Excessive hydraulic conductivity variation and PEST can mask structural and stratigraphic errors.  Over-parameterization and excessive local sub-catchment calibration adjustment versus “parsimony” ►
  15. 15. 15 GSFLOW: Dunnian Runoff ► Model representation of GW discharge areas, wetlands and shallow aquifers ► Runoff can occur off fully saturated soils (rejected recharge)  Occurs when the water table is at or near surface  Not sensitive to surficial material K ► Can create runoff from saturated gravels  Spatially controlled: Tends to occur in stream valley areas  Seasonally controlled: Tends to occur in spring when water table is high 15 Unsaturated zone StreamStream Gravity drainage Recharge Ground-water flow
  16. 16. 16 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!
  17. 17. 17 GW Discharge to the Soil Zone (Daily) Click for Animation Daily GW discharge to soil zone
  18. 18. 18 Step 4: Integration ► Re-assess the shallow conceptual model  Review soil zone properties, thickness, storage, drainage, interflow  Develop compatible groundwater layer 1 geometry and properties ► GW modellers must avoid over-simplifying the shallow system.  Resist “old habits” previously used to avoid dry GW cells ► MODFLOW NWT – stable representation of shallow complexity ► Watch the terminology  Infiltration – percolation - recharge
  19. 19. 19 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
  20. 20. 20 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
  21. 21. 21 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
  22. 22. 22 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
  23. 23. 23 Step 5: Integrated Calibration ► Get the models and the team working together ► Continue to re-conceptualize, as necessary ► Write a draft report to formulate your understanding (and impress your boss/client with your progress)
  24. 24. 24 Integrated Model Development Flowchart 24 1. DATA COMPILATION AND SYNTHESIS 2.DEVELOPMENT OF CONCEPTUAL MODELS -STRATIGRAPHY -HYDROSTRAT -SOIL ZONE -WETLANDS -RIVER SYSTEMS 3. SUB-MODEL CONSTRUCTION, PRE-CALIBRATION AND PRE-PEST 4.INTEGRATION APPLICATION AND INSIGHT 5. CALIBRATION PRECIPITATION RUNOFF SUB-MODEL GROUNDWATER FLOW SUB-MODEL ONGOING DATA EVALUATION
  25. 25. 25 Integration Time Step ► The timing of the SW and GW processes is very different, and can be a source of contention ► Daily integration time step:  Too fine for GW modelers  Too coarse for SW modelers  No one is happy, so it must be right! Click for Animation
  26. 26. 26 Iterate: Sub-model Refinement ► After the first fully integrated simulations, it is likely that the uncoupled models could use another round of 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 refinement phase, the modellers must recognize that model response will not reflect interaction
  27. 27. 27 Step 5x: 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)
  28. 28. 2828 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
  29. 29. 29 GW Recharge ► GSFLOW provides ground water recharge estimates on a daily basis Click for Animation
  30. 30. 30 Non-Technical Issues and Strategies ► Expect to do a lot of education: clients and peer reviewers  Include 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 someone who knows a little about everything to oversee integration  A polymath or renaissance hydrogeologist is needed for mediation, and “compromise”
  31. 31. 31 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 a number of fully-integrated models:  It’s hard, but it’s worth it.
  32. 32. 1. DATA COMPILATION AND SYNTHESIS 2.DEVELOPMENT OF CONCEPTUAL MODELS -STRATIGRAPHY -HYDROSTRATIGRAPHY -SOIL ZONE -WETLANDS -RIVER SYSTEMS 3. SUB-MODEL CONSTRUCTION, PRE-CALIBRATION 4.INTEGRATION APPLICATION AND INSIGHT 5. CALIBRATION VALIDATION PRECIPITATION RUNOFF SUB-MODEL GROUNDWATER FLOW SUB-MODEL ONGOING DATA EVALUATION

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