1) Integrated modeling of surface water and groundwater systems poses numerous technical and non-technical challenges. The shallow subsurface where integration occurs is highly complex and transient. 2) A general strategy for integrated model development involves identifying areas of strong interaction, integrating data and tools, conceptualizing the shallow system, developing sub-models, conducting initial and refined simulations, and achieving a final integrated calibration. 3) Key technical issues include compensating errors between models, limitations in conceptual models, and the need to consider dynamic feedback between surface water and groundwater. Non-technical issues include knowledge limitations between disciplines and effective project management.

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
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
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
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
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
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
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
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
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
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
GW Discharge to the Soil Zone (Daily)
Click for Animation
Daily GW discharge to soil zone

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
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
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
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
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
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
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
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
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
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. 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
VL-GSFLOW GW Recharge
► GSFLOW provides
ground water
recharge estimates
on a daily basis
Click for Animation

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
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.