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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.

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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

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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

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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

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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”

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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

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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

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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!

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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

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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!

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

Daily GW discharge to soil zone

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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”

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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”

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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

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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

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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 Deposits Till Upland

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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.

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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

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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

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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

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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)

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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

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VL-GSFLOW GW Recharge

► GSFLOW provides ground water recharge estimates on a daily basis

Click for Animation

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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”

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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.