Numerical Modeling Workflow - Finite Difference Grids > Define Objectives

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Define Modeling Objectives


The first step in the workflow for developing and running numerical models in Visual MODFLOW Flex is to define the modeling objectives. This entails selecting the desired flow and transport simulation options.  The selected combination of flow options (e.g. saturated groundwater flow) and transport options (e.g. reaction and sorption models) will narrow the list of available flow and transport engines and generate associated input variables (including units and default values).  The purpose of this step is to develop the necessary inputs required to build the model based on your understanding of the relevant physical processes present within the study area.

The numerical workflow supports the following engines:


Engines supported in the Numerical Workflow
























1 - MODFLOW-SURFACT is a commercial groundwater modeling engine that supports coupled groundwater flow and transport simulations.

2 - SEAWAT is a coupled variable density groundwater flow and transport model




Understanding Default Values at the Modeling Objectives Step

Values set at the Define Modeling Objectives workflow step are used by Visual MODFLOW Flex to populate the applicable arrays for the flow and transport model consistent with the modeling objectives upon reaching the Define Properties Step for the first time. Constant values (discussed below) are the exception to this and are discussed below.


Once the applicable arrays have been created in the Model Explorer, changes to the default values at the Define Modeling Objectives will not be reflected in the Flow and Transport Models.  To reset values for a group of properties (e.g. Flow/Conductivity or Transport/Initial Concentrations) delete the relevant node from the Model Explorer Tree, update the default value at the Modeling Objectives step and proceed to the Define Property Zones step to regenerate the deleted arrays with the updated defaults.



Constant values that are not saved to distributed grid arrays can be changed at any time at the Modeling Objectives Step and subsequent changes will have be reflected in the applicable translated flow/transport models.  Examples include:


Species Parameters:

Reference Concentrations and the density and viscosity slopes for SEAWAT models


Model Parameters:

Stoichiometric Yield coefficients for RT3D reactions




Flow Objectives

Under the left panel of Define Modeling Objectives, you can specify a Start Date for the project. Pumping well data and calibration data (head and concentration observation wells) are entered and save as absolute (calendar) dates.  Therefore, it is important to choose a relevant start date at this step that coincides with the start time of your simulation.

Flow and Simulation Type

There are several flow condition options available for flow modeling in the numerical workflow:


Flow Engines and Modeling Objectives Supported in Visual MODFLOW Flex

Flow Type

Saturated Flow

Variably Saturated Flow1


Zone Budget






Constant Density

Variable Density


Vapor Flow




















































1 - Abbreviations for variably saturated flow processes in the Vadose Zone:

     UZF = unsaturated zone flow package is based on 1-D unsaturated flow of a diffusion wave pseudo soil

     V = Van Genuchten relative permeability and saturation relationship

     BC = Brooks-Corey relative permeability and saturation relationship



Under Property Settings, you can modify the default flow parameters.


Start Date

The start date will be used to retrieve pumping well and head/concentration observation data for the model run.  When you define well data with absolute (calendar) dates, it is important that your start date reflects the actual start time for the model run. The well data must fall on  or after that start date. Otherwise, these data will not be included in the simulation.


Also the start date cannot be changed once it has been set. If you inadvertently set the wrong start date, you can import your pumping well data and observation data in relative times (eg. starting at 0), and you will see no difference in the numerical model inputs/outputs.


Start Date

The model start date for this exercise should be set to 1/1/2000. Visual MODFLOW Flex uses a standard Windows date picker; a few tips are shown below on how to use this.  Click on the button shown below, to load the Windows date picker.


The standard Windows calendar will appear.  Click on the month in the header (as shown below):


All months for the current year will appear as shown below. Click on the year in the header:


A range of years will then appear as shown below. Click on the range of years in the header:


A list of years for the previous decade will appear. You can then use the  <  or > buttons to change the year. Once you have reached the desired year (2000 for this example), select this on the calendar as shown below:


A list of months will then appear for that year. Select January for this example, as shown below.


Finally, select "1" from the calendar as shown below:


The selected date will then appear for the Start Date.


Transport Objectives

The transport objectives are defined on the right panel of the Define Modeling Objectives window.  When you select the "Transport Active" check box at the top of the screen, then the options can be defined as explained and shown below.


Retardation Model

Transport engine retardation model options are currently supported in Visual MODFLOW Flex as follows:






No Sorption

Linear sorption (equilibrium controlled)

Freundlich (equilibrium-controlled)

Langmuir (equilibrium-controlled)


First order kinetic sorption (non-equilibrium)



Dual-domain mass-transfer without sorption



Dual-domain mass-transfer with linear sorption in mobile domain



Dual-domain mass-transfer
(with the same linear sorption in mobile and immobile domains)




Dual-domain mass-transfer
(with different linear sorption in mobile and immobile domains)




Dual-domain mass-transfer with nonlinear sorption in mobile domain




Dual-domain mass-transfer
(with the same nonlinear sorption in mobile and immobile domains)




Dual-domain mass-transfer
(with different linear sorption in mobile and immobile domains)







Transport engine reaction model options are currently supported in Visual MODFLOW Flex as follows:






No kinetic reactions

First-order irreversible decay

Zeroth-order irreversible decay


Instantaneous aerobic degradation of of BTEX




Six-Species, First-Order, Rate-Limited, BTEX Degradation using Sequential Electron Acceptors




Rate-Limited Sorption




Double-Monod Model




Sequential First-Order Decay




Aerobic/Anaerobic PCE/TCE Dechlorination




Instantaneous biodegradation of BTEX





Note: MODFLOW-SURFACT supports additional reactions listed in the table above; however, these are not fully supported in the current version of Visual MODFLOW Flex.


The selections you make for Sorption and Reaction options will determine what parameters you need to define for your model. Default values can be defined in the "Species Parameters" and "Model Parameter" tabs.  After the model is created, you can define additional zonation for Transport Properties at the "Define Properties" step. Detailed information regarding these parameters can be found in the MT3DMS, RT3D, SEAWAT, and SURFACT Users Manuals.

The New Species and Delete Species buttons allow you to add and remove species to your Transport simulation. This option is available only for certain transport engines.

When you Species Parameter tab is selected, use the add button () to add a new species, or use the remove button () to remove the selected species. Note: the remove button will remove the last species in the list.

The reaction model parameters window is shown below. For more information, see the individual sections describing Species Parameters and Model Parameters.


When you are finished, click (Next Step) to proceed.



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