GMS:BCF Package

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MODFLOW
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Packages
Flow: BCF6, HUF, LPF, UPW
Solvers:

DE4, GMG, NWT, PCG,

PCGN, LMG, SIP, SOR,

SMS
Other:

BAS6, BFH, CHD1, CLN,

DRN1, DRT1, EVT1, ETS1,

GAGE, GHB1, GNC, HFB1,

HUF, LAK3, MNW1, MNW2,

OUT1, RCH1, RIV1, SFR2,

STR1, SUB1, SWI2, WEL1,

UZF1

NOTE: The observation, sensitivity, and parameter estimation processes do not support the BCF package.

The MODFLOW Block Centered Flow Package dialog.

Once the data in the Global Options/Basic package are initialized, the data for the flow package can be defined. The Block-Centered Flow (BCF) package is one of three flow packages that can be used. The BCF package computes the conductance between each of the grid cells and sets up the finite difference equations for the cell to cell flow. It also computes the terms that determine the rate of movement of water to and from storage. The BCF Package dialog is accessed through the BCF Package command in the MODFLOW menu.

Data Required

Data arrays required for a particular layer are dependent on the layer type. The layer arrays required for each layer type are shown in the following table. The leakance array is not required for the bottom layer and the storage coefficients are only required for transient simulations.

Layer Type Required Arrays
Confined (LAYCON=0) Transmissivity, Leakance, Primary Storage Coefficient (Storativity)
Unconfined (LAYCON=1) Bottom Elevation, Hydraulic Conductivity, Leakance, Primary Storage Coefficient (Specific yield)
Confined/Unconfined (LAYCON=2) Top Elevation, Transmissivity, Leakance, Primary Storage Coefficient (Storativity), Secondary Storage Coefficient (Specific yield)
Confined/Unconfined (LAYCON=3) Top Elevation, Bottom Elevation, Hydraulic Conductivity, Leakance, Primary Storage Coefficient (Storativity), Secondary Storage Coefficient (Specific yield)

Each of the required input arrays must be entered by the user for each layer. Some of the layer arrays can be directly entered. However, some are dependent on the layer geometry. For example, leakance is a function of the layer thickness and the vertical hydraulic conductivity. The transmissivity is equal to the horizontal hydraulic conductivity multiplied by the layer thickness.

The BCF Package can be used for simple models with a single layer for multiple layers with simple stratigraphy. In such cases, many of the parameters are constant for an entire layer and can be entered directly. For more complex models, the following steps can be taken to prepare the input arrays (or the user can use the LPF package):

  1. Import a set of scatter points defining the elevations of the stratigraphic horizons.
  2. Interpolate the top and bottom elevations of each unit to a 2D grid which matches the 3D computational grid.
  3. Compute the desired parameter arrays using the interpolated elevation arrays and the Data Calculator.
  4. Copy the parameter arrays into the appropriate MODFLOW arrays in the BCF Package dialog.

Since this approach can be quite time-consuming, the LPF Package is recommended for most models.

Layer Data

The aquifer properties are entered in the layer data section.

Layer

The layer edit field is used to select the layer. The buttons on the right side of the dialog apply to the active layer only.

Layer Data Arrays

The six buttons on the right portion of the dialog represent layer data arrays such as elevations and hydraulic conductivity. Each of the six buttons brings up a dialog for entering an array of values. The dialog can be used to edit individual values, assign a constant value to the entire array, or to copy a dataset generated by interpolating from a scatter point set to the array.
Not all of the data arrays need to be specified for each layer. Some arrays are only required for transient models. The required arrays depend on the layer type.
Layer data arrays can also be edited using other tools in GMS. The array values can be edited by selecting a set of cells and using the Cell Properties command. The values in the layer parameter arrays can be initialized using coverages of a conceptual model defined in the Map module. A set of tools for rapidly defining top and bottom elevations is provided (See Defining Layer Elevations). Layer data can also be assigned using material zones.

Layer Type

Each layer must be assigned a layer type (LAYCON). By default, the top layer is defined to be an unconfined layer and all other layers are initialized as confined. The data arrays required by a layer may be dependent on the layer type. Only one layer type can be assigned to each grid layer.

Interblock Transmissivity

The method used for computing interblock transmissivity is specified using the pull-down list in the middle right portion of the BCF Package dialog.

Anisotropy Factor

This edit field allows the user to adjust the anisotropy that should be used in the model. This value is defaulted to 1.0.

Head Assigned to Dry Cells

This edit field allows the user to adjust the default head value assigned to dry cells. This value is defaulted to -888.0 or -999.0.

Cell Rewetting Parameters

The controls related to rewetting cells in the lower left portion of the BCF Package dialog are only activated if the BCF2 or BCF3 package has been specified. If wetting of cells is to be allowed, a wetting factor, wetting iteration interval, and wetting equation must be specified.

Storage coefficients

Name Description Units
Storativity "the volume of water released from storage per unit decline in hydraulic head in the aquifer, per unit area of the aquifer"[1] unitless
Specific Storage (Ss) "the amount of water that a portion of an aquifer releases from storage, per unit mass or volume of aquifer, per unit change in hydraulic head"[1] 1/L
Specific Yield (Sy) "also known as the drainable porosity, is a ratio, less than or equal to the effective porosity, indicating the volumetric fraction of the bulk aquifer volume that a given aquifer will yield when all the water is allowed to drain out of it under the forces of gravity"[1] unitless

References

  1. 1.0 1.1 1.2 Specific Storage. (n.d.) In Wikipedia. Retrieved August 18, 2017, from http://en.wikipedia.org/wiki/Specific_storage