BCF6 - Block-Centered Flow Package

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Abbreviation in Name file

BCF6

Purpose

The Block-Centered Flow package is used to specify properties controlling flow between cells.

Documentation

•Harbaugh, A.W., Banta, E.R., Hill, M.C., and McDonald, M.G., 2000, MODFLOW-2000, the U.S. Geological Survey modular ground-water model -- User guide to modularization concepts and the Ground-Water Flow Process: U.S. Geological Survey Open-File Report 00-92, 121 p.

•Harbaugh, A.W., 2005, MODFLOW-2005, the U.S. Geological Survey modular ground-water model -- the Ground-Water Flow Process: U.S. Geological Survey Techniques and Methods 6-A16. https://doi.org/10.3133/tm6A16

•Goode, D.J., and Appel, C.E., 1992, Finite-difference interblock transmissivity for unconfined aquifers and for aquifers having smoothly varying transmissivity: U.S. Geological Survey Water-Resources Investigations Report 92-4124, 79 p.

•McDonald, M.G., Harbaugh, A.W., Orr, B.R., and Ackerman, D.J., 1992, A method of converting no-flow cells to variable-head cells for the U.S. Geological Survey modular finite-difference ground-water flow model: U.S. Geological Survey Open-File Report 91-536, 99 p.

•McDonald, M.G., and Harbaugh, A.W., 1988, A modular three-dimensional finite-difference ground-water flow model: U.S. Geological Survey Techniques of Water-Resources Investigations, book 6, chap. A1, 586 p.

Related Packages

The Layer-Property Flow package (LPF) and the Hydrogeologic-Unit Flow package (HUF2) are two other packages that can be used to specify properties controlling flow between cells.

The Horizontal Flow Barrier (HFB) package modifies the horizontal conductances between certain, specified pairs of cells.

Supported in

•MODFLOW-2000

•MODFLOW-2005

•MODFLOW-LGR

•MODFLOW-CFP

•MODFLOW-NWT

•MODFLOW-OWHM

Other Notes

•Every model must  use one and only one of the four packages (BCF6, LPF, HUF2, and UPW) that are used to specify properties controlling flow between cells. UPW is only available in MODFLOW-NWT and MODFLOW-OWHM.

•If the BCF package is used together with the UZF package, IUZFOPT in the UZF package must be set to 1.

•

See the Frequently Asked Questions for information on how to read data from binary files generated by MODFLOW.

•The SOR solver doesn't work well with the wetting capability.

Input Instructions

This version of the Block-Centered Flow (BCF) Package combines the capabilities documented in McDonald and Harbaugh (1988), McDonald and others (1992), and Goode and Appel (1992). Input for the BCF Package is read from the file that is type "BCF6" in the name file. The file type has been changed from that used in MODFLOW-96, which was “BCF”, because the input data are not compatible.

FOR EACH SIMULATION

Data Set 1

IBCFCB HDRY IWDFLG  WETFCT IWETIT IHDWET

These 6 variables are free format if the option “FREE” is specified in the Basic Package input file; otherwise, the variables all have 10-character fields.

IBCFCB—is a flag and a unit number.

•If IBCFCB > 0, it is the unit number to which cell-by-cell flow terms will be written when "SAVE BUDGET" or a non-zero value for ICBCFL is specified in Output Control. The terms that are saved are storage, constant-head flow, and flow between adjacent cells.

•If IBCFCB = 0, cell-by-cell flow terms will not be written.

•If IBCFCB < 0, cell-by-cell flow for constant-head cells will be written in the listing file when "SAVE BUDGET" or a non-zero value for ICBCFL is specified in Output Control. Cell-by-cell flow to storage and between adjacent cells will not be written to any file.

The flow terms that will be saved are the flows through the right, front, and lower cell face. Positive values represent flows toward higher column, row, or layer numbers.

HDRY—is the head that is assigned to cells that are converted to dry during a simulation. Although this value plays no role in the model calculations, it is useful as an indicator when looking at the resulting heads that are output from the model. HDRY is thus similar to HNOFLO in the Basic Package, which is the value

IWDFLG—is a flag that determines if the wetting capability is active.

•If IWDFLG = 0, the wetting capability is inactive.

•If IWDFLG is not 0, the wetting capability is active.

WETFCT—is a factor that is included in the calculation of the head that is initially established at a cell when it is converted from dry to wet. (See IHDWET.)

IWETIT—is the iteration interval for attempting to wet cells. Wetting is attempted every IWETIT iteration. If using the PCG solver (Hill, 1990), this applies to outer iterations, not inner iterations. If IWETIT is less than or equal to 0, it is changed to 1.

IHDWET—is a flag that determines which equation is used to define the initial head at cells that become wet:

•If IHDWET = 0, equation 33A (equation 3a in McDonald and others, 1992) is used:

h = BOT + WETFCT (hn - BOT)

•If IHDWET is not 0, equation 33B (equation 3b in McDonald and others, 1992) is used:

h = BOT + WETFCT (WETDRY)

Data Set 2

Ltype(NLAY)

Read one value for each layer. These values are free format if the word FREE is specified in Item 1 of the Basic Package input file; otherwise, they have fixed-format fields. The fixed format fields are each 2 characters wide with 40 values per line. Use only as many lines as required for the number of model layers.

In MODFLOW-2005 v 1.5 and earlier versions of MODFLOW-2005, Ltype must always be in free format.

Ltype—contains a combined code for each layer that specifies both the layer type (LAYCON) and the method of computing interblock conductance. Use as many records as needed to enter a value for each layer. Values are two-digit numbers:

The left digit defines the method of calculating interblock transmissivity. The methods are described by Goode and Appel (1992).

•0 or blank—harmonic mean (the method used in MODFLOW-88). (This is most appropriate for confined and unconfined aquifers with abrupt boundaries in transmissivity at the cell boundaries or for confined aquifers with uniform hydraulic conductivity.)

•1—arithmetic mean (This is most appropriate for unconfined aquifers with uniform hydraulic conductivities.)

•2—logarithmic mean (This is most appropriate for confined aquifers with gradually varying transmissivities.)

•3—arithmetic mean of saturated thickness and logarithmic-mean hydraulic conductivity. (This is most appropriate for unconfined aquifers with gradually varying transmissivities.) If the transmissivity method is 3 and LAYCON (see below) is 0 or 2, the transmissivity method will be changed to 2 (logarithmic).

The right digit defines the layer type (LAYCON), which is the same as in MODFLOW-88:

•0—confined—Transmissivity and storage coefficient of the layer are constant for the entire simulation.

•1—unconfined—Transmissivity of the layer varies. It is calculated from the saturated thickness and hydraulic conductivity. The storage coefficient is constant. This type code is valid only for layer 1.

•2—confined/unconfined—Transmissivity of the layer is constant. The storage coefficient may alternate between confined and unconfined values. Vertical flow from above is limited if the layer desaturates.

•3—confined/unconfined—Transmissivity of the layer varies. It is calculated from the saturated thickness and hydraulic conductivity. The storage coefficient may alternate between confined and unconfined values. Vertical flow from above is limited if the aquifer desaturates.

Data Set 3

TRPY(NLAY) -- U1DREL

TRPY—is a one-dimensional variable containing a horizontal anisotropy factor for each layer. It is the ratio of transmissivity or hydraulic conductivity (whichever is being used) along a column to transmissivity or hydraulic conductivity along a row. Set to 1.0 for isotropic conditions. This is a single variable with one value per layer. Do not read a variable for each layer—that is, include only one array control record for the entire variable. Note that the LPF Package can be used to represent horizontal anisotropy that varies in magnitude within a model layer.

A subset of the following two-dimensional variables are used to describe each layer. The variables needed for each layer depend on the layer-type code (LAYCON, which is defined as part of the Item-2 Ltype), whether the simulation has any transient stress periods (at least one stress period defined in the Discretization File specifies Ss/Tr as “TR”), and if the wetting capability is active (IWDFLG not 0). If a variable is not needed, it must be omitted. In no situation will all variables be required. The required variables (Items 4-9) for layer 1 are read first; then the variables for layer 2 and so forth.

Data Set 4

[Sf1(NCOL,NROW)]--U2DREL

If there is at least one transient stress period.

Sf1—is the primary storage coefficient. Read only if there are one or more transient stress periods specified in the Discretization File. For LAYCON equal to 1, Sf1 will always be specific yield, whereas for LAYCON equal to 2 or 3, Sf1 will always be confined storage coefficient. For LAYCON equal to 0, Sf1 would normally be confined storage coefficient; however, a LAYCON value of 0 can also be used to simulate water-table conditions where drawdowns are expected to remain everywhere a small fraction of the saturated thickness, and where there is no layer above, or flow from above is negligible. In this case, specific yield values would be entered for Sf1. (The confined storage coefficient is specific storage times the layer thickness.)

Data Set 5

[Tran(NCOL,NROW)] -- U2DREL

If LAYCON is 0 or 2 (see Ltype ), then read item 5.

Tran—is the transmissivity along rows. Tran is multiplied by TRPY to obtain transmissivity along columns. Read only for layers where LAYCON is 0 or 2.

Data Set 6

[HY(NCOL,NROW)] -- U2DREL

Otherwise, if LAYCON is 1 or 3 (see Ltype), read item 6.

HY—is the hydraulic conductivity along rows. HY is multiplied by TRPY to obtain hydraulic conductivity along columns. Read only for layers where LAYCON is 1 or 3.

Data Set 7

[Vcont(NCOL,NROW)] -- U2DREL

If not the bottom layer.

Vcont—is the vertical hydraulic conductivity divided by the thickness from a layer to the layer below. The value for a cell is the hydraulic conductivity divided by thickness for the material between the node in that cell and the node in the cell below. Because there is not a layer beneath the bottom layer, Vcont cannot be specified for the bottom layer.  See p 5-16 to 5-17 of the MODFLOW-2005 documentation for more details.

Data Set 8

[Sf2(NCOL,NROW)] -- U2DREL

If there is at least one transient stress period and LAYCON (see Ltype) is 2 or 3.

Sf2—is the secondary storage coefficient. Read only for layers where LAYCON is 2 or 3 and only if there are one or more transient stress periods specified in the Discretization File. The secondary storage coefficient is always specific yield.

Data Set 9

[WETDRY(NCOL,NROW)] -- U2DREL

If IWDFLG is not 0 and LAYCON is 1 or 3 (see Ltype).

WETDRY—is a combination of the wetting threshold and a flag to indicate which neighboring cells can cause a cell to become wet.

•If WETDRY < 0, only the cell below a dry cell can cause the cell to become wet.

•If WETDRY > 0, the cell below a dry cell and the four horizontally adjacent cells can cause a cell to become wet.

•If WETDRY is 0, the cell cannot be wetted.

The absolute value of WETDRY is the wetting threshold. When the sum of BOT and the absolute value of WETDRY at a dry cell is equaled or exceeded by the head at an adjacent cell, the cell is wetted. Read only if LAYCON is 1 or 3 and IWDFLG is not 0.

The Lake package requires that WETDRY be set to 0 in Lakes.