Online Guide to MODFLOW-2005

BAS6 - Basic Package

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BAS6 - Basic Package

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BAS6 - Basic Package

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Abbreviation in Name file



The Basic package is used to specify certain data used in all models. These include

1.the locations of active, inactive, and specified head cells,
2.the head stored in inactive cells, and
3.the initial heads in all cells.


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.

Related Packages

Supported in


Common Errors

There should be a hydraulic connection between all active cells.

Other Notes

The Basic package input file is required in all models.
IBOUND may require special treatment in some cases.
Specified heads specified in the Basic package can be overridden by those specified in the Time-Variant Specified-Head package and the Flow and Head Boundary package.

Input Instructions

Basic Package Input for the Groundwater Flow Process Basic (BAS) Package except for output control input is read from the file that is specified with “BAS6” as the file type. The file type has been changed from that used in MODFLOW-96, which was “BAS”, because the input data are not compatible.


Data Set 0


Item 0 is optional—“#” must be in column 1. Item 0 can be repeated multiple times.

Text—is a character variable that starts in column 2. The first two comment lines will become variable HEADNG, which is used as a printout title throughout the program. (If there are no comment lines, then HEADNG will be blank.) HEADNG is limited to 80 columns, but subsequent Text lines can be up to 199 columns. Any characters can be included in Text. The “#” character must be in column 1. Except for the name file, lines beginning with # are restricted to these first lines of the file. Text is printed when the file is read.

Data Set 1

Options (199 text characters)

If there are no options to specify, then a blank record must be included for Item 1.

Options—is a character variable that is scanned for words (separated by one or more spaces) that specify program options. Three options are currently recognized. Unrecognized words are ignored, and a word may be specified in either uppercase or lowercase. A blank record is acceptable and indicates no options.

XSECTION indicates that the model is a 1-row cross section for which STRT and IBOUND should each be read as single two-dimensional variables with dimensions of NCOL and NLAY. Likewise, head and drawdown should be printed and saved in disk files as single two-dimensional variables.

CHTOCH indicates that flow between adjacent constant-head cells should be calculated.

FREE indicates that free format is used for input variables throughout the Basic Package and other packages as indicated in their input instructions. Be sure that all variables read using free format have a non-blank value and that a comma or at least one blank separates all adjacent values.

PRINTTIME indicates that the start time, end time, and elapsed time required to execute MODFLOW-2000 are to be written to the global output file.  When the program is used in batch mode, these times are determined and written for each simulation, that is, for each name file listed in  This capability is expected to be particularly useful in evaluating performance of the program in different serial- and parallel-processing environments.

SHOWPROGRESS indicates that the stress period, time step, and the equation currently being solved should be displayed on the monitor.  Unfortunately, support for this option is processor-dependent; as a result, when this option is selected on some systems, lines showing progress will not be overwritten by subsequent lines.  In these cases, use of this option will cause the display to scroll as the progress line is updated. (MODFLOW-2000 only)

STOPERROR is an optional keyword that can be specified in the options list.  The real number, STOPER, must follow the keyword.  STOPER is a percent discrepancy that is compared to the budget percent discrepancy when the solver convergence criteria are not met.  Execution will continue unless the budget percent discrepancy is greater than STOPER.  See also the CONTINUE option in the NWT solver.


Data Set 2


If not a cross section, a layer variable is read for each layer in the grid. If a cross section, NLAY rows of NCOL values are read.

IBOUND—is the boundary variable. One value is read for every model cell. Usually, these values are read a layer at a time; however, when the XSECTION option is specified, a single variable for the cross section is read. Note that although IBOUND is read as one or more two-dimensional variables, it is stored internally as a three-dimensional variable.

If IBOUND(J,I,K) < 0, cell J,I,K has a constant head.
If IBOUND(J,I,K) = 0, cell J,I,K is inactive.
If IBOUND(J,I,K) > 0, cell J,I,K is active.

Note: Some packages (CHD and FHB) change the value of IBOUND used by the program.  In addition the Groundwater Transport (GWT) process uses the absolute value of IBOUND as an index into an array of concentrations and thus uses it to specify the concentration at the cell.  MODPATH  also uses a copy of IBOUND in it's input and uses the absolute value of IBOUND to indicate special zones.

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

If the UZF package is used, any cells with IBOUND values less than or equal to 0 should have IUZFBND set to zero.


Data Set 3

HNOFLO—is the value of head to be assigned to all inactive (no flow) cells (IBOUND = 0) throughout the simulation. Because head at inactive cells is unused in model calculations, this does not affect model results but serves to identify inactive cells when head is printed. This value is also used as drawdown at inactive cells if the drawdown option is used. Even if the user does not anticipate having inactive cells, a value for HNOFLO must be entered.

Data Set 4


If not a cross section, a layer variable is read for each layer in the grid. If a cross section, NLAY rows of NCOL values are read.

STRT—is initial (starting) head—that is, head at the beginning of the simulation. STRT must be specified for all simulations, including steady-state simulations. One value is read for every model cell. Usually, these values are read a layer at a time. When the XSECTION option is specified, however, a single array for the cross section is read. For simulations in which the first stress period is steady state, the values used for STRT generally do not affect the simulation (exceptions may occur if cells go dry and (or) rewet). The execution time, however, will be less if STRT includes hydraulic heads that are close to the steady-state solution.