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.

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 modflow.bf.  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. STOPERROR is only available in MODFLOW-2005, MODFLOW-NWT and MODFLOW-OWHM.

MAXPARAM allows the modeler to specify the maximum number of parameters (MXPAR), parameter clusters (MXCLST) and parameter instances (MXINST). This option is only available in MODFLOW-OWHM. This is specified in Data Item 1 (Options) of the BAS package with the Option “MAXPARAM” followed by MXPAR, MXCLST, and MXINST. The minimum allowable size for the three arrays is 1. MXPAR is the maximum size of the parameter names and values used by parameters; MXCLST is the maximum size of the clusters associated with parameters (for example, zone number, multiplier name, layer/row/col); MXINST is the maximum size of the instance parameter names. If MAXPARAM is not specified then the default values are MXPAR=2000, MXCLST=2000000, MXINST=50000.

An important note to prevent problems—MXCLST should be greater than MXPAR + MXINST, and MXPAR should be greater than MXINST (that is, MXCLTST ≥ MXPAR + MXINST and MXPAR ≥ MXINST). This is advantageous for highly parameterized models that require additional parameter storage space or models that do not use parameters/instances to save memory. For example, to save memory, a model that does not use instances can set MXINST to 1. If there are any problems with the storage requirements, an error flag is raised printing the current inadequate storage, and the program is stopped.

PERCENTERROR This option is only available in MODFLOW-OWHM. The Basic package (BAS) was modified to calculate the rate mass balance at every time step. A new integer variable, PDIFFPRT, represents the maximum acceptable rate mass balance error before triggering a warning to the command prompt and prints the rate mass balance to the list file. The default value of PDIFFPRT is set to 50 (that is, 50 percent) and can be adjusted on Data Item 1 (Options) of the BAS package with the Option “PERCENTERROR” followed by PDIFFPRT. Acceptable values for PDIFFPRT are any integer greater than 0. If PDIFFPRT is set to zero, then a volumetric budget is printed, without the command prompt warning, for every time step.

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.

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.

In MODFLOW-LGR, the value of IBOUND in child grids at the interface to the parent grid should be set to IBFLG.

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.

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.

In MODFLOW-NWT, if the head solution from one simulation will be used as starting heads for a subsequent simulation, or if the Observation Process is used (Harbaugh and others, 2000), then HDRY should not be printed to the output file for dry cells (that is, the UPW Package input variable should be set as IPHDRY=0).