NLAY—is the number of layers in the model grid.  In MODFLOW-2000, the maximum number of layers is 999. In MODFLOW-2005, there is no fixed limit to the number of layers.

NROW—is the number of rows in the model grid.

NCOL—is the number of columns in the model grid.

NPER—is the number of stress periods in the simulation.

ITMUNI—indicates the time unit of model data, which must be consistent for all data values that involve time except for certain variables in the DAFLOW package. For example, if years is the chosen time unit, stress-period length, time-step length, transmissivity, and so on, must all be expressed using years for their time units. Note that the program will still run even if “undefined” time units are specified because the fundamental equations used in MODFLOW do not require that the time unit be identified. But be sure to use consistent units for all input data even when ITMUNI indicates an undefined time unit. When the time unit is defined, MODFLOW uses it to print a table of elapsed simulation time:

•0 - undefined

•1 - seconds

•2 - minutes

•3 - hours

•4 - days

•5 - years

If the Farm process is used and IRTFL is set to 3 or ICUFL is set to 3, ITMUNI must be set to 4.

LENUNI—indicates the length unit of model data, which must be consistent for all data values that involve length except for certain variables in the DAFLOW package. For example, if feet is the chosen length unit, grid spacing, head, hydraulic conductivity, water volumes, and so forth, must all be expressed using feet for their length units. Note that the program will still run even if “undefined” length units are specified because the fundamental equations used in MODFLOW do not require that the length unit be identified. But be sure to use consistent units for all input data even when LENUNI indicates an undefined length unit:

•0 - undefined

•1 - feet

•2 - meters

•3 - centimeters

If using the DAFLOW package, make LENUNI consistent with the value specified for IENG.

the X Cartesian coordinate the model cell center at Row 1, Column 1. This option is only available in MODFLOW-OWHM.

the Y Cartesian coordinate the model cell center at Row 1, Column 1. This option is only available in MODFLOW-OWHM.

the Polar angle in degrees of the model grid measured in the counterclockwise direction.  This option is only available in MODFLOW-OWHM.

COORD_OPTIONS is a character variable that is scanned for keywords (separated by one or more spaces) that specify coordinate options. Unrecognized words are ignored, and a word may be specified in either uppercase or lowercase. A blank record is acceptable and indicates no options. The following are recognized keywords.

•CORNERCOORD is an optional coordinate keyword, without quotes, that when present indicates that XFIRSTCORD, and YFIRSTCORD refer to the model cell’s outer most corner (that is, NOT the cell center).

•LLCOODRINATE is an optional coordinate keyword, without quotes, that when present indicates that  XFIRSTCORD, and YFIRSTCORD refer to the cell center or outer most cell corner of Row NROW and Column 1 (that is, the model’s lower left corner)

•PRINTCOORD is an optional coordinate keyword, without quotes, that when present signifies that the coordinate arrays are to be printed to the list file.

This option is only available in MODFLOW-OWHM.

PERLEN—is the length of a stress period.

NSTP—is the number of time steps in a stress period.

TSMULT—is the multiplier for the length of successive time steps. The length of a time step is calculated by multiplying the length of the previous time step by TSMULT. The length of the first time step, Dt1, is related to PERLEN, NSTP, and TSMULT by the relation

Ss/tr—is a character variable that indicates if the stress period is transient or steady state. The only allowed options are “SS” and “TR”, but these are case insensitive.

If the SWI2 package is used, it is important to recognize that the steady-state head option does not mean that the steady-state density distribution is computed. The steady-state density distribution can only be computed by running the simulation until the density distribution is no longer changing.

STARTTIME The Discretization package (DIS) was modified to optionally read an initial decimal year that represents the start of the simulation. This option is only available in MODFLOW-OWHM. This starting date is updated and printed along with an estimate of the month based on leap/non-leap year to the list file’s Time Summary. To set the starting decimal year, the keyword, STARTTIME, followed by the year is added to the end of the first read of Data Set 7. The update to the decimal year makes the appropriate conversions based on the time step scale and assumes there are 365.242 days in a year. Below is a DIS example of Data Set 7 with a starting date of 1949.7479 (Oct, 1, 1949). Note that the transient simulation then starts in October of 1950.

365.0 1 1 ss STARTTIME 1949.7479 #PERLEN NSTP TSMULT ss/tr => Oct 1949

31.00 10 1 tr #Oct 1950

30.00 10 1 tr #Nov 1950

31.00 10 1 tr #Dec 1950

31.00 10 1 tr #Jan 1951

For general reference the following is a reference table for each month’s fraction of a year.