DIS - Discretization File
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DIS - Discretization File

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DIS - Discretization File

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Abbreviation in Name file

DIS

Purpose

The Discretization File  is used to specify certain data used in all models. These include

1.the number of rows, columns and layers
2.the cell sizes
3.the presence of Quasi-3D confining beds
4.the time discretization

Documentation

Supported in

MODFLOW-2000
MODFLOW-2005
MODFLOW-LGR
MODFLOW-CFP
MODFLOW-NWT
MODFLOW-OWHM

Other Notes

The Discretization File is required in all models.

Certain packages or processes including HUF2 and GWT may place restrictions on the allowable discretization.

Input Instructions

Discretization information is read from the file that is specified by "DIS" as the file type.

FOR EACH SIMULATION

Data Set 0

[#Text]

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

Text—is a character variable (199 characters) that starts in column 2. 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

NLAY NROW NCOL NPER  ITMUNI LENUNI {XFIRSTCORD YFIRSTCORD GRIDROTATION {COORD_OPTIONS}}

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.

 

 

Data Set 2

LAYCBD—is a flag, with one value for each model layer, that indicates whether or not a layer has a Quasi-3D confining bed below it. 0 indicates no confining bed, and not zero indicates a confining bed. LAYCBD for the bottom layer must be 0.

Data Set 3

DELR(NCOL) - U1DREL

DELR—is the cell width along rows. Read one value for each of the NCOL columns. This is a multi-value one-dimensional variable with one value for each column.

Finite difference grid illustrating DELR and DELC.

Finite difference grid illustrating DELR and DELC.

Data Set 4

DELC(NROW) - U1DREL

DELC—is the cell width along columns. Read one value for each of the NROW rows. This is a multi-value one-dimensional variable with one value for each row.

Finite difference grid illustrating DELR and DELC.

Finite difference grid illustrating DELR and DELC.

Data Set 5

Top(NCOL,NROW) - U2DREL

Top—is the top elevation of layer 1. For the common situation in which the top layer represents a water-table aquifer, it may be reasonable to set Top equal to land-surface elevation.

Data Set 6

BOTM(NCOL,NROW) - U2DREL

Item 6 is repeated for each model layer and Quasi-3D confining bed in the grid. These layer variables are read in sequence going down from the top of the system. Thus, the number of BOTM arrays must be NLAY plus the number of Quasi-3D confining beds.

BOTM—is the bottom elevation of a model layer or a Quasi-3d confining bed.

FOR EACH STRESS PERIOD

Data Set 7

PERLEN NSTP TSMULT Ss/tr [STARTTIME StartingYear]

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

 

Time step formula

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.

Month

Non leap year

Leap year

January

0

0

February

0.08493

0.08470

March

0.16164

0.16393

April

0.24658

0.24863

May

0.32877

0.33060

June

0.41370

0.41530

July

0.49589

0.49727

August

0.58082

0.58197

September

0.66575

0.66667

October

0.74795

0.74863

November

0.83288

0.83333

December

0.91507

0.91530