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A Data Input Program (MFI) for the U.S. Geological Survey Modular Finite-Difference Ground-Water Flow Model
U.S. Geological Survey Water-Resources Investigations Report 94-468

Modifying MFI to Specify Which Options Use Which File Units

In MODFLOW, the IUNIT array specifies which options are active and the file units used to read the input data for those options. Many of the IUNIT members were assigned to specific options in McDonald and Harbaugh (1988), but other options have been documented with conflicting IUNIT assignments. Thus some versions of MODFLOW may use different IUNIT values for some options. The MFI file units must match the values used in MODFLOW. Also, the units used for the Basic Package and the listing file are specified in the main MODFLOW program. These may vary according to the specific computer used. The following list shows the file unit assignments for all the options of MODFLOW that are included in MFI as well as for the Basic Package and listing file.

unit 5 -- Basic Package file
unit 6 -- Listing file
IUNIT(1) -- Block-Centered Flow Package, version 2 (McDonald and others, 1992) and version 3 (Goode and Appel, 1992)
IUNIT(2) -- Well Package
IUNIT(3) -- Drain Package
IUNIT(4) -- River Package
IUNIT(5) -- Evapotranspiration Package
IUNIT(7) -- General-Head Boundary Package
IUNIT(8) -- Recharge Package
IUNIT(9) -- Strongly Implicit Procedure Package
IUNIT(11) -- Slice-Successive Overrelaxation Package
IUNIT(12) -- Output Control Option
IUNIT(13) -- Preconditioned Conjugant Gradient Package (Hill, 1990)
Further, MFI recognizes several options for which it cannot prepare input data. MFI will put the proper unit number into the IUNIT array (in the Basic Package file) for these options even though it will not actually prepare the data files. The IUNIT assignments for these additional options are:

IUNIT(6) -- Transient Leakage (Leake and others, 1994)
IUNIT(14) -- Generalized Finite-Difference Package (Harbaugh, 1992)
IUNIT(16) -- Horizontal Flow Barrier Package (Hsieh and Freckleton, 1993)
IUNIT(18) -- Stream Package (Prudic, 1989)
IUNIT(19) -- Interbed Storage Package (Leake and Prudic, 1991)
IUNIT(20) -- Time-Varying Specified Head Package (Leake and Prudic, 1991)
To change the unit assignments, the CUNIT array in the MFI main program must be modified. The CUNIT array contains a 3-character file type that corresponds to each IUNIT element. For example, IUNIT(2) corresponds to the Well Package, and CUNIT(2) has the value "WEL." The CUNIT values that correspond to unused IUNIT elements should be blank. Note that MODPATH does not have an IUNIT assignment within MODFLOW because MODPATH is a separate program; however, within MFI, MODPATH is assigned to IUNIT(40) with a file type of "MP."

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