MODFLOW is a block-centered finite difference code for steady-state and transient simulation of two-dimensional, quasi-three-dimensional, and fully three-dimensional saturated, constant density flow problems in combinations of confined and unconfined aquifer-aquitard systems above an impermeable base. The simulated flow processes are described by the governing partial differential equation which includes anisotropy, but does not include cross-terms for hydraulic conductivity. Porous media heterogeneity is introduced during the formulation of the finite difference equations, as are the various source and sink terms, and flow and head boundary conditions. Additional capabilities are handled by specifically designed solution algorithms, such as time varying stresses using stress periods, preparation of conductance terms, mass balance calculations, and dewatering/ rewetting of cells. Other features are simulated by careful manipulation of boundary conditions and code functions (e.g., water table position and seepage face position).

The MODFLOW program consists of a main program (MAIN) and a large number of subroutines, called modules. These modules are grouped into "packages." The "standard" MODFLOW program (McDonald and Harbaugh, 1988) include 10 packages:

• BAS (Basic Package) handles those tasks that are part of the model as a whole. Among those tasks are specification of boundaries, determination of time-step length, establishment of initial conditions, and printing of results;
• BCF (Block-Centered Flow Package) calculates terms of finite difference equations which represents flow within porous medium; specifically flow from cell to cell and flow into storage;
• WEL (Well Package) adds terms representing flow to wells to the finite difference equations;
• RCH (Recharge Package) adds terms representing areally distributed recharge to the finite difference equations;
• RIV (River Package) adds terms representing flow to rivers to the finite difference equations;
• DRN (Drain Package) adds terms representing flow to drains to the finite difference equations;
• EVT (Evapotranspiration Package) adds terms representing evapo-transpiration to the finite difference equations;
• GHB (General-Head Boundary Package) adds terms representing general-head boundaries to the finite difference equations;
• SIP (Strongly Implicit Procedure Package) iteratively solves the system of finite difference equations using the Strongly Implicit Procedure; and
• SOR (Slice-Successive Over-Relaxation Package) iteratively solves the system of finite difference equations using the Slice-Successive Over-Relaxation technique (SSOR).

An "extended" version has been prepared by IGWMC incorporating additional packages. These packages are:

• PCG2: Preconditioned Conjugate-Gradient 2 Solution Procedure Package (Hill, 1990) iteratively solves the finite difference equations using a variant of the Preconditioned Conjugate-Gradient technique.
• STR1: Streamflow Routing Package (Prudic, 1989) to facilitate interaction between ground water and surface water while accounting for the amount of flow (runoff) in streams using the Manning formula.
• BCF3: Block-Centered Flow 3 Package; includes converting no-flow cells to variable-head cells (BCF2; McDonald et al., 1991), and formulating alternative interblock transmissivities (BCF3; Goode and Appel, 1992); an update of the original BCF package and the BCF2 package included in previous IGWMC versions.
• HFB1: Horizontal Flow Barrier Package (Hsieh and Freckleton, 1993) for simulation of thin, vertically oriented, low permeability geologic features that impede the horizontal flow of ground water.
• TLK1: Transient Leakage Package (Leake et al., 1994) for simulating transient leakage and ground-water storage changes in compressible confining units.
• GFD1: General Finite Difference Flow Package (Harbaugh, 1992); a generalized finite-difference formulation to facilitate user input of conductance terms in stead of MODFLOW calculating them; if used, the package replaces the BCF package (only one internal flow package can be in use at a time).
• IBS1: Interbed Storage Package (Leake and Prudic, 1991); simulating compaction of compressible fine-grained beds within or adjacent to aquifers due to aquifer pumping.
• CHD1: Time-Varient Specified Head Boundaries Package (Leake and Prudic, 1991).

IGWMC distributes, in a single package, five implementations of MODFLOW:

The "Standard" version is implemented using the Microsoft FORTRAN Optimizing Compiler, v. 5.10. It has been compiled to run on any Intel 80i86-based computer with math co-processor and at least 540 Kb free conventional RAM with the X-array in MODFLOW set at 90,000.

The "Extended" version is implemented to run on an Intel 80386-based computer with math coprocessor or compatible. This version has been compiled for 4 different RAM sizes (4 Mb, 8 Mb, 12 Mb and 16 Mb total RAM) using the Lahey F77L EM/32 FORTRAN compiler, version 5.20, and the Phar-Lab 386|DOS Extender/Linker version 5.1. The X-array for these different memory sizes is set at 517,000, 800,000, 1,000,000, and 1,500,000, respectively.

The IGWMC interactive preprocessor PREMOD is included with the program package to facilitate user-friendly data entry and editing. Currently, the preprocessor, developed by GeoTrans, Inc. for IGWMC, supports the the "Standard" version with up to 80 rows by 80 columns by 20 layers, and a single "Extended" version with up to 250 rows by 250 columns by 60 layers requiring 4 Mb RAM. The preprocessor supports the packages of the Standard version including the BCF2 part of the BCF3 package, as well as the PCG2 package. Input data files for larger size versions and for the STR1, HFB1, GHD1, TLK1, BCF3 and IBS packages can be prepared using a text editor, or a commercial preprocessor, such as MODPAK.

Also, this package include the IGWMC-prepared postprocessor POSTMOD for reformatting the model output of head, drawdown, and cell-by-cell flow terms. The reformatted output is compatible with graphical contouring packages such as SURFER (TM) (not included). POSTMOD comes in three implementations: the "Standard" version sized for 80 * 80 *20 cells, and two "Extended" versions: 1) for 160 * 160 * 30 cells requiring at least 4 Mb RAM, and 2) for 250 * 250 * 60 cells requiring at least 16 Mb RAM.

SYSTEM REQUIREMENTS

PC version:

• Intel 8088/80286 based computer
• DOS 3.0 or higher
• 640 Kb RAM
• about 500 Kb free disk space to install executable and example files
• math coprocessor

EXT Version:

• Intel 80386/80486 based computer
• DOS 3.0 or higher
• 4 Mb RAM (3 Mb extended memory)
• about 1.5 Mb free disk space to install executable and example files
• math coprocessor

Authors: M.G. McDonald and A.W. Harbaugh (U.S. Geological Survey)