


U.S. Geological Survey                                modflow(1)



NAME
     modflow - Modular three-dimensional finite-difference
               ground-water model

ABSTRACT
     MODFLOW is a three-dimensional finite-difference ground-
     water model that was first published in 1984.  It has a modular
     structure that allows it to be easily modified to adapt the
     code for a particular application.  Many new capabilities have
     been added to the original model.  OFR 00-92 (complete reference
     below) documents a general update to MODFLOW, which is called
     MODFLOW-2000 in order to distinguish it from earlier versions.

     MODFLOW-2000 simulates steady and nonsteady flow in an
     irregularly shaped flow system in which aquifer layers can
     be confined, unconfined, or a combination of confined and
     unconfined.  Flow from external stresses, such as flow to
     wells, areal recharge, evapotranspiration, flow to drains,
     and flow through river beds, can be simulated.  Hydraulic
     conductivities or transmissivities for any layer may differ
     spatially and be anisotropic (restricted to having the
     principal directions aligned with the grid axes), and the
     storage coefficient may be heterogeneous.  Specified head
     and specified flux boundaries can be simulated as can a head
     dependent flux across the model's outer boundary that allows
     water to be supplied to a boundary block in the modeled area
     at a rate proportional to the current head difference
     between a "source" of water outside the modeled area and the
     boundary block.  MODFLOW is currently the most used
     numerical model in the U.S. Geological Survey for ground-
     water flow problems.

     In addition to simulating ground-water flow, the scope of
     MODFLOW-2000 has been expanded to incorporate related
     capabilities such as solute transport and parameter estimation.


METHOD
     The ground-water flow equation is solved using the finite-
     difference approximation.  The flow region is subdivided
     into blocks in which the medium properties are assumed
     to be uniform.  In plan view the blocks are made from
     a grid of mutually perpendicular lines that may be
     variably spaced.  Model layers can have varying thickness.
     A flow equation is written for each block, called a cell.
     Several solvers are provided for solving the resulting matrix
     problem; the user can choose the best solver for the particular
     problem.  Flow-rate and cumulative-volume balances from each
     type of inflow and outflow are computed for each time step.


HISTORY

     MODFLOW-2000 Version 1.19.01 2010/03/25 - This release corrects a
        minor bug in the GMG Package, and the Unix distribution includes
        a revised openspec.inc file to correct a problem introduced at
        version 1.19.00.

     MODFLOW-2000 Version 1.19.00 2010/03/15 - This release includes the
        revised Multi-Node Well (MNW2) Package, and other changes and bug
        fixes, as described in release.txt.

     MODFLOW-2000 Version 1.18.01 2008/06/20 - This release includes bug
        fixes and minor changes.  Bug fixes and other changes are described 
        in release.txt.

     MODFLOW-2000 Version 1.18.00 2007/08/23 - Added the SWT Package to 
        support simulation of subsidence in water-table aquifers.  It also
        includes bug fixes.

     MODFLOW-2000 Version 1.17.04 2007/08/03 - This release includes bug
        fixes and minor changes.  Bug fixes and other changes are described 
        in release.txt.
        
     MODFLOW-2000 Version 1.17.02 2007/02/14 - This release includes bug
        fixes and minor changes.  Bug fixes and other changes are described 
        in release.txt.

     MODFLOW-2000 Version 1.17.01 2006/09/22 - This is a bug-fix release.
        Bug fixes are described in release.txt.

     MODFLOW-2000 Version 1.17.00 2006/07/25 - The LAK3, SFR2, and GAG5
        Packages have been updated to support simulation of new 
        capabilities.  The GMG Package has been updated to support a new 
        adaptive-damping option.  This version also includes a change in the
        SUB Package and fixes for bugs in the ADV, MNW, and DRT Packages.  
        Please see release.txt and files referenced therein for details on 
        the new options and changes.

     MODFLOW-2000 Version 1.16.00 2006/03/28 - The SFR2 Package replaces
        the SFR1 Package of version 1.14.00, adding support for simulation
        of unsaturated flow beneath streams.  The ADV2 Package has been
        made more compatible with MODPATH.  Other changes and bug fixes
        are described in release.txt.

     MODFLOW-2000 Version 1.15.01 2005/04/05 - This is a bug-fix release.
        Bug fixes are described in release.txt.

     MODFLOW-2000 Version 1.15.00 2004/08/06 - Added the GMG Solver Package.
        Modifications were made to the SFR Package.

     MODFLOW-2000 Version 1.14.00 2004/07/01 - Added the SFR Package to 
        allow enhanced capabilities for simulation of streams.  The maximum
        number of layers is increased from 200 to 999.

     MODFLOW-2000 Version 1.13.00 2004/01/22 - Includes revisions to the
        LAK3 Package and bug fixes for the DRT, SUB, HUF2, and PCG2
        Packages.

     MODFLOW-2000 Version 1.12.01 2003/10/03 - This is a bug-fix release.
        Bug fixes are described in release.txt.

     MODFLOW-2000 Version 1.12 2003/09/08 - Added the SUB Package to allow 
        simulation of subsidence and aquifer-system compaction.  The HUF 
        Package was upgraded from version 1 to version 2, providing support 
        for the new Layer Variable-Direction Horizontal Anisotropy (LVDA) 
        capability, an alternative specification of storage properties for
        the uppermost active cells, calculation of flow in  hydrogeologic
        units, and the new Hydraulic-Conductivity Depth-Decay (KDEP)
        capability.  The ADV Package was updated from version 2.3 to version 
        2.4, which includes support for porosity parameters.  Additional 
        enhancements and bug fixes are described in release.txt.
        
     MODFLOW-2000 Version 1.11 2003/04/10 - Added the DAF Package to allow
        coupling of MODFLOW with the Diffusion Analogy Surface-Water Flow 
        Model, and the MNW Package to allow simulation of well completed in
        intervals represented by multiple model cells.  New options for
        showing progress of the simulation and controlling the calculation
        of drawdowns have been added. Modifications to the PCG2 Package
        affect the way DAMP works.  Please see release.txt for information
        on using these new packages, new options, and DAMP.  This version
        also includes a number of bug fixes and minor output changes, also
        described in release.txt.

     MODFLOW-2000 Version 1.10 2002/07/26 - Fixes bugs encountered when 
        compiling the source code distributed with version 1.9 and a
        problem in a seldom-used part of the LMG Package.

     MODFLOW-2000 Version 1.9 2002/07/15 - Updates were made to the LAK3,
        GAGE5, and HUF1 Packages: (1) The LAK Package was modified to add
        compatibility with the LPF and HUF packages and MODPATH, to clarify
        output, and report cumulative lake budget; (2) Modifications to the
        GAGE Package allow optional additional output related to lake gages;
        and (3) A problem related to the calculation of horizontal-
        conductance sensitivities was fixed in the HUF Package.  Some
        problems related to omission of observations (from the sum of
        squared, weighted residuals objective function and other statistics
        and the underscore ("_") output files) were corrected.  Please see
        the Release Notes in the file release.txt for more information
        related to these issues.

     MODFLOW-2000 Version 1.8 2002/05/01 - Added support for an option
        that can be used to suppress printing of cell lists in any or all
        of the RIV, GHB, DRN, DRT, HFB, WEL, and CHD Packages.  This
        version also includes fixes to (1) problems in the ADV Package
        related to stagnant particles and incorrect sensitivities for 
        observation locations in confining units; (2) a bug that caused an
        invalid value to be put into a hydraulic-property array when the
        OBS Process was active and the PES Process was inactive; (3) 
        problems in LPF and HUF related to the sensitivity of vertical 
        conductance to head in the calculation of sensitivities for 
        unconfined cells; (4) a problem in HUF related to the rewetting 
        capability; (5) a problem in LMT related to use of the STR Package
        and and the Extended-header option of LMT; and (6) problems in in
        the RESAN2K post-processing program that affected ordering of
        observation names and plot symbols.

     MODFLOW-2000 Version 1.7 2001/12/04 - Added support for time-varying
        parameters.  Time-varying parameters are useful when the user
        desires to estimate one parameter but vary properties included in
        the parameter definition from one stress period to another.
        Please see files Time-varying-parameters.pdf and str6.pdf in the
        doc directory for details.

     MODFLOW-2000 Version 1.6 2001/10/19 - Fixed two problems.  The first
        caused simulation time to be truncated, for runs that involve
        calculation of sensitivities, to a simulation period one time step
        too short when the last flow observation in a transient simulation
        falls in a later time step than the final head observation.  The
        second problem caused observation sensitivities for constant-head
        flow observations to be incorrect for log-transformed parameters.

        In addition, support for new options for the name file was added to
        allow users to designate files of type DATA, DATA(BINARY), DATAGLO,
        or DATAGLO(BINARY) as either input or output files.  Please see the
        Release Notes in the file release.txt for a description of these
        options.

     MODFLOW-2000 Version 1.5 2001/08/16 - Added the LMT Package to
        support generation of the link file required by the contaminant-
        transport program MT3DMS.

     MODFLOW-2000 Version 1.4 2001/07/10 - Added the LMG Package for
        solving matrix equations using an algebraic multigrid solver.

     MODFLOW-2000 Version 1.3 2001/06/11 - Fixed problem in LPF where if
        a lower cell goes dry while the cell above is still wet, there
        is a non-zero vertical conductance to a dry cell, causing
        calculated heads and flows to be in error.

     MODFLOW-2000 Version 1.2 2001/04/12 - Added support for using
        vendor-specific options for opening unformatted (binary)
        files.  On personal computers, this can make it possible
        for programs compiled by different compilers to write files
        that have the same structure.  The formula used to calculate
        dimensionless, scaled sensitivities of simulated equivalents
        to observations for log-transformed parameters is changed.
        Other changes include bug fixes and clarification of output.
        The MODFLOW-2000 distribution now includes Release Notes in
        the file release.txt in the doc directory -- this file
        describes in detail changes to MODFLOW-2000 for each version.
        
     MODFLOW-2000 Version 1.1 2001/01/17 - Added IBS, HUF, LAK, ETS,
        and DRT Packages.  Added HYDMOD option.  The EVT Package was
        modified as described in the LAK documentation to include
        the option (NEVTOP=3) to apply ET to the uppermost wet cell
        in a vertical column of cells as done in the RCH Package. 

     MODFLOW-2000 Version 1.0 2000/07/20 - Enhanced modular structure
        that facilitates the addition of broader capabilities such as
        parameter estimation and solute transport.  New data input
        methods are included to support parameter estimation.
        Vertical cell dimensions are specified in a separate
        discretization file.  A new package called the LPF Package
        has been added as an alternative to the BCF Package.

        The MODFLOW-2000 documentation report (OFR 00-92) does not
        include the STR Package; however, this code has been upgraded
        to use parameters and is included in this version of
        MODFLOW-2000.  Revised STR input instructions can be found
        in file str6.pdf, which is included in the doc directory that
        is part of the distribution for MODFLOW-2000.

        The ADV Package has been modified for use with MODFLOW-2000.
        Changes in input instructions are contained in file adv.txt,
        which is also in the doc distribution directory.

        The RES and FHB Packages are included in MODFLOW-2000 without
        modification. These will work as before, but they do not
        incorporate parameters or observations.

        The IBS, TLK, and GFD Packages are not included in this
        release. They require modification in order to work within
        MODFLOW-2000.

        Note that MODFLOW-2000 sends some messages to the computer
        display.  Some Fortran compilers make use of a reserved file
        unit, for example unit 6, for output to the display.  If such
        compilers are used, it is important to avoid using a reserved
        unit for any files in the MODFLOW name file.

     MODFLOW-96 Version 3.3 2000/05/02 - Error fixed in IBS code that
        caused incorrect values for critical head to be written when
        the option is invoked to save these values in an external
        file (variable IHCSV>0).  The error did not affect the
        actual interbed storage calculations, and none of the
        other model results, such as computed head and budget
        flows, are affected by the correction. There are no
        changes to any other packages.

     MODFLOW-96 Version 3.2 1998/01/09 - The Flow and Head Boundary Package,
        Version 1, (FHB1) has been added. This package allows for
        assignment of transient specified-flow and specified-head
        boundaries in applications of MODFLOW-96.  The FHB1
        package is documented in U.S. Geological Survey Open-File
        Report 97-571.

     MODFLOW-96 Version 3.1 1997/03/11 - Calls to the Horizontal Flow
        Barrier (HFB) Package have been corrected in the main
        program.  The main program for Version 3.0 incorrectly
        calls the HFB1RP module where it should call the HFB1FM
        module.

     MODFLOW-96 Version 3.0 1996/12/03 - Updated version of overall model,
        which is called MODFLOW-96.  A number of changes were
        made to make MODFLOW easier to use and easier to enhance.
        MODFLOW-96 can use existing input data sets, and has the
        same computational methods.  Any package added to the
        original model (now referred to as MODFLOW-88) will also
        work with MODFLOW-96.

        The Preconditioned Conjugate Gradient Package, Version 2
        (PCG2) has been updated.  This requires a minor change in
        input data compared to the documentation in U.S.
        Geological Survey Water-Resources Investigations Report
        (WRIR) 90-4048 (full reference below).  WRIR 90-4048
        specifies two lines of input data.  All of the data are
        the same except the seventh value on the 2nd line.  This
        value is named IPCGCD in WRIR-4048, but it has been
        replaced by a value named DAMP.  That is, IPCGCD is no
        longer part of the input data.  DAMP can be used to
        reduce oscillation when the solver is having difficulty
        converging due to excessive oscillation.  The value of
        DAMP is multiplied times the head change calculated each
        iteration at all cells.  Thus, if DAMP is 0.5, the head
        change is cut in half.  If DAMP is 1.0, then PCG2 behaves
        as it did prior to the addition of this capability.  DAMP
        should be set equal to 1.0 except when there is
        indication of excessive oscillation.  If the value of
        DAMP is specified as 0.0 or less, it is automatically
        changed to 1.0.

        Also the sign of the C.B. STORAGE cell-by-cell budget
        data in the TLK1 Package was changed to match the
        standard sign convention in MODFLOW.  The change to TLK1
        does not impact computed heads or the overall volumetric
        budget; it only affects data written to a cell-by-cell
        budget file.

     MODFLOW-88 Version 2.6 1996/09/20 - Added Reservoir package (RES1) as
        documented in U.S. Geological Survey Open-File Report
        96-364.  Problem fixed for IBS package.  Although
        subsidence is only meant to be active for layers in which
        IBQ>0, sometimes MODFLOW performed subsidence
        calculations when IBQ<0.  Note that this was a problem
        only if negative IBQ values were specified.  That is, the
        code has always worked correctly for IBQ=0 and IBQ>0.

     MODFLOW-88 Version 2.5 1995/06/23 - Added direct solution package
        (DE45).

     MODFLOW-88 Version 2.4 1995/06/15 - Added transient leakage package
        (TLK1).

     MODFLOW-88 Version 93/08/30 - Release with PCG2, BCF3, STR1, HFB1,
        ISB1, CHD1, and GFD1 additions.

     MODFLOW-88 Version 87/07/24 - Fortran 77 version published in U.S.
        Geological Survey Techniques of Water-Resources
        Investigations 6-A1.

     MODFLOW Version 83/12/28 - Fortran 66 version published in U.S.
        Geological Survey Open-File Report 83-875.

DATA REQUIREMENTS
     In order to use MODFLOW, initial conditions, hydraulic
     properties, and stresses must be specified for every model
     cell in the finite-difference grid.

OUTPUT OPTIONS
     Primary output is head, which can be written to the listing
     file or into a separate file.  Other output includes the
     complete listing of all input data, drawdown, and budget
     data.  Budget data are printed as a summary in the listing
     file, and detailed budget data for all model cells can be
     written into a separate file.

SYSTEM REQUIREMENTS
     MODFLOW-2000 generally is written in Fortran 77 with the 
     following extensions:  use of variable names longer than 6
     characters, Fortran 90 statements for dynamic memory
     allocation, a call to SUBROUTINE GETARG to retrieve command-
     line arguments, and two calls to DATE_AND_TIME, which is a
     Fortran 90 intrinsic subroutine. Generally, the program is
     easily installed on most computer systems.  The code has
     been used on UNIX-based computers and DOS-based 386 or
     greater computers having a math coprocessor and 4 mb of 
     memory.
     
     Starting with version 1.12, a Fortran-90 or later compiler 
     is required to compile one or more of the source-code files, 
     including gwf1sub1.f.

PROCESSES
     This version of MODFLOW includes the following processes:

         GWF1 -- Ground-Water Flow Process
         SEN1 -- Sensitivity Process
         OBS1 -- Observation Process
         PES1 -- Parameter-Estimation Process

PACKAGES
     This version of MODFLOW includes the following packages:

         BAS6 -- Basic Package
         BCF6 -- Block-Centered Flow Package
         LPF1 -- Layer-Property Flow Package
         RIV6 -- River Package
         DRN6 -- Drain Package
         WEL6 -- Well Package
         GHB6 -- General Head Boundary Package
         RCH6 -- Recharge Package
         EVT6 -- Evapotranspiration Package
         CHD6 -- Time-Variant Specified-Head Package
         HFB6 -- Horizontal Flow Barrier Package
         SIP5 -- Strongly Implicit Procedure Package
         SOR5 -- Slice Successive Over-Relaxation Package
         PCG2 -- Version 2 of Preconditioned Conjugate Gradient
                 Package
         DE45 -- Direct solver
         LMG1 -- Multigrid solver (for USGS use only)
         STR6 -- Streamflow-Routing Package
         ADV2 -- Advective-Transport Observation Package
         RES1 -- Reservoir Package (RES is the file type in the
                 MODFLOW name file)
         FHB1 -- Flow and Head Boundary Package (FHB is the file
                 type in the MODFLOW name file)
         IBS6 -- Interbed Storage (subsidence) Package (IBS is the
                 file type in the name file)
         HUF2 -- Hydrogeologic-Unit Flow Package
         LAK3 -- Lake Package
         ETS1 -- Evapotranspiration with a Segmented Function Package
         DRT1 -- Drains with Return Flow Package
         LMT6 -- Link to MT3DMS contaminant-transport model
         MNW1 -- Multi-Node Well Package
         DAF1 -- Diffusion Analogy Surface-Water Flow Package
         SUB1 -- Subsidence and Aquifer-System Compaction Package
         SFR2 -- Stream-Flow Routing Package, version 2
         GMG1 -- Geometric MultiGrid Solver Package
         SWT1 -- Subsidence and Aquifer-System Compaction Package
                 for Water-Table Aquifers
         MNW2 -- Revised Multi-Node Well Package

ADDITIONAL CAPABILITIES:

         HYDMOD -- Hydrograph option
         GAGE -- Hydrograph option for lakes (LAK3 Package) and streams 
                 (SFR Package)
            

DOCUMENTATION
  The basic documentation is contained in the following five
  reports:

     Harbaugh, A.W., Banta, E.R., Hill, M.C., and McDonald, M.G., 2000,
     MODFLOW-2000, the U.S. Geological Survey modular ground-water model --
     User guide to modularization concepts and the Ground-Water Flow
     Process: U.S. Geological Survey Open-File Report 00-92, 121 p.
     
     Hill, M.C., Banta, E.R., Harbaugh, A.W., and Anderman, E.R., 2000,
     MODFLOW-2000, the U.S. Geological Survey modular ground-water model --
     User guide to the Observation, Sensitivity, and Parameter-Estimation
     Processes and three post-processing programs: U.S. Geological Survey
     Open-File Report 00-184, 210 p.

     Harbaugh, A.W., and McDonald, M.G., 1996, User's
     documentation for MODFLOW-96, an update to the U.S.
     Geological Survey modular finite-difference ground-water
     flow model: U.S. Geological Survey Open-File Report
     96-485, 56 p.

     Harbaugh, A.W., and McDonald, M.G., 1996, Programmer's
     documentation for MODFLOW-96, an update to the U.S.
     Geological Survey modular finite-difference ground-water
     flow model: U.S. Geological Survey Open-File Report
     96-486, 220 p.

     McDonald, M.G., and Harbaugh, A.W., 1988, A modular three-
     dimensional finite-difference ground-water flow model:
     U.S. Geological Survey Techniques of Water-Resources
     Investigations, book 6, chap. A1, 586 p.

  The BCF6 code is documented in several places.  It includes
  the capabilities of BCF1, BCF2, and BCF3 Packages.  The
  primary documentation is in the basic model documentation
  (TWRI 6-A1, OFR 96-485, and OFR 96-486).

  BCF2 documentation describes the addition of the capability
  to convert dry cells to wet:

     McDonald, M.G., Harbaugh, A.W., Orr, B.R., and Ackerman,
     D.J., 1992, A method of converting no-flow cells to
     variable-head cells for the U.S. Geological Survey
     modular finite-difference ground-water flow model: U.S.
     Geological Survey Open-File Report 91-536, 99 p.

  BCF3 documentation describes the addition of alternate
  interblock transmissivities.

     Goode, D.J., and Appel, C.E., 1992, Finite-difference
     interblock transmissivity for unconfined aquifers and for
     aquifers having smoothly varying transmissivity: U.S.
     Geological Survey Water-Resources Investigations Report
     92-4124, 79 p.

  Version 2 of Preconditioned Conjugate Gradient Package is
  documented in:

     Hill, M.C., 1990, Preconditioned conjugate-gradient 2
     (PCG2), a computer program for solving ground-water flow
     equations: U.S. Geological Survey Water-Resources
     Investigations Report 90-4048, 43 p.

  The Streamflow-Routing (STR) Package is documented in:

     Prudic, D.E., 1989, Documentation of a computer program to
     simulate stream-aquifer relations using a modular,
     finite-difference, ground-water flow model: U.S.
     Geological Survey Open-File Report 88-729, 113 p.

  The Time-Variant Specified-Head Package is documented in:

     Leake, S.A., and Prudic, D.E., 1991, Documentation of a
     computer program to simulate aquifer-system compaction
     using the modular finite-difference ground-water flow
     model:  U.S. Geological Survey Techniques of Water-
     Resources Investigations, book 6, chap. A2, 68 p.

  The Horizontal-Flow Barrier Package is documented in:

     Hsieh, P.A., and Freckleton, J.R., 1993, Documentation of a
     computer program to simulate horizontal-flow barriers
     using the U.S. Geological Survey modular three-
     dimensional finite-difference ground-water flow model:
     U.S. Geological Survey Open-File Report 92-477, 32 p.

  The DE45 Package is documented in:

     Harbaugh, A.W., 1995, Direct solution package based on
     alternating diagonal ordering for the U.S. Geological
     Survey modular finite-difference ground-water flow model:
     U.S. Geological Survey Open-File Report 95-288, 46 p.

  The RES1 Package is documented in:

     Fenske, J.P., Leake, S.A., and Prudic, D.E., 1996,
     Documentation of a computer program (RES1) to simulate
     leakage from reservoirs using the modular finite-
     difference ground-water flow model (MODFLOW): U.S.
     Geological Survey Open-File Report 96-364, 51 p.

  The FHB1 Package is documented in:

     Leake, S.A., and Lilly, M.R., 1997, Documentation of a
     computer program (FHB1) for assignment of transient
     specified-flow and specified-head boundaries in
     applications of the modular finite- difference ground-
     water flow model (MODFLOW): U.S. Geological Survey Open-
     File Report 97-571, 50 p.

  The ADV Package is documented in:

     Anderman, E.R. and Hill, M.C., 1997, Advective-transport
     observation (ADV) package, a computer program for adding
     advective-transport observations of steady-state flow
     fields to the three-dimensional ground-water flow
     parameter-estimation model MODFLOWP: U.S. Geological
     Survey Open-File Report 97-14, 67 p.
     
     Anderman, E.R. and Hill, M.C., 2001, MODFLOW-2000, the U.S. 
     Geological Survey modular ground-water model -- Documentation 
     of the  Advective-Transport Observation (ADV2) Package, 
     version 2: U.S. Geological Survey Open-File Report 01-54,
     69 p.
     
     File MODPATH-ADV_compatibility_3-2006.pdf, which is included 
     in the doc directory of the MODFLOW-2000 distribution.

  HYDMOD is documented in:

     Hanson, R.T. and Leake, S.A., 1999, Documentation of HYDMOD,
     a program for extracting and processing time-series data
     from the U.S. Geological Survey's modular three-dimensional
     finite-difference ground-water flow model: U.S. Geological
     Survey Open-File Report 98-564, 57 p.

  The IBS Package is documented in:

     Leake, S.A. and Prudic, D.E., 1991, Documentation of a
     computer program to simulate aquifer-system compaction using
     the modular finite-difference ground-water flow model: U.S.
     Geological Survey Techniques of Water-Resources
     Investigations, Book 6, Chapter A2, 68 p.

  The HUF Package is documented in:

     Anderman, E.R., and Hill, M.C., 2000, MODFLOW-2000, the U.S.
     Geological Survey modular ground-water model -- Documentation
     of the Hydrogeologic-Unit Flow (HUF) Package: U.S. Geological
     Survey Open-File Report 00-342, 89 p.
     
     Anderman, E.R., Kipp, K.L., Hill, M.C., Valstar, Johan, and 
     Neupauer, R.M., 2002, MODFLOW-2000, the U.S. Geological Survey
     modular ground-water model -- Documentation of the Model-Layer 
     Variable-Direction Horizontal Anisotropy (LVDA) capability of
     the Hydrogeologic-Unit Flow (HUF) Package: U.S. Geological
     Survey Open-File Report 02-409, 60 p.

     Anderman, E.R., and Hill, M.C., 2003, MODFLOW-2000, the U.S. 
     Geological Survey modular ground-water model -- Three additions 
     to the Hydrogeologic-Unit Flow (HUF) Package: Alternative storage
     for the uppermost active cells, Flows in hydrogeologic units, and
     the Hydraulic-coductivity depth-dependence (KDEP) capability:
     U.S. Geological Survey Open-File Report 03-347, 36 p.

  The LAK Package is documented in:

     Merritt, L.M. and Konikow, L.F., 2000, Documentation of a
     computer program to simulate lake-aquifer interaction using
     the MODFLOW ground-water flow model and the MOC3D
     solute-transport model:  U.S. Geological Survey Water-Resources
     Investigations Report 00-4167, 146 p.

  The ETS and DRT Packages are documented in:

     Banta, E.R., 2000, MODFLOW-2000, the U.S. Geological Survey
     modular ground-water model -- Documentation of packages for
     simulating evapotranspiration with a segmented function (ETS1)
     and drains with return flow (DRT1): U.S. Geological Survey
     Open-File Report 00-466, 127 p.

  The LMG Package is documented in:

     Mehl, S.E. and Hill, M.C., MODFLOW-2000, the U.S. Geological
     Survey modular ground-water model -- User guide to the LINK-AMG
     (LMG) Package for solving matrix equations using an algebraic
     multigrid solver: U.S. Geological Survey Open-File Report 01-177,
     33 p.

  The LMT Package is documented in:

     Zheng, Chunmiao, Hill, M.C., and Hsieh, P.A., 2001, MODFLOW-2000,
     the U.S. Geological Survey modular ground-water model - User
     guide to the LMT6 package, the linkage with MT3DMS for multi-
     species mass transport modeling: U.S. Geological Survey Open-File
     Report 01-82, 43 p.

  The MNW1 Package is documented in:

     Halford, K.J. and Hanson, R.T., 2002, User guide for the drawdown-
     limited, multi-node well (MNW) package for the U.S. Geological
     Survey's modular three-dimensional finite-difference ground-water
     flow model, versions MODFLOW-96 and MODFLOW-2000: U.S. Geological
     Survey Open-File Report 02-293, 33 p.
     
  The MNW2 Package is documented in:
  
     Konikow, L.F., Hornberger, G.Z., Halford, K.J., and Hanson, R.T.,
     2009, Revised multi-node well (MNW2) package for MODFLOW ground-
     water flow model: U.S. Geological Survey Techniques and Methods 
     6-A30, 67 p., available at: http://pubs.usgs.gov/tm/tm6a30/

  The DAF Package is documented in:

     Jobson, H.E. and Harbaugh, A.W., 1999, Modifications to the
     diffusion analogy surface-water flow model (DAFLOW) for coupling
     to the moddular finite-difference ground-water flow model (MODFLOW):
     U.S. Geological Survey Open-File Report 99-217, 107 p.
     
  The SUB Package is documented in: 

     Hoffmann, J., Leake, S.A., Galloway, D.L., and Wilson, A.M., 2003,
     MODFLOW-2000 Ground-Water Model--User Guide to the Subsidence and
     Aquifer-System Compaction (SUB) Package: U.S. Geological Survey 
     Open-File Report 03-233, 44 p.

  The SFR Package is documented in:

     Prudic, D.E., Konikow, L.F., and Banta, E.R., 2004, A new stream-
     flow routing (SFR1) package to simulate stream-aquifer interaction
     with MODFLOW-2000: U.S. Geological Survey Open-File Report
     2004-1042, 95 p.
     
     Niswonger, R.G. and Prudic, D.E., 2005, Documentation of the 
     Streamflow-Routing (SFR2) Package to include unsaturated flow 
     beneath streams--A modification to SFR1: U.S. Geological Survey 
     Techniques and Methods, Book 6, Chap. A13, 47 p.

  The GMG Package is documented in:

     Wilson, J.D. and Naff, R.L., 2004, The U.S. Geological Survey modular
     ground-water model -- GMG linear equation solver package 
     documentation:  U.S. Geological Survey Open-File Report 2004-1261, 
     47 p.  <http://pubs.water.usgs.gov/ofr2004-1261/>
     
  The SWT Package is documented in:
  
     Leake, S.A., and Galloway, D.L., 2007, MODFLOW ground-water model -- 
     User guide to the Subsidence and Aquifer-System Compaction Package 
     (SUB-WT) for Water-Table Aquifers: U.S. Geological Survey Techniques
     and Methods, Book 6, Chap. A23, 42 p. 
     <http://pubs.usgs.gov/tm/2007/06A23/pdf/TM6a23.pdf>


CONTACTS
     Operation:
        U.S. Geological Survey
        Office of Ground Water
        Arlen Harbaugh
        411 National Center
        Reston, VA 20192

        harbaugh@usgs.gov


     The USGS MODFLOW fact sheet (a short description of the MODFLOW
     model) can be retrieved using the World-Wide Web at site:

             http://water.usgs.gov/public/pubs/FS/FS-121-97/

     See

             http://water.usgs.gov/software/ordering_documentation.html

     for information on ordering printed copies of USGS publications.


SEE ALSO

     modpath(1) - Particle-tracking postprocessor program for the
                  modular three-dimensional finite-difference
                  ground-water flow model

     zonebudget(1) -- Program for computing subregional water budgets
                      for MODFLOW ground-water flow models
