modflw96.txt

Modflow96地下水数值模拟软件

modflw96(1)      U.S. Geological Survey (wrdapp)      modflw96(1)


NAME
       modflw96 - Modular three-dimensional finite-difference ground-water
                  flow model

ABSTRACT
       MODFLOW is a three-dimensional finite-difference ground-water flow
       model.  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 96-485
       (complete reference below) documents a general update to MODFLOW,
       which is called MODFLOW-96 in order to distinguish it from earlier
       versions.

       MODFLOW 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 direction aligned with the grid axes and the anisotropy
       ratio between horizontal coordinate directions is fixed in any one
       layer), and the storage coefficient may be heterogeneous.  The model
       requires input of the ratio of vertical hydraulic conductivity to
       distance between vertically adjacent block centers.  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.  An efficient contouring
       program is available (Harbaugh, 1990) to visualize heads and
       drawdowns output by the model.

METHOD
       The ground-water flow equation is solved using the finite-difference
       approximation.  The flow region is considered to be subdivided into
       blocks in which the medium properties are assumed to be uniform.
       The plan view rectangular discretization results from a grid of
       mutually perpendicular lines that may be variably spaced.  The
       vertical direction zones of varying thickness are transformed into a
       set of parallel "layers".  Several solvers are provided for solving
       the associated matrix problem; the user can choose the best solver
       for the particular problem.  Mass balances are computed for each
       time step and as a cumulative volume from each source and type of
       discharge.

HISTORY
       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.



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modflw96(1)      U.S. Geological Survey (wrdapp)      modflw96(1)


       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.

       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.

       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.

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

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

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

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




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modflw96(1)      U.S. Geological Survey (wrdapp)      modflw96(1)


       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-96 is written in Fortran 77 with the following extension:
       use of variable names longer than 6 characters.  By default, the
       software is dimensioned for use with models having up to 90,000
       cells.  MODFLOW-96 requires that file names needed for the
       simulation be defined prior to execution.  A Name File is used for
       this purpose.  Each record of the Name File specifies a file type,
       unit number, and file name for each file used in the MODFLOW
       simulation.  The format of this file is described in the
       documentation.  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.

PACKAGES
       This version of MODFLOW includes the following packages:

           BAS5 -- Basic Package
           BCF5 -- Version 3 of Block-Centered Flow Package
           RIV5 -- River Package
           DRN5 -- Drain Package
           WEL5 -- Well Package
           GHB5 -- General Head Boundary Package
           RCH5 -- Recharge Package
           EVT5 -- Evapotranspiration Package
           SIP5 -- Strongly Implicit Procedure Package
           SOR5 -- Slice Successive Over-Relaxation Package
           UTL5 -- Utility Package
           PCG2 -- Version 2 of Preconditioned Conjugate Gradient Package
           STR1 -- Stream Package
           IBS1 -- Interbed-Storage Package
           CHD1 -- Time-Variant Specified-Head Package
           GFD1 -- General Finite Difference Flow Package
           HFB1 -- Horizontal Flow Barrier Package
           TLK1 -- Transient Leakage Package
           DE45 -- Direct solver
           RES1 -- Reservoir Package
           FHB1 -- Flow and Head Boundary Package




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modflw96(1)      U.S. Geological Survey (wrdapp)      modflw96(1)


       The user must specify the file type for each file in the name file.
       File types for packages not in the original model are:

           TLK -- Transient leakage
           DE4 -- D4 Direct solver
           PCG -- Preconditioned Conjugate Gradient solver
           GFD -- General Finite-Difference Package
           HFB -- Horizontal-Flow Barrier Package
           STR -- Stream Package
           IBS -- Interbed Storage Package
           CHD -- Time-Variant Specified-Head Package
           RES -- Reservoir Package
           FHB -- Flow and Head Boundary Package


DEPENDENCIES AMONG PACKAGES
       As documented in Open-File Report (OFR) 94-59, the Transient Leakage
       (TLK) Package does not simulate flow through a confining unit at any
       horizontal grid location at which a cell on either side of the
       confining unit is dry.  When this situation occurs as a result of
       initial conditions, the user can determine if this is appropriate
       before making a simulation.  However, a cell can go dry at any time
       during a simulation when using the water-table or convertible layer
       options in the Block-Centered Flow (BCF) Package.  When a cell goes
       dry on either side of a confining unit, the transient leakage
       through the confining unit immediately becomes zero at that
       horizontal location.  Users should check simulations to see if cells
       on either side of a confining unit are going dry at any time during
       a simulation and determine if it is acceptable for the transient
       leakage to switch to zero.  Further complications can result when
       using the wetting capabilities of version 2 of the BCF Package.  If
       dry cells convert to wet so that cells on both sides of a confining
       unit are wet, then transient leakage calculations will be started;
       however, the equations will not be properly formulated to simulate
       the previous conditions, so the transient flow will not be correct.
       Thus, the wetting capability should not be used for any model layers
       that connect to a confining unit that is being simulated with the
       TLK Package.

       The Time-Variant Specified-Head (CHD) Package can potentially cause
       the TLK Package to operate incorrectly if the CHD Package is being
       used to specify constant heads at cells on either side of a
       confining unit.  The TLK Package relies on initial head as defined
       by the Basic Package to setup initial parameters.  If the data for
       the CHD Package define initial heads (i.e., head for the first time
       step of the simulation) on either side of a confining unit to be
       different than defined by the Basic Package, the transient leakage
       calculations will be incorrect.  To avoid this conflict, do not use
       the CHD Package to define constant head cells on either side of a
       confining unit, or be sure that the initial head in the Basic
       Package exactly matches the initial head defined by the CHD Package.

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



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