Water Resources Applications Software
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Summary of MODFLW96
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.
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.
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
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:
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 BCF5 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 Stream 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 Interbed-Storage and Time-Variant Specified-Head Packages are
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 General Finite Difference Flow Package is documented in:
Harbaugh, A.W., 1992, A generalized finite-difference formulation
for the U.S. Geological Survey modular three-dimensional finite-
difference ground-water flow model: U.S. Geological Survey Open-
File Report 91-494, 60 p.
The HFB1 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 Transient-Leakage Package (TLK1) is documented in:
Leake, S.A., Leahy, P.P., and Navoy, A.S., 1994, Documentation of a
computer program to simulate transient leakage from confining
units using the modular finite-difference ground-water flow
model: U.S. Geological Survey Open-File Report 94-59, 70 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.
RELATED DOCUMENTATION
Harbaugh, A.W., 1990, A simple contouring program for gridded data:
U.S. Geological Survey Open-File Report 90-144, 37 p.
REFERENCES
MODFLOW is widely used in the USGS and throughout the world.
Belitz, K., and Phillips, S.P., 1993, Numerical simulation of
ground-water flow in the central part of the western San Joaquin
Valley, California: U.S. Geological Survey Water-Supply Paper
2396, 69 p.
Prince, K.R., Franke, O.L., and Reilly, T.E., 1988, Quantitative
assessment of the shallow ground-water flow system associated
with Connetquot Brook, Long Island, New York: U.S. Geological
Survey Water-Supply Paper 2309, 28 p.
TRAINING
Modeling of Ground-Water Flow Using Finite-Difference Methods
(GW2096TC), offered annually at the USGS National Training Center.
Advanced Finite-Difference Modeling of Ground-Water Flow (GW3099TC),
offered annually at the USGS National Training Center.
CONTACTS
Operation:
U.S. Geological Survey
Office of Ground Water
Arlen Harbaugh
411 National Center
Reston, VA 20192
harbaugh@usgs.gov
Distribution:
U.S. Geological Survey
Hydrologic Analysis Software Support Program
437 National Center
Reston, VA 20192
h2osoft@usgs.gov
Official versions of U.S. Geological Survey water-resources analysis
software are available for electronic retrieval via the World Wide
Web (WWW) at:
http://water.usgs.gov/software/
and via anonymous File Transfer Protocol (FTP) from:
water.usgs.gov (path: /pub/software).
The WWW page and anonymous FTP directory from which the MODFLOW
software can be retrieved are, respectively:
http://water.usgs.gov/software/modflow.html
--and--
/pub/software/ground_water/modflow
The WWW page from which the USGS MODFLOW fact sheet (a short
description of the MODFLOW model) can be retrieved is:
http://water.usgs.gov/public/pubs/FS/FS-121-97/
If you would like to obtain the price of and (or) order paper copies
of USGS reports, contact the USGS Branch of Information Services at:
U.S. Geological Survey
Branch of Information Services
Denver Federal Center, Box 25286
Denver CO 80225-0286
To inquire about Open-File Reports or Water-Resources Investigations
Reports:
Tel: 303-202-4210; Fax 303-202-4695
To inquire about other USGS reports:
Tel: 303-202-4700; Fax 303-202-4693
SEE ALSO
contour - A contouring program for gridded data
mmsp - Modular Model Statistical Processor
moc - Two-dimensional method-of-characteristics
ground-water flow and transport model
moc3d - Three-dimensional method-of-characteristics
ground-water flow and transport model
modfe - Modular finite-element model for areal and
axisymmetric ground-water flow problems
Modflowp - Parameter-estimation version of the modular model
modpath - Particle-tracking postprocessor program for
the modular three-dimensional finite-difference
ground-water flow model
sutra - Saturated and (or) unsaturated, constant or
variable-density fluid flow, and solute or energy
transport (2-dimensional finite-element code)
vs2dt - Model for simulating water flow and solute
transport in variably saturated porous media
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Page created: 02/15/2000