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 3.3 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.
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/
See
http://water.usgs.gov/software/ordering_documentation.html
for information on ordering printed copies of USGS
publications.
SEE ALSO
contour(1) - A contouring program for gridded data mmsp -
Modular Model Statistical Processor
moc(1) - Two-dimensional method-of-characteristics ground-
water flow and transport model
moc3d(1) - Three-dimensional method-of-characteristics
ground-water flow and transport model
modfe(1) - Modular finite-element model for areal and
axisymmetric ground-water flow problems
Modflowp(1) - Parameter-estimation version of the modular
model
modpath(1) - Particle-tracking postprocessor program for the
modular three-dimensional finite-difference
ground-water flow model
sutra(1) - Saturated and (or) unsaturated, constant or
variable-density fluid flow, and solute or energy
transport (2-dimensional finite-element code)
vs2di(1) - A graphical software package for simulating fluid
flow and solute or energy transport in variably
saturated porous media