NAME
modflow - 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. This version
includes all the major capabilities that were documented as of
September 1996.
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 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-88 is written in Fortran 77 with the following extensions:
use of variable names longer than 6 characters and the
CARRIAGECONTROL option in OPEN statements. By default, the software
is dimensioned for use with models having up to 90,000 cells.
MODFLOW-88 uses preallocated files, which means that the file names
are not assigned by Fortran OPEN statements. Instead, the compiler
or operating system must provide a way to open the files. Example
script files are provided to illustrate this procedure. 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:
BAS1 -- Basic Package
BCF3 -- Version 3 of Block-Centered Flow Package
RIV1 -- River Package
DRN1 -- Drain Package
WEL1 -- Well Package
GHB1 -- General Head Boundary Package
RCH1 -- Recharge Package
EVT1 -- Evapotranspiration Package
SIP1 -- Strongly Implicit Procedure Package
SOR1 -- Slice Successive Over-Relaxation Package
UTL1 -- 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
The user must specify values for the appropriate IUNIT array element
to include packages not in the original model. The IUNIT array is
interpreted according to the following table:
BCF3 -- IUNIT(1) -- same IUNIT as used for BCF1 because BCF3
replaces BCF1
TLK1 -- IUNIT(6)
DE45 -- IUNIT(10)
PCG2 -- IUNIT(13)
GFD1 -- IUNIT(14)
HFB1 -- IUNIT(16)
RES1 -- IUNIT(17)
STR1 -- IUNIT(18)
IBS1 -- IUNIT(19)
CHD1 -- IUNIT(20)
The input unit for the Basic Package is unit 5, which is defined by
the assignment of variable INBAS in the MAIN program.
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
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.
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., 1988, Documentation of a computer
program to simulate aquifer-system compaction using the modular
finite-difference ground-water flow model: U.S. Geological Survey
Open-File Report 88-482, 80 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 Version 2 of the Block-Centered Flow Package is documented in:
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.
The BCF3 Package is documented in three pieces. It builds on two
previous versions of the Block-Centered Flow (BCF) Package.
Documentation for the BCF1 Package describes the fundamental
function of all BCF Packages. This documentation is contained in
the basic model documentation (McDonald and Harbaugh, 1988). 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. The BCF3 code includes the capabilities of BCF1
and BCF2:
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.
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.
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
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
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