U.S. Geological Survey                                             vsf(1)

NAME
     VSF - Three-dimensional variably saturated ground-water flow model

ABSTRACT
The Variably Saturated Flow (VSF) Process enables MODFLOW (MF2K v15) to simulate three
dimensional variably saturated flow in porous media.  The VSF Process includes five packages 
unique to unsaturated flow; the Richards' Equation Flow (REF1) Package, the Seepage Face (SPF1)
Package, the Surface Ponding (PND1) Package, the Surface Evaporation (SEV1) Package, and the 
Root Zone Evapotranspiration (RZE1) Package.  Additionally a new Adaptive Time-Stepping (ATS1)
Package is introduced for use by both the Ground-Water Flow (GWF) Process and VSF.
Processes may be active or inactive, depending on simulation objectives; however, not all 
processes are compatible. For example, the Observation, Sensitivity and Parameter Estimation 
Processes are not compatible with VSF. The VSF computer code was tested against two published 
one-dimensional simulations, two published two-dimensional simulations and against an existing
two-dimensional model (VS2DT).

METHOD
The saturated ground-water flow equation is expanded to include unsaturated flow using
Richards' Equation and solved using a finite-difference approximation similar to the one 
solved by MODFLOW-2000. 

HISTORY

VSF Version 1.01  - Initially released by R. Brad Thoms, Richard L. Johnson (OHSU) 
and Richard W. Healy (USGS)

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

OUTPUT OPTIONS
Primary output is head and soil saturations, which can be written to the listing 
file or to separate ASCII / binary files.  Other output includes the complete listing
of all input data, drawdown, and flow budget.  Flow 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.  All binary output is 
written to files that are opened with the FORM='BINARY' option.  This means that 
some pre- and post-processors may not be able to read the output unless the open 
statements are modified, the program is recompiled, and the model is rerun.

SYSTEM REQUIREMENTS
VSF 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 
DOS-based 386 or greater computers having a math coprocessor and 4 mb of memory.  Parts
of the GMG solver are written in the C programming language.

PROCESSES
GLO1 -- Global Process
GWF1 -- Ground-Water Flow Process
VSF1 -- Variably Saturated Flow Process
OBS1 -- Observation Process
SEN1 -- Sensitivity Process
PES1 -- Parameter-Estimation Process

Process compatibility issues are described in the user's manual.  In general, the VSF
Process is not compatible with OBS, SEN and PES.  VSF is an alternative to GWF, and thus
the two cannot be used concurrently in a simulation. 

PACKAGES COMPATIBLE WITH THE VARIABLY SATURATED FLOW (SDF) PROCESS
BAS -- Basic 
BCF -- Block-Centered Flow 
LPF -- Layer Property Flow
REF -- Richards' Equation Flow 
HFB -- Hydraulic Flow Barrier
DRN -- Drain 
GHB -- General-Head Boundary 
WEL -- Well
RCH -- Recharge 
CHD -- Time-Variant Constant Head 
FHB -- Flow and Head Boundary
MNW -- Multi-Node Well
ATS -- Adaptive Time-Stepping
SPF -- Seepage Face 
PND -- Surface Ponding
SEV -- Surface Evaporation
RZE -- Root Zone Evapotranspiration
SIP -- Strongly Implicit Procedure
SOR -- Slice-Successive Overrelaxation
PCG -- Preconditioned Conjugate Gradient Solver 
DE4 -- Direct Solver
GMG -- Geometric Multi-Grid Solver
OC -- Output Control Option*
LMT -- Linkage with MT3DMS
GCG -- Generalized Conjugate Gradient Solver

*NOTE:  The Adaptive Time-Stepping (ATS1) Package is only compatible with the alphabetic output control.

PACKAGES REQUIRING USER-DISCRETION WHEN USED WITH VSF (see documentation for details)
RIV -- River
LAK -- Lake 
STR -- Stream
SFR -- Streamflow Routing

PACKAGES NOT COMPATIBLE WITH THE VARIABLY SATURATED FLOW (VSF) PROCESS

HUF2 -- Hydrogeologic Unit Flow

Other packages (distributed with MODFLOW-2000) are included in MF2K-VSF, however,
these additional packages may not be compatible with the VSF Process.

DOCUMENTATION

The basic documentation is contained in the following report:

Thoms, R.B., Johnson, R.L. and Healy, R.W., 2006, User's guide to the 
Variably Saturated Flow (VSF) Process for MODFLOW: U.S. Geological Survey
Techniques and Methods 6-A18, 58 p.

Users will also need documentation for MODFLOW-2000, and possibly the
references cited in those reports.

CONTACTS
Operation:

R. Brad Thoms
OHSU Center for Groundwater Research
20000 NW Walker Rd.
Beaverton, OR 97201
thoms@ebs.ogi.edu

Richard L. Johnson
OHSU Center for Groundwater Research
20000 NW Walker Rd.
Beaverton, OR 97201

Richard W. Healy
U.S. Geological Survey
Mail Stop 413 Box 25046
Denver Federal Center
Lakewood, CO 80225

See
http://water.usgs.gov/software/ordering_documentation.html
for information on ordering printed copies of USGS publications.