NAME
vs2dt - Model for simulating water flow and solute transport in
variably saturated porous media
ABSTRACT
Computer program VS2DT solves problems of water and solute movement
in variably saturated porous media. The finite difference method is
used to approximate the flow equation, which is developed by
combining the law of conservation of fluid mass with a nonlinear
form of Darcy's equation, and the advection-dispersion equation.
The model can analyze problems in one and two dimensions with planar
or cylindrical geometries. There are several options for using
boundary conditions that are specific to flow under unsaturated
conditions: infiltration with ponding, evaporation, plant
transpiration, and seepage faces. Solute transport options include
first-order decay, adsorption, and ion exchange. The program is
written in standard Fortran 77. Extensive use of subroutines and
functions subprograms provides a modular code that can be easily
modified for particular applications.
METHOD
For the flow equation, spatial derivatives are approximated by
central differences written about grid-block boundaries. Time
derivatives are approximated by a fully implicit backward scheme.
Nonlinear conductance terms, boundary conditions, and sink terms are
linearized implicitly. Relative hydraulic conductivity is evaluated
at cell boundaries by using full upstream weighting, the arithmetic
mean, or the geometric mean of values from adjacent cells.
Saturated hydraulic conductivities are evaluated at cell boundaries
by using distance-weighted harmonic means. Nonlinear conductance
and storage terms can be represented by algebraic equations or by
tabular data.
For the advection-dispersion equation, either central or backward
differences may be selected for the spatial and time derivatives.
Equilibrium adsorption can be described by either Freundlich or
Langmuir isotherms. Four types of ion exchange can be simulated.
The matrix equations produced for both the flow and transport
equations are solved using the strongly implicit procedure.
HISTORY
Version 2.5 1996/09/11 - Added interactive file name designation
procedure. Hard-coded file unit numbers were replaced with
common block variables.
1996/04/09 - Uses Freundlich solution of slope of sorption isotherm
instead of Langmuir solution and incorporates several bug fixes.
1991/09/01 - Supersedes VS2D and allows simulation of solute
transport.
1986/10/10 - Original VS2D release.
DATA REQUIREMENTS
A conceptual model of the geometry and boundaries of the region to
be simulated is of prime importance. Initial conditions in terms of
pressure heads or moisture contents for flow simulations and
concentrations for transport simulations are needed. Hydraulic and
transport properties of the porous media are also required. These
values can be different for different sediments. Flow simulations
require values for saturated hydraulic conductivity and for relative
hydraulic conductivity and moisture content as functions of pressure
head. Transport simulations require values for dispersivity and
molecular diffusion. Other information may be needed, depending on
the program options that are selected.
OUTPUT OPTIONS
Simulation results can be output in terms of pressure head, total
head, volumetric moisture content, saturations, velocities, and
solute concentrations. Time histories and spatial profiles of the
data can be obtained. In addition, the user may opt to view time
histories of up to 72 mass balance parameters.
SYSTEM REQUIREMENTS
VS2DT is written in Fortran 77 with the following extensions: use
of include files and variable names longer than 6 characters.
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.
DOCUMENTATION
Healy, R.W., 1990, Simulation of solute transport in variably
saturated porous media with supplemental information on
modifications to the U.S. Geological Survey's Computer Program
VS2D: U.S. Geological Survey Water-Resources Investigations
Report 90-4025, 125 p.
Lappala, E.G., Healy, R.W., and Weeks, E.P., 1987, Documentation of
computer program VS2D to solve the equations of fluid flow in
variably saturated porous media: U.S. Geological Survey Water-
Resources Investigations Report 83-4099, 184 p.
REFERENCES
Healy, R.W., 1987, Simulation of trickle irrigation, an extension to
the U.S. Geological Survey's Computer Program VS2D: U.S.
Geological Survey Water-Resources Investigations Report 87-4086,
61 p.
VS2DT is being used by personnel in several districts, as well as by
scientists at various universities and consulting companies. An
application is described in:
Mills, P.C., and Healy, R.W., 1993, Water and tritium movement
through the unsaturated zone at a low-level radioactive-waste
disposal site near Sheffield, Illinois, 1981-85: U.S. Geological
Survey Water-Supply Paper 2386, 72 p.
TRAINING
Advanced Modeling of Subsurface Flow and Transport (GW3086TC), USGS
National Training Center.
CONTACTS
Operation and 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 VS2DT
software can be retrieved are, respectively:
http://water.usgs.gov/software/vs2dt.html
--and--
/pub/software/ground_water/vs2dt
SEE ALSO
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
modflow(1) - Modular three-dimensional ground-water flow
model
Modflowp(1) - Parameter-Estimation Version of the Modular Model
sutra(1) - Saturated and (or) unsaturated, constant or
variable-density fluid flow, and solute or energy
transport (2-dimensional finite-element code)