Water Resources Applications Software

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Summary of VS2DT

       vs2dt - Model for simulating water flow and solute transport in
               variably saturated porous media

       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.

       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.

       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

       1986/10/10 - Original VS2D release.
       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.

       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.

       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.

       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.

       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.

       Advanced Modeling of Subsurface Flow and Transport (GW3086TC), USGS
       National Training Center.

       Operation and Distribution:
          U.S. Geological Survey
          Hydrologic Analysis Software Support Program
          437 National Center
          Reston, VA 20192

       Official versions of U.S. Geological Survey water-resources analysis
       software are available for electronic retrieval via the World Wide
       Web (WWW) at:


       and via anonymous File Transfer Protocol (FTP) from:

         (path: /pub/software).

       The WWW page and anonymous FTP directory from which the VS2DT
       software can be retrieved are, respectively:


       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
       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)

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