NAMEvs2dt - Model for simulating water flow and solute transport in variably saturated porous mediaABSTRACTComputer 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.METHODFor 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.HISTORYVersion 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 REQUIREMENTSA 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 OPTIONSSimulation 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 REQUIREMENTSVS2DT 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.DOCUMENTATIONHealy, 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.REFERENCESHealy, 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.TRAININGAdvanced Modeling of Subsurface Flow and Transport (GW3086TC), USGS National Training Center.CONTACTSOperation 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/vs2dtSEE ALSOmoc(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)

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