1. General Model Information

Name: Simulating water and solute movement in two-dimensional variably saturated media

Acronym: SWMS_2D

Main medium: terrestrial
Main subject: hydrology, biogeochemistry
Organization level: ecosystem
Type of model: partial differential equations (finite elements,2D), ordinary differential equations
Main application:
Keywords: water flow, solute transport, unsaturated, porous media, saturated, richards equation, convection-dispersion equation, equilibrium adsorption, zero-order production, first order degradation, water uptake


Simunek, J.
U. S. Salinity Laboratory, USDA, ARS
450 West Big Springs Road
Riverside, CA 92507-4617
Walter Russell
U.S. Salinity Laboratory
450 West Big Springs Road
Riverside, CA 92507-4716

Tel: 909-369-4850
FAX: 909-342-4964
Email: wrussell@ussl.ars.usda.gov


Simunek. J., Vogel T., van Genuchten M.Th
U.S. Salinity Laboratory, USDA/ARS, Riverside, California


Simunek. J., Vogel T., van Genuchten M.Th
U.S. Salinity Laboratory, USDA/ARS, Riverside, California

SWMS_2D is a model for simulating water and solute movement in two-dimensional variably saturated media. Horizontal, vertical, inclined or 3-D-radial symmetric flow-problems can be treated. Meanwhile a 3D-version SWMS_3D (which can be applied for 2D-problems as well) is available. SWMS_2D is the model used by the Windows-based modeling package HYDRUS_2D . The program numerically solves the Richards' equation for saturated-unsaturated water flow and the convection-dispersion equation for solute transport. The flow equation incorporates a sink term to account for water uptake by plant roots. The transport equation includes provisions for linear equilibrium adsorption, zero-order production, and first order degradation.
The program may be used to analyze water and solute movement in unsaturated, partially saturated, or fully saturated porous media. SWMS_2D can handle flow regions delineated by irregular boundaries. The flow region itself may be composed of nonuniform soils having an arbitrary degree of local anisotropy. The water flow part of the model can deal with prescribed head and flux boundaries, as well as boundaries by atmospheric conditions. For solute transport, the code supports both (constant and varying) prescribed concentration (Dirichlet or first-type) and concentration flux (Cauchy or third-type) boundaries. The code can also handle a seepage face boundary through which water leaves the saturated part of the flow domain, and free drainage boundary conditions. Nodal drains are represented by a simple relationship derived from electric analog experiments. The dispersion tensor includes a term reflecting the effects of molecular diffusion and tortuosity. The unsaturated soil hydraulic properties are described by a set of closed-form equations resembling the 1980 van Genuchten equations. Modifications were made to improve the description of hydraulic properties near saturation. SWMS_2D implements a scaling procedure to approximate the hydraulic variability in a given area by means of a set of linear scaling transformations which relate the individual soil hydraulic characteristics to reference characteristics.
The governing flow and transport equations are solved numerically using Galerkin-type linear finite element schemes. Depending upon the size of the problem, the matrix equations resulting from discretization of the governing equations are solved using either Gaussian elimination for banded matrices, or a conjugate gradient method for symmetric matrices and the ORTHOMIN method for asymmetric matrices. Additional measures are taken to improve solution efficiency in transient problems, including automatic time step adjustment and checking if the Courant and Peclet numbers do not exceed preset levels. The water content term is evaluated using the mass-conservative method proposed by Celia et al.(1990). To minimize numerical oscillations upstream weighing is included as an option for solving the transport equation.

II. Technical Information

II.1 Executables:

Operating System(s):

II.2 Source-code:

Programming Language(s): ANSI standard FORTRAN 77 The source code is available upon request from the authors, please email: wrussell@ussl.ars.usda.gov or jsimunek@ussl.ars.usda.gov The program comes with a user manual giving detailed instructions for data input preparation. Example input and selected output files are also provided.

II.3 Manuals:

see section II.2

II.4 Data:

see section II.2

III. Mathematical Information

III.1 Mathematics

III.2 Quantities

III.2.1 Input

III.2.2 Output

IV. References

Simunek. J., Vogel T., van Genuchten M.Th., 1994. The SWMS_2D code for simulating water flow and solute transport in two-dimensional variably saturated media, Version 1.21. Research Report No. 132, 197 p., U.S. Salinity Laboratory, USDA, ARS, Riverside, California.

V. Further information in the World-Wide-Web

VI. Additional remarks

Last review of this document by: T. Gabele: Dec 17 1997
Status of the document:
last modified by Tobias Gabele Wed Aug 21 21:44:51 CEST 2002

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