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