Simunek, J.
U. S. Salinity Laboratory, USDA, ARS
450 West Big Springs Road
Riverside, CA 92507-4617
email:jsimunek@ussl.ars.usda.gov
Walter Russell
U.S. Salinity Laboratory
USDA, ARS
450 West Big Springs Road
Riverside, CA 92507-4716
Tel: 909-369-4850
FAX: 909-342-4964
Email: wrussell@ussl.ars.usda.gov
SWMS_3D is a model for simulating water and solute movement in three-dimensional variably saturated media.
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_3D 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 p
rescribed 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 dispersion tensor includes a term reflecting the effects of molecular diffusion and
tortuosity.
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.