SWMS_2D is a computer program for simulating water and solute movement in two-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_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. Flow and transport can occur in the vertical plane, the horizontal plane, or in a three-dimensional region exhibiting radial symmetry about the vertical axis. The water flow part of the model can deal with prescribed head and flux boundaries, as well as boundaries controlled by atmospheric conditions. New features of the present version 1.21 include the implementation of free drainage boundary conditions, and a simplified representation of nodal drains using results of electric analog experiments. 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 equation are solved using either Gaussian elimination for banded matrices, or a conjugate gradient method for symmetric matrices and the ORTHOMIN method for asymmetric matrices. The program is written in ANSI standard FORTRAN 77. Computer memory is a function of the problem definition. This report serves as both a user manual and reference document. The program comes with a manual containing instructions for data input preparation. Example input and selected output files are also provided as is a listing of the source code.
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