PARSWMS: A Parallelized Model for Simulating Three-Dimensional Water Flow and Solute Transport in Variably Saturated Soils

ith the increasing computational power in the 1980s, numerical three-dimensional models for water fl ow and solute transport have been developed. The objective was to build codes capable of solving the nonlinear Richards’ and convec-tion–dispersion equations (CDE) for any domain geometry and for complex boundary and initial conditions. Since then, three-dimensional models have been used for a wide range of problems in terms of scale, boundary conditions, and processes involved. Nowadays, processes such as biodegradation, nonlin-ear sorption, geochemical reactions, and runoff, which render the numerical problem even more complex, have been added to the original Richards’ equation and the CDE. From simple and local-scale problems (Kasteel et al., 2000), the current use of three-dimensional fl ow and transport models has been extended to much larger scales and to bigger number of nodes. Three-dimensional vadose zone models are being used more and more for solving hydrological problems at the catchment scale (Herbst et al., 2006). Yet, a key issue of the distributed modeling is the maximum grid resolution at which such models are valid. Theoretically, the grid resolution should be of the same order of magnitude as the Darcy (or elementary representative volume) scale, that is, between 10