Technical Abstract: Many models of varying degree of complexity and dimensionality have been developed during the past several decades to quantify the basic physical and chemical processes affecting water flow and pollutant transport in the unsaturated zone and these models are now increasingly being used for a wide range of applications in research and management of natural subsurface systems. Modeling approaches range from relatively simple analytical and semi-analytical models, to more complex numerical codes that permit consideration of a large number of simultaneous nonlinear processes. While analytical and semi-analytical solutions are still used for relatively simple applications, the ever-increasing power of personal computers, and the development of more accurate and numerically stable solution techniques have given rise to the much wider use of numerical models in recent decades