Progress in unsaturated flow and transport modeling

This paper reviews recent progress in modeling water flow and solute transport in the unsaturated (vadose) zone. Much progress has been attained in the analytical and numerical description of vadose zone transfer processes. A variety of deterministic models are currently available for describing and predicting these processes. The most popular ones continue to be the classical Richards equation for unsaturated flow and the Fickian‐based convection‐dispersion equation for solute transport. While deterministic solutions of these equations remain useful tools in research and management, their practical utility for predicting actual field water and solute concentration is increasingly being questioned. Problems caused by preferential flow through soil macropores and by spatial and temporal variability in soil hydraulic properties have caused some disillusionment with the classical models. A number of alternative deterministic and stochastic formulations have been proposed to specifically deal with preferential flow and/or spatial variability. Those models have greatly increased our understanding of field scale transport processes, and have in some cases led to better practical tools for management purposes.