Modelling water flow and solute transport in heterogeneous soils: A review of recent approaches

A quantitative description of water flow and solute transport in the unsaturated zone of the soil is required to predict the impact of human influences on the environment. This paper starts with the basic concepts of the mathematical descriptions of transport processes in homogeneous media. However, water flow and solute transport in natural soils are significantly influenced by the occurrence of (1) macropores and structured elements (micro-heterogeneity), (2) spatial variability of soil properties (macro-heterogeneity) or (3) a combination of (1) and (2). In these cases,, the classical representations of water flow and solute transport are not adequate. The paper presents an overview of some recent modelling concepts dealing with water flow and solute transport in heterogeneous media. For each model, we first introduce the underlying physical concept, and then translate the concept into a mathematical model. Each model is illustrated for a specific water flow and solute transport problem. Finally, some applications of the models are discussed. At this moment, it is difficult to specify which model should be used to solve a particular problem since no extensive validation of the models has been performed. Additional research is required to develop accurate and rapid measurement techniques for the necessary input parameters. To be useful in real environmental problems, modelling concepts for micro- and macro-heterogeneity should be coupled in one overall mathematical framework. (C) 1998 Silsoe Research Institute.