MODELING WATER-FLOW AND SOLUTE TRANSPORT IN HETEROGENEOUS SOILS - A REVIEW OF RECENT APPROACHES

A quantitative description of water flow and solute transport in the u nsaturated zone of the soil is required to predict the impact of human influences on the environment. This paper starts with the basic conce pts of the mathematical descriptions of transport processes in homogen eous media. However, water flow and solute transport in natural soils are significantly influenced by the occurrence of (1) macropores and s tructured elements (micro-heterogeneity), (2) spatial variability of s oil properties (macro-heterogeneity) or (3) a combination of (1) and ( 2). In these cases,, the classical representations of water flow and s olute transport are not adequate. The paper presents an overview of so me recent modelling concepts dealing with water flow and solute transp ort in heterogeneous media. For each model, we first introduce the und erlying physical concept, and then translate the concept into a mathem atical 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 sho uld be used to solve a particular problem since no extensive validatio n of the models has been performed. Additional research is required to develop accurate and rapid measurement techniques for the necessary i nput parameters. To be useful in real environmental problems, modellin g concepts for micro- and macro-heterogeneity should be coupled in one overall mathematical framework. (C) 1998 Silsoe Research Institute.