Poor soil structure, i.e. aggregation and porosity, is widely acknowledged as a major limitation to infiltration, redistribution and storage of water in a soil profile, leading to more runoff and erosion, reduced available water for plants and reduced crop production. Models of soil-crop systems are useful tools for evaluating interactions between soil physical condition, climate, management and crop growth. An outline of the principal components of soil structure which influence the water balance, hence crop growth, is given. Models that represent these components are reviewed and classified according to their representation of soil structure and the water balance. Few of the models reviewed explicitly represented soil structure and none attempted to simulate long-term changes in soil physical properties. About half the models used a USDA Curve Number approach to represent runoff and a 'tipping bucket' approach to represent soil water redistribution, and were classed as 'functional'. Functional models were mostly used for predicting long-term trends in erosion, soil fertility and crop growth. The remainder were termed 'mechanistic' as they used more physically based representations of the water balance. Mechanistic models were typically used for detailed simulations of soil water dynamics and solute movement and were more complex than functional models. No one model or modelling approach suites all applications; the problem the model is being used to resolve define the features and complexity needed. Future development of models of the soil-crop system would benefit from better methods of parameterising and applying models, extension of point-source models to simulate spatial variability and to handle varying catchment scales, and a more complete representation of soil physical properties. (C) 1998 Elsevier Science B.V. All rights reserved.
