SIMULATION MODELING AND SOIL BEHAVIOR

The increasing power and availability of computers has encouraged both the development of computer-based simulation models and their use in making decisions. It has therefore become increasingly important to understand what a model is, when it can be used validly and in what circumstances it can be said to predict. There are also several types of models not all of which are suitable for all purposes. This paper offers some guidelines for soil scientists. Models must be properly validated, particularly when used in decisions, and a recently published validation scheme is summarized. It is suggested that models should be analysed not only for sensitivity to changes in their parameters but in some circumstances to changes in parameter variance, particularly where the model may not be linear with respect to its parameters. Non-linearity may be a problem where parameters are subjected to spatial averaging or interpolation. Models are sometimes developed and validated at one scale and then used at a larger scale. Problems may result, and some questions are suggested that should be asked when a change of scale is envisaged. Some of the modelling challenges implicit in the evolution of sustainable forms of agriculture are discussed.