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 un derstand what a model is, when it can be used validly and in what circ umstances it can be said to predict. There are also several types of m odels not all of which are suitable for all purposes. This paper offer s some guidelines for soil scientists. Models must be properly validat ed, particularly when used in decisions, and a recently published vali dation scheme is summarized. It is suggested that models should be ana lysed not only for sensitivity to changes in their parameters but in s ome circumstances to changes in parameter variance, particularly where the model may not be linear with respect to its parameters. Non-linea rity may be a problem where parameters are subjected to spatial averag ing or interpolation. Models are sometimes developed and validated at one scale and then used at a larger scale. Problems may result, and so me questions are suggested that should be asked when a change of scale is envisaged. Some of the modelling challenges implicit in the evolut ion of sustainable forms of agriculture are discussed.