Modelling the fate of crop nutrients in the environment: Problems of scale and complexity

Computer modelling has an important role in the establishment of a satisfactory balance between agricultural production and environmental health. This role seems likely to involve the use of models at larger scales than previously, making both the choice of model and the way it is used a matter of increased importance. One factor that needs to be considered is the interaction between any non-linearity in the model with respect to its parameters and the variances of these parameters, because such interactions can affect adversely both the parameterisation and use of models. Spatial variability of parameters therefore needs to be handled with care. For use at larger scales, there is much to be said for simple models that are largely linear with respect to their parameters and whose parameters have small variances. For transport processes this implies the use of capacity-type models. A list of questions that need to be asked when a model is translated from one scale to another is suggested.