Past problems

Simulation modelling is an important aspect of research in the earth, environmental and life sciences. Models can guide the research process; integrate the knowledge coming from a variety of studies; permit the testing of hypotheses; and enable predictions to be made. Many major research programmes have the development of such models as a central component of the whole programme.

In the recent past, most models are implemented in conventional programming languages, such as Fortran and C. We believe that in many cases this greatly reduces the effectiveness of the modelling effort. For example:

1. It requires either that the researcher learns a programming language (which they may be unwilling or unable to do), or that a programmer is employed to implement the model (which is a significant cost and adds a barrier between the model as conceived and the model as implemented).
2. Developing a reliable programmed implementation is a time-consuming hence costly operation, not only in the initial implementation of the model but also in its subsequent debugging and maintenance.
3. It is difficult for others to fully understand the model, since they will be reluctant to wade through many lines of computer code.
4. The written description of the model is often incomplete, sometimes ambiguous, and it has no guaranteed correspondence with the model as implemented.
5. Shareability of model components with other researchers is severely limited, thus leading to considerable duplication of effort. This applies both to submodels and to support routines, such as data input facilities and tools for visualising model behaviour.
6. Implementing model in a procedural programming language ignores the fact that running simulations is just one of the things we can do with a model: there are many other activities associated with the modelling process (such as comparing two models, generating descriptions of a model in a variety of formats, and transforming models) that could in principle be aided by a computer, but not if the model is implemented as a program.

These problems have been recognised for a good number of years in the modelling community, and various solutions have been proposed. First, there has been some development of modular modelling environments (such as APSIM), but these tend to make rigid assumptions about the way that submodels should integrate, and thus can at best only work within a small, well-defined community. Second, there are now a number of simulation languages (such as ACSL), and visual modelling environments (such as Stella, ModelMaker and PowerSim). These certainly reduce the programming overhead, but tend to restrict the modeller to a certain modelling approach (such as differential equations or System Dynamics modelling). Third, there is now considerable interest in the adoption of object-oriented or component-based technologies such as C++ or COM. These have the potential to permit re-use of model components between groups, but are still tied into a programming paradigm for the implementation of the individual model components.