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WIMOVAC Mathematical Model & AlgorithmModular program design and model construction has received considerable attention from software engineers {Timlin, 1996 #1981}. The importance of modularity comes only partially from the ability to decompose complex systems into manageable parts, more important is the advantage that modularity has of isolating the information needed to construct a module from other modules. Hodges et al., (1992) described a modular structure for the International Benchmark Sites for Agrotechnology Transfer (IBSNAT) models that enhances the ability to modify the model for new applications or improvements. A modular structure can be developed following accepted guidelines for modular software design or object orientated (OOP) paradigms. Wimovac is coded as a series of inter-linked model objects which represent real World processes. These objects are coded as procedural modules with tightly defined internal and external properties expressed in the form of driving, auxiliary and output variables. Each code module consists of one or more subroutines relating to individual sub-processes within an object. The subroutines therefore represent the mechanisms of the object. The "scope" and "visibility" of variables in wimovac model objects are set to reflect the hierarchical structure of the model and the information flows expressed within the model (Figure 19). Detailed leaf biochemical properties for example are stored in internal leaf variables that have a scope and visibility that is restricted to the leaf only. These properties are not visible and cannot be used or modified outside of the leaf level (Figure 19). Careful attention to the scope and visibility of program variables and therefore informational flows in a model is paramount to maintaining the integrity of a complex model. Where informational flows between hierarchical levels are required in wimovac, global scope variables are used. These are visible at all model levels and whilst this is not ideal because the scope of the variables does not adequately reflect the informational flows of the model it is the only practicable approach and does not affect the functionality of the program. Whilst it is programmatically possible to modify the visibility of variables by declaring them all locally and preferentially passing local variable values to the specific subroutines that require them as part of the subroutine or function definition this approach is both time consuming and difficult to administer in large projects with many subroutines. This approach would also be a step further away from the concept of clearly defined objects with identifiable properties at the program code level. Property based object linked data structures have been established in wimovac which provide an identifiable structure for global scope variables. The object/property orientated concept of constructing a model programmatically is very important in determining the re-usability of an object within a model. For example the multiple layer canopy model in wimovac is made up of up to 20 leaf layers. Each layer consists of sunlit and shaded leaves. Whilst it would be possible to create forty individual leaf classes in code to accomplish this, it is much simpler to define the leaf programmatically as a generic object whose modifiable properties, such as Jmax, Vcmax and incident solar radiation reflect the model properties of the leaf and informational flows between the leaf and other objects in the model. This generic leaf object may then be called repeatedly, varying its properties in accordance with information from the canopy level, to give the forty different leaf classes. From a practical point of view constructing a model in an hierarchical manner in which lower hierarchical layers may become objects with a small number of externally visible properties conforms well with modern object orientated/property lead programming techniques used in languages such as C++, Pascal (Delphi) and Visual Basic. Figure 20 provides a simplified schematic review of the modules in wimovac with an indication of the literature sources of individual components. The wimovac model schematic indicating the main model components and the iterative nature of leaf photosynthesis, conductance, evapo/transpiration and energy budget calculations.
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