Traditionally, agricultural system models have been developed and implemented in a code-based modeling approach, that is, constructing a model as a sequence of lines of code in a common programming language such as Fortran, Visual Basic, C, C++, or Java. However, it is becoming increasingly more common to construct and link model components in a visual or 'icon-based' modeling environment. Proprietary visual modeling environments (e.g., Stella (isee systems, Inc.), Vensim (Ventana Systems, Inc.), POWERSIM (Powersim Software AS), and Model Builder (ESRI, Inc.)) are systems where models are constructed by assembling and linking icon-based model components using a sophisticated button- and menu-driven Graphical User Interface. There are three advantages in using these systems for agricultural model development: (1) they typically contain a large number of built-in functions (e.g., mathematical, logical, and statistical including risk and Monte Carlo analysis) that greatly simplify model development and evaluation; (2) they are easy to learn, intuitive to use, and familiar to users of Windows-based software; and (3) they can readily import and export data, generate graphs and tables, and work well together with other commercial applications through dynamic linking with spreadsheets and other software programs. In some respects, the icon-based modeling systems have many of the same capabilities of the more comprehensive modular frameworks described above; however, they are somewhat constrained in their flexibility and effectiveness in producing complex and mul-tifaceted models. These limitations have been somewhat alleviated through a hybrid linkage of code- and icon-based modeling approaches. This concept has been successfully demonstrated by using Vensim to develop a new seed bank module and linking the module (through dynamic linked library support) to the APSIM farming systems model.
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