Figure 6 Odum's formalism for energy-based conceptual diagrams: (a) source of energy, (b) sink (loss of energy from system), (c) storage tank, (d) production unit (takes in energy and information to create other quality of energy), (e) consumption unit, and (f) energy mixer of work gate.
subwatersheds, river reaches, and reservoirs. The Stella interface can be used as a drawing board to put together various conceptual diagrams and discuss them with other people in a process known as participatory modeling. In
this case the major value of the interface is that one can easily add or delete variables and processes, and immediately see what impact this may have on the model performance. The model itself becomes a tool for deliberations and consensus building.
Very similar diagrams can be put together using other system dynamics software such as Madonna, Vensim, Powersim, or Simile. In these software packages the 'stock-and-flow' formalism is used to describe the system. The diagrams are also known as flow diagrams because they represent how material flows through the system.
A somewhat different formalism is used in packages such as GoldSim, Simulink, and Extend. Here we have more flexibility in describing what we wish to do in the model and the model does not need to be presenting only the stocks and flows. Groups of processes can be defined as submodels and encapsulated into special icons that become part of the icon set used to put together the diagrams. As usual, we get more functionality and versatility at the expense of a steeper learning curve and higher complexity of design.
Yet another option in building conceptual diagrams is given by the Universal Modeling Language (UML), which is a standardized specification language for object modeling. It is designed as a diagrammatic tool that can be used to build models as diagrams, which can be then automatically converted into a number ofobject-oriented languages, such as Java, C++, Python, etc. In this case, one is actually almost writing computer code when developing the conceptual model. Once again, even more universality and almost infinite flexibility is achieved at a price of yet more efforts one will need to make mastering the tool. Figure 8 presents a sample conceptual diagram created in UML to formulate an agent-based model of a landscape used by sheep farmers, foresters, and National Park rangers that are interacting on very different temporal and spatial scales with different development objectives (sheep production, timber production, and nature conservation).
There are several types ofdiagrams that one can create using UML. One of them is the activity diagram, which describes the temporal dimension of the model. The class
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