project, because the questions to be answered by the simulation determine the 

 detail reflected in the systems components (Figure 5). 



STEP 5: IDENTIFICATION OF INTERACTIONS BETWEEN SUBSYSTEMS AND SOURCES 



In the Hillsborough County example (Figure 5), interactions between the 

 subsystems and sources are shown by energy flow pathways. A matrix may be 

 helpful to systematically identify these flows. The energy sources with in- 

 ternal sources such as the output of the urban system followed by the external 

 sources in order of increasing energy concentration are listed on the vertical 

 axis. The internal energy sinks followed by the external sinks are listed on 

 the horizontal axis. An agricultural production unit is an example of an 

 internal sink. Once the input/output matrix is completed an "X" may be used 

 to indicate a significant energy flow pathway. The completed matrix now forms 

 a guide to the necessary energy flow pathways to diagram the system, i.e., one 

 energy flow pathway on the model will be represented by one "X" in the input/ 

 output matrix. If each energy flow in the input/output matrix was evaluated 

 and the corresponding energy flow quantity used to replace the "X" in the 

 matrix, an energy input/output model would result. For researchers fainiliar 

 with economic input/output models, this may be a familiar arrangement with 

 which to work. 



STEP 6: ENERGY FLOWS WITHIN THE SUBSYSTEMS 



A researcher can incorporate more detail into the model by further exam- 

 ining energy flows within individual system components. For example, Figure 6 

 shows the system detail for the production systems. Farms, salt marshes, and 

 forests are typical production systems. The "producer" system shown by the 

 bullet-shaped symbol contains a storage tank, which is an energy accumulator, 

 or "counting" device and a feedback loop. 



Once all subsystem diagrams showing energy flows and storages are com- 

 pleted, the energetics model is complete. The actual flows in the model must 

 now be measured or calculated. To facilitate this, each flow pathway and 

 storage symbol is assigned a unique identifier. These identifiers for a nat- 

 ural subsystem model , such as a forest, are shown in Figure 6. 



STEP 7: EVALUATION OF THE ENERGETICS MODEL 



Each storage and flow of energy identified in the previously drawn ener- 

 getics diagram must now be quantified, or evaluated, as the quantification 

 process is also called. The evaluation of the model can be done at a broad 

 level, but it is much simpler to undertake this step at the subsystem level 

 because the interdisciplinary nature of systems tends to make model evaluation 

 difficult. Evaluation of energy flows and storages in the natural system can 

 be based on information found in ecological literature (Lieth and Whittaker 

 1975), just as information on agricultural systems can be found in the agri- 

 cultural literature. All flows of energy must adhere to the laws of thermo- 

 dynamics. That is, energy may not be created or destroyed in any process, and 



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