Natural 



Producer 



System 



T 



7-^ 



Agricultural 



Producer 



System 



Figure 5. Basic Hillsborough County model 



This step in the modeling process is fulfilled by drawing a large rec- 

 tangle around the system. The flows of energy across the boundary are repre- 

 sented as energy sources (circles, Step 2). The more dilute energy sources 

 such as the sun, wind, and rain are customarily located in the lower left of 

 the rectangle, whereas the more concentrated sources such as fossil fuel, 

 petroleum, and infomation are shown on the top or right side of the rec- 

 tangle. The energy quality increases from left to right. 



STEP 4: IDENTIFICATION OF THE PRINCIPAL SUBSYSTEMS WITHIN THE SYSTEM 



In the example of Hillsborough County, both natural and agricultural sub- 

 systems are shown (Figure 5). If agriculture were relatively unimportant, it 

 might logically be included with the energy flows of the natural subsystem 

 component. -Examples of natural systems are estuaries, ponds, tropical 

 forests, or grass prairies. The distinction between natural and agricultural 

 systems is that natural systems are self-organizing and self-maintaining 

 whereas agricultural systems require maintenance and organization. The im- 

 portant balance is to include all necessary detail in the energetics simula- 

 tion without including detail of unnecessary subsystems. The identification 

 of the subsystems to be modeled is dependent on the goals of the research 



287 



