col /yr 



cal/yr 



Figure 7. An evaluated model of the Hillsborough County natural system (see 

 Figure 6 for energy flow pathway names). 



plied by the mean weight of the particular ecosystem biomass. The energy 

 stored in the biomass is computed by multiplying the cal /g of biomass dry 

 weight by 4.25. The total energy stored in the biomass is computed by 

 the individual ecosystem energy storage values. Similarly, the gross 

 production of the boundaries area is computed and then summed. This 

 illustrated by Table 2. Our experience has shown that splitting gross 

 production equally between the work required for respiration and 

 synthesis is a good first estimate. 



summing 

 primary 

 i s al so 

 primary 

 photo- 



The energy 

 multiplying the 



value of the harvest from the natural system was computed by 

 dollar value of the stumpage (total volume of wood harvested. 



i.e., 8.7 X 105) from Hillsborough County's natural system by the 1978 energy 

 to dollar ratio from Figure 4 (1.6 x 10^ cal/$) yielding 1.4 xlOlO cal/yr. 

 The harvest is small when compared to the total energy stored in the natural 

 system. 



STEP 8: TRANSLATION OF ENERGETICS DIAGRAMS TO DIFFERENTIAL EQUATIONS 



An energetics diagram is actually a differential equation in a pictorial 

 form. Figure 8 is an example of an energy circuit model with its correspond- 

 ing differential equation. The storage symbol in the diagram represents the 

 equation state variable. The rate of change of the storage of energy is cal- 

 culated by summing of all of the flows of energy into and out of the storage. 

 Energy flows leaving the storage are given a minus sign. The differential 

 equation for the natural system of Hillsborough County is given in Figure 8. 



290 



