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2. Modeling in STELLA
DcC»O \
FIGURE 2.2
The Diagram Layer displays the following icons which represent-from
left to right in Figure 2.2-stocks, flows, converters , and connectors (information arrows).
Click on the globe (Figure 2.3) to access the modeling mode . In modeling mode you can specify your model 's initial conditions and functional relationships.
We begin with the first tool, a stock (rectangle) . In our example model ,
the stock will represent the number of whales in your population. Click on
the rectangle with your mouse , drag it to the center of the screen , and click
again. Type in WHALES. Figure 2.4 shows what you should get.
This is the first state variable in our model. Here we indicate and document a state or condition of the system. In STELLA, this stock is known as a
reseruoir. In our model , this stock represents the number of humpback
whales in our sample population. If we assume that the whales are in an
area measuring 1 square kilometer, the value of the state variable WHALES
is also its density, which will be updated and stored in the computer's
memory at every step of time (DT) throughout the duration of the model.
The whale population is a stock, something that can be contained and conserved in the reservoir; density is not a stock, it is not conserved. Nonetheless, both of these variables are state variables. So, if we are studying a population of whales in a specific area (l square kilometer), the population
size and density are represented by the same rectangle .
Inside the rectangle is a question mark. This is to remind us that we need
an initial or starting value for all state variables. Double-click on the rectangle. A dialog box will appear. The dialog box is asking for an initial value .
Add the initial value you choose, such as 200, using the keyboard or the
mouse and the dialog keypad. When you have finished, click OK to close
the dialog box. The question mark will have disappeared.
FIGURE 2.3
WHALES FIGURE 2.4
2. Modeling in STELLA
DcC»O \
FIGURE 2.2
The Diagram Layer displays the following icons which represent-from
left to right in Figure 2.2-stocks, flows, converters , and connectors (information arrows).
Click on the globe (Figure 2.3) to access the modeling mode . In modeling mode you can specify your model 's initial conditions and functional relationships.
We begin with the first tool, a stock (rectangle) . In our example model ,
the stock will represent the number of whales in your population. Click on
the rectangle with your mouse , drag it to the center of the screen , and click
again. Type in WHALES. Figure 2.4 shows what you should get.
This is the first state variable in our model. Here we indicate and document a state or condition of the system. In STELLA, this stock is known as a
reseruoir. In our model , this stock represents the number of humpback
whales in our sample population. If we assume that the whales are in an
area measuring 1 square kilometer, the value of the state variable WHALES
is also its density, which will be updated and stored in the computer's
memory at every step of time (DT) throughout the duration of the model.
The whale population is a stock, something that can be contained and conserved in the reservoir; density is not a stock, it is not conserved. Nonetheless, both of these variables are state variables. So, if we are studying a population of whales in a specific area (l square kilometer), the population
size and density are represented by the same rectangle .
Inside the rectangle is a question mark. This is to remind us that we need
an initial or starting value for all state variables. Double-click on the rectangle. A dialog box will appear. The dialog box is asking for an initial value .
Add the initial value you choose, such as 200, using the keyboard or the
mouse and the dialog keypad. When you have finished, click OK to close
the dialog box. The question mark will have disappeared.
FIGURE 2.3
WHALES FIGURE 2.4