Fire is a pulsing model, which includes a switch. When the grass (Q) has grown up to a critical mass (G1), then the fire switches on. When the grass has burned down so low (G2) there is no longer enough to burn, the fire switches off. As the fire burns, it releases nutrients, which stimulate growth and are bound up in the grass again. (Figure III-13).
In the program (Table III-13) the fire switch is turned on when Q>G1 (see statement). G2 is the quantity of grass left after the fire. G1 and G2 are thresholds and are shown at the top of the switch symbol in the figure.
The total nutrients in the system (TN) include the nutrients in the soil (N) plus those bound up in the grass (F*Q). When the grass (Q) is burned, all the nutrients in the burned grass return to the soil: that is, the nutrients in the soil are the total nutrients minus the nutrients bound-up in the unburned grass (N-TN-F*Q).
Statements draw lines from the values Q and N had right before the fire to their values right after the fire. When these are red, they illustrate the fire dramatically.
Examples of Switch Models
Many natural systems have fire as part of their normal pattern. Pine forests in southeastern U.S., chaparral in southern California and the Mediterranean, and grasslands around the world have this pattern. Sometimes fire comes every 4 or 5 years as in our Southeast; sometimes it's every hundred or so as in the lodgepole pine forest in Yellowstone National Park. A spurt of growth follows the fire, as the nutrients are recycled into the soil.
An economic example is a buildup of inventories in a store to a certain amount, followed by a sale to sell most of them. N might be the empty hangers and shelves ready to take more clothes.
"What if" Experimental Problems
COMPUTER MINIMODELS AND SIMULATION EXERCISES
FOR SCIENCE AND SOCIAL STUDIES
Howard T. Odum* and Elisabeth C. Odum+
* Dept. of Environmental Engineering Sciences, UF
+ Santa Fe Community College, Gainesville
Center for Environmental Policy, 424 Black Hall
University of Florida, Gainesville, FL, 32611
Autorização concedida gentilmente pelos autores para publicação na Internet
Laboratório de Engenharia Ecológica e Informática Aplicada - LEIA - Unicamp
Mileine Furlanetti de Lima Zanghetin
Campinas, SP, 20 de julho de 2007