Subject: Loop of the Week

Posted by Jay Forrester on 11/4/2003
In Reply To:Loop of the Week Posted by John Sterman on 11/1/2003

Message:

It seems to me that there are various responses that one can give to the example of water evaporating from the ocean. The issue is not that the process starts with evaporation, but rather in the use of the word "cycle."


The author of the book uses "cycle" as a circular process where the water starts from the ocean and returns to the ocean. We tend to think of the word as describing a circular process in which each element in the circle makes a dynamics contribution to the process. It seems to me that the ocean in this example is inactive in the dynamic movement of water.


A system dynamics model would show the process of evaporation as drawing water from an infinite source, a cloud in the flow diagram, and returning the water to a cloud. The point is that the evaporation does not dynamically change the size of the ocean or cause the ocean to have an effect on the evaporation. From the viewpoint of dynamics, the process is unidirectional and starts with evaporation. Only if the amount of water evaporated were to change the ability of the ocean to supply water would be have a true system dynamics cycle.


I can argue in favor of the interpretation that was given in the book.


Jay Forrester.

Follow Ups:

Loop of the Week - John Sterman 11/5/2003 
Loop of the Week - Bill Barroway 11/6/2003
Loop of the Week - Peter Welles 11/6/2003
Loop of the Week - Richard Turnock 11/7/2003
Loop of the Week - Dexter Chapin 11/7/2003
Loop of the Week - Kevin Williamson 11/5/2003
Loop of the Week - John Heinbokel 11/6/2003