Chapter 12. Simulating the Flow of DDT

System dynamics may be used to simulate the flow of material through the environment. The flows can involve multiple media such as water, soil, air and the bodies of animals and man. The models can help us understand the time delays between the application of a controlled flow and its arrival at "target" or "non target" organisms. The models are normally constructed in a step by step manner based on our understanding of material flows in each medium. The goal is to improve our instincts for managing the controlled flows to achieve the desired impacts on the target organism with minimal indirect effects on non target organisms. This chapter illustrates a general approach to simulating multi-media flows with an example of DDT. This pesticide is well known to students of the environment, and the example will reveal the usefulness of system dynamics in the field of toxicology. The ideas are useful wherever scientists wish to simulate material flows through several media (i.e., in pharmacokinetic modeling of drug flows in the body).

Figure 12.1 shows a flow diagram of a system dynamics model constructed by Jorgen Randers (1973) to study the long time delays between the application of DDT on crops and the appearance of the chemical in fish. The flow diagram follows the dynamo conventions explained in Appendix D. The DDT model is thoroughly documented in the Randers' chapter in Toward Global Equilibrium. You may work with the model directly in Dynamo, or you could choose to convert the model into any of the stock and flow programs. This chapter explains the DDT model in a step by step manner using progressively more complicated Stella models. By the end of the exercise, you will appreciate the applicability of system dynamics modeling to simulating material flows through multiple media. You will also appreciate some of the challenges that may arise when simulating material flows with a wide disparity in time constants.