Nitrogen & Systems Thinking

Goals for this Class Period

1. Learn about different ways to represent and understand systems:

   1. • Rich Pictures

      • Causal Loop Diagrams

      • Stock and Flow Diagrams

2. Create a "Stock-and-Flow" diagram for (i) Jamie Scott's rotation and (ii) Amish Aquaponics' -- we will pay special attention to Nitrogen for this exercise. We will use these to examine where and how we might want to change the system.

Rich Pictures

A rich picture is a drawing of a situation that illustrates the main elements and relationships that need to be considered in trying to intervene in order to create some improvement. ... It is called a rich picture because it illustrates the richness and complexity of a situation. Emphasis is on understanding a system without "solving" it.

Causal Loop Diagrams

A causal loop diagram consists of four basic elements: the variables, the links between them, the signs on the links (which show how the variables are interconnected), and the sign of the loop (which shows what type of behavior the system will produce). By representing a problem or issue from a causal perspective, you can become more aware of the structural forces that produce puzzling behavior.

• https://docs.google.com/document/d/1L_PniyKJ7sBb794DONebLeLGgmmj2cwEjhOOedIdlls/edit?usp=sharing

• https://docs.google.com/presentation/d/1FAD6mnlIMvrASKVcWrpJRGosDzZmrzOWJQr_ONeSjeY/edit?usp=sharing

When a Causal Loop Diagram is Not Enough

• A causal loop diagram is a great first step for wrapping your head around a system.

• However, sometimes we need to go further...to do that we need a technique that:

  • visually distinguishes between the parts of the system and what causes them to change

  • allows for precise – quantitative – specification of all the system’s parts and their interrelation

  • can provide a basis for simulating the behavior of the system over time

• Stock and Flow Diagrams enable us to create a prototype of the system that will allow us to explore its behavior and to test the effect of changes to the system’s structure. Stock and flow diagrams, along with their mathematical expressions, provide such a technique

Stock and Flow Diagrams

Stock and flow diagrams provide a richer visual language than causal loop diagrams, we distinguish between six main kinds of elements: stocks, flows, converters, connectors, sources and sinks.

Let’s handle the basics of stock and flow diagrams

• Stocks

• Flows

• Source/Sink

• Converters

• System Boundaries

Stocks

• Stocks are the foundation of any system and are the elements that you can see, feel, count, or measure

• Stocks do not have to be physical

Flows

• Stocks change over time through the actions of a flow

• A stock is the present memory of the changing flows within the system

Sources/Sinks

Sources and sinks are stocks that lie outside of the model's boundary – they are used to show that a stock is flowing from a source or into a sink that lies outside of the models boundary. On diagrams, sources and sinks are represented by small cloudish things.

Converters

Converters either represent parts at the boundary of the system (i.e. parts whose value is not determined by the behavior of the system itself) or they represent parts of a system whose value can be derived from other parts of the system at any time through some computational procedure.

• Birth Rate can be calculated as the numbers of births per unit time...this can change!

• Mortality Rate can be calculated as the number of deaths per unit time...this can change!

Loops

• A feedback loop is formed when changes in a stock affect the flows into or out of that same stock

• Balancing feedback loops are stability seeking and try to keep a stock at a certain level or within a certain range

• Reinforcing feedback loops occur when a system element has the ability to reproduce itself or grow at a constant fraction of itself

Sometimes we think Systems are Really Simple...

The key feature of a stock and flow diagram is that each construct can be precisely specified using a mathematical expression – viewed from a mathematical perspective, such fully specified stock and flow models are just a way of visualizing a corresponding set of integral equations.

Next Step with Stock and Flow Diagrams is to actually assign mathematical equations to the processes….

Below are examples of Stock and Flow Diagrams for Industrial Ag Row Cropping