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Implications of Flow
From SystemsWiki
Not only do flows flow, they accumulate and they have to come from somewhere because they are conserved. One of the great failings of Causal Loop Diagrams is that they don't explicitly identify Stocks & Flows. Not only don't they explicitly identify Flows, there are no flows contained on a Causal Loop Diagram. Stock & Flow models overcome this shortcoming, though often when we create Stock & Flow models we use clouds as the source or destination for the Flow indicating we're not concerned with where/what it is specifically. One understanding developed from Systems Thinking [Senge 1990] is that there is not away. With the interconnectedness of everything the source and destination of flows are often more relevant than we estimate.
Here are three Flow examples [Ford 2009].
Contents |
[edit] CO2 in the Atmosphere
For years we never worried much about the CO2 emissions from our constructions, e.g., cars, machinery, manufacturing plants, etc., though now it seems that our short-sightedness is becoming something we should be concerned about, which becomes evident in the Fig. 2 Stock & Flow Diagram.
The flows are measured in gigatons of carbon (GTC) per year and the initial value of co2 in atmosphere in 2000 was 750 GTC. The flows vary as follows.
Anthropogenic Emissions
This inflow, resulting mainly from the combustion of fossil fuels, is represented in Fig. 2. This figure also demonstrates a capability of most Simulation Software to depict a Flow, or a Variable, as a table of values, in this case a step table that changes every 10 years.
Biomass & Soil Removal
Fig. 3 depicts the estimated removal of CO2 from the atmosphere through absorption in GTC/yr.
Ocean Removal
The ocean removal of CO2 is estimated to be constant at 2.0 GTC/yr over the model period.
Implications
When this model is run over the period form 2000 to 2100 the result is depicted in Fig. 4.
So while the inflow of CO2 per year never exceeded 18 CTG/yr during the period of this model the amount of CO2 in the atmosphere more than doubled from 750 GTC to 1,576 GTC. As such it is important that when dealing with Flows & Stocks it is very important to realize that even small changes can have a significant effect over a period of time.
[edit] Water in a Reservoir
Reservoirs are generally constructed to better manage the water flow to the area downstream from the reservoir. As a result of this control, while the inflow can vary greatly during the year the outflow can be maintained relatively constant. Fig. 5 is a reservoir model with a composite inflow and a composite outflow.
While there is only one inflow and one outflow, measured in thousands of acre-feet (KAF), each one represents multiple sources.
Inflow
The inflow for Fig. 5, measured in KAF/month, is represented as a graph, similar to that in Fig. 2, though Fig. 6 is a point graph rather than a step graph, another common feature of simulation packages.
Outflow
The outflow is managed to be a relative constant of 5 KAF/month.
Implications
If we assume 5,000 KAF as an initial value for the water in the reservoir and run the model it produces the graph in Fig. 7.
The implication is that while the inflow varies substantially, 400% from low to high, and outflow is constant, the water in the reservoir only changes by 1.6% over the 12 months. In this particular instance the stock provides a stability not available from the flow directly.
[edit] Population Growth
Population growth is a situation where the flow, growth, is defined based on feedback from a stock, population, in conjunction with a growth rate as depicted in Fig. 8.
Note that this structure is a typical Balancing Loop Systems Archetype.
If this model is simulated with an initial population of 100 million and a growth rate of 20% the results are depicted in Fig. 9.
Notice that with a 20% growth rate the population goes from 100 millin to 270 million in just 5 years, which is quite substantial. The resultant curve is a typical exponential growth curve which is characteristic in reinforcing feedback structures. Note that the growth curve is the same shape as the population curve, just on a different scale. And this growth happens while the growth rate is a constant 20%.
[edit] Conclusions
- Whenever you use a cloud as a source or destination for a flow in a model stop and ask yourself if a cloud is really appropriate? Think about the longer term implications of the interactions in the model and the effect they may have on what's in that cloud.
- The behavior of structures when feedback loops are operating are often very unintuitive. Simulation is a sound approach to understanding the operation of the structure
[edit] References
- Translating Causal Loop Diagrams to Stock & Flow Diagrams
- Ford, Andrew (2009) Modeling the Environment: Second Edition. Island Press
- Senge, Peter (1990) The Fifth Discipline: The Art & Practice of the Learning Organization, Doubleday Currency
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