Environmental Engineering Reference
In-Depth Information
be expressed in other terms or divided further into (sub-) components. The compo-
nents should have a minimum of internal structure. If a component has a pronounced
internal structure it should be considered to disaggregate it further. Disaggregation of
the overall context into its components should be continued to approximate a
manageable number of components (or elements). Frequently, manageable problems
are limited to around ten components. If a higher number seems inevitable, it should
be discussed whether the modelling task eventually requires a more specific focusing.
The first step is to write the single components so that relations between them can be
drawn in the form of arrows. The arrow between two components means there is an
influence of one component on another component concerning quantitative changes.
The criterion for where to set an arrow is a specific one: if the component increases in
quantity, the other component should either increase as well or decrease in quantity.
The influence must not be necessarily in exact proportion, only the direction of increase
or decrease should hold. If the influence is mutual, two arrows should be drawn.
For a cause-effect diagram, a relation exists between two components if the
quantity of a component changes as a result of a change in another component.
A positive influence would exist if component A increases, and then, as a result
component B increases too. Also, a positive relation would be considered if B
decreases as a consequence of a decrease in A.
A negative influence exists, if element A increases in quantity, and then element B
decreases as a result (or B increases as a result of a decrease in A).Thenextstepisto
draw arrows between the elements as relations (influences). This may require shifts of
the location of the components so that the overview is optimized and the possible
crosses of the arrows are minimized. The quality of the influence, whether it is positive
or negative, is indicated by assigning the arrow with an according symbol (
).
The cause-effect diagram (see Fig.
4.1
for an example) facilitates a conceptual
overview. Discussing the result can give rise to further refinements and is the
crucial step in an identification of the important elements (variables) that are used
in further development steps in modelling. Further steps include a quantitative
specification of each of the influences as a mathematical function.
The selection of a system analytical interaction network is not as trivial as it may
seem at first glance. The selection of each of the components (elements) and
influences (relations) requires careful discussion to capture the crucial interaction.
It forces the modeller to be explicit about the ideas of how things work together.
þ
or
The Flow-Diagram (Flow Chart): Compartment, Connections,
Controls: The “3 Magic C's”
Cause-effect diagrams provide the first overview. In a following step, they can be
refined into flow-diagrams. Flow diagrams are an important tool deriving from the
classical systems approach, which deals with the development of differential equa-
tion models where time-dependent quantitative changes occur as a result of the
