Environmental Engineering Reference
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in the system. The simple model used a less complicated
observation level but used more sophisticated explanation
of assumption for the causal links.
Inflow of
phosphorus
+
+
+
+
Phosphorus in
waterbody
R
Fish
Sediment
+
+
+
−
B
R
B
−
17.5 Sorting
Plankton
+
−
Building models is a long process especially if the model
is designed to answer many questions. The general rule
of thumb is that a specific question requires one model.
If a problem involves many questions, then many differ-
ent models may be required to address them and so the
'model' actually represents a cluster of models. Neverthe-
less, constructing a model with many components can be
costly (Figure 17.3). The art is to construct a model that
is robust, answers the desired questions and is simple.
The performance need not be perfect; it should only be
sufficient, and nothing more. Such models can both save
time and money, and be useful building blocks for further
model developments.
The process starts with sorting the causalities in the
problem in relation to their importance for answering the
specific question. Then they are sorted according to their
contribution to performance. Causalities with obvious
driving forces explicitly related to the problem have the
Outflow of
water
Figure 17.1
A simple conceptual model of the phosphorus
cycle in lakes that is powerful enough to illustrate and predict
the phosphorous dynamics between the key elements;
phytoplankton, fish and sediment. The different fish species
have been simplified to one 'fish'. All plankton and
zooplankton have been simplified to one general 'plankton'.
The shaded area illustrates the core-driving loop that runs the
cycle. If lake water transparency is the issue, then this will be a
fully sufficient model.
It is evident that model complexity depends directly
on the question asked, as illustrated in Figures 17.1
and 17.2. In the case of the phosphorus cycle, the com-
plicated model did not enhance the understanding of the
overall behaviour but increased the uncertainty that was
involved by increasing the number of observation levels
Predator
fish
Inflow of
phosphorus
−
+
B
++
+
−
+
+
Fishing
+
Phosphorus in
waterbody
R
−
Prey fish
Sediment
+
+
++
−
−
B
B
−
Burial
Zooplankton
+
−
B
−
Outflow of
water
Phytoplankton
−
+
R
Figure 17.2
A complex model of the phosphorous cycle, which is based on the simple model but includes components that add
details to the driving loop for eutrophication. The additional components do not necessarily enhance the performance of the model
with respect to the water-transparency issue; the simple model will be good enough. If the issue is fish-population dynamics, then the
added details will be required.
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