McKay, Donald. "Front matter" Multimedia Environmental Models ...

Figure 7.11 Schematic representation of the results corresponding to computed output.
check that the balance is correct (i.e., input equals output for each compartment)
and identify the primary processes which control environmental fate. It may then
be appropriate to examine these processes in more detail, seeking more accurate
parameter values. Usually, the chemical’s fate is controlled by a few key processes,
but these are not always obvious until a Level III calculation is performed.
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7.11 LEVEL IV CALCULATIONS
It is relatively straightforward to extend the Level III model to unsteady-state
conditions. Instead of writing the steady-state mass balance equations for each
medium, we write a differential equation. In general, for compartment i, this takes
the form
V iZ idf i/dt = I i + S(D jif j) – D Tif i