Use the draining process whenever you
want to represent the draining, passive
decay, or aging of some Stock. The two inputs to the draining flow are the Stock (or
its alternate measure) and a draining fraction. The flow (an outflow from the Stock)
is generated as the product of the two. You might also think of the draining fraction
as a "loss" or "decay" fraction. This fraction is the fraction of the Stock that
is lost or
decays per unit of time. In some instances, you may want to think of the draining
fraction as a "time constant." The time constant is simply the reciprocal of the loss
fraction. It tells you the average length of time that a unit spends in the Stock, when
the Stock is in "steady state."
Algebraic Form:
production=Stock * decay fraction (units/lime)=(units)
* (units/unii/time)
or
production = Stock / time constant (units/time)
= (units) / (time), where
time constant = 1 / loss fraction
(time) = l/(units/unit/time)
The behavior pattern of the draining process,
in isolation, is the mirror image of the
compounding process. This behavior pattern is shown below:
This asymptotic pattern says that at first,
lots of material is in the Stock. Since the
volume of the outflow is determined by how much material is in the Stock, initial
outflows are large. Initially, then, the magnitude of the Stock decreases rapidly. But,
as the magnitude of the Stock declines, so does the outflow! Smaller values for the
Stock produce smaller flow values. The magnitude of the Stock thus declines by a
smaller and smaller amount each cycle. This yields the "fizzle-out" pattern of goal-
seeking activity characteristic of the draining process. Larger values for the loss
fraction result in faster draining of the Stock than smaller values.
As the examples shown below illustrate,
the "pure" draining process, with a
constant loss fraction and a direct linkage between Stock and flow, is sufficient to
depict a wide variety of circumstances. As with the compounding process, you
should always think carefully about whether the loss fraction is a variable (either a
Converter or a Stock), and about any intervening Converters that may exist
between a Stock and its outflow.
