We are in danger of creating some confusion here if we talk about
residence time and response time without exploring their relationship
in a bit more detail. Imagine a simple system - one reservoir, one
inflow, one outflow, where the outflow is defined as a typical
draining flow. If you recall from our discussion above that:
and also from above, we established that:
Looking at the equation for the residence time, if we write it
with symbols instead of words, we can see that:
So, in this case, they are the same. Why then, do we have these
two terms? Part of the reason is that the concept of a response time
is relevant to systems in which flows are defined as typical draining
flows. The concept of response time, as defined here, is not relevant
to cases where the inflows and outflows are defined differently - as
constant values, for instance. In those cases, the concept of a
residence time is still applicable and a useful means of
characterizing a reservoir.
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