In all of the fundamental equations which will follow we will find
it possible to distinguish two important modes of transport of
physical quantities. These are the advective transport, which is
that related directly to the velocity of the fluid; and the
non-advective transport, which represents phenomena observable
on the macroscopic scale but associated with motions on the
microscopic scale. We have already seen examples of this in the
preceding article. The advective transport of salt is
; while the non-advective transport is the
diffusive flux 
.
Of the set of fundamental hydro-thermodynamic equations, the only one which does not contain a non-advective flux is the equation of continuity of total mass. This is because of the nature of the definition of the fluid velocity.