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.