variations on the equation of continuity

98.4F

Hi

How is the equation of continuity for a layer written. There some different way of writing it using radial volume fluxes and thickness of the layer but how is it done and how do they get an equation like this

d h/dt = d(psi_1)/rdr + d(psi_0)/rdr - (Q2 - Q1)

where d is the partial derivative h is the thickness and psi are the radial volume fluxes per unit radian which are defined by

psi = -h*u*r where u is the radial component of velocity.

Thanks