Furthermore, we have that m = \rho V_p, where V_p is the pore volume of the medium between x and x+\Delta x and \rho is the density.
\frac{d(\rho q)}{d x} + \frac{d(\rho q)}{d y} + \frac{d(\rho q)}{d z} = \frac{-d(\rho \phi)}{d t} .