  When a magnetised, thermal plasma containing a shock wave efficiently
accelerates Cosmic Rays (CR) a reaction is exerted on the fluid structure. The
simplest description of this process involves three conservation equations for
the background fluid, with the CR pressure gradient included as an extra force,
plus a hydrodynamic equation for the CR energy density. Since the CR spectrum
is not determined in this approach two quantities are introduced, the ratio of
CR pressure to energy density and a mean diffusion coefficient, that must be
specified to close the system of equations. Physical arguments are presented
for how these quantities should evolve and then compared with time dependent
numerical solutions of acceleration at a plane parallel, piston driven shock.
