Before revision:  The biophysical model is a reaction-advection-diffusion partial differential equation (PDE) with only two unknown parameters representing the spreading (the diffusion part of the PDE) and  growth of tau (the reaction part of the PDE).
After revision:  The biophysical model is a reaction-advection-diffusion partial differential equation (PDE) with only two unknown parameters representing the spreading (the diffusion part of the PDE) and the other one the growth of tau (the reaction part of the PDE).