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 scalar unknown parameters, one representing the spreading (the diffusion part of the PDE) and growth of tau (the reaction part of the PDE).