I have learnt that diffusion/heat equation can be used to model groundwater flow in confined conditions. Recently I read a paper where they used linear Boussinesq equation (equation 1 in linked paper) to model groundwater flow in unconfined aquifer. Then in another paper, the auther said, he is using non-linear boussinesq equation (equation 1 in 2nd linked paper) to model groundwater flow. While looking apparently at these equations they look almost same. My question is what is difference between using linear Boussinesq equation and non-linear Boussinesq equation for gw flow modeling as compared to diffusion equation.