What is the difference between the Hydraulic diffusion equation and the Richards equation in groundwater dynamics? I want also to understand the difference between the different storage coefficients (Ss, S,..).
Both are differential equations:
Diffusivity equation: $$\ Ss. ∂h/ ∂t = −∇ ·(− K \nabla h) − G$$.(for Incompressible liquid)(the flux (q), source terms (G), the specific storage (Ss), hydraulic head (h)) (source)
Richards equation (headed based form (there two other forms of this equation)): $$C(h) . ∂h/∂t = ∇ · K(h)∇H$$ (specific moisture capacity C(h), (H) total hydraulic head, h is the matric head) (source)
Aren't they supposed to describe unsaturated flow ???