The governing flow equation for three-dimensional saturated flow in saturated porous media is:
where Kxx,Kyy,Kzz the hydraulic conductivity along the x, y and z axes of the model, which are assumed to be parallel to the principle axes of hydraulic conductivity tensor, h is the hydraulic head, Q represents the source/sink terms, and Ss is the specific storage coefficient.
Two special features of this apparently straightforward elliptic equation should be noted. First, the equations are non-linear when flow is unconfined and, second, the storage coefficient is not constant but switches between the specific storage coefficient for confined conditions and the specific yield for unconfined conditions.
