  The spreading of an incompressible viscous liquid over an isotropic
homogeneous unsaturated porous substrate is considered. It is shown that,
unlike the dynamic wetting of an impermeable solid substrate, where the dynamic
contact angle has to be specified as a boundary condition in terms of the
wetting velocity and other flow characteristics, the `effective' dynamic
contact angle on an unsaturated porous substrate is completely determined by
the requirement of existence of a solution, i.e. the absence of a nonintegrable
singularity in the spreading fluid's pressure at the `effective' contact line.
The obtained velocity dependence of the `effective' contact angle determines
the critical point at which a transition to a different flow regime takes
place, where the fluid above the substrate stops spreading whereas the wetting
front inside it continues to propagate.
