  It is generally accepted that the effective velocity of a viscous flow over a
porous bed satisfies the Beavers-Joseph slip law. To the contrary, in the case
of a forced infiltration of a viscous fluid into a porous medium the interface
law has been a subject of controversy. In this paper, we prove rigorously that
the effective interface conditions are:
  (i) the continuity of the normal effective velocities;
  (ii) zero Darcy's pressure and
  (iii) a given slip velocity.
  The effective tangential slip velocity is calculated from the boundary layer
and depends only on the pore geometry. In the next order of approximation, we
derive a pressure slip law. An independent confirmation of the analytical
results using direct numerical simulation of the flow at the microscopic level
is given, as well.
