Under fairly general assumptions, this paper shows that for periodic p
orous media, whose period is of the same order as that of the inclusio
n, the nonlinear correction to Darcy's law is quadratic in terms of th
e Reynolds number, i.e. cubic with respect to the seepage velocity. Th
is claim is substantiated by reinspection of well-known experimental r
esults, a mathematical proof (restricted to periodic porous media), an
d numerical calculations.