  The intermittent burst dynamics during the slow drainage of a porous medium
is studied experimentally. We have shown that this system satisfies a set of
conditions known to be true for critical systems, such as intermittent activity
with bursts extending over several time and length scales, self-similar
macroscopic fractal structure and $1/f^\alpha$ power spectrum. Additionally, we
have verified a theoretically predicted scaling for the burst size
distribution, previously assessed via numerical simulations. The observation of
$1/f^\alpha$ power spectra is new for porous media flows and, for specific
boundary conditions, we notice the occurrence of a transition from $1/f$ to
$1/f^2$ scaling. An analytically integrable mathematical framework was employed
to explain this behavior.
