  We report the experimental observation of $1/f^{\alpha}$ noise in
quasi-bidimensionnal turbulence of an electromagnetically forced flow. The
large scale velocity $U_L$ exhibits this power-law spectrum with $\alpha \simeq
0.7$ over a range of frequencies smaller than both the characteristic turn-over
frequency and the damping rate of the flow. By studying the statistical
properties of sojourn time in each polarity of $U_L$, we demonstrate that the
$1/f^{\alpha}$ noise is generated by a renewal process, defined by a two-state
model given by the polarities of the large scale circulation. The statistical
properties of this renewal process are shown to control the value of the
exponent $\alpha$.
