  Context: The dusty debris disk around the $\sim$20 Myr old main-sequence
A-star {\beta} Pictoris is known to contain gas. Evidence points towards a
secondary origin of the gas as opposed to being a direct remnant form the
initial protoplanetary disk, although the dominant gas production mechanism is
so far not identified. The origin of the observed overabundance of C and O
compared to solar abundances of metallic elements, e.g. Na and Fe, is also
unclear. Aims: Our goal is to constrain the spatial distribution of C in the
disk, and thereby the gas origin and its abundance pattern. Methods: We used
the HIFI instrument onboard Herschel to observe and spectrally resolve CII
emission at 158 $\mu$m from the {\beta} Pic debris disk. Assuming Keplerian
rotation, we use the spectrally resolved line profile to constrain the spatial
distribution of the gas. Results: We show that most of the gas is located
around $\sim$100 AU or beyond. We estimate a total C gas mass of
$1.3\times10^{-2}$ M$_\oplus$. The data suggest that more gas is located on the
southwest side of the disk than on the northeast side. The data are consistent
with the hypothesis of a well-mixed gas (constant C/Fe ratio throughout the
disk). Assuming instead a spatial profile expected from a simplified accretion
disk model, we found it to give a significantly worse fit to the observations.
Conclusions: Since the bulk of the gas is found outside 30 AU, we argue that
the cometary objects known as "falling evaporating bodies" are unlikely to be
the dominant source of gas; production from grain-grain collisions or
photodesorption seems more likely. The incompatibility of the observations with
a simplified accretion disk model could favour a preferential depletion
explanation for the overabundance of C and O. More stringent constraints on the
spatial distribution will be available from ALMA observations of CI at 609
$\mu$m.
