  The high spectral resolution and sensitivity of Herschel/HIFI allows for the
detection of multiple rotational water lines and accurate determinations of
water production rates in comets. In this letter we present HIFI observations
of the fundamental 110-101 (557 GHz) ortho and 111-000 (1113 GHz) para
rotational transitions of water in comet 81P/Wild 2 acquired in February 2010.
We mapped the extent of the water line emission with five point scans. Line
profiles are computed using excitation models which include excitation by
collisions with electrons and neutrals and solar infrared radiation. We derive
a mean water production rate of $1.0 \times 10^{28}$ molecules s$^{-1}$ at a
heliocentric distance of 1.61 AU about 20 days before perihelion, in agreement
with production rates measured from the ground using observations of the 18-cm
OH lines. Furthermore, we constrain the electron density profile and gas
kinetic temperature, and estimate the coma expansion velocity by fitting the
water line shapes.
