  Water is key in the evolution of protoplanetary disks and the formation of
comets and icy/water planets. While high excitation water lines originating in
the hot inner disk have been detected in several T Tauri stars (TTSs), water
vapor from the outer disk, where most of water ice reservoir is stored, was
only reported in the closeby TTS TW Hya. We present spectrally resolved
Herschel/HIFI observations of the young TTS DG Tau in the ortho- and para-
water ground-state transitions at 557, 1113 GHz. The lines show a narrow
double-peaked profile, consistent with an origin in the outer disk, and are
~19-26 times brighter than in TW Hya. In contrast, CO and [C II] lines are
dominated by emission from the envelope/outflow, which makes H2O lines a unique
tracer of the disk of DG Tau. Disk modeling with the thermo-chemical code
ProDiMo indicates that the strong UV field, due to the young age and strong
accretion of DG Tau, irradiates a disk upper layer at 10-90 AU from the star,
heating it up to temperatures of 600 K and producing the observed bright water
lines. The models suggest a disk mass of 0.015-0.1 Msun, consistent with the
estimated minimum mass of the solar nebula before planet formation, and a water
reservoir of ~1e2-1e3 Earth oceans in vapour, and ~100 times larger in the form
of ice. Hence, this detection supports the scenario of ocean delivery on
terrestrial planets by impact of icy bodies forming in the outer disk.
