  Context: observations of water lines are a sensitive probe of the geometry,
dynamics and chemical structure of dense molecular gas. The launch of Herschel
with on board HIFI and PACS allow to probe the behaviour of multiple water
lines with unprecedented sensitivity and resolution. Aims: we investigate the
diagnostic value of specific water transitions in high-mass star-forming
regions. As a test case, we apply our models to the AFGL2591 region. Results:
in general, for models with a constant water abundance, the ground state lines,
i.e., 1_(10)-1_(01), 1_(11)-0_(00), and 2_(12)-1_(01), are predicted in
absorption, all the others in emission. This behaviour changes for models with
a water abundance jump profile in that the line profiles for jumps by a factor
of ~10-100 are similar to the line shapes in the constant abundance models,
whereas larger jumps lead to emission profiles. Asymmetric line profiles are
found for models with a cavity outflow and depend on the inclination angle.
Models with an outflow cavity are favoured to reproduce the SWAS observations
of the 1_(10)-1_(01) ground-state transition. PACS spectra will tell us about
the geometry of these regions, both through the continuum and through the
lines. Conclusions: it is found that the low-lying transitions of water are
sensitive to outflow features, and represent the excitation conditions in the
outer regions. High-lying transitions are more sensitive to the adopted density
and temperature distribution which probe the inner excitation conditions. The
Herschel mission will thus be very helpful to constrain the physical and
chemical structure of high-mass star-forming regions such as AFGL2591.
