On the influence of collisional rate coefficients on the water vapour
  excitation
  Water is a key molecule in many astrophysical studies. Its high dipole moment
makes this molecule to be subthermally populated under the typical conditions
of most astrophysical objects. This motivated the calculation of various sets
of collisional rate coefficients (CRC) for H$_2$O (with He or H$_2$) which are
necessary to model its rotational excitation and line emission. We performed
accurate non--local non--LTE radiative transfer calculations using different
sets of CRC in order to predict the line intensities from transitions that
involve the lowest energy levels of H$_2$O (E $<$ 900 K). The results obtained
from the different CRC sets are then compared using line intensity ratio
statistics. For the whole range of physical conditions considered in this work,
we obtain that the intensities based on the quantum and QCT CRC are in good
agreement. However, at relatively low H$_2$ volume density ($n$(H$_2$) $<$
10$^7$ cm$^{-3}$) and low water abundance ($\chi$(H$_2$O) $<$ 10$^{-6}$), these
physical conditions being relevant to describe most molecular clouds, we find
differences in the predicted line intensities of up to a factor of $\sim$ 3 for
the bulk of the lines. Most of the recent studies interpreting early Herschel
Space Observatory spectra used the QCT CRC. Our results show that although the
global conclusions from those studies will not be drastically changed, each
case has to be considered individually, since depending on the physical
conditions, the use of the QCT CRC may lead to a mis--estimate of the water
vapour abundance of up to a factor of $\sim$ 3.
