  We consider the excitation of water in the Photon Dominated Region (PDR).
With the use of a three-dimensional escape probability method we compute the
level populations of ortho- and para-H_2O up to 350 K (i.e., 8 levels), as well
as line intensities for various transitions. Homogeneous and inhomogeneous
models are presented with densities of 10^4-10^5 cm^{-3} and the differences
between the resulting intensities are displayed. Density, temperature, and
abundance distributions inside the cloud are computed with the use of a
self-consistent physi-chemical (in)homogeneous model in order to reproduce the
line intensities observed with SWAS, and to make predictions for various lines
that HIFI will probe in the future. Line intensities vary from 10^{-13} erg
cm^{-2} s^{-1} sr^{-1} to a few times 10^{-6} erg cm^{-2} s^{-1} sr^{-1}. We
can reproduce the intensity for the 1_{10}-1_{01} line observed by the SWAS
satellite. It is found that the 2_{12}-1_{01} line is the strongest, whereas
the 3_{12}-2_{21} line is the weakest, in all the models. It is found that the
1_{10}-1_{01} line probes the total column, while higher excitation lines probe
the higher density gas (e.g., clumps).
