  The changes in the spectral line profile of the 22GHz H2O maser are
calculated as a function of emerging maser flux. We address not only the
narrowing and re-broadening of the maser lines, but also the deviations of
Gaussian symmetry as a result of the various hyperfine components of the 22 GHz
maser line. Non-LTE models of the H2O maser transition, including hyperfine
structure and cross-relaxation, are compared with high spectral resolution Very
Long Baseline Interferometry (VLBI) observations of the H2O masers in the
circumstellar envelopes (CSEs) of a sample of late-type stars. This yields
estimates on the thermal width in the maser region as well as the emerging
maser flux and thus the level of saturation. We also discuss the effect of a
velocity gradient along the maser path on the line widths and shapes of the
line profile and the effect of the geometry of the maser region. We find that
the typical velocity shift along the maser path is of the order of 1.0 km/s.
The effect of this shift on the shape of the maser spectrum is difficult to
distinguish from the effect of the hyperfine components.
