  Through polarization observations water masers are excellent probes of
magnetic fields in the maser region. Magnetic field strengths, such as those in
the water masers regions of the envelopes of late-type stars and star-forming
regions, are typically determined using a direct relation between the field
strength and the observed circular polarization. Here it is shown that velocity
and magnetic field gradients along the maser have a significant effect on the
field strengths obtained from circular polarization observations. Due to
velocity gradients the actual magnetic field strength could be up to 100%
higher than the field strength derived from the observations. Additionally,
when a magnetic field gradient is present, the resulting circular polarization
derived is caused predominantly by the average magnetic field in the
unsaturated maser core. Measurements of the fractional linear polarization are
not affected by velocity or magnetic field strength gradients, though changes
in the magnetic field angle along the maser do quench the linear polarization
intensity when the maser saturates.
