  In this paper, we present the distribution of $H_2O$ masers associated with
the semi-regular variable star W Hydrae (W Hya). We have collected the radio
interferometric data of the maser distribution taken with the Very Large Array
(VLA), the Kashima-Nobeyama InterFErometer (KNIFE), the Multi-Element Radio
Link Network (MERLIN), the VLBI Exploration of Radio Astrometry (VERA), and the
combined array of the Korean VLBI Network (KVN) and VERA (KaVA) in order to
trace the maser distribution variation in two decades. Even though differences
in the sensitivities and angular resolutions of the interferometric
observations should be taken into account, we attempt to find possible
correlation of the maser distribution with the stellar light curve. Our failure
in the measurement of the annual parallax of the masers with VERA is likely
caused by the properties of the maser features, which have been spatially
resolved by the synthesized beam and survived for only half a year or less. No
dependence of the maser spot flux density on its size is found in the KNIFE
data, suggesting that maser spot size is determined by the physical boundary,
as is expected for a clump affected by outward propagation of a stellar
pulsation shock wave, rather than the (spherical) geometry of maser beaming in
the maser gas clump.
