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In this paper, we present the distribution of $H_2O$ masers associated withthe semi-regular variable star W Hydrae (W Hya). We have collected the radiointerferometric data of the maser distribution taken with the Very Large Array(VLA), the Kashima-Nobeyama InterFErometer (KNIFE), the Multi-Element RadioLink Network (MERLIN), the VLBI Exploration of Radio Astrometry (VERA), and thecombined array of the Korean VLBI Network (KVN) and VERA (KaVA) in order totrace the maser distribution variation in two decades. Even though differencesin the sensitivities and angular resolutions of the interferometricobservations should be taken into account, we attempt to find possiblecorrelation of the maser distribution with the stellar light curve. Our failurein the measurement of the annual parallax of the masers with VERA is likelycaused by the properties of the maser features, which have been spatiallyresolved by the synthesized beam and survived for only half a year or less. Nodependence of the maser spot flux density on its size is found in the KNIFEdata, suggesting that maser spot size is determined by the physical boundary,as is expected for a clump affected by outward propagation of a stellarpulsation shock wave, rather than the (spherical) geometry of maser beaming inthe maser gas clump.

Overall variation of the $H_2O$ masers around W Hydrae in 28 years