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Within the 'Medicina/Effelsberg H2O maser monitoring program' we haveobserved the maser emission of R Crt and RT Vir for more than two decades. Toget insight in the distribution and longevity of maser spots in thecircumstellar envelopes, we have collected interferometric data, taken in thesame period, from the literature.  We confirm short-time variations of individual maser features on timescalesof months to up to 1.5 years. Also decade-long variations of the generalbrightness level independent from individual features were seen in both stars.These are due to brightness variations occurring independently from each otherin selected velocity ranges, and are independent of the optical lightcurves.Expected drifts in velocity of individual features are usually masked byblending. However, in RT Vir we found an exceptional case of a feature with aconstant velocity over 7.5 years (<0.06 km/s/yr).  We attribute the long-term brightness variations to the presence of regionswith higher-than-average density in the stellar wind, which host several cloudswhich emit maser radiation on the short time scales. These regions typicallyneed ~20 years to cross the H2O maser shell, where the right conditions toexcite H2O masers are present. The constant velocity feature (11 km/s) islikely to come from a single maser cloud, which moved through about half of RTVir's H2O maser shell without changing velocity. From this we infer that itspath was located in the outer part of the H2O maser shell, where RT Vir'sstellar wind apparently has already reached its terminal outflow velocity. Thisconclusion is corroborated by the observation that the highest H2O maseroutflow velocity in RT Vir approaches the terminal outflow velocity as given byOH and CO observations. This is generally not observed in other semi-regularvariable stars.

Water vapour masers in long-period variable stars II. The semi-regular variables R Crt and RT Vir