  Water ice-rich patches have been detected on the surface of comet
67P/Churyumov-Gerasimenko by the VIRTIS hyperspectral imager on-board the
Rosetta spacecraft, since the orbital insertion in late August 2014. Among
those, three icy patches have been selected, and VIRTIS data are used to
analyse their properties and their temporal evolution while the comet was
moving towards the Sun. We performed an extensive analysis of the spectral
parameters, and we applied the Hapke radiative transfer model to retrieve the
abundance and grain size of water ice, as well as the mixing modalities of
water ice and dark terrains on the three selected water ice rich areas. Study
of the spatial distribution of the spectral parameters within the ice-rich
patches has revealed that water ice follows different patterns associated to a
bimodal distribution of the grains: ~50 {\mu}m sized and ~2000 {\mu}m sized. In
all three cases, after the first detections at about 3.5 AU heliocentric
distance, the spatial extension and intensity of the water ice spectral
features increased, it reached a maximum after 60-100 days at about 3.0 AU, and
was followed by an approximately equally timed decrease and disappearanceat
about ~2.2 AU, before perihelion. The behaviour of the analysed patches can be
assimilated to a seasonal cycle. In addition we found evidence of short-term
variability associated to a diurnal water cycle. The similar lifecycle of the
three icy regions indicates that water ice is uniformly distributed in the
subsurface layers, and no large water ice reservoirs are present.
