  Combining new HI data from a synergetic survey of ASKAP WALLABY and FAST with
the ALFALFA data, we study the effect of ram pressure and tidal interactions in
the NGC 4636 group. We develop two parameters to quantify and disentangle these
two effects on gas stripping in HI-bearing galaxies: the strength of external
forces at the optical-disk edge, and the outside-in extents of HI-disk
stripping. We find that gas stripping is widespread in this group, affecting
80% of HI-detected non-merging galaxies, and that 41% are experiencing both
types of stripping. Among the galaxies experiencing both effects, the two types
of strengths are independent, while two HI-stripping extents moderately
anticorrelate with each other. Both strengths are correlated with HI-disk
shrinkage. The tidal strength is related to a rather uniform reddening of
low-mass galaxies ($M_*<10^9\,\text{M}_\odot$) when tidal stripping is the
dominating effect. In contrast, ram pressure is not clearly linked to the
color-changing patterns of galaxies in the group. Combining these two stripping
extents, we estimate the total stripping extent, and put forward an empirical
model that can describe the decrease of HI richness as galaxies fall toward the
group center. The stripping timescale we derived decreases with distance to the
center, from $\mathord{\sim}1\,\text{Gyr}$ beyond $R_{200}$ to
$\mathord{\lesssim}10\,\text{Myr}$ near the center. Gas-depletion happens
$\mathord{\sim}3\,\text{Gyr}$ since crossing $2R_{200}$ for HI-rich galaxies,
but much quicker for HI-poor ones. Our results quantify in a physically
motivated way the details and processes of environmental-effects-driven galaxy
evolution, and might assist in analyzing hydrodynamic simulations in an
observational way.
