  We use the EAGLE suite of cosmological hydrodynamical simulations to study
how the HI content of present-day galaxies depends on their environment. We
show that EAGLE reproduces observed HI mass-environment trends very well, while
semi-analytic models typically overpredict the average HI masses in dense
environments. The environmental processes act primarily as an on/off switch for
the HI content of satellites with stellar mass Mstar>10^9 Msun. At a fixed
Mstar, the fraction of HI-depleted satellites increases with increasing host
halo mass M200 in response to stronger environmental effects, while at a fixed
M200 it decreases with increasing satellite Mstar as the gas is confined by
deeper gravitational potentials. HI-depleted satellites reside mostly, but not
exclusively, within the virial radius r200 of their host halo. We investigate
the origin of these trends by focussing on three environmental mechanisms: ram
pressure stripping by the intra-group medium, tidal stripping by the host halo,
and satellite-satellite encounters. By tracking back in time the evolution of
the HI-depleted satellites, we find that the most common cause of HI removal is
satellite encounters. The timescale for HI removal is typically less than 0.5
Gyr. Tidal stripping occurs in halos of M200<10^{14} Msun within half r200,
while the other processes act also in more massive halos, generally within
r200. Conversely, we find that ram pressure stripping is the most common
mechanism that disturbs the HI morphology of galaxies at redshift z=0. This
implies that HI removal due to satellite-satellite interactions occurs on
shorter timescales than the other processes.
