  The next generation of galaxy surveys will provide more precise measurements
of galaxy clustering than have previously been possible. The 21-cm radio
signals that are emitted from neutral atomic hydrogen (HI) gas will be detected
by large-area radio surveys such as WALLABY and the SKA, and deliver galaxy
positions and velocities that can be used to measure galaxy clustering
statistics. But, to harness this information to improve our cosmological
understanding, and learn about the physics of dark matter and dark energy, we
need to accurately model the manner in which galaxies detected in HI trace the
underlying matter distribution of the Universe. For this purpose, we develop a
new HI-based Halo Occupation Distribution (HOD) model, which makes predictions
for the number of galaxies present in dark matter halos conditional on their HI
mass. The parameterised HOD model is fit and validated using the Dark Sage
semi-analytic model, where we show that the HOD parameters can be modelled by
simple linear and quadratic functions of HI mass. However, we also find that
the clustering predicted by the HOD depends sensitively on the radial
distributions of the HI galaxies within their host dark matter halos, which
does not follow the NFW profile in the Dark Sage simulation. As such, this work
enables -- for the first time -- a simple prescription for placing galaxies of
different HI mass within dark matter halos in a way that is able to reproduce
the HI mass-dependent galaxy clustering and HI mass function simultaneously and
without requiring knowledge of the optical properties of the galaxies. Further
efforts are required to demonstrate that this model can be used to produce
large ensembles of mock galaxy catalogues for upcoming surveys.
