  We evaluate recent and upcoming low-redshift neutral hydrogen (HI) surveys as
a cosmological probe of small scale structure with a goal of determining the
survey criteria necessary to test ultra-light axion (ULA) dark matter models.
Standard cold dark matter (CDM) models predict a large population of low-mass
galactic halos, whereas ULA models demonstrate significant suppression in this
small-scale regime, with halo mass cutoffs of $10^{12}\, \mathrm{M}_{\odot}$ to
$10^{7}\, \mathrm{M}_{\odot}$ corresponding to ULA masses of $10^{-24}\,$eV to
$10^{-20}\,$eV, respectively. We generate random, homogeneously populated mock
universes with cosmological parameters adjusted to match CDM and ULA models. We
simulate observations of these mock universes with hypothetical analogs of the
mass-limited ALFALFA and WALLABY HI surveys and reconstruct the corresponding
HI mass function (HIMF). We find that the ALFALFA HIMF can test for the
presence of ULA DM with $m_{a}\lesssim 10^{-21.5}\,$eV, while WALLABY could
reach the larger window $m_{a}\lesssim 10^{-20.9}\,$eV. These constraints are
complementary to other probes of ULA dark matter, demonstrating the utility of
local Universe HI surveys in testing dark matter models.
