  Prior high angular resolution spectroscopic observations of the
Low-ionization nuclear emission-line region (Liner) in NGC 3998 obtained with
the Space Telescope Imaging Spectrograph (STIS) aboard the Hubble Space
Telescope (HST) revealed a rich UV-visible spectrum consisting of broad
permitted and broad forbidden emission lines. The photoionization code XSTAR is
employed together with reddening-insensitive emission line diagnostics to
constrain a dynamical model for the broad-line region (BLR) in NGC 3998. The
BLR is modelled as a large H$^+$ region ${\sim}$ 7 pc in radius consisting of
dust-free, low density ${\sim}$ 10$^4$ cm$^{-3}$, low metallicity ${\sim}$ 0.01
$Z/Z_\odot$ gas. Modelling the shape of the broad H${\alpha}$ emission line
significantly discriminates between two independent measures of the black hole
mass, favouring the estimate of de Francesco et al. (2006). Interpreting the
broad H${\alpha}$ emission line in terms of a steady-state spherically
symmetric inflow leads to a mass inflow rate of 1.4 ${\times}$ 10$^{-2}$
M$_\odot$/yr, well within the present uncertainty of calculations that attempt
to explain the observed X-ray emission in terms of an advection-dominated
accretion flow (ADAF). Collectively, the model provides an explanation for the
shape of the H${\alpha}$ emission line, the relative intensities and
luminosities for the H Balmer, [O III], and potentially several of the broad UV
emission lines, as well as refining the initial conditions needed for future
modelling of the ADAF.
