  Semi-analytic models (SAMs) are a promising means of tracking the physical
processes associated with galaxy formation, but many of their approximations
have not been rigorously tested. As part of the SMAUG (Simulating Multiscale
Astrophysics to Understand Galaxies) project, we compare predictions from the
FIRE-2 hydrodynamical "zoom-in" simulations to those from the Santa Cruz SAM
run on the same halo merger trees, with an emphasis on the global mass flow
cycle. Our study includes 13 halos spanning low-mass dwarfs (M_vir~10^10 M_sun
at z=0), intermediate-mass dwarfs (M_vir~10^11 M_sun) and Milky Way-mass
galaxies (M_vir~10^12 M_sun). The SAM and FIRE-2 predictions agree relatively
well with each other in terms of stellar and interstellar mass, but differ
dramatically on circumgalactic mass (the SAM is lower than FIRE-2 by ~3 orders
of magnitude for dwarfs). Strikingly, the SAM predicts higher gas accretion
rates for dwarfs compared to FIRE-2 by factors of ~10-100, and this is
compensated for with higher mass outflow rates in the SAM. We argue that the
most severe model discrepancies are caused by the lack of preventative stellar
feedback and the assumptions for halo gas cooling and recycling in the SAM. As
a first step towards resolving these model tensions, we present a simple yet
promising new preventative stellar feedback model in which the energy carried
by supernova-driven winds is allowed to heat some fraction of gas outside of
halos to at least the virial temperature such that accretion is suppressed.
