  Star-forming galaxies are believed to replenish their atomic gas reservoir,
which is consumed in star-formation, through accretion of gas from their
circumgalactic mediums (CGMs). However, there are few observational constraints
today on the gas accretion rate in external galaxies. Here, we use our recent
measurement of the scaling relation between the atomic hydrogen (HI) mass
$M_{HI}$ and the stellar mass $M_*$ in star-forming galaxies at $z \approx
0.35$, with the relations between the star-formation rate (SFR) and $M_*$, and
the molecular gas mass $M_{Mol}$ and $M_*$, and the assumption that
star-forming galaxies evolve along the main sequence, to determine the
evolution of the neutral gas reservoir and the average net gas accretion rate
onto the disks of star-forming galaxies over the past 4 Gyr. For galaxies with
$M_* \gtrsim 10^9 M_{\odot}$ today, we find that both $M_*$ and $M_{HI}$ in the
disk have increased, while $M_{Mol}$ has decreased, since $z \sim 0.35$. The
average gas accretion rate onto the disk over the past 4 Gyr is similar to the
average SFR over this period, implying that main-sequence galaxies have
maintained a stable HI reservoir, despite the consumption of gas in
star-formation. We obtain an average net gas accretion rate (over the past 4
Gyr) of $\approx 6 M_{\odot} yr^{-1}$ for galaxies with the stellar mass of the
Milky Way. At low redshifts, $z \lesssim 0.4$, the reason for the decline in
the cosmic SFR density thus appears to be the inefficiency in the conversion of
atomic gas to molecular gas, rather than insufficient gas accretion from the
CGM.
