  Galactic winds are a key physical mechanism for understanding galaxy
formation and evolution, yet empirical and theoretical constraints for the
character of winds are limited and discrepant. Recent empirical models find
that local star-forming galaxies have a deficit of oxygen that scales with
galaxy stellar mass. The oxygen deficit provides unique empirical constraints
on the magnitude of mass loss, composition of outflowing material and metal
reaccretion onto galaxies. We formulate the oxygen deficit constraints so they
may be easily implemented into theoretical models of galaxy evolution. We
parameterize an effective metal loading factor which combines the uncertainties
of metal outflows and metal reaccretion into a single function of galaxy virial
velocity. We determine the effective metal loading factor by forward-fitting
the oxygen deficit. The effective metal loading factor we derive has important
implications for the implementation of mass loss in models of galaxy evolution.
