  The radially averaged metallicity distribution of the ISM and the young
stellar population of a sample of 20 disk galaxies is investigated by means of
an analytical chemical evolution model which assumes constant ratios of
galactic wind mass loss and accretion mass gain to star formation rate. Based
on this model the observed metallicities and their gradients can be described
surprisingly well by the radially averaged distribution of the ratio of stellar
mass to ISM gas mass. The comparison between observed and model predicted
metallicity is used to constrain the rate of mass loss through galactic wind
and accretion gain in units of the star formation rate. Three groups of
galaxies are found: galaxies with either mostly winds and only weak accretion,
or mostly accretion and only weak winds, and galaxies where winds are roughly
balanced by accretion. The three groups are distinct in the properties of their
gas disks. Galaxies with approximately equal rates of mass-loss and accretion
gain have low metallicity, atomic hydrogen dominated gas disks with a flat
spatial profile. The other two groups have gas disks dominated by molecular
hydrogen out to 0.5 to 0.7 isophotal radii and show a radial exponential
decline, which is on average steeper for the galaxies with small accretion
rates. The rates of accretion (<1.0 x SFR) and outflow (<2.4 x SFR) are
relatively low. The latter depend on the calibration of the zero point of the
metallicity determination from the use of HII region strong emission lines.
