It is well-established that velocities in the immediate surroundings of solar magnetic elements produce an asymmetry in the Stokes V profiles emerging from the magnetic feature. Conversely, the observed Stokes V asymmetry can be used to infer the velocity field. Taking as constraints the area asymmetries of the Stokes V profiles of two lines of neutral magnesium (lambda 457nm and lambda 517nm) observed near the center of the solar disk, the (vertical) component of the velocity field below the magnetic canopy of flux tubes is investigated. We find that the strong Mg I b_2 line at 517nm qualitatively extends the diagnostic capabilities of Stokes V asymmetry, mainly due to the fact that it is sensitive to velocities over a large range of heights and hence also at relatively large distances from the flux tube axis. In order to retrieve the observed area asymmetry of both lines, up- as well as downflows have to be introduced in the models. If the temperature differences between the two flows are neglected then a downflow of 1.5 - 2km s(-1) close to the edge of the flux tube and an almost equally strong upflow at greater distances (corresponding to the central part of a granule) reproduces the observed area asymmetries. If we take into account that the temperature in the downflow is lower than in the upflow, we can only reproduce the observations if the downflow is fast (>= 5 km s(-1) ) and concentrated into narrow lanes.