To compute discharge using image velocimetry, the measured surface velocity must be related to the depth-averaged velocity; this is typically done using a simple coefficient known as the alpha coefficient. The alpha coefficient is the most significant input to the computation of discharge from imagery that is often not empirically determined, but the resulting discharge value is directly proportional to the value chosen. The USGS database contains more than half a million ADCP measurements - the profiles in these measurements can be used to inform selection of alpha. Results from a compilation of profiles at a variety of sites will be presented.
