Delta outflow is calculated from the velocities recorded continuously by UVM's at various sites in the California water system. This requires calibration of vUVM against complete velocity profiles. Here we discuss the way the calibration is done, and analyze the errors involved in using vUVM as an index for discharge. We have fit the shape of the observed rating curves to a multi-parameter model. We have estimated the error incurred in calculating the channel discharge due to approximating the rating curve in various ways. We have also estimated the error due to various other effects.