Under laboratory conditions, Parshall Flumes can be to be accurate to within +/-2%. However, practical considerations such as approach flow, installation, and dimensional tolerances result in free-flow accuracies of +/-5% (per ASTM D1941).  

Installations where the upstream / downstream / installation conditions are less than optimal or where the flume is out of dimension will exhibit accuracies less than above and may require field calibration.

For Parshall Flumes that have settled (or been installed at a slope), corrections have been developed to correct the indicated flow rate.   

The above is true for Parshall Flumes experiencing free-flow.  For installations where downstream conditions restrict the flow out of the flume, submergence may become a factor.  Above the submergence transition, S_t, for the flume size, submerged flumes should not use free-flow discharge equations, as they will over-indicate the flow rate.