Where should the flow be measured at a Parshall flume?

In a Parshall flume, the most accurate location for measuring flow is ⅔ of the length of the converging section upstream from the throat. This specific measurement point is chosen because it allows for a stable flow profile and minimizes potential turbulence or disturbances caused by the flume's geometry.

By positioning the measurement at this location, operators can achieve a more consistent reading that accurately reflects the flow rate. The design of the flume dictates that at this point, the flow is less influenced by any hydraulic changes occurring at the point of maximum velocity in the throat, thus enhancing measurement reliability.

The other options, although they may seem plausible, do not take full advantage of the hydraulic principles at play in a Parshall flume. For example, measuring at the throat could result in more variability due to flow turbulence, while measuring at the beginning of the converging section or at one-third of the converging section does not provide the same stability as the two-thirds point.