For example, the measuring station of a Parshall flume is in the convergence water surface drawdown. In the computer derived ratings, drawdown and friction losses caused by the gage location are an integral part of the calculation. Therefore, the gage should be located as indicated in the precomputed long-throated structure design and selected tables. The success or failure of the structure and the value of the collected data depend closely on the proper selection of a suitable head measurement device. The listed errors are higher than the expected random errors, partly to compensate for the effects of several systematic errors, such as zero-setting, instrument lag, reading error, temperature, and stilling well leakage.
Examples are canals where the flow changes are gradual.
The gage should be placed so that the water level can be read from the canal bank. The gage should be easy to clean. The value for measuring head on the sloping walls of trapezoidal-shaped canals must be appropriately converted to vertical head values before entering the discharge tables. These tables are usually made for stilling well use or vertical gage applications. The sloping gage can be marked to read direct values or equivalent values of vertical head.