Furrow Flow Measurement Accuracy

The primary source of error in properly calibrated, constructed, and installed flow measurement devices is due to reading error or uncertainty. Head reading uncertainty in small V‐notch flumes and submerged orifices is measured in the field as $\pm 3\mathrm{mm}$ with no consistent variation with reading. Elapsed time measurement uncertainty for volumetric measurements increases with the square root of the time. The sensitivity of flow measurement uncertainty to head or time reading uncertainty is proportional to the ratio of the device discharge equation exponent to the reading. Furrow flow measurement uncertainty varies with the device and flow rate, but generally exceeds $\pm 5\%$ and often exceeds $\pm 10\%\text{.}$ Maintaining uncertainty below $\pm 10\%$ requires flume measurements in the upper 50% of their range, orifice measurements with head readings greater than 13mm, and volumetric measurement elapsed times greater than 4 seconds.