Accuracy of Round Meter Gates for On-Farm Deliveries

Recent California legislation requires irrigation water agencies larger than 25,000 acres to measure volumetric water deliveries within specified levels of relative uncertainty. Although the meter gate is one of the most widely used flow measurement devices in California, little investigation has been conducted into the accuracy, limitations, and uncertainties of the rating tables developed over 60 years ago. A meter gate testing facility was constructed at the Cal Poly Irrigation Training and Research Center’s Water Resources Facility capable of testing gates up to 0.76 m (30 in.). The facility was constructed with gates oriented perpendicular to the supply channel flow to match actual field conditions. Three commonly used gate sizes of 0.30 m (12 in.), 0.46 m (18 in.), and 0.61 m (24 in.) were examined under a variety of upstream head, head loss, and gate opening conditions. Based on the limitations, guidelines are included to assist users and lower the uncertainty of these devices. It was found that meter gates can be an accurate flow measurement device if installed and operated correctly according to guidelines provided. Limitations were found: upstream head above the top of the turnout pipe must be maintained equivalent to at least half the pipe diameter, and gate openings less than 25% open can lead to large uncertainties. Using the new rating tables for the three gates examined, the relative uncertainty is less than ±5 to ±7% at the 95% confidence level with the new rating tables, as compared with less than ±10% at a 95% confidence level using common published tables. Uncertainties are lower than the required estimated 10.7% instanta­ neous flow rate uncertainty that will be needed to meet current SB X7-7 requirements.