Accurate Irrigation Water Flow Measurement in Pipes

Accurate Irrigation Water Flow Measurement in Pipes Burdette Barker, Utah State University Extension irrigation specialist Cody Zesiger, Utah State University Extension assistant professor, Weber and Morgan counties Matt Yost, Utah State University Extension agroclimate specialist

Introduction Accurate flow measurement is important to irrigation water management and water rights accounting and protection. Accurate flow measurement is essential in ensuring the equitable water distribution to water rights holders and shareholders within irrigation companies. Accurate flow measurement can be used to resolve or even prevent conflicts. And it can help producers ensure they can best use their limited water resources. Many methods can be used to measure water flow, and different methods and technologies are used for pipe flow compared to open channel (canal) flow. In this fact sheet, we cover only pipe flow and for that, only technologies that are most common in conventional irrigation practice. We also address some basic flow measurement principles and the pros and cons of different measurement technologies.

Flow Measurement Principles Water Flow Rate Water flow rates are measured and reported in terms of the volume of water moved per amount of time--for example, gallons per minute (gpm) or cubic feet per second (cfs). A cubic foot per second is sometimes called a second-foot. There are 449 gpm in 1 cfs. Another way to think about water flow is that the flow rate is equal to the average velocity (or speed) of water multiplied by the flow area: 𝐹𝑙𝑜𝑤 𝑅𝑎𝑡𝑒 = (𝐴𝑣𝑒𝑟𝑎𝑔𝑒 𝐹𝑙𝑜𝑤 𝑉𝑒𝑙𝑜𝑐𝑖𝑡𝑦) × (𝐹𝑙𝑜𝑤 𝐴𝑟𝑒𝑎) Example A pipe has an average flow velocity of 4 feet per second and a cross-sectional area of 0.7 square feet. Find the flow rate. The answer is 4 feet per second × 0.7 square feet = 2.8 cubic feet per second (cfs). This is the same as about 2.8 cfs × 449 gpm/cfs = 1,260 cfs.

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