3: Measuring Water Applications

Irrigation Systems Management (AET 021) · LibreTexts

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    • 3.1: Introduction
      Have you ever wondered what it would be like to drive an automobile without a speedometer and an odometer? You might feel somewhat lost. You would not know how fast you were going, nor how far you've traveled. Irrigating without water measurement is much the same way. Without knowing the water flow rate, you do not know how fast you are applying water. And, without measured volumes, you cannot determine the depth of application.
    • 3.2: Basic Principles of Flow Measurement
      With most water measuring devices, the fundamental measurement is the velocity of the flowing water. Using the continuity principle (Equation 3.6), flow velocity is converted to flow rate. There are many methods used to estimate flow velocity. These include mechanical devices such as impellers, paddle wheels, bucket wheels, vanes, floats, and the measurement of pressure differences within hydraulic structures to infer the flow velocity.
    • 3.3: Flow Measurement in Pipelines
      There are many water measuring devices available for both pressurized pipelines and open channels. We will discuss only a few of them. For more detailed discussions of water measurement, the reader is referred to the following: ASME (1971), Bos et al. (1984), Miller (1996), Replogle et al. (1990), and USBR (1997).
    • 3.4: Flow Measurement in Open Channels
      Open channel flow is distinguished from pipeline flow by the fact that the water surface is at atmospheric pressure. With closed conduit or pipeline flow, the surface of the water is contained by the conduit's wall which causes the water pressure to exceed atmospheric pressure.
    • 3.5: Summary
      Flow measurement is important in irrigation so that both the rate that water is being applied and the depth of application are known. Without this information it is difficult to be a good water manager. Flow measurement typically relies on the principle of continuity. Flow rate is related to the velocity of water flow and the cross-sectional area of flow (Equation 3.6).
    • 3.6: Questions
      Why are long sections of straight pipe and straight channel, free of obstructions, required of upstream flow measuring devices? Show how Equation 3.3 can be rearranged to determine the flow rate required to apply a desired depth in a given time period.
    • 3.7: References
      American Society of Mechanical Engineers. (1971). Fluid meters, their theory and application (6th ed.). New York, NY: ASME. Bos, M. G., Replogle, J. A., & Clemmens, A. J. (1984). Flow measuring flumes for open channels. New York, NY: John Wiley & Sons.

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