Fundamentals of Irrigation and On-farm Water Management: Volume 1 || Measurement of Irrigation Water

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  • Chapter 10

    Measurement of Irrigation Water

    One of the major problems in surface irrigation system is the low efficiency of

    irrigation water. A measure that is essential for reducing waste and ensuring more

    efficient use among farmers is metering the volumes of water supplied and charge

    for it. There is a wise saying, You cannot manage something if you can not

    measure it.

    The amount of water that moves or passes a point in a given time period is

    termed as water flow rate. Measurement and determination of flow rate is a critical

    component of irrigation water management. Various techniques and tools are

    available to measure the flow rate. These range from simple indigenous methods

    to high-tech automated systems. Each of these methods has some advantages

    over the others as well as some limitations. This is the subject matter of this chapter.

    The techniques have been clarified with examples.

    10.1 Basics of Water Measurement

    10.1.1 Purpose of Water Measurement

    Measurement of irrigation water is essential for making recommendations on farm

    irrigation practices to reduce water loss. Measuring, monitoring, and control of flow

    at scheme levels are important for a fair distribution of irrigation water. Other

    purposes of water flow measurement (irrigation pumps, stream, main or tertiary

    canal, etc.) are as follows:

    l For hydrological water budgetingl To document water requirementl To charge for water usedl To develop water management plans for the effective use of waterl To document water availability baseline for evaluating any future changes and

    its impact on land-use and environment

    M.H. Ali, Fundamentals of Irrigation and On-farm Water Management: Volume 1,DOI 10.1007/978-1-4419-6335-2_10,# Springer ScienceBusiness Media, LLC 2010

    453

  • l To document water-use or discharge-rate for performance evaluation of water

    development projectl To give the irrigation provider the possibility to control the volume of water that

    are supplied to the schemel To give the Water User Associations the possibility to check the delivered flows

    10.1.2 Types of Flow Measuring Devices

    The methods or approaches used to measure water supplies can be broadly categor-

    ized as follows:

    l Volumetric or direct measurement methodl Area-velocity methodl Tracer measurement method

    In the area-velocity method, the volume of water flowing per unit time is

    determined with the area of flow and velocity of flow. But in the volumetric method,

    the discharge rate can be measured directly: the area of flow need not be deter-

    mined; only the height of water over the particular section of the instrument is

    essential. Automated flow meters are available for different types of flow measure-

    ment, and especially for pipe flow, it is common.

    Flow rate through a small opening can be measured simply by collecting the

    outflow for a particular time period, and then bymeasuring the volume of the collected

    water. Alternatively, a stopwatch may be used to record the time required to fill in a

    large container of known volume (or a particular volume). Then the flow rate,

    Q volume of water collected=time required

    But this approach is not suitable for large discharge or open channel flow.

    The tools or devices used for measuring water flow can be categorized as follows:

    1. Volumetric flow measuring devices

    2. Velocity measuring devices

    3. Water level (height) measuring devices

    The volumetric flow measuring devices are referred to as primary devicesas water flow can be known from their measured data. On the other hand, velocity

    and water-height measuring devices are referred to as secondary devices as waterflow cannot be determined directly from these readings.

    10.1.2.1 Volumetric Flow Measuring Devices

    This category includes weirs, flumes, V-notch, venturimeter, orifice meter, magnetic

    meter, ultrasonic meter, etc. Magnetic and ultrasonic meters require an electronic

    454 10 Measurement of Irrigation Water

  • processor. Flumes may be of different types: Parshall flume, cut-throat flume, long-

    throated flume, trapezoidal flume, etc.

    10.1.2.2 Velocity Measuring Devices

    The instruments that fall under this category are current meter, pitot tube, float, etc.

    10.1.2.3 Water Height Measuring Devices

    This category includes staff gauge, meter scale, water level sensor, ultrasonic water

    level sensor, etc.

    10.1.3 Selection of a Water Measuring Device

    Various devices and tools are available for measuring discharge or flow rate. Each

    type of water measuring devices has advantage and disadvantage relative to the

    other type. Selection of a flow measuring device for a particular application or site

    depends on the following:

    l The need of precisionl Economic factor (cost of the device)l Easiness or limitations of the devicel Hydraulically appropriatenessl Site-specific factorsl Technical sophistication or technical know-how of the instrumentation

    The type of the device that is to be selected for a particular purpose largely

    depends on the precision or accuracy needed for the intended use. The second factor

    that comes into consideration is the cost of the device, its maintenance, and

    operational easiness. The hydraulic factor include stream size, depth of flow, flow

    type such as free or submerged flow, width of the channel, characteristics of

    channel bed, layout and bend of the channel, turbidity of water, etc. Appropriate-

    ness of the device for the intended use is also important. The site-specific factor

    may include the economic condition of the farmer, social acceptance, reliance, etc.

    A convenient low-cost device is desirable to obtain reliable flow data.

    Traditionally, cutthroat flumes have been used in open drain systems or ditches

    because of the ease of fabrication and installation. However, due to the extended

    transitional length and width associated with the cutthroat flume, transportation of

    the flume requires special facilitation or permits. In monitoring storm and irrigation

    water, plant effluent and sewage water, the Parshall flume is most widely used for

    permanent installation. For extremely severe industrial effluents, flume materials

    may be stainless steel or other special materials as needed.

    10.1 Basics of Water Measurement 455

  • The selection process often involves reaching a compromise among several

    features.

    10.1.4 Basic Terminologies and Hydraulic Principlesof Flow Measurement

    10.1.4.1 Flow Rate or Discharge

    The quantity of fluid flowing per second through a channel or pipe is termed as flow

    rate or discharge (Q). It is usually expressed in m3/s or liter/s.

    10.1.4.2 Head of the Fluid

    Energy per unit weight of fluid is termed as head.

    10.1.4.3 Velocity of Approach

    It is the velocity with which the water approaches (i.e., reaches) the flow measuring

    device. Mathematically, it can be expressed as:

    Va Q= Area of flow : (10.1)

    10.1.4.4 Continuity Equation

    Continuity equation is based on the principle of conservation of mass. It states that

    the quantity of fluid flowing per second through a conduit at all the cross-section is

    constant (considering no loss from the system or gain outside the system). It can be

    written as:

    Q A1V1 A2V2 A3V3; (10.2)

    where A1, A2, A3 are the area at section 1, 2, 3 and V1, V2, V3 are the velocity at thesections, respectively.

    10.1.4.5 Fluid Pressure: Pascals Law

    A fluid at rest (static) or in motion (dynamic) exerts pressure on its surface. In a

    static fluid, the intensity of pressure at a point is equal in all directions, i.e.,

    px py pz: (10.3)

    456 10 Measurement of Irrigation Water

  • 10.1.4.6 Steady and Unsteady Flow

    In an open channel flow, if the velocity, depth of flow, and flow rate do not change

    with time, the flow is termed as steady flow. On the other hand, if the velocity, depth

    or flow rate changes with time, the flow is referred to as unsteady flow.For steady flow,

    dv

    dt 0; dh

    dt 0; dQ

    dt 0; and for unsteady flow; dv

    dt6 0; dh

    dt6 0; dQ

    dt6 0:

    10.1.4.7 Bernoullis Equation

    Bernoullis theorem states that, in a steady flow of an incompressible fluid, the total

    energy at any point of the fluid is constant.

    Bernoullis equation for nonviscous fluid between two points of a flowing path

    can be written as:

    P1 V21

    2g Z1 P2 V

    22

    2g Z2; (10.4)

    where P1 is the pressure head (i.e., pressure energy per unit of fluid) p/rg, V21=2gis the kinetic head (i.e., kinetic energy per unit of fluid), V1 is the velocity of flow,and Z1 and Z2 are the elevations of the two points.

    The orifice, nozzle, pitot tube, and venture flow rate meters use the Bernoullis

    equation to calculate the fluid flow rate using pressure difference through obstruc-

    tion in the flow.

    10.1.4.8 Specific Energy

    Specific energy of a flowing fluid is defined as the energy per unit weight of fluid.

    It is given by:

    E h V21

    2g: (10.5)

    10.1.4.9 Critical Depth, Critical Velocity, and Critical Flow

    Critical depth (hc) is the depth of flow when the specific energy is minimum, that is

    hc q2

    g

    1=3: (10.6)

    10.1 Basics of Water Measurement 457

  • Velocity of flow at critical depth is known as critical velocity. If the velocity offlow is critical, the flow is termed as critical flow. Alternatively, critical f

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