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

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<ul><li><p>Chapter 10</p><p>Measurement of Irrigation Water</p><p>One of the major problems in surface irrigation system is the low efficiency of</p><p>irrigation water. A measure that is essential for reducing waste and ensuring more</p><p>efficient use among farmers is metering the volumes of water supplied and charge</p><p>for it. There is a wise saying, You cannot manage something if you can not</p><p>measure it.</p><p>The amount of water that moves or passes a point in a given time period is</p><p>termed as water flow rate. Measurement and determination of flow rate is a critical</p><p>component of irrigation water management. Various techniques and tools are</p><p>available to measure the flow rate. These range from simple indigenous methods</p><p>to high-tech automated systems. Each of these methods has some advantages</p><p>over the others as well as some limitations. This is the subject matter of this chapter.</p><p>The techniques have been clarified with examples.</p><p>10.1 Basics of Water Measurement</p><p>10.1.1 Purpose of Water Measurement</p><p>Measurement of irrigation water is essential for making recommendations on farm</p><p>irrigation practices to reduce water loss. Measuring, monitoring, and control of flow</p><p>at scheme levels are important for a fair distribution of irrigation water. Other</p><p>purposes of water flow measurement (irrigation pumps, stream, main or tertiary</p><p>canal, etc.) are as follows:</p><p>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</p><p>its impact on land-use and environment</p><p>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</p><p>453</p></li><li><p>l To document water-use or discharge-rate for performance evaluation of water</p><p>development projectl To give the irrigation provider the possibility to control the volume of water that</p><p>are supplied to the schemel To give the Water User Associations the possibility to check the delivered flows</p><p>10.1.2 Types of Flow Measuring Devices</p><p>The methods or approaches used to measure water supplies can be broadly categor-</p><p>ized as follows:</p><p>l Volumetric or direct measurement methodl Area-velocity methodl Tracer measurement method</p><p>In the area-velocity method, the volume of water flowing per unit time is</p><p>determined with the area of flow and velocity of flow. But in the volumetric method,</p><p>the discharge rate can be measured directly: the area of flow need not be deter-</p><p>mined; only the height of water over the particular section of the instrument is</p><p>essential. Automated flow meters are available for different types of flow measure-</p><p>ment, and especially for pipe flow, it is common.</p><p>Flow rate through a small opening can be measured simply by collecting the</p><p>outflow for a particular time period, and then bymeasuring the volume of the collected</p><p>water. Alternatively, a stopwatch may be used to record the time required to fill in a</p><p>large container of known volume (or a particular volume). Then the flow rate,</p><p>Q volume of water collected=time required</p><p>But this approach is not suitable for large discharge or open channel flow.</p><p>The tools or devices used for measuring water flow can be categorized as follows:</p><p>1. Volumetric flow measuring devices</p><p>2. Velocity measuring devices</p><p>3. Water level (height) measuring devices</p><p>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</p><p>and water-height measuring devices are referred to as secondary devices as waterflow cannot be determined directly from these readings.</p><p>10.1.2.1 Volumetric Flow Measuring Devices</p><p>This category includes weirs, flumes, V-notch, venturimeter, orifice meter, magnetic</p><p>meter, ultrasonic meter, etc. Magnetic and ultrasonic meters require an electronic</p><p>454 10 Measurement of Irrigation Water</p></li><li><p>processor. Flumes may be of different types: Parshall flume, cut-throat flume, long-</p><p>throated flume, trapezoidal flume, etc.</p><p>10.1.2.2 Velocity Measuring Devices</p><p>The instruments that fall under this category are current meter, pitot tube, float, etc.</p><p>10.1.2.3 Water Height Measuring Devices</p><p>This category includes staff gauge, meter scale, water level sensor, ultrasonic water</p><p>level sensor, etc.</p><p>10.1.3 Selection of a Water Measuring Device</p><p>Various devices and tools are available for measuring discharge or flow rate. Each</p><p>type of water measuring devices has advantage and disadvantage relative to the</p><p>other type. Selection of a flow measuring device for a particular application or site</p><p>depends on the following:</p><p>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</p><p>The type of the device that is to be selected for a particular purpose largely</p><p>depends on the precision or accuracy needed for the intended use. The second factor</p><p>that comes into consideration is the cost of the device, its maintenance, and</p><p>operational easiness. The hydraulic factor include stream size, depth of flow, flow</p><p>type such as free or submerged flow, width of the channel, characteristics of</p><p>channel bed, layout and bend of the channel, turbidity of water, etc. Appropriate-</p><p>ness of the device for the intended use is also important. The site-specific factor</p><p>may include the economic condition of the farmer, social acceptance, reliance, etc.</p><p>A convenient low-cost device is desirable to obtain reliable flow data.</p><p>Traditionally, cutthroat flumes have been used in open drain systems or ditches</p><p>because of the ease of fabrication and installation. However, due to the extended</p><p>transitional length and width associated with the cutthroat flume, transportation of</p><p>the flume requires special facilitation or permits. In monitoring storm and irrigation</p><p>water, plant effluent and sewage water, the Parshall flume is most widely used for</p><p>permanent installation. For extremely severe industrial effluents, flume materials</p><p>may be stainless steel or other special materials as needed.</p><p>10.1 Basics of Water Measurement 455</p></li><li><p>The selection process often involves reaching a compromise among several</p><p>features.</p><p>10.1.4 Basic Terminologies and Hydraulic Principlesof Flow Measurement</p><p>10.1.4.1 Flow Rate or Discharge</p><p>The quantity of fluid flowing per second through a channel or pipe is termed as flow</p><p>rate or discharge (Q). It is usually expressed in m3/s or liter/s.</p><p>10.1.4.2 Head of the Fluid</p><p>Energy per unit weight of fluid is termed as head.</p><p>10.1.4.3 Velocity of Approach</p><p>It is the velocity with which the water approaches (i.e., reaches) the flow measuring</p><p>device. Mathematically, it can be expressed as:</p><p>Va Q= Area of flow : (10.1)</p><p>10.1.4.4 Continuity Equation</p><p>Continuity equation is based on the principle of conservation of mass. It states that</p><p>the quantity of fluid flowing per second through a conduit at all the cross-section is</p><p>constant (considering no loss from the system or gain outside the system). It can be</p><p>written as:</p><p>Q A1V1 A2V2 A3V3; (10.2)</p><p>where A1, A2, A3 are the area at section 1, 2, 3 and V1, V2, V3 are the velocity at thesections, respectively.</p><p>10.1.4.5 Fluid Pressure: Pascals Law</p><p>A fluid at rest (static) or in motion (dynamic) exerts pressure on its surface. In a</p><p>static fluid, the intensity of pressure at a point is equal in all directions, i.e.,</p><p>px py pz: (10.3)</p><p>456 10 Measurement of Irrigation Water</p></li><li><p>10.1.4.6 Steady and Unsteady Flow</p><p>In an open channel flow, if the velocity, depth of flow, and flow rate do not change</p><p>with time, the flow is termed as steady flow. On the other hand, if the velocity, depth</p><p>or flow rate changes with time, the flow is referred to as unsteady flow.For steady flow,</p><p>dv</p><p>dt 0; dh</p><p>dt 0; dQ</p><p>dt 0; and for unsteady flow; dv</p><p>dt6 0; dh</p><p>dt6 0; dQ</p><p>dt6 0:</p><p>10.1.4.7 Bernoullis Equation</p><p>Bernoullis theorem states that, in a steady flow of an incompressible fluid, the total</p><p>energy at any point of the fluid is constant.</p><p>Bernoullis equation for nonviscous fluid between two points of a flowing path</p><p>can be written as:</p><p>P1 V21</p><p>2g Z1 P2 V</p><p>22</p><p>2g Z2; (10.4)</p><p>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.</p><p>The orifice, nozzle, pitot tube, and venture flow rate meters use the Bernoullis</p><p>equation to calculate the fluid flow rate using pressure difference through obstruc-</p><p>tion in the flow.</p><p>10.1.4.8 Specific Energy</p><p>Specific energy of a flowing fluid is defined as the energy per unit weight of fluid.</p><p>It is given by:</p><p>E h V21</p><p>2g: (10.5)</p><p>10.1.4.9 Critical Depth, Critical Velocity, and Critical Flow</p><p>Critical depth (hc) is the depth of flow when the specific energy is minimum, that is</p><p>hc q2</p><p>g</p><p> 1=3: (10.6)</p><p>10.1 Basics of Water Measurement 457</p></li><li><p>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 flow isdefined in relation to Froude number. That is, the flow is said to be critical if the</p><p>Froude number (FN) is one (1.0). Froude number,</p><p>FN VgD</p><p>p ; (10.7)</p><p>where V is the mean velocity, D is the hydraulic depth of the channel (A/T), A is thewetted area, and T is the wetted perimeter.</p><p>If FN&lt; 1, the flow is subcritical, and if FN&gt; 1, the flow is super-critical. In otherway, if the depth of flow is less than the critical depth, the depth is referred to as</p><p>subcritical depth. Similarly, if the depth of flow is greater than the critical depth, thedepth is referred to as super-critical depth.</p><p>10.1.4.10 Flow Through Pitot Tube</p><p>Pitot tube is a device used for measuring the velocity of flow in an open channel or</p><p>pipe (Fig. 10.1). It works based on the principle of momentum of energy. If no</p><p>energy loss occurs between two points in a flow path, the total energy will be the</p><p>same, that is</p><p>p1rg</p><p> V21</p><p>2g Z1 p2rg</p><p>V222g</p><p> Z2: (10.8)</p><p>Here, Z1 Z2 (two points are in the same level)p1=rg H (say) Therefore, p2=rg H h</p><p>h</p><p>VFig 10.1 Schematic of apitot tube</p><p>458 10 Measurement of Irrigation Water</p></li><li><p>Thus,</p><p>H V21</p><p>2g H h V</p><p>22</p><p>2g</p><p>Velocity head (V2=2g) at point 2 is zero, as no kinetic head exists in the tube.Thus,</p><p>H V21</p><p>2g H h</p><p>or</p><p>h V21</p><p>2g;</p><p>or</p><p>V1 2gh</p><p>p: (10.9)</p><p>Here, V1 is the theoretical velocity. Actual velocity can be expressed as:</p><p>V1act Cv2gh</p><p>p: (10.10)</p><p>Here, Cv is the co-efficient of velocity for the pitot tube (there is some loss ofenergy due to entrance in pitot tube). The Cv &lt; 1.0, normally 0.950.98.</p><p>10.1.4.11 Empirical Equation of Discharge Over Rectangular Weir</p><p>Empirical equation for discharge rate (Q) over rectangular weir (Fig. 10.2) is:</p><p>Q 23Cd L</p><p>2g</p><p>p h3=2; (10.11)</p><p>where Cd is the co-efficient of discharge, L is the length of weir, and h is the head ofwater (or water depth).</p><p>Length</p><p>Water depth</p><p>Fig. 10.2 Schematic of aweir</p><p>10.1 Basics of Water Measurement 459</p></li><li><p>For a particular weir, L is constant and thus Q is dependent on h. Expressing witha single coefficient,</p><p>Q Ch3=2: (10.12)</p><p>Considering velocity of approach, (10.11) can be expressed as:</p><p>Q 23Cd L</p><p>2g</p><p>p h ha3=2 h3=2; (10.13)</p><p>where ha is the velocity of approach.</p><p>10.1.4.12 Empirical Equation of Flow Over V-Notch</p><p>Empirical equation for discharge rate (Q) over V-notch (Fig. 10.3) is:</p><p>Q 815</p><p>Cd tan y2</p><p>2g</p><p>p h5=2; (10.14)</p><p>where, y is the angle of notch.For a 90 notch,</p><p>tany2 tan 45 1:</p><p>Thus,</p><p>Q 815</p><p>Cd h5=2 2g</p><p>p Kh5=2;</p><p>(10.15)</p><p>where K is the combined coefficient.If Cd 0.60, then (10.15) reduces to:</p><p>Q 1:417 h5=2:</p><p>h</p><p>Fig. 10.3 Schematic of aV-notch</p><p>460 10 Measurement of Irrigation Water</p></li><li><p>10.1.5 Permanent Flow Measuring Structure</p><p>Permanent flow measuring structures are specially designed channel shapes that</p><p>characterize the flow. Common types are rectangular weirs and parshall flumes. The</p><p>choice of flume or weir depends on the application flow rate, channel shape, solid</p><p>content of the flowing water, etc.</p><p>Flumes and weirs are designed to force a transition from subcritical to super-</p><p>critical flow. In case of flume, the transition is caused by designing it to have a</p><p>narrowing at the throat and a drop in the channel bottom. Such a transition causes</p><p>flow to pass through critical depth in the flume throat. At the critical depth, energy is</p><p>minimized and there is a direct relationship between water depth, velocity, and</p><p>flow-rate.</p><p>10.1.5.1 Characteristics of Flow Measuring Structure</p><p>Flow measuring structures for small irrigation schemes should meet the following</p><p>requirements:</p><p>l Easy and solid constructionl Low maintenance needsl Quick and accurate readability of gaugesl Water level drop as small as possible</p><p>Long-throated flumes fit well to these requirements. The same applies to mea-</p><p>suring weirs, but the difference in water level up- and down-stream from a weir is</p><p>substantially bigger than in case of a measuring flume.</p><p>10.1.5.2 General Rules for Setting a Flow Measuring Structure</p><p>l From the hydraulic point of view, it is important to install a structure in a straight</p><p>channel section with a uniform cross section, slope, and roughness. The channel</p><p>must be straight upstream from the structure for a distance of 1020 times of the</p><p>water head, depending on the measuring instrument/structure.l By using a canal drop in the design, the risk of submergence of the flow in the</p><p>control section can be eliminated.</p><p>10.1.5.3 Setting Permanent Structure/Flume in the Canal</p><p>The procedure for the installation of a permanent measuring structure consists of</p><p>the following phases:</p><p>l Selection of the proper locationl Evaluation of canal characteristics</p><p>10.1 Basics of Water Measurement 461</p></li><li><p>l Formulation of design criterial Design and construction</p><p>Selection of Location</p><p>The flow measuring structure should be installed at the head of the main canals, as</p><p>close as possible to the intake structure and on a well accessible site.</p><p>Canal Characteristics</p><p>The upstream water level of the structure is influenced by the shape and level of the</p><p>control section of the structure, whereas the corresponding downstream water level</p><p>is determined by the characteristics of the canal, namely bed width, inclination of</p><p>the canal banks, longitudinal slope, and canal roughness.</p><p>Critical flow at the control section of the structure is essential. This requires</p><p>a control section with an elevated and/or narrow bed than the canal in which</p><p>the structure is placed. With respect to back water curve, a high and/or narrow</p><p>control section causes a high upstream water level, and this may result in upstream</p><p>overtopping of the embankments. On the other hand, a less high and/or less narrow</p><p>control section may result in downstream water level that the flow is submerged</p><p>and noncritical. So, a solution should be found in which the upstream water level</p><p>is lower than the maximum level permitted, and the water level in the control</p><p>section is enough for a free flow (i.e., not being influenced by a high downstream</p><p>water level).</p><p>Formulation of Design Crit...</p></li></ul>

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