Irrigation Water Measurement M. U. Kale Assistant Professor Deptt. of Irrigation and Drainage Engineering, Dr. PDKV, Akola

Importance of water : Water is essential for achieving triple goal of ensuring Food security, Reducing poverty and Conserving ecosystems. Water is a limited resource It is estimated that with 10% increase in the present level of water use efficiency in irrigation projects, an additional 14 Mha area can be brought under irrigation from the existing irrigation capacities. Only if we act to improve water use in agriculture we will meet the acute freshwater challenges facing humankind over the coming 50 years.

Measurement of Irrigation Water :- “Good water management begins with water measurement” Water is the most valuable asset of irrigated agriculture and can be detrimental if used improperly. An accurate measurement permits an intelligent use. The methods to use for measurement should depend on the flow, environmental conditions and the degree of accuracy required.

Water Measurement Units :- Water is measured in terms of Volume It is water “at rest”. It is measured in terms of ‘Quantity of water’. m3, ft3, litres, hectare cm, hectare-inch etc. Volume = Area x depth 1 ha-cm = 10000 x 0.01 = 100 m3 = 100000 lit 1 cm3 = 0.001 lit; 1m3 = 1000 lit Depth Rainfall is measured as depth likewise irrigation as well Inch, foot, millimeter, centimeter etc. Depth = Volume / Area, mm Flow It is water “in motion” It is measured as volume of water per unit time; cubic meter per minute, cubic feet per second, acre-inches per day, liters per second etc. Q = Volume / time

Methods of Irrigation Water Measurement Direct Discharge Methods Using instruments like Weirs, Orifices Flumes Velocity - Area Methods Float method Current meter Tracer method

Methods of Measuring Irrigation Water Direct Discharge Methods Direct method: Collect water in a container of known volume e.g. bucket. Measure the time required for water from an irrigation source e.g. siphon to fill the bucket. Flow rate = Volume/time, m3/hr or lit/sec Method can use to determine the discharge rate of pumps and other water lifts like parsian wheel and leather bucket lift (charsa).

Methods of Measuring Irrigation Water Direct Discharge Methods Weirs: Weirs are regular notches over which water flows. A weir is an overflow structure installed perpendicular to open channel flow They are used to regulate floods through rivers, overflow dams and open channels. Made from concrete, timber or metal Named for cross-section as rectangular, trapezoidal or triangular. Weirs can be sharp or broad crested; Sharp crested rectangular or triangular sections are commonly used on the farm.

Important Terminology Nappe or Vein is the sheet of water flowing through a notch or over weir. Crest or Sill – is the top edge of a weir over which water flows. Weir Pond – Head is measured upstream 4 X maximum head on crest

Important Terminology Velocity of approach is the velocity with which water approaches the weir. Trapezoidal weir with 1 horizontal to 4 vertical slope is known as cipolletti weir. Approach must be kept free of sediment deposits

Weirs Advantages Disadvantages Simple to construct Fairly good at passing trash 1 head measurement Disadvantages High head loss Susceptible to sedimentation problems Sensitive to approach and exit conditions 10

Weirs Weirs can be either contracted or suppressed. If the length of crest is equal to width of approach channel, then there is no contraction. Suppressed weir No side contraction occurs Contracted weir When approach channel width is greater than size of weir crest, then the weir is called as contracted weir 11

Rectangular weir with no end contraction Rectangular weir with two end contraction Triangular weir with two end contraction Trapezoidal weir with two end contraction

Weirs The discharge through a weir is usually expressed as: Q = C L Hm where Q = discharge; C = coefficient dependent on the nature of weir crest and approach conditions; L = length of crest; H = head on the crest and m = exponent depending on weir opening. Weirs should be calibrated to determine these parameters before use. More in Chapter 6.

Weirs Franci’s formula generally used to determine flow rate through weirs Type of weir Formula Rectangular Q = 0.0184 L H3/2 Medium discharge for two end contraction Q = 1.84 [L – 02 H] H3/2 for without contraction Q = 1.84 [L – 01 H] H3/2 2) Triangular Q = 0.0138 H5/2 Small discharge 3) Trapezoidal (cipolletti) Q = 0.0186 L H3/2 Large discharge Where, Q = discharge in lit/sec, L = length of crest of weir, cm H = Head over the crest of weir, cm. Triangular weirs permit more accurate measurement of lower discharge than horizontal crested weirs since head recorded in this is higher.

Cipolleti weir installed in canal

Methods of Measuring Irrigation Water Direct Discharge Methods Orifices: An orifice is an opening in the wall of a tank containing water. The orifice can be circular, rectangular, triangular or any other shape.

The discharge through an orifice is given by: Q = C A 2 g h where Q = discharge rate; C = coefficient of discharge (0.6 - 0.8); A = area of the orifice; g = acceleration due to gravity and h = head of water over an orifice.

Methods of Measuring Irrigation Water Direct Discharge Methods Orifices must have sharp edges, else it is treated as tube. The main advantage of submerged orifice is the suitability for measuring flows in channels having very small slopes where it is difficult to obtain enough fall for the use of weirs. It is not accurate as it collects debris, sand and silt.

Methods of Measuring Irrigation Water Direct Discharge Methods Flumes: Hydraulic flumes are artificial open channels or sections of natural channels. Two major types of hydraulic flumes are Parshall Flume Cut-Throat Flume Flumes need to be calibrated before use.

Flumes are based on concept of specific energy and critical flow in channel. For greater depth, velocity is low and flow is sub -critical. For lesser depth, velocity is high; and flow is super Principle - When subcritical flow passes through a structure changing the flow into critical and supercritical state, it is found that the depth of flow upstream of structure is independent of that downstream of structure. In such situation discharge is a single valued function of upstream depth only.

Methods of Measuring Irrigation Water Parshall Flume Ralph L. Parshall, an irrigation research engineer, in 1922 invented the flume now known by his name. The parshall flume is an open channel type measuring device that operates with a small drop in head. The loss of head for free flow limit is only about 25% of that for weir. It consists of converging section, throat and diverging section. The size of parshall flume is defined by the width of its throat.

Parshall flume

Methods of Measuring Irrigation Water This flume can be used under free flow and submerged flow condition. Reasonably accurate measurement of flow is possible with this flume. It is self cleaning device. Meaurement of upstren water level is required to dertemine discharge under free flow condition, and upstream and downstream water level observations are required under submerged flow condition.

Parshall Flumes Flow occurs under two conditions Free flow Downstream water surface does not reduce discharge Requires only 1 head reading (Ha) 24

Parshall Flumes Submerged flow Downstream flow is high enough to reduce discharge 2 head readings required 50% submergence (Hb/Ha) on 1-3 inch flumes 80% submergence (Hb/Ha) ≥8 feet flumes After 90% submergence, flume is no longer effective Ha Hb 25

Parshall Flumes Disadvantages Advantages Relatively low head loss (1/4 of sharp crested weir) Handle some trash and sediment Well accepted Many sizes are commercially available Disadvantages Complicated geometry for construction Tight construction tolerances Aren’t amenable to fluid flow analysis 26

Methods of Measuring Irrigation Water Cut Throat Flume These flumes do not have throat section. These flumes have flat bottom and vertical walls only. It is developed by Skogerboe, Hyatt, Anderson and Eggleston in 1967. The advantage of this flume is economy.

Methods of Irrigation Water Measurement Velocity Area Method Float Method: A floating object is put in water and observe the time it takes the float e.g. a cork to go from one marked area to another. Assuming the float travels ‘D’ meters in ‘t’ secs Velocity of water at surface = (D/t) m/s Average velocity of flow = 0.8 (D/t) Flow rate, Q = Cross sectional x velocity. area of flow Object D

Methods of Irrigation Water Measurement Velocity Area Method Current meter Current meter is a small instrument containing a revolving wheel or vane that is turned by the movement of water. Insert the current meter to the depth = 0.6 d (d = depth of flow). Record the number of rotations (rpm) and determine the velocity of flow from calibrated chart. Current meter gives average of discharge at depth 0.2 d and 0.8 d. A current meter is used to measure velocity at 0.2 and 0.8 Depth or at only 0.6 depth.

Anemometer type current meter

Methods of Irrigation Water Measurement Velocity Area Method Tracer Method Insert the tracer at desired point. Determine the concentration of tracer, C1 Collect the sample at marked point. Determine the concentration of tracer, C2. Original concentration of tracer is C0. q1 is the flow rate of tracer into the canal Determine the flow rate using following equation Where, F = Counts per unit of radioactivity / unit volume of water/ time. A = Total units of radioactivity introduction. N = Total counts. Geiger counters/scintillation counters measure gamma rays emission

Methods of Irrigation Water Measurement Water meters Water meter consist of multiple blades that rotates on horizontal axle due to flowing water through pipe. Must have full pipe flow Basically operate on Q=VA principle Rotations of blades are calibrated in terms of flow rate. Thus water meter directly shows flow rate. Accuracy can be ±2-5% of actual flow

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