🧩 Measurement

Irrigation Water Measurement

Several devices are commonly used for measuring irrigation water. They grouped into four categories.

• Volumetric Methods
• Velocity Area Methods
  • Float Method
  • Water Meters
• Measuring Structures/Devices
  • Orifices
  • Weirs
  • Flumes
• Tracer Methods

1) Volumetric Methods (Using a container)

• A simple method of measuring a small irrigation stream is to collect the flow in container of known volume for a measured period. An ordinary bucket or barrel is used as container. The time required to fill the container is recorded with a stopwatch or with seconds on wristwatch.
• The rate of flow is measured as below:

Problem

A 24 liter capacity bucket is filled in 10 seconds by discharge from a Persian wheel. What is rate of flow?

Solution

Discharge ratio lt/sec = 24/10 = 2.4 lt/sec or 144 lt./min.

2) Velocity Area Method

(a) Float Method

To determine the velocity of water at the surface of the channel, the length of the trial section is divided by the average time taken by the float to cross it. Since the Velocity of the float on the surface of the water will be greater than the average velocity of the stream; it is constant factor, which is usually assumed to be 0.85. To obtain the rate of flow, this average velocity (measured velocity x coefficient) is multiplied by the average cross sectional area of the stream.

Q = A × V

Where, Q: Discharge rate in m³/sec. V: Velocity of flow in m/s A: Cross sectional area of channel in m²

(b) Water Meters

• Water meters utilize a multi blade propeller made of metal, plastic or rubber, rotating in a vertical or horizontal plane and geared to a totalizer in such a way that a numerical counter can totalize the flow in any desired volumetric units, water meters are available for a range of sizes suiting the pipe size commonly used on the farm. Basic requirements for accurate operation of the water meter are:
  • The pipe must flow full at all times.
  • The rate of flow must exceed the minimum for the rated range.
• Meters are calibrated in the factory and field adjustments are usually not required. When water meters are installed in open channels, the flow must be brought through the pipes of known cross sectional area. Care must be taken that no debris or other foreign materials obstruct the propeller.

3) Water Measuring Devices

Orifices

• Orifices in open channel are usually circular or rectangular openings in vertical bulk head through which water flows.
• The edges of opening are sharp and often constructed of metal. The cross sectional area of orifice is small in relation to the stream cross section.
• Orifice may operate under free flow or submerged flow conditions.

Weirs

• Weir means a notch in a well built across a stream which is used to measure the flow in an irrigation channel, or the discharge of a well or canal outlet at a source.
• The notch may be
  • (a) Rectangular
  • (b) Trapezoidal and
  • (c) 90 degree V (Triangular) notch or weir
• Weirs measures the flow in an irrigation channel or discharge of a well or channel outlet.
• Common weir type for water measurement in canal - Broad crested weir, Parshall flume, Cut throat weir
• Measuring uniform flow of water - Rectangular and Trapezoidal weir
• For highly variable water flow and desired greater - Triangular and Parabolic weir accuracy
• 90º V notch weirs measure small and medium size streams.

Parshall Flume or (Venturi flume)

• Parshall flume is a device in which the discharge is obtained by measuring the loss in the head caused by forcing a stream of water through a throat or converged section of a flume with a depressed boom. The loss in head is very small in this device. The accuracy of measurement in the Parshall flume is within allowable limits of 5 per cent the flumes ranging from 3 inches to 10 feet throat width are used, which gives the range of discharge of 1/30 to 200 cusecs. The flumes of 3, 6 and 9 inch size are generally used in field measurement.
• Parshall/Venturi flume is the most common water flow measuring device which measures water flow in open conduit.
• Cut-throat flumes: Cut-throat flumes are developed for measurement of water. Since there is no throat section (Zero throat flumes), the flumes have been given the name as out throat flumes by the designers.

4) Tracer Methods

• These methods are independent of stream cross section and are suitable for field measurements without installing fixed structures. In these methods, a substance (tracer) is concentration form is introduced into flowing water and allowed to thoroughly mix.
• The concentration of the tracer is measured at downstream section. Since only the quantity of water is necessary to accomplish the dilution is involved, there is no need to measure velocity, depth, and head, cross sectional or any other hydraulic factor usually considered in discharge measurement.
• The relationship between size of stream, time of application, area to be irrigated and depth of water to be applied is as below:

Qt = ad

Where,

• Q: Size of stream or discharge (liter/second) or (ha-cm per hour)
• t: The time of application of water (seconds or hour)
• a: Area (sq. m or hectare)
• d: Depth in cm that the volume of water used would cover the land irrigated, if quickly spread uniformly over its surface.