Irrigation water is measured because it is a valuable resource and scarce commodity and measurement helps to reduce excessive use, wastage and allows optimum water use, uniform distribution, increases conveyance, distribution, application and usage efficiencies. Water measurement is essential in the operation and maintenance of any irrigation system, lay out of irrigation structures, layout and planning of irrigation projects and for drawing cropping programmes.
Methods Measurement of irrigation water may be done by
• Volumetric method,
• Velocity area method,
• Direct discharge method, and
• Tracer method.
A. Volumetric method
Materials required – Plastic bucket, alkathene pipe and stop watch.
Procedure – A known volume of bucket or barrel (210 litre) is taken and placed under the delivery end of a pump or pipe. The time taken to fill the bucket/barrel is recorded using stopwatch. The rate of low of water in a water pipe or a pump set is worked out by using the formula.
Volume of bucket (lit)= Discharge rate (lit sec)/ Time taken to fill the bucket (sec)
B. Velocity area method
This method is used to determine the discharge rate in a pipe or open channel by multiplying the cross-sectional area of flow at right angles to the direction of flow by the average velocity of water.
Rate of flow/Discharge rate = Area (a) × velocity (v) (in m3/sec)
a = Area of cross-section of a channel or pipe (m2)
v = Velocity of flow (m/sec)
There are important methods under the velocity area method to find out the velocity of flowing water.
• Float method,
• Using current meter, and
• Water meters,
(a) Float method – Here, the rate of movement of floating body over flowing water is equated to the velocity of running water with a co-efficient of 0.85.
Materials required – A rubber ball or a closed empty plastic bottle or a block of wood or any floating material, measuring tape, stop watch etc.
Procedure – Measure 40 m length in a straight channel and mark the upstream (A) and downstream (B) points. Allow the float to float on the running water at A, the upstream point. Note the time when it touches the upstream point and let this be the initial time. Also note the time when it reaches the down stream point (B) which will be the final time. Repeat the procedure several times and find out the mean time to travel this 40 m distance. The velocity is determined by the following relationship.
Velocity= Length of channel (m) / Average time taken by float (sec) x 0.85 = m/sec
The average velocity is calculated by multiplying a co-efficient factor (0.85) as above. The flow rate of the water is worked out using the formula
Q = a × v Rate of flow
(Q) = average velocity × cross sectional area of the channel.
(b) Using current meters – It is a small instrument containing a revolving wheel or vane that is rotated by the movement of water. The number of revolutions of the wheel in a given time is noted and corresponding velocity is reckoned from a calibration table/graph.
(c) Using water meters – Water meters utilize a multiplied propeller made of metal, plastic or rubber, rotating in a vertical plane and geared to a totalizer, which totalizes the flow in any desired volumetric units. To use the water meter at all times accurately, the flow of water should be full and the rate of flow must exceed the minimum for the rated range. Meters are calibrated and no field adjustments are necessary. Care should be taken to avoid obstruction due to foreign materials in the propeller.
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