Measurement of irrigation water

A. Volumetric Method:

• It is used for small irrigation system.
• An empty Siphon or an empty bucket of known volume is taken
• The time taken to fulfill the empty bucket through siphon is recorded with stop watch.
• The rate of flow is measured by the formula:

Discharge rate (ltr/sec) = Volume of container in liter / time taken to fill in seconds

B. Velocity-Area Method:

• The rate of flow passing a point in a pipe or open channel is determined by multiplying the cross-sectional area of the flow section at right angles to the direction of flow by the average velocity of water.

i.e. Discharge(Q) = Velocity (m/s) x Area (m²)

• The cross section area is determined by direct measurements.
• It is further divided into 2 types:

i. Current meter method :

• Calculated by calculating the no. of rotation of impeller in a given time interval and corresponding velocity is calibrated in graph.

ii. Float method :

• Time taken by a empty bucket to reach a certain distance is calculated .
• Several trial is made before calculation is done.
• Consists of noting the rate of movement of a floating body.
• The time the float needs to pass from the upper to the lower section is recorded.

Velocity(v) = Cf x Vf

Where,

Cf = correction factor (0.85)

Vf = float velocity; m/s.

• Since the velocity of water on the surface of the water will be greater than the average velocity of the stream, it is necessary to correct the measurement by multiplying by a correction factor (0.85).