Irrigation System Application Rates and Bench Marks

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Calculating Average Pressure

Sprinkler (or dripper) performance needs to be tested about every 3 - 5 years or more often depending on the water quality.

Pressures and discharge need to be periodically measured to ensure that the performance of the system remains at an acceptable standard. Decreasing the pressures through the build up of slime, or rust in the risers, alters the discharge and reduces the radius that sprinklers cover. The tendency is insufficient water in the centre of the diamond pattern resulting in weaker growth in that area.

Pressure can be measured in psi (pounds per square inch), kilopascals or centibars.

Some Important Conversions

1 Centibar (cb) = 1 kilopascal (kpa)

1 pound per square inch (psi) = 7 kpa (approx)

1 bar = 100 kpa

1 atmosphere is also = 1 bar

There are acceptable operating ranges for the different classes of sprinkler as seen in the Application Rate/Pressure Bench Marks table.

Application Rate/Pressure Bench Marks

System Type
Optimum Pressure Range (kpa)
Discharge Range (l/hr)

Drippers
80 - 100
2 - 8

Microjets
100 - 150
25 - 200

Mini-sprinklers
125 - 200
35 - 350

Low-level
200 - 300
300 - 1200

Overhead
250 - 400
700 - 3000

(Source: Swan Hill Irrigation Managment Course, Book 1, Chapter 3)

Measuring Sprinkler Pressures
Measuring selected sprinklers in each valve unit and performing a few simple calculations will tell

If the sprinklers are within their correct operating pressure range
What depth of water is applied at each irrigation
How much variation in depth of water applied there is across the system

The operating pressures of sprinklers need to be measured. Take measurements spread across each irrigation shift, while the irrigation system is operating under normal conditions.

For an irrigation system with a single sub-main, at least nine sprinklers should be checked. Systems with more sub-mains require more sprinklers to be checked.

The RULE is to check sprinklers nearest and furthest from the valves, at the start and end of laterals, and at points of high and low elevation.

Step 1:
Operate the irrigation system as per usual. For each section test sprinkler pressures and also record information on the sprinkler brand/model, nozzle size, crop type and spacing, and sprinkler spacing. Also record what other units are operating at the same time and the pressure at the pump.

Step 2a: Small Low-Level Sprinklers
Select the sprinklers, pinch off the lead tubing and unscrew the sprinkler from the stand or stake. Screw the pressure gauge and fitting onto the stand or stake and replace the sprinkler on top of the fitting. Release the lead tube and allow the sprinkler to operate normally and record the operating pressure of each sprinkler as indicated by the pressure gauge.

Step 2b: Large Under tree Sprinklers or Overhead Sprinklers
Select your sprinklers for measurement. Position the pitot tube and gauge on the main outlet of the sprinkler, with the point of the tube 3mm (1/8") from the nozzle in the stream of the water and record the value obtained from the gauge. For more information on pitot tubes visit the Oregan State University website (external site).

Calculating Average Pressure
Add up all the pressures and divide the total by the number of readings.

Example:

Emitter	Pressure
1	150
2	155
3	150
4	160
5	170 (Highest)
6	160
7	145 (Lowest)
8	155
9	145
Total	1390 kPa
Average pressure = 1390 / 9	154 kPa

Pressure Variation
To calculate Pressure Variation, you need the Midpoint Pressure and the Difference. To get the Midpoint Pressure, add the highest pressure and lowest pressure and divide the result by two.

Using the example above:

Highest + Lowest	170 + 145 = 315
Divided by	2
Midpoint Pressure	= 157.5

To find the difference, subtract the lowest pressure from the midpoint pressure.

Using the same example agian:

Midpoint - Lowest	157.5 - 145
	= 12.5

Finally, to find the pressure variation as a percentage, divide the Difference by the Midpoint and multiply by one hundred.

Concluding the example from above:

Difference / Midpoint	12.5 / 157.5 = .079
By	100
Variation	= �7.9%

This is less than �10%, so it is acceptable.

(Source: Swan Hill Irrigation Managment Course, Book 1, Chapter 3)

Also visit the Information Note AG0137: Maintenance of Trickle Irrigation Systems

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