Irrigation Systems - Underrated Farm Assets

To grow an even crop using water efficiently, the pressure within your irrigation system must be uniform and correct for your sprinkler types. This is very important to remember when you are installing or maintaining an irrigation system.

Matching your sprinklers to their recommended pressure and checking to make sure you have even pressure in your lines is the least that should be done to make sure your irrigation system is contributing to efficient irrigation.
A catch-can test is the next step in making sure your irrigation is performing well and providing water uniformly to the crop. The sprinkler uniformity can be described mathematically by simple terms known as the coefficient of uniformity (CU) and the distribution uniformity (DU).

The uniformity of your irrigation system can be measured using catch containers placed in a grid pattern between sprinklers and laterals to measure application rates (Image 1 below). The measurements are then used to calculate the CU and DU and can also be graphed to show a surface map of water applied (Image 2 below). The green colour represents the average application of water; the orange to red shows areas that receive less water and the blue colours show areas that are receiving more water than the average.

Often there can be two to three times difference in application from the driest to the wettest area. It is preferable to have more areas receiving the same amount or very close to the average (Image 3 below). The higher the DU and CU the more likely this is to occur, reducing the need to compensate for dry areas by overwatering.

Application efficiency, while very useful, is not the only indication of irrigation efficiency. A more detailed system assessment will also look at pressure losses through the system. Large pressure losses result in extra pumping time and therefore higher running costs. Most large losses can be overcome through correctly designing and installing systems.

A way of checking the losses in your system is to read the pressure gauge at the pump and then at the first and last valves in the system. Elevation will contribute to losses, but these are easily accounted for. A 1 metre increase in height will result in about 10 kPa loss. If on a level site and the pumps are operating at 600 kPa but the first sprinkler after the valve is only receiving 280 kPa, the pressure loss through the system is 320 kPa, which is excessive. Generally, no more than 20% loss should be accepted. If losses are greater, then a full assessment should be performed to diagnose the cause of the problem.

An irrigation system is a large investment and can be one of the most long-lived pieces of equipment on a farm. Without it, growing horticultural crops relying on rain would not be economical. If poorly designed, installed and operated, irrigation systems will have a shorter life and will cost you more than necessary. This reduces profit and will more than likely result in higher water use and lower water use efficiency.

So, next time you are irrigating your crop grab a pressure gauge and check the sprinklers pressures at the start and end of the laterals and also at the pump to see how much difference there is in your system.
Also keep an eye out for the flyer for the next Good Practice field walk in March where irrigation systems, uniformity and efficiency will be discussed in detail in the Myalup area.

For further information contact Rohan Prince at the Department of Agriculture and Food. Email: [email protected].

This article was taken from The Overflow (#28, Summer 2012), Irrigational Australia Ltd - WA Region's quarterly journal. To view the journal click here, or you can visit the Irrigation Australia website at www.irrigation.org.au

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