The IRRIG8Quick Irrigation Calibration worksheets calculate a number called the Excess Water Fraction. A couple of recent queries have asked for more explanation, so here it is.

The Excess Water Factor uses Applied Depth and Distribution Uniformity (DU) to determine how much extra water is required to adequately water the area. This is compared to a perfect system (DU=1).

IRRIG8Quick uses DU_lq, which is based on how much water the lowest quarter of the irrigated area receives compared to the average across the whole area. DU_lq was (I think) introduced in the 1950s by the US Soil Conservation Service.

DU_lq can be used to determine the Scheduling coefficient, or how much extra water should be applied to adequately irrigate most of the area. The Scheduling coefficient is just the reciprocal (1/DU_lq). Alternatively, divide your target application depth by your system’s DU_lq and apply that much irrigation.

Let’s say you aim to apply 20mm of irrigation. If your system has a DU_lq of 0.80, then pump to apply 25mm (20 / 0.80 = 25). This way, 7/8^th of the area will get at least the targeted 20mm. Some will get quite a bit more and some will get a wee bit less. But it is a good trade-off between getting enough watered and not wasting too much.

This explains why getting uniform application is so important. A system with a DU_lq of 0.80 uses 25% extra water. A system with a DU_lq of 0.50 would need 100% extra water.

That is what the Excess Water Fraction is working out.

The formula ((Depth ÷ DU) – Depth) ÷ Depth x 100 firstly calculates the total water that must be applied (depth / DU gives total depth needed). That minus depth gives the extra depth needed. That divided by the depth gives the percentage extra water needed.

An example.

• 20mm average depth
• DU_lq 0.80

Solution

• 20 / 0.80 = 25 (average depth / DU_lq)
• 25 – 20 = 5 (Total – average depth)
• 5 / 20 = 0.25 (Extra / average depth)
• 0.25 x 100 = 25% (convert to  percentage)

You can use this to approximate how much extra money you spend on power and water (if you pay for it). It also shows how much water you could use elsewhere if the system was perfect.

You could also calculate the EWF for a very good system (say DU = 0.9 for a pivot), and compare how much extra water you are using compared to that ideal system.

The standard irrigation system evaluation protocol for Centre Pivot irrigators has been amended. The main change is the removal of the Circular Uniformity test.

The updated protocol can be downloaded from: http://www.pagebloomer.co.nz/wp-content/uploads/2010/04/COP%204-7%20Centre%20Pivot.pdf

The Circular Uniformity test was adopted on recommendation from the Irrigation Association publication, “Center Pivot Design” 2000, pp179 – 181.

Our experience found the test to be problematic; much variability noted during such tests has been due to the radial variation (differences as you move along the length of the machine) rather than elevation difference or hysteresis effects.

The Evaluation Code Centre Pivot Evaluation protocol retains the Radial Uniformity test, and recommends multiple radial tests with and without any end gun, corner arm or other major variable operating. Repeating radial tests in up-slope and down-slope positions would also appear more useful than a circular uniformity test.

The most common failings we have identified to date are associated with incorrect nozzle selection and or insufficient system pressure.

Nozzle package selection should provide a machine with high performance. Unfortunately, too often this is not the case. The ‘blame’ is usually identifiable and there is no one answer.

In some cases, system purchasers (farmers) provide water / well flow rate information that is optimistic. Perhaps the information originates with well drillers, perhaps it is simple misunderstanding or forgetfulness. Regardless, once built and the true flow measured, it becomes obvious that the possible flow will never match the demand of the nozzle package supplied.

In other cases the specifications for the machine length have changed but the nozzle selection not redone. And in a couple, the package was just wrong!

Low (unsatisfactory) end pressure is also relatively common. Often there is insufficient flow available to fill the system, as noted above. Other times we find incorrect settings in variable speed drive controllers. It is good to set systems to minimise energy consumption, but excessively low pressure is false economy. Make sure the pumps are working to design specs so the system does the job for which you bought it.

We like to see a pressure test point or pressure gauge mounted above the regulator on the last dropper on the main machine. If the pressure there is less than 40kPa higher than the regulator setting, we expect to find low performance.