Category Archives: Water Resources

Excess Water Factor

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

DUlq 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/DUlq). Alternatively, divide your target application depth by your system’s DUlq and apply that much irrigation.

Let’s say you aim to apply 20mm of irrigation. If your system has a DUlq of 0.80, then pump to apply 25mm (20 / 0.80 = 25). This way, 7/8th 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 DUlq of 0.80 uses 25% extra water. A system with a DUlq 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
  • DUlq 0.80

Solution

  • 20 / 0.80 = 25 (average depth / DUlq)
  • 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.

 

Pivot Evaluation Protocol Amended

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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.

Excessive surface ponding

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I’ve worked a lot and with many people on irrigation efficiency and on application of effluent to land. We keep coming up against the question, “What actually constitutes surface ponding?”

I’d love to get some agreement on this.

In the last year or so:

  • I’ve heard that ponding only applies to durations lasting 4 hours or more, and I’ve heard it applies to anything from a few minutes duration.
  • I’ve heard it has to be at least a pretty big area and I’ve heard anything at all counts.
  • I’ve been told the Environment Court determined any duration mattered. A regional council had applied a four hour minimum when assessing ponding, but the court said the consent said ‘no surface ponding’ and that meant no surface ponding.
  • I’ve seen lots of it and I’ve seen evidence that excessive application rates are problematic – both for irrigation and for land applied effluent.

There will almost always be some surface ponding; even drip systems micro-pond. So applying a concept of excessive surface ponding seems better. But what is excessive surface ponding?

Excessive surface ponding – defined

In the interests of sparking debate, I propose a definition of excessive surface ponding:

“Excessive surface ponding means the presence of surface water pooled in contiguous areas of greater than 0.04m2 found, one hour after application starts,  at more than four of forty sample points selected at random over at least 25% of the application area, with each point being more than two metres apart.”

  • Surface water pooled may need definition, but I’d say means clearly visible puddling – i.e. not just wet soil
  • contiguous areas means connected
  • 0.04m2 (about 20x20cm) is bigger than a hoof print (my first intent) but is maybe too small – the auction starts now, your bids please . . .  .
  • one hour after the start of application covers both travellers and stationary nozzles but is the time right?
  • Four of forty sample points means 10% and hopefully is a big enough sample that is not too onerous to take
  • Random is random – let’s specify a method e.g. tossing a 0.25 m2 quadrat or ring backwards over your head with your eyes shut having spun twice clockwise
  • 25% of the application area means you have to look around a bit
  • More than 2 m apart spreads it out a bit and avoids sampling landing in the same contiguous, but relatively small, pond. Pick a number (and justify it!)

I don’t know that I agree with my proposed definition.

For a start, I’ve watched high application rate water (and effluent too) disappear very rapidly, and upon digging significant holes, couldn’t find sign of it in the rootzone. So maybe, if you can’t see ponding for more than 10 minutes, that’s when you should get worried!?

Please, have a think, then post a comment below. Let’s try to have a reasoned and enlightening debate!

Regards

Dan

Measure Irrigation Energy Efficiency

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IRRIG8Quick Irrigation Energy Efficiency guidelines and worksheets have been loaded on the Page Bloomer website: See http://www.pagebloomer.co.nz/resources/tools/irrigation-energy-efficiency/

These guidelines and worksheets were funded by the Energy Efficiency and Conservation Authority, EECA.

There are two separate guidelines, one for the pumping plant (pump and motor) and one for the delivery system (headworks and mainline). Ideally you’ll do both – they are designed to work together.

Why check pump performance?

Profitability – Incorrectly sized or physically deteriorated pumps will waste energy and money. A good pumping system saves money!

Sustainability – efficient pumping minimises energy use and carbon emissions. A good pumping system saves the environment!

Pump and motor selection are important system design considerations. Incorrectly sized pumps and/or motors will not operate at their most efficient points. So they will waste energy.

Low pressure is a common cause of poor irrigation uniformity which reduces overall system effectiveness and efficiency. The pump must provide adequate pressure and flow to ensure the system operates as designed.

Excessive pressure affects performance and wastes energy. Pump selection will usually allow about 5% extra pressure capacity to allow for slippage with time. But excessively oversized pumps are major energy wasters.

Why check delivery system performance?

Profitability – Incorrectly sized or physically deteriorated components can waste energy and money. A good system saves money!

Sustainability – energy efficient irrigation minimises energy use and carbon emissions. A good system saves the environment!

Pipe and component selection are important system design considerations. Selecting smaller options may reduce up front capital cost, but increases ongoing energy costs as bigger pumps are required. The correct selections optimise the necessary trade-offs.