Category Archives: Irrigation

Excess Water Factor

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_lqwas (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_lqand 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.

Protocols for Compliance Checks of Farm Dairy Effluent Irrigators

I spent a day with staff from a regional council discussing farm dairy effluent application. In particular, they were interested in protocols for monitoring the performance of travelling irrigators. They need efficient and acceptable ways to monitor compliance with regional plan rules and resource consent conditions.

It is an interesting topic.

For the purpose of effluent application, the regional council has defined high and low risk soils, and different consent conditions apply. And of course, there are variations on conditions depending on when consents were issued, the location of the site and so on. And some consent conditions apply to nutrient loadings, including both 24 hour and annual time frames. Yet others relate to soil moisture status, and others to applied depths.

We discussed a lot of things:

soil water physics; including application depths, rates, and uniformity
surface ponding and over-land flow or run-off
the Farm Dairy Effluent System Design Code of Practice and Design Standards,
the Irrigation Evaluation Code of Practice,
current approaches for assessing dairy effluent irrigators
results from system evaluations
accuracy and confidence of monitoring results

Then we went out to a farm and physically measured travelling effluent irrigator performance. And we debriefed.

And so to a protocol: what elements are required?

A protocol needs to be:

recognised – look for existing procedures and processes that are suitable
defensible – ensure sufficient sampling and fair statistics
cost-effective – be efficient, not overly time consuming
meaningful – so everyone can understand the process and the outcomes

Most, if not all, of these are available. The Irrigation Evaluation Code of Practice and the Farm Dairy Effluent System Design Code of Practice and Design Standards, cover field sampling, statistics and standards reasonably well.

But they were not developed as tools to audit performance of Farm Dairy Effluent irrigation performance (equipment and management) against the full range of regional plan rules and individual resource consent conditions that are found within one region, let alone across the country.

And as a country, we still have not agreed on how to assess surface ponding, or interpret our findings. What is acceptable in what environments, and over what time frames?

Pivot Evaluation Protocol Amended

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.

(C) Page Bloomer Associates Ltd. DISCLAIMER This website and any downloaded information are intended to provide accurate and adequate information relating to the subject matters contained in them and are based on information current at the time of publication. Information contained in this website is general in nature and not intended as a substitute for specific professional advice on any matter and should not be relied upon for that purpose. No endorsement of named products is intended nor is any criticism of other alternative, but unnamed products. It has been prepared and made available to all persons and entities strictly on the basis that Page Bloomer Associates Ltd., its researchers and authors are fully excluded from any liability for damages arising out of any reliance in part or in full upon any of the information for any purpose.