Total irrigation system scrutinised

01-07-2009 | Sandra Taylor

Making irrigation water go further is the objective of a Canterbury irrigation efficiency project.

AgriBusiness Group is running the two-year Sustainable Farming Fund project now entering its second year.

One of the project's co-ordinators, Geoff Dunham, says it is not just about water application but the total irrigation system including energy, labour and operating costs.

The project uses focus farms throughout the province which have irrigation systems typical of many seen throughout the country.

The problems and issues identified on these farms will be similar to those faced by other irrigating farmers.

Ian McIndoe, a soil and water engineer from Aqualinc, says factors that need to be taken into account when assessing an irrigation system is its design and day-to-day operation and management.

Most importantly farmers need to know what their irrigation system is supposed to do in terms of application rates and efficiencies, information that should be provided by the company installing the system.

McIndoe says unless the design of an irrigation system is efficient, the system won't be. So design is the first thing looked at when assessing an irrigation system.

One of the focus farms is Roel and Diana Wobben's 530ha dairy operation near Rangiora in North Canterbury.

The couple's home block of 310ha is watered by a1km long centre-pivot and long laterals.

A second 105ha block is watered with a hard-hose gun and a third block has a half-pivot, a Roto-Rainer, K-lines and long laterals.

The centre-pivot is driven by water drawn from the unreliable Waimakariri scheme at 180L/sec. When the scheme is switched off the pivot uses water from a 120m deep well at the reduced rate of 120-130L/sec.

When the pivot is run on well water every third sprinkler is tied up in order to keep the pressure in the system.

The Roto-Rainer, which is on a seven-day return, draws water from a 35m well at 30-35L/sec.

Guns are run on scheme water and are on a 12-day return while lateral sprinklers, which water the corners, are shifted every second day.

McIndoe says technicians carried out a detailed evaluation of the pivot and gun on the Wobbens' farm, measuring pressures and flows at critical points.

They carried out an application test measuring the application uniformity of the water, looked at the pump and measured pump efficiency. They also looked at electricity use, and assessed operation and system maintenance.

The Wobbens' irrigation system has been designed to apply 5mm/day, an amount based on soil types and evapotranspiration (ET) rates.

The gross application of the centre-pivot was found to be 5.7mm/day while the guns were applying 4.4mm/day.

But the net effectiveness, once other losses were taken into account, were 5.2mm/day from the pivot and 3.9mm/day from the gun.

So the gun was falling short of the ideal.

Soils over the Wobben's farm are Lismore stony silt loams with a water holding capacity of around 60mm.

The pivot is applying small amounts of water, often-23mm over 3.5 days, while the gun is applying 31mm every nine days. The quantity of water applied by the pivot is better suited to the farm's soils.

The application rate, defined as the rate at which the water is being applied to the soils every hour, ranged between 51-87mm/hour along the length of the pivot.

As the soil infiltration rate was 20-60mm/hour there were problems with ponding, particularly under the 80-90m corner arms.

McIndoe says this is a manufact­uring fault and manufacturers seem unable to address the problem.

There is potential to improve water use efficiency by reducing the flow rate into the corner arm in the areas between corners, when the arms are folded in, but there was a high likelihood that there have been pasture production losses due to the variability of the application rates.

Different irrigation systems have different application uniformities (DU) and average application uniformities (CU).

Ideally centre pivots have a CU of 85%. The main part of Wobbens' pivot had a CU of 83%, but CU was found to be poor under the corner arms. The guns had a CU of 76%, which McIndoe says was not too bad considering the measurements were taken on a windy day.

The poor CU of the corner arms on the centre-pivot was identified as the biggest problem in whole system.

McIndoe says while the industry doesn't have all the answers in regards to this variation in application uniformity under centre-pivots, the simplest solution is shorter pivots.

"Anything bigger than 500-600m you want to take a hard look at."

The effects of poor application uniformity in long centre-pivots have only become obvious in recent dry seasons, but there are thoughts that the production losses due to this problem may be significant.

One possible solution would be to apply less water more often, but McIndoe says this can be difficult and shortening rounds can be problematic as other issues come into play.

Boom backs can be retro-fitted to spread the water over a bigger area and help prevent ponding, but ideally the system should be designed to avoid the problem.

Blair Miller, an irrigation management consultant from Streat Instruments, pointed out that it was important to get the DU of irrigation systems correct as farmers can end using a lot more water if the DU is low.

For example, if farmers are aiming to apply 5mm of water but the DU of their system is only 30%, they need to apply 16.7mm to achieve that 5mm.

This is because the water is not being applied to where it should be.

Miller says there are huge production gains to be made by correcting the DU.

Tony Daveron from HydroServices says from all the centre-pivots they have evaluated they have not found one with a DU of better than 85%.

"Which is pretty average."

Most are running at around 80%, which is not much better than a Roto-Rainer.

The best machines Daveron ever tested were linear systems and these were achieving DUs of 88-90%.

Looking at energy efficiency, the guns used double the energy to apply a cubic square metre of water than that needed by the centre-pivot.

The pumps feeding the centre-pivot on the Wobbens' property were very efficient, while the pump feeding the gun was performing 10% below the recommended pumping efficiency.

Typically the higher the pressure the higher the energy use, but there can be other factors that affect energy use.

McIndoe points out that farmers have a choice in the type of pump they buy and he urges them to invest in one with a high efficiency rating.

In the Wobbens' operation the labour efficiency around the irrigation system was found to be average and little could be done to improve that.

The long laterals, which were shifted twice a day, were the most labour intensive. However they needed to be shifted frequently to match the soil type.

The operating costs of the gun, at $120/1000m3 of water, were nearly three times that of the centre-pivot, which came to $45/1000m3 of water.

Energy costs were 80% of the total irrigation operating costs, which was typical of these systems and highlights the benefits of gravity systems.

One of the downfalls of the Wobbens' irrigation system was its unreliability due to the nature of the Waimakariri scheme and this makes total system efficiency more difficult to achieve.

Like many farmers on unreliable water schemes, they tend to irrigate when the water is available whether the soils need water or not.

Farmers with a 100% reliable supply can make decisions around when to irrigate and will often use less water over a season than those with unreliable supply.

The evaluation of the irrigation system on the Wobbens' farm centred around the centre-pivot and guns, but the couple also use K-lines and a Roto-Rainer.

While these weren't looked at in detail, McIndoe says K-Line uniformity is very poor at under 50%.

K-line sprinklers are typically stretched as far apart as possible to keep the depth of application down over a 24 hour period.

Many soils cannot hold that amount of water and there is even more loss when this lack of uniformity is taken into account.

In order to improve efficiency, McIndoe suggests shifting K-lines twice a day and on every second rotation the sprinklers should be placed in the gaps, although this is a lot more involved than just lining the K-line up against a fence post.

More frequent shifts make a huge difference to the performance of a K-line system, but the labour costs involved can be significant.

K-Lines were designed for rolling downland and they work well in these situations, however in most other applications travelling irrigators and guns are all better options than K-lines.

If set-up correctly, Roto-Rainers can have uniformities of 85%, but the big issue with these is the application rates on the end jets which usually exceeds the ability of the soil to absorb the water.

Roto-Rainers are easy to maintain and shift and potentially good at growing grass, but the infiltration rate is the issue with these.

Soil types and soil properties have a big influence on water use efficiency and irrigation management.

As Daveron points out, stones don't hold water and the volume of stones in the soil needs to be taken into account when deciding on water application rates.

Wormholes and root hairs are both indicators of soil health as they indicate good soil porosity.

Daveron says one of the biggest issues with these long centre-pivots is the breakdown of soil macroporosity due to the poor application uniformity.

"In 10 years time it will be interesting to see how these very long pivots are going to perform."

Aquaflex and neutron probes are two commonly used methods of measuring soil moisture and it is the information provided by these instruments that dictates irrigation management.

Miller says Aquaflex is a 3m tape which is buried in the root zone on a slope to 400mm.

It measures soil moisture and temperature and can either feed the information telemetrically back to a computer or data, which has been saved in a box, can be downloaded to a palm pilot.

Neutron probes allow the soil moisture to be measured to a depth of one metre.

Daveron says it shows the moisture available in the root zone, which is 300mm for most grasses and clovers, but also indicates that water is being lost through the soil profile.

He believes the industry under-estimates the amount of drainage that is occurring under irrigators.

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