Irrigation Systems - Don't Accept Second Best by Steve Goodchild (Source: Australian Olive Grower Issue 11, April 1999)
Australia's olive groves are creating a  new market for irrigation companies. Steve  Goodchild outlines the essentials of a  system designed to last for years and increase the grove's  profits.

Over 5,000 kilometres of irrigation lines will be laid in  Australia's olive groves this year. While not a huge quantity compared with  other horticultural industries, irrigation companies are all looking for their  share of the business.

I feel quite fortunate in having the opportunity to work with investors and growers involved in taking the Australian Olive Industry to a  new level. Involvement in a large number of projects both small and large gives  me the opportunity to see a wide range of growers' experiences. From this exposure, I've come to realise that there are some serious problems with the advice and design of many irrigation systems.

I'm quick to admit that irrigation is not my immediate consulting field, but I've seen and worked with enough good and bad irrigation  systems to understand what an olive grower needs. In a nutshell, the grower is  looking for the best possible job at the best possible price for the best possible return ... over many years. Olive growers must constantly remind  themselves that there is only one opportunity to do things right when it  involves underground irrigation systems.

Thousands of dollars are spent to ensure that your trees have every opportunity to yield to their full potential. During my travels I see some second rate irrigation systems and realise that the unsuspecting grower has  been presented with a cheap, short term design. The design is clearly geared towards winning a quote and selling more replacement materials in years to come. Also, in all fairness to the irrigation specialists, growers will often ignore  their advice and choose to go 'cheap' to their own detriment.

Low Density Pipes

One of the most common problems is the use of Low Density  Polythene buried underground. The reason that I am not keen on burying low density poly is because of the intense root system of the olive tree. I've seen  trees as young as two and a half years starting to crush the pipe. This in turn restricts the flow through the line and the result is uneven watering throughout the grove. There are several problems associated with this, the loss of yield being the worst.

Burying your pipes underground, in my opinion, is the  ideal way to have lateral lines installed with just the sprinkler supply hose protruding from the ground. This allows unrestricted access throughout the grove for maintenance and machinery.

As the pipe is buried, it is also insulated against  expansion and contraction as daily and seasonal temperatures rise and fall. In some cases I've seen the pipe move to the extent that it has either pulled the sprinkler out of the ground or disconnected the sprinkler supply hose. In drip irrigation systems the emitter is often dragged well away from the tree. Both are dangerous situations for young trees.

Another serious problem is with unburied lateral lines being exposed to the summer sun. Scalding temperatures can be achieved due to  the incredible amount of heat that the black polythene absorbs when exposed to  direct sunlight. I can't see any benefit in putting scalding water on a newly  planted tree.

My advice is to use Rural or Class B polythene buried  approximately 80mm-100mm underground to give adequate insulation and allow easy access throughout the grove for maintenance purposes.

Sprinklers vs. Drip

This debate will go on for many years due to the fact  that both systems can work effectively when properly designed. However, my experience shows that on most soils, the benefit of under-tree sprinklers outweighs the cost savings of drippers in the long term.

Under-tree drippers are inexpensive and water the tree's root zone reasonably well for up to twelve months. The dripper produces a  football shaped water pattern beneath the soil and the roots tend to concentrate  in that area with little encouragement to move out further. (Soil types and rainfall regularity can affect this.) As the tree grows, many more drippers or additional drip lines need to be added to give a reasonably even water supply to the spreading roots.

Some growers place a dripper on either side of a mature  tree and think they have adequately irrigated it - not so! In nature, the olive  tree has a wide and strong root system that can handle a man-made tree shaker.  One or two drippers won't encourage the development of the extensive root system  needed and root damage can result.

You also need to consider what happens if you miss a watering, or worse still, run out of water? Here is what manufacturers of the  drip irrigation equipment tell us happens. The tree's foliage needs the water  and the water in the concentrated root zone at the dripper position is quickly  sucked up to feed the tree. If another application of water doesn't arrive reasonably soon, the tree becomes stressed. If the water shortage goes on for  more than a short time, the tree can be badly knocked back in its growth or even die. Consequently, the confined fibrous root system dries out sometimes beyond repair.

If you start irrigating with drippers, after about one year it is important to consider changing the drip irrigation to under-tree sprinklers. This encourages the roots to form a wider, stronger, more  stress-free base for the olive tree to stand on. Most irrigation manufacturers produce both drip lines and sprinklers so don't be afraid to question them on  the best products for your site ... they'll still get a sale.

Also, don't skimp on costs by using a cheap inadequate filter to feed water to your under-tree sprinklers or drippers - you will regret it. Insects and ants are often blamed for the blocking of irrigation outlets,  when more often the real culprit is an inadequate filter system. Suitable filters for under-tree sprinklers are more expensive than the cheaper strainers  but if you don't fancy wasting hours each watering while you unblock sprinklers,  then you will be well rewarded by the extra expense of a suitable  filter.

The Irritrol Water Bird® Olive Sprinkler is especially  designed for under-tree watering and delivers an even amount to every tree, regardless of whether the trees are growing on flat land or slopes.

The Irritrol Olive Sprinkler has a built in pressure compensation facility which stabilises flow characteristics. Without this  device, trees higher up the hill would receive less water than trees at the base of the hill where the water pressure in the line is much greater. With these pressure compensated sprinklers, as long as the water pressure variation in the lines stays between 1.3 and 4.0 bar (20 and 60 psi), then each tree, regardless  of its position on the hill, will receive the same amount of water.

It is not sensible to use a sprinkler that produces a  fine mist because much of the water can be blown into the air and not used where  it is needed at the tree's roots. The Irritrol Olive Sprinkler produces a reasonably large water particle and therefore very little of the water is  wasted.

The Irritrol Olive Sprinkler has a nominal flow rate of  35 litres per hour which is quite adequate for supplying an olive tree's water  requirements.

The next very important benefit of this particular Irritrol sprinkler is the snap off deflector.

The olive sprinkler offers three spray stages so that  water is not wasted. The stages are designed to take an olive tree from the  initial planting through to maturity.

The first stage (bubbler) waters an area of approximately  400mm in diameter. This setting is simply achieved by removing the spinner from  the sprinkler head and replacing the bearing cap. This setting will meet the  tree's requirements for approximately six months.

The second stage is achieved by replacing the spinner  that has been previously removed. This setting will water an area of at least 1.5 metres in diameter. The tree's water requirements can be met from this  setting until the tree is approximately 18 months old.

For the third stage of watering, snap off the little plastic deflector to allow the water to spray slightly upwards. This will mean  that a ground diameter of about 4-4.5 metres will now be watered, which is suitable for a mature tree. All of the three stages mentioned occurred without the capital outlay of changing to a new sprinkler.

One last point. To fit the barbed fitting into your  lateral rural poly line that runs past the tree, use a tool that has been designed for punching the hole and pushing the barb into the pipe. Your local  irrigation shop should be able to supply this. If not, Olive Agencies can supply you with the polythene hole punch which will reduce leakage from roughly made holes.

Another problem worth mentioning is that some irrigation designs are recommending rubber grommets and push-in barbed fittings to seal where the lateral lines are connected to the larger supply line/sub main. One of the problems with using rubber grommets is that they have a limited life. Even though the grommets are buried underground, the exposure to the elements has an  effect on the life of the rubber.

Some irrigation equipment suppliers have told me of cases  where rubber grommets have had to be replaced approximately 10 years after being  installed. Imagine having to dig down later to repair something that could have  been avoided at the planning phase of your grove. As the saying goes, "If you  haven't got the money or the time to do it right in the first place, wherever do  you think you're going to get the time and the money to fix it later?"

Other problems associated with the considerably cheaper  rubber grommets and push-in barbed fittings is that they have a much lower  blow-out pressure than the standard rural class polythene fittings. Water hammer and pressure fluctuation can easily occur when stopping and starting pumps or opening and closing valves. It is at this stage when blow-outs occur and you start to realise you've installed an underground liability instead of an  asset.

I have been involved in a project where the irrigation design recommended that I use a 13mm (half inch) lateral line to supply several Irritrol Waterbird® Olive Sprinklers. To even consider the recommendation has disaster written all over it. To any of you who know the Irritrol sprinkler, you would soon realise that if we were to insert its micro-tube's barbed fitting into the tiny 13mm lateral line, it would restrict the flow quite considerably. This would create problems for trees further along the row. I believe it unwise to use polythene smaller in diameter than 19mm on sprinkler systems and  preferably 25mm.

Bulk Buying

There are considerable savings to be had if you can buy  irrigation equipment in bulk amounts. If for example, you plan to set up your grove in three stages, assess the option of buying enough irrigation materials  to install the entire amount required to complete the project at the start. This  will naturally depend on your initial finances but get a quote on the total amount anyway.

PVC Mains

PVC water mains are by far the most economical way to  move large volumes of water. PVC pipe is semi-flexible and permanent which is  what you are looking for in an orchard situation. Some growers have been advised  to use solvent joint (glue) PVC when rubber ring joint PVC is available at practically the same price. The problem with solvent joint PVC is that after several years of use, the bond between the two pipes can sometimes let go. The advantage of the rubber ring joint is that it allows constant movement and will not leak. Unlike rubber grommets, rubber ring joint PVC will remain a permanent fixture. The rubber seal is protected from the elements by the outer PVC pipe.

Filters, Pumps and Fittings

There are some excellent products available on the market  today for filtering irrigation water. We must remember that each water source should be reviewed according to the solids in it.

If you are drawing your water from a dam or creek where  high levels of algae are present, a media filter is commonly used. If algae is not a problem, then disc filters are an economical option. In some bores and wells sand can be quite a problem. If this is the case a hydrocyclone sand separator is a good choice. A disc filter will then be used as a secondary filter to do the final cleaning.

Filtering systems are available in an automatic back flush design or with a manual cleaning method. Your choice really depends on  your personal situation and the irrigation designer should present you with your  best options.

Several options are available in the area of pumps. A  skillful adviser will carefully choose the right pump for the right job. There's  also a wide range of products when you look at taps, valves and solenoids to water sections of your grove. Again your irrigation designer should present the  products available and advise on the advantages and disadvantages of each item.

Quite a few of the bigger groves I have inspected are  using what are called 'Pressure Compensating Valves', which I feel is a good safety precaution. Pressure compensating valves maintain an even pressure on the discharge side of the valve (tap) thus eliminating water hammer and pressure fluctuation problems. By installing such a valve, the chance of damaging water mains is greatly reduced.

It would be wise to ensure that the irrigation system is able to include the capability of introducing soluble fertilisers to the root  zone via the irrigation system. Your irrigation specialist will advise on suitable fertigation equipment.

It will pay to ask advice from more than one irrigation  supplier until you become confident of the information and services being supplied. If you find you are getting two entirely opposite sets of advice, then make sure you ask why. Your current and future bank account depends on the right advice! Don't choose one adviser over another just because he has a more appealing personality ... you need the facts and the right equipment.

Asking the necessary questions in the early stages of  developing your grove, and making calculated decisions before it is too late, will reinforce the firm foundation of the Australian Olive Industry.