Moveable Irrigation

September 2007
Water is a very important issue for dairying in Canterbury and North Otago. In these areas, irrigation by dairy farms uses a lot of water. It has been calculated that twenty 500 ha dairy farms use as much water as the whole of the city of Christchurch. Not surprisingly, farmers are under pressure to use water more efficiently, whilst at the same time, they have to maintain pasture productivity to ensure economic survival.

There is considerable scope for many dairy farmers to improve their water use efficiency. A 30% range in annual pasture production among irrigated dairy farms, observed in a recent study, equates to over $1000/hectare/year difference in income between the highest and lowest producing farms. Hence there is considerable opportunity for many dairy farmers both to improve their water use efficiency and to increase their returns by using water more efficiently.

Dr Dick Martin from Crop and Food research is running trial on irrigation. Part one is devising a benchmark for ryegrass and tall fescue yield under irrigation. The research is focused on improving water use efficiency and get better bang for your buck using for what is becoming an increasingly scarce resource. The researchers also want to develop an irrigation calculator. The trial has just completed its first year and theyve applied for another year of funding.

This kind of research looking at when to irrigate and how much to apply has already been done for arable crops. Theres a definable $ saving to made under these conditions and Dick believes same can be said of these pasture species.

What is irrigation efficiency?

Irrigation efficiency can mean different things to different people. Here are two key definitions:

1. Pasture water use efficiency. This is the amount of pasture production per mm of water applied. Optimal water use efficiency occurs when you are producing high pasture yields with low volumes of irrigation.

2. Application efficiency. This describes how efficiently your irrigation system is used to apply water. It relates the amount of irrigation applied to the amount of this water that is available for growing the pasture. High application efficiency means that a large proportion of the irrigation applied will be used by the plant, whereas a low application efficiency means that much of that water has been lost to runoff or drainage.

How to optimise pasture water use efficiency

What farmers want to know is when to start irrigating, when to stop, how much should be applied and when should it be applied.

What they need to measure is rainfall, soil depth, soil moisture, pasture response and irrigator efficiency.

With all this info at hand you can schedule your irrigation to maximise your returns.

More detail about Sunshine and soil moisture

If the daily average soil temperature at 10 cm depth is below 10oC, grass grows very slowly. Therefore there is no point irrigating when soil temperatures are that low.

Above a daily average soil temperature of 10oC, grass growth of a well managed dairy pasture is mainly related to solar radiation and soil moisture. Under New Zealand conditions, the more solar radiation or sunshine, the faster grass grows. For a well managed pasture, the major limitation to grass growth is then soil moisture.

However, soil moisture does not restrict grass growth until the soil has dried to a critical deficit (also called a trigger point or refill point or stress point, Figure 1). After this, grass growth decreases with increasing soil moisture deficits until the soil is so dry that pasture growth stops (called the wilting point). A rough rule of thumb is that the critical deficit is around half the available soil moisture (ASM) in your soil, where ASM is the amount of water the soil can hold between field capacity (when the soil is full of water, and any more water will be lost to drainage or runoff) and wilting point.

Soil depth

Soil depth to stones or to a pan is important as this determines how much water your soil can hold, and so much and how often you need to irrigate.

This means that it is important to know the depth of soil in your paddocks.

Knowing when to apply irrigation

It is helpful to understand how much water is in your soil and how this changes. The soil moisture balance is important as it gives an indication of how dry the soil is, and hence whether irrigation is needed and how much water the soil can hold. Like your bank balance, the soil moisture balance is based on your initial balance (soil moisture), credits (rainfall and irrigation) and debits (plant use/evapotranspiration, drainage or runoff).

The soil moisture balance is calculated as:

Irrigation can be scheduled by making direct measurements of the soil and determining the soil moisture deficit. There are now more accurate devices coming on the market which are cheaper and more user-friendly than in the past, and some can be linked remotely to a computer. Many farmers use a soil moisture monitoring service to measure soil moisture for them, and rely on its interpretation to schedule their irrigation. Cost usually restricts scheduling services to one or two paddocks on a farm.

Farmers can measure their rainfall, should know the amount of water their irrigator is applying, and, if they are using good irrigation practice, they should not be applying so much water that drainage occurs.

Evapotranspiration is calculated from data collected from weather stations, and is published weekly through the irrigation season in most Canterbury newspapers. A correction factor, based on the soil water holding capacity and pasture cover, is used to convert the potential soil moisture to actual soil moisture. With these figures, it is possible to calculate soil moisture deficits for every paddock on the farm.

Irrigation Calculator

Dairy Insight has funded a project to develop an Irrigation Calculator using this method to assist in irrigation scheduling and planning on farm. The Calculator has the advantage that it relates actual soil moisture to pasture growth (as in Figure 1), and can be applied to every paddock on a farm. It is designed to be run on a PC, but could be adapted to other IT devices. The Calculator is being designed to be simple to use, but it is based on sound scientific understanding. Crop & Food Research and AgResearch are undertaking the tool development and implementation.

Obviously, any calculator like this needs a starting point. In most winters, rainfall is in excess so that, at the beginning of August in New Zealand, most soils are full. Also, it is wise, occasionally, to check actual soil moistures in your paddock to make sure that consultants and calculators are tracking correctly, because there may be some key factor that is not being recorded properly. It may be that the amount applied by the irrigator is not the same as expected or that the soil does not hold as much water as assumed. Any problems can be diagnosed easily with some simple measurements.

To maximise production, water has to be applied to avoid deficits that reduce production. So, for any paddock:

Irrigation should start when the soil moisture deficit approaches the critical deficit. Exactly the same rule applies for subsequent irrigations.

At most, just enough water should be applied to bring the soil moisture deficit back to zero, although ideally a slight deficit should be maintained of at least 10 mm to allow for any subsequent rainfall.

At the end of the season, irrigation should be stopped when water is no longer limiting grass production i.e. when growth is again being limited by soil temperature.

Just how this will work out in practice will depend on knowledge of your soil and water applications, rainfall amount and distribution, irrigation system uniformity and operational constraints.

How to optimise application efficiency

Once you know the amount of water that your paddock needs and when, then you need to determine whether your irrigator is applying the right amount of water at the correct rate and evenly (known as the systems uniformity).

This is relatively easily checked and all it needs is at least 20 plastic buckets and a reasonably accurate liquid measuring device. For a travelling irrigator, the buckets are laid out across the line of travel, the irrigator is run past them, and the amount in each bucket is measured. For non-travelling systems, a grid of buckets can be laid out. The amount applied is the average contents of all the buckets, and this amount, divided by the time taken for the irrigator to pass over the buckets, gives the irrigation rate.

Poor set-up and/or maintenance of the irrigators can reduce a systems DU dramatically. Common set-up and maintenance problems include mismatches between pump pressure delivery and irrigation pressure requirement, poorly set variable speed drives, blocked lines and sprinklers, and poorly matched sprinklers fitted along the irrigator arm. Most of the simpler problems are easily fixed. Having the performance of your irrigator evaluated by qualified consultants will point out any sources of reduced performance, and rectifying the problems will soon save you the costs of the evaluation.

Even with a well set-up and maintained irrigation system, significant losses can occur through poor irrigation management, which includes over-watering and under-watering, irrigation too early or too late, and applying water too fast. Applying more water than the existing deficit will result in more water in the soil than it can hold, and so the excess will be lost through drainage and runoff. This just wastes water through drainage and increases pumping costs, as there is no additional grass growth. Irrigating too late, when soil moisture has fallen below the critical deficit, reduces pasture yield, and this cannot be compensated for by applying more water at a late irrigation. Applying water faster than it can be absorbed by the soil leads to runoff and ponding. Using scheduling tools such as the Irrigation Calculator and changing the system set-up to reduce application amounts and rates can avoid these irrigator operation problems.

How to achieve optimal water use?

Ensure your irrigation system is working to specification. Consider having the systems performance evaluated. Ensure that the system is regularly maintained.

Manage your irrigation based on the demands of the pasture by following an irrigation schedule:

Know what your critical/trigger point is before you lose yield.

Know how much your irrigator applies.

Know when to apply irrigation.

You can apply too much water! Dont irrigate more than the soil can hold.

Make best use of rainfall. Average rainfall in Canterbury over the season is worth about $300/ha. The benefits of rainfall will be mainly realised early and late in the season when water use is lowest. Irrigating too early or too late may just waste water.

Consider whether your system is providing a high application distribution uniformity (DU). If DU is low, replacing it or adjusting it to ensure a more uniform system will produce more grass with much less water.


Among results Dick says tall fescue is showing itself to be more drought tolerant.