Deciding when to start/stop irrigating

It is important to know who has the responsibility of deciding when to start and stop irrigation, and how that decision is made.

Consider these factors before irrigating:

• Soil temperature – check soil temperature before irrigating. Grass growth is slow below 10 °C at 10 cm depth. Drier soils warm more quickly than wet soils. Also, applying water can cool the soil further.
• Soil moisture status – Check soil moisture status is between refill point and field capacity (with room for irrigation and rainfall).
• Weather forecast – check forecast for rain.
• Water restrictions – check for water supply restrictions, annual volume allocation limits or water delivery roster.
• Effluent – in areas where effluent is applied, ensure irrigation does not result in ponding.
• Stock – Preferable graze in advance of the irrigator. This means the soil is at its driest and minimises soil compaction and pugging.
• How much water the irrigation system can apply.
• What the evapotranspiration (ET) rate is.

Irrigation scheduling

When irrigating, the objective is to:

• apply water when the plant needs it, to maximise plant growth
• not overfill soil, which wastes water.

There is no value (only cost) in applying more water than the soil can store, but if water isn’t applied before critical soil water deficit is reached, pasture growth will slow down. If the irrigation system and water supply arrangements allow, irrigation should happen when the soil moisture level reduces to refill point. Up until this point, moisture levels will not limit pasture growth and no visual signs of plant stress will be occurring.

The diagram below illustrates how plant growth is affected at each level of soil moisture.

Refill point for your farm needs to be customised, as it is dependent on soil and crop type. However, a simple rule of thumb is 50% of plant available water. In the early and later months (the ‘shoulders’ of the irrigation season) soil moisture levels can be kept closer to refill point because ET rates are lower, and irrigation and rainfall are able to keep up with plant demand for water.

When soil moisture levels fall below refill point, plant roots have to work harder to find water, slowing down plant growth. To demonstrate how soil releases water for plant growth, it can be compared to a sponge. The sponge is the soil and the hand represents the energy required by the plant to extract water from the soil.

Soil moisture monitoring

Soil moisture monitoring is key to providing the necessary information for irrigation scheduling. Monitoring tells you when the soil is at field capacity to stop watering and when it is at refill point to start irrigating.

Although soil moisture can be monitored manually, the most common way is by a handheld meter such as a neutron probe or a permanently installed strip. Data can be collected by a consultant of the farmer, or sent direct to a personal computer.

IrrigationNZ has information on soil moisture monitoring equipment and how to install it. For more information, click here.

Another way to track soil moisture levels is by using a soil moisture template. The soil water balance template will aid you with irrigation scheduling decisions and assist in providing auditable evidence that you are making informed decisions of when to irrigate and how much to apply. Download template.

How much water is being put on?

It is important to know how much water is being put on and understand the way that it moves through soil. The diagram below explains some common irrigation principles.

What influences irrigation needs?

The diagram below illustrates the inputs and outputs of water in an irrigation system.

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• 1. Soil type

  Soil is a natural storage tank, holding water for pasture plants to absorb through their roots.

  Soils vary greatly in the amount of water they can hold onto. Clay soils have smaller pores and can hold more water, but hold onto the water tighter. Sandy soils have bigger pores and hold less water but make it easier for plant roots to extract the water. The amount of water a soil can hold is expressed in millimetres of water per metre depth of soil (mm/m) and is called water holding capacity (WHC). It varies from 175 – 190 mm/m for clay loams to 45 – 55 mm/m for sand.

  

• 2. Plant type

  Pastures receive the majority of the water they require through their roots. Therefore, the depth of the plant’s roots affect the amount of water a plant can uptake. A shallow rooting plant will have less water available through its root system than a deep-rooted plant. Annual and perennial pasture has a rooting depth between 0.3 – 0.8 m, compared to lucerne’s 1.2 – 1.8 m. The root zone will also be affected be the plant growth stage so a seedling will have a smaller root area than an established plant.

  

• 3. Climate

  Plants use the majority of water they require to keep themselves cool, pumping water from the roots to be transpired by the leaves.

  The rate at which they extract water from the soil is based on the evapotranspiration rate (ET). ET rates are affected by climatic conditions such as temperature, wind, humidity and growth stage of the plant. A hot windy day in the middle of summer will have a high ET because the plant needs more water to keep cool.

  Irrigating too early in the season can waste water, as plants use less water when temperatures and evaporation are low, but letting the soil get too dry before starting to irrigate may make it hard to catch up, depending on soil and irrigation type.

  

  In Canterbury, ET is high during the summer period and irrigation generally needs to proceed uninterrupted (until a significant rain). The ‘shoulders’ of the seasons are generally where much of the water savings can be made:

  Spring

  In spring, soils are generally near field capacity (full of water) and temperatures are still low. There is good potential to save water by delaying the start of irrigation until it is actually needed, i.e. when a soil moisture deficit occurs and temperatures increase. Saving water in the spring, when ET is low (risk to crops is minimal) also means there will be more water left for the peak season. But care must be taken not to let the soil get too dry, as it may be hard to catch up, especially with irrigators that have a long return period (e.g. when it takes longer than 10 days to return back to a paddock).

  Autumn

  ET rates can decrease rapidly in the autumn. This means irrigation water does not need to be applied as regularly. Minimising unnecessary irrigation in the autumn also helps minimise cooling of the soil, helping to keep plants growing longer.

  Rainfall is the best form of irrigation as it does not cost anything. Where possible, leave enough room in the soil to absorb any rain so it can be utilised to naturally increase soil moisture, saving on irrigation. If soil is fully saturated and it rains, the water has nowhere to go except run-off or drain out the bottom of the soil, so it is not utilised and may take important nutrients with it.

• 4. Irrigation system capability

  The amount of water an irrigator applies (application depth) varies hugely and depends on how the irrigation system has been designed.

  Other influences include:

  • The amount of water available for use
  • How soon an irrigator can return to a paddock
  • How much water the irrigator can apply

  The overall capability of a system is referred to as system capacity and describes that maximum amount of water able to be applied in a given timeframe (e.g. mm/day, l/s/ha or mm/wk).

  Because the main aim of irrigation is to keep up with ET rates (plants demand for water), system capacity should try to match ET levels.

  As it can be impractical or uneconomic to have an irrigation system which is able to match ET rates at their peak, decisions have to be made on how to manage this limitation.

  Long-term options include reviewing the irrigation system and making changes to improve the efficiency of the existing irrigation, increasing irrigators or installing more efficient irrigation types. To improve the reliability of water supply, development of seasonal storage or applying for additional water allocation may be required.

Application rates

If a farm’s irrigation system has been designed to return to a paddock frequently (every 3 – 4 days) then a lower application depth can be used, because it is only a short period of time before water will be applied again. Conversely if an irrigation system is designed to only return to the paddock every 15 days, then a higher application depth is required to keep the soil moisture levels above refill point and limit plant stress over the longer period of time.

Below is an example of two different irrigation systems which deliver 4 mm of water per day but at different application depths.

	Irrigation type	Return period	Application depth per irrigation pass	mm/day
System A	Centre pivot	3 days	12 mm	4 mm (12 mm/3 days)
System B	Rotary boom	15 days	60 mm	4 mm (60 mm/15 days)

Centre pivot length and application rate

With a centre pivot, the outside span needs to travel faster than the inside span in order to keep in line. Because the outer span covers greater distances, the application rate varies along the length of the pivot to achieve the same application depth.

At the centre, it will be a low application rate (light drizzle) and at the end span, a higher application rate (heavy downpour). The longer the pivot, the greater the application rate will be at the end span.

This can cause problems, especially on rolling country, if the application rate is too high and the water is being applied faster than the soil can absorb it, increasing the risk of puddles and run-off. Because of uneven infiltration into the plant zone, grass production can also be reduced. Any steps that reduce very high application rates will be beneficial. For example:

• installing sprinklers with greater coverage and which spread water over a larger area
• decreasing the application depth for each watering and returning more frequently.