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.


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.