Irrigation is the act of supplying water to meet crop water needs between rainfall events.
In order to sustainably manage your water resources, you need to calculate the best time to irrigate and also how much water to use. A generalised diagram of the relationship between plant yield and water applied is shown below. Note especially the different parts of the graph:
- Where the yield increases without irrigation.
- Where irrigation contributes to continued yield increase.
- Where more water causes no increase in yield.
- Where too much water (waterlogging) causes yield decline.
In general, the irrigator should aim for the target zone for best yield of crops (between 2 & 3 in the previous graph) but there will be times when you need to control the application of water more accurately as in watering to control sugar levels in grapes.
Irrigation Schedules tell us when to water. An irrigation schedule should take into account effective rainfall, crop water use and irrigation practices. A schedule should tell us the right time to deliver the right amount of water for each particular crop.
The Daily Water Balance is the difference between how much water has been used by the crop each day and the amount of water still held in the effective rootzone of the soil. In other words, the Daily Water Balance tells us “how much fuel is left in our tank” and how fast it is being used. You will need to calculate your daily water balance in order to work out the timing of irrigation events.
Not all rain that falls is available to be taken up by the plant. The first part of a rainfall event can be intercepted by mulch or leaf litter or run off. For this reason, it is generally accepted that the first 5mm of any rainfall event is NOT effective. Effective rainfall is therefore defined as: The proportion of any rainfall event that is stored within the crop rootzone and available for use by the crop. When estimating effective rainfall you need to take into account the time that the rainfall event occurred as well as it’s intensity.
Crop water use will vary with climate, weather patterns and the stage of development of the crop (as shown in the graph).
In the early stages of growth, water requirements are generally low. Crop water use grows as the plant matures and fruits. After fruiting or seeding, the plant usually starts dying or drying off (senescence) or shuts down for dormancy.
The amount of water that different crops use varies according to crop type and environment. At peak growth, for example, a crop of lucerne uses about twice as much water as a grape crop.
Irrigators have fine-tuned the watering of many crops over the years to improve yields and maximise use of water resources.For example;
- Rice crops are flooded at the early pollen stage to reduce the risk of sterile grains in the seed heads. Precise control of watering at leaf tillering and panicle initiation will also improve crop yields.
- With grape vines, while it is usual practice to avoid water stress at flowering, fruit quality can often be improved by stressing the plant selectively at specific growth stages.
- With many citrus crops, water stress from flowering to harvest results in fruit drop. Avocados exhibit similar growth habits and water stress at flowering can cause low fruit set.
