Irrigation design
Once you have sorted out your water supplies for irrigation, you have to then develop a well-designed irrigation system that delivers the volumes required in an efficient manner.
This is usually best performed by a competent irrigation engineer (for contact details see 'Avocado industry contacts in Western Australia'), who can design a system that delivers a crop’s water requirement in the most economical manner. Remember to design the system to cope with (or be easily expanded to cope with) your planned, full size mature orchard. To do this you will need to determine likely delivery quantities, frequencies and flow rates for your orchard when mature.
In most circumstances, it is considered that avocado trees in the South-West of Western Australia perform better with the use of under tree sprinklers rather than drip systems. While drip systems can be used to grow Avocados, such as in the Tristate region in eastern Australia and the countries Chile, Israel and Peru this is likely only done repsonse to lower water availabilty than as a means to boost productivity. Sprinkler systems can be designed to wet the entire orchard floor, as is common in the sandy soils north of Perth or a portion of the under tree ‘canopy area’, the more common set up.
Full coverage systems use more water as you are throwing water out beyond the trees main root zone. However, this does help maintain the between row sod (the vegetative cover growing between rows) which helps reduce heat reflection. Aiming to apply a relatively even coverage over 100% of the orchard floor is a little excessive. A more efficient alternative is to design the system so that your sprinklers have 100% overlap along the rows, but only limited overlap between rows, Figure 1. This can be achieved with the use of wide diameter mini-sprinklers set out along the rows, usually one per tree. The benefit of this system, as opposed to 100% even coverage, is that it delivers more water (and nutrients if you are fertigating) to your trees than the inter row sod yet should still keep your sod growing to maintain the benefits. A Full coverage sprinkler design will allow for better frost control and climate modification applications if required than an under canopy system.
In cooler locations where the climate satisfactorily maintains your inter-row sod, or heavier soil situations or you need to minimise water use, then designing your system to only wet the under tree ‘canopy area’ is more common and more efficient. In this situation, the aim is to try and deliver the water in an even strip under the trees that does not extend into the inter-row beyond the trees canopy. These systems often require a stepped design approach. Initially a single sprinkler is installed per tree, with a deflector tab to reduce delivery diameter, near the tree. Once the tree grows beyond the wetted area of the mini-sprinkler, the deflector can be removed to increase the delivery diameter. When the trees grow larger and start to fill the row space, the mini-sprinkler position is often shifted slightly along and a second one installed. The mini-sprinklers should end up being in line with the tree row, evenly spaced along the row with the throw of each sprinkler reaching the base of the next one, Figure 2. The size of the mini-sprinklers will be determined by the tree spacing — a 7 x 4m tree spacing would be best served by a mini-sprinkler with a diameter throw of about 4m, an 8 x 5m spacing a 5m diameter throw.
Orchard blocks
You should break your orchard up into manageable sized blocks, and should be able to provide different irrigation rates and frequencies to different soil types, different sized trees and ideally different varieties and/or rootstocks (particularly if you are fertigating). This is because these factors will impact on the volume and frequency of irrigation and fertiliser required.
It is important that your irrigation design and operation results in a uniform distribution of water within each block and root zone. If you do not have an acceptable delivery uniformity of at least 85%, within a block, you will be delivering excessively different volumes of water to different trees within the block. This leads to uneven growth and yields and inefficiency.