How much irrigation to apply and when
Avocados are sub-tropical trees. They are a wintergreen type of tree (half way between deciduous and evergreen) with large leaves and shallow root systems and require water year round. Irrigation best practice is to not supply water in excess of what the tree requires. This prevents water being pushed beyond the root system, which can have negative environmental consequences (nutrient pollution) and increased operational costs due to wasted water, nutrients and power costs.
In essence, there are three steps to working out irrigation needs for your avocado orchard. These are to:
- determine how much readily available water (RAW) is available to your avocado tree — often referred to as the ‘bucket size’
- determine how much water has been used by your tree as a result of the climatic conditions and growth stage — or how quickly your tree is depleting the bucket
- know what your irrigation output is so you can determine how long to irrigate to replace the used water — or how long is required to refill the bucket
Determining Readily Available Water
A key component of efficient irrigation is to know how much water your soil can hold and how much of this will be available to your crop, referred to as the Readily Available Water (RAW) — or what is the size of the bucket of water available. To determine this you need to identify several components; soil type, root depth and irrigation strategy.
Firstly you need to identify the soil type as this plays a critical part in determining how much water the soil can hold and how easy it will be for the plants to extract. A Factsheet ‘Soil Texture – Measuring in the Field’ by Soil Quality, provides detailed directions on determining soil type.
Next, a reasonable calculation of the average depth of the feeding root mass has to be calculated. This is important because water located below the root depth is not readily available to the plant. The aim is to maintain moisture in this root zone to within your chosen strategy parameters. This requires you to dig a hole and look at your root system. Avocados grown under mini-sprinklers will most commonly have a relatively shallow root mass depth, with most roots being in the top 60 centimetres (cm) and the bulk of fine roots (those that take up the bulk of water and nutrients) located in the top 20cm.
Your irrigation strategy relates to the level of water ‘stress’ you are prepared to allow the tree to experience before you irrigate. Water is held in the soil in the pore spaces between the soil and by attraction forces between the soil particles and the water. To extract this water, plants must exert some suction power. Water stress is measured as the amount of effort (suction) required for extracting the water from the soil and is measured in kilopascals (kPa) of suction pressure. As the soil dries, the tree must use more suction effort to extract the water which increases the stress on the tree. Different plants have differing capacities to extract water and can handle different levels of water stress — mild stress -20kPa, moderate stress -40kPa and high stress -60kPa and higher. Experiments have been conducted to determine the RAW for one cubic metre (the Gross RAW) of a range of soil types under different suction pressures.
Traditionally with avocados, the irrigation strategy is for mild stress, for the purposes of determining the RAW figure it is recommended growers use the -20kPa water suction figure. At water suction pressures lower than -20kPa avocado trees roots are unable to remove much water from the soil.
Also, don’t forget to take into account the percentage of gravel in the soil when calculating your final figure as gravel is a poor holder of water and considered inert.
To calculate your RAW you multiply the gross RAW with the soil proportion (after removing the % of gravel) and the depth of water extracting roots, see Calculation 1 for an example. ‘Calculating readily available water’ provides detailed directions on calculating RAW and provides a table to show the approximate amount of gross RAW in millimetres per metre (mm/m) of soil depth for different types of soil and irrigation strategies.
Calculation 1 Determining RAW available for avocado trees in mm
Soil type = sandy clay loam
Irrigation strategy = mild stress (-20kPa)
Gross RAW (mm/m) for soil type and irrigation strategy (GRAW) = 39mm/m
Measured average effective root-zone depth (RD) = 0.2m
Gravel content of soil = 15%, therefore soil content (S) = 85% (0.85)
Readily Available Water
= GRAW x S x RD
= 39 x 0.85 x 0.2
How much water has been used daily?
As mentioned earlier, good irrigation practice is to replace the water that has been used in the effective root zone — how much of the bucket has been used. The volume of water used by plants varies not only due to the climate but also depends on their size, crop load, growth stages and evolutionary traits.
There are essentially two methods of determining the water usage of plants; evaporation replacement (refer to ‘Evaporation-based irrigation scheduling’) and direct soil moisture monitoring (refer to ‘Soil moisture monitoring to fine-tune irrigation scheduling’). Both methods aim to efficiently replace the water used through plant transpiration and lost through environmental evaporation or drainage, are relatively complex and require full explanation in their own publications.
Duration of irrigation
The idea of efficient irrigation is to replace the water used from the root zone — how much water is required to re-fill the bucket. Therefore, the length of irrigation depends on how dry you let your soil get before you re-irrigate. Let us assume here that you are following the mild stress crop strategy between irrigations and you are on a sandy clay loam. Using calculation 1, we can see that we have 6.6mm of RAW in our example. If we allow the tree to use all this, then this is how much we need to replace.
Firstly you need to know the average output from your sprinklers in mm per hour (mm/hr). To determine this you need to test your irrigation set up. It is important to note that this figure can change over time, as it is affected by the condition of your system, so should be checked annually. A simple method to determine your sprinkler output in mm/hr is to measure the output from several emitters and average this figure. Then divide this figure by the area wet by the sprinkler (for simplicity, just square the diameter of the sprinkler throw), see Calculation 2 for an example.
Calculation 2 Determining sprinkler delivery rate in millimetres per hour
Measured flow rate average (FR) = 75L/hr
Sprinkler throw diameter = 4m
The sprinkler wets a circular area therefore:
Formula for area = Pi(3.14) x radius2
Wetted area (WA) = 3.14 x (2m)2 = 12.56m2
Sprinkler delivery rate
= FR ÷ WA
= 75 ÷ 12.56
= 5.97 mm/hr
Once you have calculated your delivery rate, to calculate the duration of irrigation required to replace used RAW, assuming all the RAW has been used, you simply divide the RAW by the sprinkler output and then multiply by 60 to provide the duration in minutes, see Calculation 3 for an example.
Calculation 3 Determining irrigation duration required to replace RAW
RAW = 6.6 mm
Sprinkler output (SO) = 5.97mm/hr
Irrigation length (I) = minutes
= (RAW / SO) X 60
= (6.6 / 5.97) X 60
= 66.33 minutes (round down to 66 minutes-if it was 66.5 minutes you would round up to 67)
If you are scheduling to commence irrigations whenever the RAW is fully depleted, then this is the length of the irrigation required to replace this used water. Irrigating for longer periods will result in more water being applied than can be held by your root zone. This excess water will drain beyond your plants root zone and not be available for plant uptake and will be wasted. Along with wasted water can also travel nutrients, which will also be lost to your plants and potentially provide a source of pollution. This is wasteful and inefficient in terms of lost nutrients and water and excess power from pumping costs.
If on the other hand you do not irrigate for long enough to replace the depleted RAW, you will be reducing the RAW that will be available to your plants after the irrigation. If this is a regular occurrence, you will be steadily reducing the depth to which your irrigation reaches, which will reduce the depth of your feeding root mass.
Monitoring with soil moisture sensors can help keep a watch on the occurrence of excess or inadequate water replacement. Scheduling based on evaporation requires careful and accurate adherence to the water use and replacement calculations. Regular monitoring of sprinkler operations, tree health for signs of water stress and occasional digging in the root zone wetted area is essential back up for any irrigation scheduling.
Frequency of irrigation
The frequency of irrigation is dependent on two factors; the RAW and the climate (which influences water use by the tree). The generally held view is that in our hot dry summers, rapid onset of water stress can occur, which can lead to reduced production. As a result, a ‘mild stress’ strategy is preferable for avocados, using a -20kPa benchmark. The limited research in WA has generally shown that well watered avocado trees (but not overwatered) produce larger crops. Some recent work suggests that the tree may have a greater ability to extract water than this, but there are growth stages when water stress should be avoided. Periods such as flowering and early fruit set in spring and the rapid fruit growth phase in late summer are adversely affected by moisture stress.
Monitor soil moisture all year
The avocado tree is an evergreen tree of a semi-tropical origin and as such it can suffer from moisture stress at any stage during the year if your soil dries out, this includes winter. How fast your soil dries out depends on your soil’s water holding capacity and the weather. This will obviously occur much faster on lighter sandy soils and during hot dry conditions. So careful monitoring must occur in these circumstances. It is recommended that you try and avoid subjecting the tree to moisture stress at any time, but there are two particular periods when avoiding moisture stress is critical. The first stage is from the onset of flowering until after the initial fruit drop. The second period coincides with the stage of rapid fruit growth, a period when many growers experience what is called ‘summer fruit shed’, this usually occurs from mid January to the end of March.
A common mistake is to forget about irrigation during winter. While an avocado tree certainly slows down in its growth rate during the winter in the south west of Western Australia, it is still sizing fruit and you can still find actively growing roots that are taking up water throughout winter. Recent research has shown that the avocado tree will be taking up water on any day the sun shines. The only days the tree does not pump water is when it is totally over cast. Obviously evapotranspiration is much less during cooler weather, so less water is being used compared with hotter weather, but extended periods of dry, sunny days in winter can lead to water stress. As your mature trees will still be carrying a crop during this period, it is recommended that you avoid moisture stress during this period as well.