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Guidelines
for
managing
under
limited
water
supplies
Avocados
1
Purpose of Guidelines
and identify areas for future work by the
industry.
Water resource availability varies
enormously across the Australian avocado
industry. While some regions currently
have adequate water for production (e.g.
parts of Queensland), other areas have
experienced limited supplies due to the
drought (especially in Western Australia, the
tri-state region of the Murray Darling Basin
and southern Queensland).
The guidelines are a starting point to
provide avocado growers with practical
information on how to improve water use
efficiency. This information can be used
to help improve management under both
limited and non-limited water supply
scenarios.
Critical Growth Stages
Managing avocado production under
limited water availability is now a challenge
faced by many growers. It is predicted that
the number of growers experiencing this
challenge will increase, as pressures on
water availability for irrigated horticulture
rise. Understanding the best way to use
low water allocations is the key to farm
sustainability in the long-term.
There are six events/stages in the growth
cycle of avocado trees. They include:
1)Bud break
2)Flowering
3)First leaf flush and root flush
4)Fruit drop (time of harvest) – avocado
fruit don’t ripen on the tree. Several
fruit are picked and tested to
determine if they are ready, prior to
harvesting the remainder of the crop.
5)Second leaf flush
6)Dormancy and second root flush.
However, current information on how
best to manage avocados with limited
water is anecdotal and has not yet been
verified by science. These guidelines aim to
summarise the current information available
Time of Year
Aug
Sep
Oct
Nov
Dec
Jan
Feb
Mar
Apr
May Jun
Jul
100
90
80
70
%
Growth
60
50
Shoot
40
Fruit
Root
30
20
10
0
Major
Growth
Stages
2
Bud break
1st leaf flush
/root flush
Flowering
2nd leaf flush
Fruit drop (harvest)
Dormancy/2nd root flush
Figure 1: Concept diagram of avocado tree growth curve
(Based on AVOMAN Avocado Orchard Management Software, QLD DPI&F 1998-2003)
The timing of these stages varies depending
on the region, however a general concept
diagram of the avocado growth curve is
shown below (Figure 1).
The irrigation requirement during
each avocado growth stage varies.
Understanding the growth stages that
are most susceptible to water availability,
enables irrigation management to be
tailored and optimised throughout the
season.
These guidelines focus on highlighting
the approximate irrigation needs for
each growth stage, based on the latest
information.
Irrigation Requirements
Importance of adequate irrigation
Avocados are highly sensitive to limited
water availability. Their shallow root
systems exhibit little control over moisture
loss. Furthermore, they tend to be planted
on lighter soils with low water holding
capacity.
Water stress can lead to poor fruit set,
reduced yield and quality, increased fruit
drop, ‘ringneck’ (browning of the fruit stalk
that leads to smaller and poorer quality
fruit), poor fruit shape and poor internal
quality due to limited uptake of boron
and calcium. It is therefore essential that
irrigation be carefully managed so that
the water needs of avocado trees are
adequately met.
Factors affecting irrigation demand
Several factors influence the volume and
timing of water required by avocado trees.
Soil
The type of soil will influence the waterholding capacity and the subsequent
Figure 2: Ringneck is an indication of water stress
(Photo source: Alec McCarthy, Department of Agriculture
& Food, Western Australia)
irrigation regime. Lighter sandy soils will
hold less water and will require a higher
frequency, but lower volume of irrigation
water, compared to heavier clay soils. The
depth, organic matter content and surface
condition (e.g. level of compaction) will all
influence the volume of irrigation required.
Climate
Climate varies dramatically across
Australia’s avocado growing regions
(Figures 3 and 4) and influences the volume
of water required. Higher humidity and
lower temperatures and wind will reduce
evaporation and consequently reduce the
water requirements of avocados. Rainfall
will also reduce the need for irrigation.
The climate in parts of Queensland results
in a lower irrigation demand compared
to other areas in Western Australia,
South Australia (Riverland) and Victoria
(Sunraysia).
3
Orchard Management
The size and condition of the avocado trees
will also influence water demand. Water
requirements increase with tree size and as
the canopy density increases. The majority
of the root system is found in the upper
30cm of soil, meaning irrigation demand is
highest at a shallow depth.
Management decisions will also affect the
volume of irrigation water required. For
example, mulching and windbreaks can
greatly reduce the tree water demand.
However, planting trees on soil mounds
to improve drainage can lead to rapid soil
drying and up to 20% extra water being
needed.
Regions
These guidelines focus on three key
avocado growing regions – tropical,
subtropical and mediterranean. Included in
these regions are the following areas, which
have various irrigation demands according
to previous work:
• Tropical – Mareeba, Atherton and
Bundaberg in Queensland ~6-12ML/ha.
• Subtropical – coastal Queensland (e.g.
Nambour), north coast of NSW e.g.
Ballina ~3-5ML/ha
• Mediterranean – Riverland in South
Australia (e.g. Loxton), Sunraysia in
Victoria (e.g. Mildura), Western Australia
(e.g. Pemberton and Perth), Central
Burnett in Queensland, Dareton district of
NSW ~8-18ML/ha.
4
Irrigation needs for
different growth stages
and levels of water
availability
Background
While avocados require irrigation all year
round, their water needs are critical during
certain periods. In particular two growth
stages that are particularly susceptible to
low water availability include:
1) Onset of flowering when avocados begin
flowering, their water requirement rises
due to the warmer weather increasing
evapotranspiration and the flowers
increasing the surface area of the plant
by up to 90 per cent (which increases
the area that can lose water). Limited
water during this time can reduce the
proportion of flowers setting to fruit
(to as low as 0.4 per cent in some
instances). Adequate water during
flowering is therefore vital for fruit set.
2) During the main fruit drop period
(January to March or later in WA) which
coincides with high temperatures. This
is the most critical growth stage for
water availability as the final fruit yield is
determined in this period.
Adequate water is also required following
fruit set and development for regenerating
purposes. If a tree is to continue fruiting in
coming seasons, it is essential vegetative
growth and roots are allowed to regenerate
free of stress.
When water is non-limiting, growers tend
to irrigate their trees with the aim of
maximising production. However, when
water is limited the production goals may
change.
Figure 3: Average annual rainfall across Australia (Bureau of Meteorology)
Figure 4: Average annual evapotranspiration across Australia (Bureau of Meteorology)
5
While maintaining some production during
drought is desirable, an extreme scenario
may involve irrigating to keep trees alive
while sacrificing yield in the short-term.
This would allow for rapid yield recovery
when water availability improves, rather
than replanting trees after the drought
and waiting for them to mature. However,
irrigating for tree survival only is not
suggested as a long-term strategy.
The spectrum of possible options is shown
below.
No
Limitations
Irrigation
water
availability
Maximise
yield
Production
goal
Major
Limitations
Keep trees
alive
(minimal yield
if any)
This section examines the irrigation needs
over the six growth stages of avocados for
the two extreme scenarios:
1. When water supplies are non-limiting;
2. When major water shortages occur.
Calculations have been based on a tropical,
sub-tropical and mediterranean climate, to
capture the major differences in demand
across Australia.
requirements for avocado trees enables
more accurate assessments to be made
regarding the likely impacts of water supply
shortages on tree survival and production.
More informed management decisions can
be made at an earlier stage and on-farm
drought management strategies can be
implemented more efficiently as a result of
such information.
However, the FAO approach requires
validation for Australian conditions.
Therefore, the below calculations are
intended as a starting point only.
They also assume water quality is not an
issue. A potential issue with highly reduced
water applications in areas with poor
water quality, is salt accumulation in the
soil profile. The impacts of this need to be
considered when determining the irrigation
regime.
It is recommended growers undertake a
similar process (highlighted under example
A and B) for their own orchards, so that
their specific irrigation requirements for
different scenarios can be determined.
By understanding the water requirements
for avocado trees when water is nonlimited, irrigation can be used to optimise
growth and production with minimal
wastage. This helps improve irrigation
efficiency and reduce costs associated with
water losses.
Unfortunately, information on the water
requirement to achieve various avocado
yields is currently limited. This is an area to
focus future research. In the interim these
guidelines have outlined the estimated
water requirement for tree survival using an
approach adopted overseas (FAO56 1998).
6
Understanding the minimum irrigation
Figure 5: Water stressed leaves on an avocado tree
(Photo source: Alec McCarthy, Department of Agriculture
& Food, Western Australia)
Estimated irrigation
needs
Estimated irrigation requirements for
the different growth stages in Figure 1
are outlined below. These estimates use
single crop coefficients (Kc) when water is
unlimited and water stress coefficients (Ks)
when limitations occur. Both are taken from
the FAO56 ‘Guidelines for computing crop
water requirements’.
ETo measurements were calculated from
SILO using the FAO Penman-Monteith
formula. Tropical ETo figures were based
on Mareeba data, while subtropical and
mediterranean ETo figures were based on
Nambour and Mildura, respectively.
Refer to the end of this section for
assumptions and example calculations.
Stage 1:
Bud break (predominantly August)
The irrigation demand during bud break
(usually August) to maximise production
and for tree survival in the three major
climate zones is shown below.
Table 1: Bud break (August) irrigation demand
for two extreme scenarios in three climates
Stage 2:
Flowering (predominantly September)
The irrigation demand during flowering
(usually September) to maximise production
and for tree survival in the three major
climate zones is shown below.
Table 2: (right) Flowering (September) irrigation
demand for two extreme scenarios in three climates
7
Stage 3: 1st leaf flush/root flush
(predominantly October/November)
Stage 5: 2nd leaf flush
(predominantly February/ March)
The irrigation demand during the first
leaf flush/root flush (usually October to
November) to maximise production and
for tree survival in the three major climate
zones is shown below.
The irrigation demand during the second
leaf flush (usually February to March) to
maximise production and for tree survival
in the three major climate zones is shown
below.
Table 3: 1st leaf/root flush (Oct/Nov) irrigation
demand for two extreme scenarios in three climates
Table 5: 2nd leaf flush (Feb/Mar) irrigation demand
for two extreme scenarios in three climates
Stage 4: Fruit drop/harvest
(predominantly December/January)
The irrigation demand during the fruit
drop/harvest period (usually December to
January) to maximise production and for
tree survival in the three major climate
zones is shown below.
Table 4: Fruit drop (Dec/Jan) irrigation demand for two
extreme scenarios in three climates
8
Figure 7: Avocado fruit shed in summer
(Photo source: Alec McCarthy, Department of Agriculture
& Food, Western Australia)
Stage 6: 2nd root flush/dormancy
(predominantly April to July)
The irrigation demand during the second
root flush and dormancy (usually April to
July) to maximise production and for tree
survival in the three major climate zones is
shown below.
EFFECTIVE RAINFALL
The amount of irrigation required will
be influenced by the amount of effective
rainfall. Effective rainfall is generally
considered to be an event that is greater
than 5mm. The amount and timing of
effective rainfall varies significantly between
the three regions.
Tropical
– most rainfall occurs in summer with
minimal effective rain in winter and spring.
The contribution to water demand would
range from 2-6ML/ha.
Subtropical
– rainfall peaks in summer and autumn
with lower rainfall in winter and spring.
Total contributions to water demand range
from 4-8ML/ha. In a wet year, irrigation
would be minimal.
Mediterranean
– rainfall is fairly consistent over winter and
spring with less in summer and autumn.
Total contributions to water demand would
be in the order of 1-2ML/ha.
Table 6: 2nd root flush/dormancy (Apr-Jul) irrigation
demand for two extreme scenarios in three climates
9
TOTAL WATER REQUIREMENT
and IRRIGATION DEMAND
Region
Total Water
Requirements
(ML/ha/season)
Unlimited water
Major water
shortages
Total Irrigation
Demand
(ML/ha/season)
Unlimited water
Major water
shortages
Tropical
11.9
4.2
2-6
6-10
4
Subtropical
10.1
3.8
4-8
2-6
4
Mediterranean
11.4
3.8
1-2
9-11
4
The total water requirement per season
for the different avocado growing regions
is shown below (based on the previous
assumptions).
Total irrigation demand will be reduced by
the volume of effective rainfall. This must
be considered in calculating irrigation
requirements.
CALCULATIONS/
ASSUMPTIONS
10
Effective
Rainfall
(ML/ha)
Formula used: Average daily water
requirement = crop coefficient (Kc – for
unlimited conditions and Ks - for stressed
conditions) x average daily ETo
Assumptions (from FAO56 1998):
1. Crop coefficients based on subhumid
climate
2. Long-term average ETo from Mareeba
(Tropical), Nambour (Subtropical),
Mildura (Mediterranean) (Bureau of
Meteorology Silo 2006).
3. Assumes sandy loam with:
- soil water content at field capacity 0.23
m3/m3
- soil water content at wilting point 0.11
m3/m3
- soil water content during water stress
0.12 m3/m3.
IRRIGATION SCHEDULE
Information in the previous irrigation table
can be used to determine the irrigation
schedule for avocado trees at a specific
time (Murray Valley Citrus Board 2005).
For example, the approximate maximum
irrigation hours required can be determined
by dividing the Readily Available Water
(RAW) by the irrigation application rate.
Example A:
RAW = 0.59mm/cm x 30cm soil = 17.7mm
(for sandy loam with a water tension
between -8 to -40kPa, source: NSW DPI)
Application rate (drip) = 3mm/hour
Max irrigation hours needed =
RAW ÷ application rate (mm/hr)
=17.7mm ÷ 3mm/hr = 5.9 hours
The approximate irrigation interval can then
be determined for each month by dividing
the RAW by the average daily water
requirement for each month (as shown
below)
Example B:
January irrigation interval (mediterranean
climate) = 17.7mm ÷ 5.6mm/d = 3.2 days
Therefore for January the approximate
irrigation schedule would be six hours of
irrigation every three days.
Drought Management
Strategies
Limited water has been found to
proportionally reduce yield and the
physiological activity of avocado trees.
A variety of tools exist to help avocado
growers manage their trees during drought
periods. Several of these tools are outlined
below.
Reduce Competition for Water
Weeds and groundcover plants established
intentionally between tree rows can
compete with avocados for soil moisture.
This can have a negative impact on the
avocado trees if moisture is limited.
Therefore regular mowing of the inter row,
weed elimination and using herbicides to
kill off inter-row plants after heavy rainfall
periods (e.g. winter rains) can reduce
competition while maintaining some of the
groundcover benefits.
Reduce Soil Evaporation
Soil water is not only removed through
plant processes, but can be evaporated
directly from the soil surface.
Mulching around trees during dry
periods can reduce soil temperatures and
evaporation. A 10-15cm loose layer of
mulch should be distributed beyond each
tree’s drip line, but should not accumulate
around the trunk. Coarsely cut crops such
as oats, sorghum, setaria (possibly mixed
with a legume such as lupins) are desirable.
However, finely cut soft material should not
be used due to the increased risk of fungal
disease.
Irrigating at night can also minimise
evaporation and increase the proportion
of water entering the soil column for use
by the trees. However, it is important
to regularly check the condition and
distribution uniformity of the irrigation
system during daylight to ensure no
unexpected water losses are occurring.
Avoiding tree skirting (the pruning of
underlying branches) temporarily, can also
reduce evaporative losses by reducing
air flow under the trees. However, it is
important that the irrigation distribution
remains uniform and is not impacted by any
low hanging branches.
Figure 8: Mulching is an excellent way to reduce water
loss (Photo source: Alec McCarthy, Department of
Agriculture & Food, Western Australia)
Efficient Irrigation Systems
Irrigation efficiency varies dramatically
across different types of irrigation systems.
It is currently recommended under tree
mini-sprinklers be used as these allow
optimal root hydration. However, it is
important that the chosen sprinklers
irrigate the main root area only. In doing
so, irrigation water is focused on the plant
uptake zone (drip zone) rather than being
applied onto non-productive parts of the
orchard that may also be more exposed to
evaporation.
Furthermore, misting from the irrigation
system should be avoided by instead using
high output, low-pressure sprinklers. This
reduces water loss through evaporation and
11
improves irrigation efficiency.
Pulse irrigation can also have production
benefits when water is limiting. More
frequent, but shorter irrigation intervals
(‘pulses’) have been shown to optimise
nutrient and water uptake over longer
periods and consequently improve
production and fruit quality for a given
volume of water. The water shortage in
Western Australia has encouraged the
adoption of pulse irrigation, with such
techniques now used across several
horticultural industries.
Irrigation scheduling
For efficient irrigation management,
especially when water is limited, it is
essential irrigation scheduling be used to
identify the timing and volume of water
required by avocado trees.
The irrigation schedule is determined
by closely monitoring soil moisture so
12
that optimum levels are maintained
throughout the critical growth stages.
Several techniques exist for monitoring
soil moisture as outlined below. It is
recommended the soil-based monitoring
systems be used rather than solely relying
on the climate-based systems, which
estimate evapotranspiration.
As avocado trees mainly use water from
the top 0-30cm of soil, it is important soil
moisture monitoring is focused within this
zone. Residual moisture levels deeper in
the soil (e.g. 60-100cm) should also be
checked. Expert advice should be sought
when initially planning and establishing a
monitoring system.
Tensiometers
Tensiometers are a common, cheap and
effective way of monitoring soil moisture.
Each tensiometer consists of a hollow tube
filled with water and algaecide, a ceramic
tip through which water moves, a water
reservoir and a vacuum gauge that reads
the water tension.
When the soil is saturated, the tensiometer
has a reading of 0-8kPa. As the soil loses
moisture over time, the water within the
tensiometer moves through the ceramic
tip into the soil. This results in the water
tension increasing up to 90kPa. When
the soil is re-hydrated, water moves from
the soil into the tensiometer causing the
reading to fall.
Two tensiometers are often used, measuring
soil moisture at approximately 15cm and
43cm depths. However, measurements
at 60cm depth can also be taken (i.e.
below the root zone). At least one pair of
tensiometers should be used per variety or
per block.
It is vital that tensiometers be installed in
an area wetted by the irrigation system, on
a representative tree and on the drip line
to the northeast under the canopy (i.e. the
warmest area).
According to the literature, when a reading
of 20kPa on sandy soils or 30-40kPa on
loamy soils occurs, irrigation should begin.
Some literature suggests watering should
cease once the reading reduces to 10kPa.
If the deep tensiometer readings rise
immediately after irrigation, not enough
water has been applied. If they fall to less
than 10kPa shortly after irrigation, the area
has been over irrigated.
However, the practical experience of some
growers suggests the reactive speed of
tensiometers rarely allows them to be used
to indicate when to cease an irrigation
event – they only indicate when to begin
irrigating. Tensiometers can also be
inaccurate in extremely wet, dry or sandy
soils.
Resistance blocks
Resistance blocks consist of two electrodes
embedded in a block of porous material
that is buried in the soil oil and mimics
the soil moisture conditions of their
surroundings. A pair of wires is attached
that are exposed on the soil surface and
connect to a digital ohmmeter to record
electrical resistance. This measurement is
displayed as water tension, which can then
be used to determine when to irrigate.
Soil moisture sensors are installed in
the same way as for tensiometers and
exhibit many of the same benefits and
disadvantages. However, the gypsum can
break down and may require replacement
after 18 months.
Capacitance probes
These probes measure the dielectric
constant of the soil, which is proportional
to the water content. Two main types of
probe exist:
(1) T hose with a single sensor that take
a reading at 100mm intervals down a
vertical PVC tube and are recorded with
a hand held logger (e.g. Gopher and
Diviner; and
(2) T hose that have multiple sensors and
are often left in an orchard for a season
to take continuous measurements
from various depths. A cable is used to
connect the probes to a data logger,
which automatically downloads data
every few days.
At least three probes are recommended
per block of avocado trees and it is very
important that the irrigation distribution
patterns are well understood prior to
installation.
More information on soil monitoring
methods is available in Soil Water
13
Monitoring – An Information Package
(Charlesworth, 2005).
Evapotranspiration
Evaporation data in the form of ETo
readings calculated from weather stations
(sourced from the Bureau of Meteorology
or government agencies) is used in
this technique to calculate the water
requirements of a crop.
While this is a relatively cheap and easy
method, it is recommended soil moisture
monitoring data be used occasionally to
validate the results.
14
Limiting Irrigation to
Specific Trees/Varieties
The University of California has
recommended only the healthiest,
productive avocado trees be irrigated as a
way of reducing water demand.
Trees exhibiting disease or severe frost
damage should not be irrigated. Border
trees that experience high winds and do not
produce fruit could have capped sprinklers.
Alternatively, they may be top-worked with
‘Bacon’ or ‘Zutano’ varieties and irrigated
with a low volume so that they flower for
cross- pollinating purposes (but do not
fruit).
Figure 9: Water stress is a problem for avocado trees
(Photo source: Alec McCarthy, Department of Agriculture & Food, Western Australia)
Stumping and Thinning
It is possible to reduce water usage by
implementing orchard canopy management
strategies. This may include implementing
planned strategies ahead of time eg
removing a crowded or unproductive
orchard with the intention of replanting
once water becomes non-limiting.
Trees that have canopied (e.g. after not
being thinned for 10 years) have a high
water use compared to other growth stages
according to Witney and Bender (1992).
However there are questions regarding
whether canopied trees have a lower leaf
surface area, reduced ground evaporation
and reduced sprinkler evaporation due to
wind reduction and increased humidity. This
would in fact see water use per hectare
being lower, despite other issues existing
around production efficiency, ease of
harvest and spraying.
The literature recommends canopied trees
be cut-off (stumped) between 1.2 to 1.8m
and immediately whitewashed (with a
50:50 water:water based latex paint) to
protect from sunburn. These trees are
then allowed to regrow using a reduced
irrigation frequency.
The University of California recommends
stumping in alternative blocks and after
three years (when fruiting has resumed),
remaining trees should then be stumped.
In situations where trees have not yet
canopied, but have grown into each other,
some current research recommends that
the orchard be thinned. i.e. every other tree
removed. Sprinklers on thinned trees can
be capped resulting in an instant water use
reduction. However, some horticulturalists
recommend against stumping alternative
trees within a block as access to sunlight of
the stumped trees becomes impeded by the
larger adjacent trees.
Further Information
For further information contact:
Avocados Australia
Lvl 1, 8/63 Annerley Rd, Woolloongabba
PO Box 8005 Woolloongabba Qld 4102
Ph: 07 3846 6566
Fax: 07 3846 6577
www.avocado.org.au
Horticulture Water Initiative
Horticulture Australia Limited
Level 1, 50 Carrington Street
Sydney NSW 2000
Ph: +61 2 8295 2300
Fax: +61 2 8295 2399
www.horticulture.com.au/water
www.horticulture.com.au/drought
Prepared by
Figure 10: Stumping can be used as a water conservation measure
(Photo source: Alec McCarthy, Department of Agriculture & Food, Western Australia)
15
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Department of Primary Industries QLD.
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http://www.lwa.gov.au
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http://www.agric.wa.gov.au
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Before You Start,
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Irrigation: A Citrus Grower’s Guide, NSW
Department of Primary Industries, NSW.
PIRSA 2006, Avocado irrigation in drought
conditions, fact sheet No. 16/06.
16
Partridge, C. J. 1997, ‘Avocado irrigation
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need, irrigation design and frequency
scheduling’, Proceedings from Joint Meeting
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Plessis, S.F. du 1991, ‘Factors important for
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NSW Agriculture. 2002, Irrigation for
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Survival Requirements: A Collation of the
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Restrictions Lasting One Season, prepared
for the Department of Natural Resources
and Environment.
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and Efficiency, www.avocadosource.com
Witney, G.W. & Bender, G.S. 1992, Water
Conservation Strategies for California
Groves, Proceedings of Second World
Avocado Congress, pp. 349-355.