Scheduling is the method by which a grower determines the most effective and efficient irrigation practice for optimising crop performance which includes when and how much water to apply. To do this properly requires more than monitoring soil moisture, it utilises a knowledge of the plant, the soil and the irrigation system.
Potential benefits
Proper irrigation scheduling can improve:
- efficiency of water use.
- yield.
- crop health and vigour.
- use of fertiliser.
- salinity management.
- decision making.
Prerequisites for success
Irrigation system
Design
Ability of the irrigation system to apply the correct amount of water at the desired time is necessary to attain the most benefit from irrigation scheduling.
Different crops or even varieties have different water requirements. Soil type also has a major bearing on the optimum quantity and timing of irrigation. If the irrigation system has not been designed to account for these factors then scheduling can not optimise irrigation for all areas of the block. Ideally each variety and soil type should be watered separately. In almost all situations some compromise in irrigation design must be made and scheduling must be based on the norm within each irrigation shift.
Condition
Often the condition of the irrigation equipment
is a limiting factor to achieving the benefits of irrigation scheduling.
Unless an even distribution of water can be applied it will never be possible
to optimise the performance of all trees in the block. Two common faults
causing uneven water distribution are inadequate pressure and worn sprinklers.
If present these problems need to be rectified. Once operating as designed,
the system should be checked regularly, at least annually, and the appropriate
maintenance carried out.
Crop growth stages
An understanding of the crop growth stages of
a deciduous tree crop and their relative water requirements is needed to
schedule irrigations effectively.
Dormancy - The tree crop will not require irrigation.
However if a cover crop or sod is planted, irrigation may be needed to
boost growth, particularly in drier winters. It is not advisable to let
the profile dry out completely, even though the trees are dormant.
Budburst - Root activity commences prior to blossoming
and if soil moisture level is low, irrigate to fill the soil profile 3-4
weeks before blossoming.
Early fruit development - As the leaf area increases
and temperatures rise so does the water demand. Trees should not be water
stressed in the period from fruit set until stone tip hardening. Cell division
in the fruit occurs in this period and water stress will reduce potential
fruit size.
Stone hardening - Fruit growth slows at this
stage and there is the opportunity to practice reduced deficit irrigation
to help limit excessive shoot growth. Seek professional advice before imposing
any water stress on trees, otherwise fruit size, yield or quality could
be reduced.
Fruit filling - This is the most critical period
to maintain soil moisture levels, particularly in the four weeks leading
up to harvest. Water stress will reduce fruit size and yield.
Postharvest - Water requirement is reduced, but
adequate levels of soil moisture must be maintained as root uptake of nutrients
continues and fruit bud formation occurs. Water stress can reduce the following
years crop.
Weed control
Weed growth can reduce the performance of an
irrigation system. Distribution of water can be disrupted by weeds surrounding
sprinklers, particularly mini-sprinklers and sprays. Control must be based
on need, rather than a routine schedule, otherwise water distribution is
bound to be affected by weed growth.
Secondly weeds can account for up to 50 per cent
of water use. Lush weed growth can lead to water stress of neighbouring
trees.
Soil Water Measurement
A range of methods are available for measuring
soil water content. Some can be carried out by the grower, but the equipment
required for others puts it in the hands of consultants.
When using soil water measurement to schedule
irrigations it is important to consider that the measurements are taken
from a small part of the orchard. These measurements are then used to make
decisions for the whole irrigation shift. To ensure maximum benefit ensure
that monitoring sites are representative of the irrigation shift.
Place monitoring sites a couple of rows into
the block and on the north-west side of the trees as this is the hottest
part and will use water the quickest.
Avoid the following areas:
- Outside rows
- Wheel tracks
- Disturbed soil
- Stunted or sick plants
- Areas with poor irrigation distribution uniformity
- Areas with shallow water tables
Feel
With experience, the water availability in a
soil sample can be estimated. To assess the need for irrigation properly
a soil auger or dig stick (gouge corer) soil sampling tube should be used
to test moisture content at several depths within the root zone. Merely
scratching the surface frequently leads to an incorrect conclusion. A number
of holes must be sampled to achieve a realistic judgment of orchard soil
moisture. It is common to find some sites high in moisture while others
are quite dry. This depends on the position and density of roots and the
distribution pattern of the irrigation system.
Tensiometers
To successfully schedule irrigations with tensiometers
requires careful installation, regular maintenance and experience for interpretation.
Tensiometers consist of a sealed tube full of
water with a porous ceramic tip at the bottom and a vacuum gauge at the
top. The ceramic tip is buried to a suitable depth in the soil. As the
soil dries out, water moves out from the tube through the porous tip into
the soil creating a vacuum in the tube. This is measured by a gauge and
is a direct measure of availability of water to the plant at the depth
which the tip has been buried. Gauges read either kilopascals (kPa) or
centibars (cb). One centibar equals one kilopascal).
Installation
At the time of installation the tube must be
completely full of water as air will effect the units accuracy. The tip
must be completely saturated which can be achieved by standing in a bucket
of water for a minimum of 24 hours. With a hand vacuum pump, pump the tensiometer
gauge up to approximately 70 kPa and tap it to release any air bubbles
in the instrument.
When placing in the soil ensure that all parts
of the ceramic tip are in contact with the soil, otherwise false reading
will be obtained. The bottom 10 centimetres of the installation hole should
be made with a coring tool or steel rod of the same diameter as the tensiometer
so that the tip of the tensiometer fits snugly in the hole. Do not force
or twist the tensiometer into position. Tensiometers are best installed
while the soil is moist.
Positioning must ensure that tensiometer sites
represent an average location in the orchard. Avoid area's of poor infiltration
and ensure its in the irrigated zone. Locating in the tree line helps to
prevent damage from slashing and other cultural operations. Depth in the
soil is also critical. At least one tensiometer must be located in the
centre of the root zone. Normally this is 30 - 50 centimetres deep and
between 0.5 to 1.5 metres from the tree trunk. Observing the abundance
of roots when installing the tensiometer will confirm correct placement.
A more detailed picture of soil moisture content
throughout the trees root zone can be obtained by installing tensiometers
at several depths at the one monitoring site. Two to three depths within
the rootzone are most commonly used.
Maintenance
If air occupies the tube then water should be
added. The frequency that topping up is required depends on the condition/quality
of the tensiometer and dryness of the soil. After topping up, remove air
bubbles with a hand vacuum pump by applying and holding the suction at
70 - 80 kPa for at least 15 to 20 seconds while tapping the side of the
tensiometer.
Rain water should be used in tensiometers. Boil
the water to remove air from it and then place in a hot water bottle to
cool. An algacide should be added to the water to prevent algal growth
in the tensiometers. Alternately methylated spirits at a rate of 50 millilitres
per litre of water can be used.
Tensiometers do not operate properly in dry soils.
Unless the trees are being water stressed this should not pose a problem
in irrigated orchards.
During winter, tensiometers should be covered
to reduce the likelihood of the water freezing and damaging the tensiometer.
Neutron probe
Neutron probes enable a rapid measurement of
soil moisture to be made. The neutron probe has a radioactive source which
releases neutrons. The neutrons are emitted into the soil when the probe
is lowered into an aluminium tube which has been installed in the ground.
Whenever a neutron collides with a hydrogen atom (part of a water molecule)
it is slowed down. A detector counts the slow neutrons that have been deflected
back to the instrument. A calibration equation is used to convert this
number into a soil moisture content.
Neutron probes allow the testing of soil moisture
at several depths at the one site. This allows the operator to determine
the plant available water, but also to monitor the effective depth of irrigations.
This information minimises the loss of water and soluble nutrients below
the root zone.
As the probe is portable it can be used to measure
soil moisture at many sites.
Access to the benefits of using this device are
available through consultants who provide irrigation scheduling services
or growers may choose to purchase their own.
Soil moisture monitors
Several devices are available that monitor soil
moisture. These devices are based on capacitance (eg Enviroscan) or heat
probes (eg DRW). The advantage of these devices over other soil moisture
measurement methods is their ability to continually take soil moisture
readings at a fixed time interval and display this information later at
the operators convenience. This makes them an easy to use tool for irrigation
scheduling by growers or consultants. Other devices such as the neutron
probe and tensiometers only provide soil moisture data at the time they
are read.
As soil moisture monitors record automatically,
a separate unit needs to be dedicated to each block, making the units expensive.
Associated software is available for growers
who wish to schedule their own irrigations.
Water application
Before the decisions of when and how much water
to apply can be made properly, a grower must have the following:
- an understanding of water holding capacity
of the existing soil types, (refer to soils section)
- a reliable method of measuring moisture content
of the soil.
- a knowledge of the irrigation systems performance
including an accurate value for its application rate.
When to irrigate
Ability to accurately measure the amount of soil
water available to the plant on a regular basis is fundamental to irrigation
scheduling. Without this there is no means of determining when irrigation
is required.
Deciding when to irrigate is based on knowing
to what level soil moisture can be allowed to deplete without causing undue
water stress to the trees. This soil moisture level varies with soil type
and water demand of the crop. A refill point needs to be set above this
point. If irrigation is delayed beyond the refill point productivity may
be reduced due to crop water stress. In practice the refill point should
be set at a point well before soil moisture becomes limiting to growth,
thus giving a margin for error.
No matter which method of soil moisture measurement
is adopted the above principle is used to determine when to irrigate.
Feel
At what soil moisture content irrigation should
be applied is dependant on the soil type and growth stage of the crop.
In the four weeks prior to harvest it is critical that soil moisture is
not limiting. Irrigation should be applied soon after the soil dries beyond
75 % available water. After harvest the extent to which the soil is allowed
to dry out can be extended to around 50 % available water. In the period
from budburst until the commencement of rapid fruit development, irrigation
should be applied between 75 and 50 % available water.
Water is more easily extracted by tree roots
from sandy soils.
Tensiometers
The first step in obtaining sensible results
for proper irrigation scheduling is regular reading of the tensiometers.
Regular means at least weekly and more frequently during periods of high
water demand and especially in the 4-6 weeks leading up to harvest.
The second step is to convert the reading into
a format which is easily understood. Plotting the readings on a graph is
the most sensible option, refer to example in Figure 1. This allows decrease
in soil moisture to be followed and gives an idea of when the next irrigation
will be required, thus it can be planned for. By graphing, irregular readings
become obvious indicating that the tensiometer is not functioning correctly.
Otherwise these inaccurate readings can lead to water stress of the trees
or unneeded irrigations.
Finally decisions on when to irrigate must be
made. As a guide for a tree crop, irrigation should be carried out when
the tensiometer reading reaches 20-30 kPa on a sand and 40-50 kPa on clay
loam soil. In the context of soil water availability these values equate
to refill points. These values need to be assessed for the particular conditions
in each block. Digging some holes and comparing the moisture content of
the soil with the tensiometer readings being obtained is a worthwhile process
when learning to use tensiometers.
Figure 1.
Neutron probes
Computer software is available to process the
data which a neutron probe generates, allowing irrigations to be scheduled.
Growers can purchase this software and do their own scheduling, but due
to the initial cost and the ongoing time requirement to measure and interpret
the data, most growers are better served engaging a irrigation scheduling
consultant.
Soil monitoring devices
Most soil monitoring devices come with associated
software to interpret the soil moisture data they record. This allows growers
to schedule their own irrigations. In some cases it is beneficial to engage
a consultant for at least the first year, as this allows a grower to assess
the suitability and benefits of these devices to their situation before
committing to the capital outlay for their own device.
Quantity
The amount of irrigation to apply is the quantity
needed to wet the soil profile to the effective rooting depth of the tree
crop. This amount is expressed in millimetres of irrigation. To convert
this figure to hours of irrigation it is divided by the application rate
of the irrigation system.
Ideally, soil in the root zone should be irrigated
to field capacity. The quantity of water required is the water holding
capacity of soil in the trees root zone minus the amount of water in the
soil profile prior to commencing irrigation. Assuming the decision to irrigate
was based on accurate soil moisture measurement a reasonable estimate of
the quantity of water in the soil profile prior to irrigation should be
known. Water holding capacity of the soil profile which does not change
dramatically over time should be known from a soil survey. Methods of determining
water holding capacity are outlined in the soils section.
Irrigation water applied above that to reach
field capacity is lost below the root zone through drainage. This is wasteful
of water, leaches out nutrients and may cause water table problems. Some
drainage is required to leach salts below the root zone.
Leaching requirement
Consistently applying irrigations which do not
result in any drainage lead to a build up of salt in the trees root zone.
The requirement for leaching irrigations depends on the salinity of the
irrigation water along with the type and distribution uniformity of the
irrigation system.
Applying a leaching fraction each irrigation
to prevent salt building up in the root zone is more effective than applying
heavy irrigations at the end of the season or in winter to remove salt
that has built up.
A leaching fraction of 5 to 10 % is normally
adequate where the distribution uniformity of the orchard is above 75 %.
This means 5 to 10 percent more water is applied in each irrigation than
is required to fill the soil profile to field capacity. This degree of
accuracy can only be achieved with reliable method of irrigation scheduling.
Where irrigation scheduling and/or distribution
uniformity of the irrigation system are poor, a leaching fraction of up
to 20 percent may be necessary to prevent build-up of salts in the root
zone. This is not desirable as it increases irrigation costs, leaches nutrients,
raises the water table if present, and has larger scale environmental impacts.
Improving the irrigation system and scheduling methods used is a better
option.
Checking effectiveness
Effectiveness of irrigation, that is depth of
penetration, can be assessed by taking soil moisture readings 1-2 days
after irrigation. If there is no increase in soil moisture level towards
the bottom of the root zone then irrigation application was insufficient.
If soil moisture level 30 centimetres below the root zone increased then
too much irrigation was applied.
If a water table is present, testwells provide
the means to check for over irrigation. If irrigation causes a rise in
the water table greater than 30 centimetres then excessive irrigation was
applied and water wasted.
Recording this information allows more correct
quantities to be applied in future irrigations.