Irrigation
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- Potential benefits
- Irrigation system prerequisites for success
- Water application
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
Irrigation system prerequisites for success
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 year's 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.
