Regulated deficit irrigation (RDI) of fruit trees in the Goulburn Valley of southeastern Australia has increased water use efficiency by approximately 60 percent with no loss in yield or substantial reductions in vegetative vigour. Original techniques to schedule RDI were based on a 12.5 percent (peach) and 20 percent (pear) replacement of US Class A pan evaporation. Subsequent research into soil moisture measurement led to a recommended soil suction of 400 kPa to trigger irrigation. To extend the application of RDI to other environments and fruit crops, practical scheduling steps have been developed. Firstly, fruit growth is measured to determine when to apply RDI. Secondly, an irrigation plan is developed to estimate irrigation run time and interval based on soil type, root distribution, wetting pattern and average daily water use. Thirdly, soil moisture sensors are installed and irrigation is applied when soil suction reaches 200 kPa. Irrigation run time is adjusted by measuring soil moisture immediately following irrigation. Finally, US Class A pan evaporation is measured or reference crop evapotranspiration is calculated to estimate irrigation interval for scheduling in later years.
Regulated deficit irrigation (RDI) was developed to improve control of vegetative vigour in high-density orchards in order to optimize fruit size, fruitfulness and fruit quality. RDI is usually applied during the period of slow fruit growth when shoot growth is rapid. However, it can also be applied after harvest in early-maturing varieties. Furthermore, RDI can generate considerable water savings. Thus, it is useful for reducing excessive vegetative vigour, and also for minimizing irrigation and nutrient loss through leaching.
Increasingly, orchards are being planted with compact, closely spaced trees. Higher density improves profitability as trees bear earlier, yields are higher, and production costs are lower (Chalmers, 1986). While the benefits of high-density orchards are well known, excessive vegetative vigour in badly managed high-density orchards can lead to shading and associated barrenness (Chalmers et al., 1981). Fruitlet retention, fruit size and fruit colour can be reduced in the current season while fruit-bud formation in the following season can be inhibited (Purohit, 1989). Therefore, when full canopy cover is reached, it is critical that excessive vegetative growth minimized.
Techniques for controlling vegetative vigour include branch manipulation, mechanical shoot and root pruning, the application of chemical growth regulators, manipulating crop load, fertilizer management, and RDI (Chalmers et al., 1984). Of these, RDI is arguably the most economical, as less water is applied with no loss in fruit size or total yield. Genetic control methods such as the use of dwarfing rootstocks will control vegetative vigour for the life of an orchard and are widely used in apple production. However, vigour management based on cultural practices ensures that trees remain inherently vigorous and are capable of rapidly filling their allotted space and producing high early yields (Chalmers et al).
Extensive research means that the effects of regulated water deficits on tree growth and development are well understood. Most studies have shown that mild water stress applied during the period of slow fruit growth controlled excessive vegetative growth while maintaining or even increasing yields. These included studies on peach (Prunus persica) (Li et al., 1989; Williamson and Coston, 1990), European pear (Pyrus communis) (Brun et al., 1985a, 1985b; Chalmers et al., 1986; Mitchell et al., 1984, 1986, 1989), Asian pear (Pyrus serotina) (Caspari et al., 1994) and apple (Malus domestica) (Irving and Drost, 1987). In addition, water stress applied after harvest reduced vegetative growth of early-maturing peach trees (Larson et al., 1988; Johnson et al., 1992). RDI applied to olives over a ten-week period following pit hardening had no adverse effect on oil production (Alegra et al., 1999). Moderate levels of water stress applied to prunes (Prunus domestica), by withholding irrigation in a deep soil during stage II of fruit growth, increased return fruit bloom, crop load, and total fruit dry matter yield (Lampinen et al., 1995).
The application of RDI improves water use efficiency (WUE). Mitchell and Chalmers (1982) found WUE, expressed as yield per unit irrigation, increased from 4.9 to 8.0 t/Ml under RDI in canning peaches that yielded 48 t/ha. Similarly, Mitchell et al. (1989) found WUE increased from 12.5 to 22 t/Ml under RDI in WBC pears that yielded approximately 90 t/ha. In the Goulburn Valley in southeastern Australia these improvements in WUE would lead to water savings of 3 Ml/ha and 2 Ml/ha for peaches and pear, respectively. Even larger water savings have been reported for peaches in China (Goodwin et al., 1998). In this case, total irrigation applied was reduced from 3.0 Ml/ha to 1.4 Ml/ha without any effect on yield. Goldhamer (1999) reported water savings of 25 percent for RDI applied to olives in California, United States of America. with no yield reduction.
Increased WUE under RDI is due largely to reductions in transpiration, which might be as much as 50 percent (Boland et al., 1993b). Reduced transpiration appears attributable to partial stomatal closure. Despite reduced transpiration, measured increases in fruit osmotic potential (Jerie et al., 1989) indicate that fruit dry weight accumulation is not impaired. This also holds for Asian pear (Behboudian et al., 1994), grapefruit (Cohen and Goell, 1988) and apple (Failla et al., 1992), and is thought to be a mechanism of adaptation to water stress (Mitchell et al., 1994).
Both the timing and level of water stress are critical to the success of RDI. These factors need to be considered in relation to what is understood of the growth and development of the species in question. In addition, it is necessary to adopt modern techniques for scheduling irrigation that allow adequate assessment of water stress in any environment. This paper describes how to determine the timing and frequency of RDI, and it presents practical scheduling techniques for estimating water application rates.
The development of RDI was not possible without first understanding patterns of tree and fruit growth. Initially, RDI experiments focused on peach and pear, and a comparison of the development of these fruits illustrates the importance of the timing of RDI application. Although patterns of growth and development may vary in other horticultural crops, the basic principle of applying RDI when fruit growth is minimal remains the same.
The growth curve of peach is double-sigmoidal with two periods of increasing growth rate. Three phases are commonly attributed to fruit growth. Stages I and III are separated by a phase of decreasing growth rate (Stage II) known as the lag phase (Chalmers and van den Ende, 1975, 1977). Changes in the relative sink strengths of the seed and pericarp govern development. Only 25 percent of total fruit growth occurs when vegetative parts are growing rapidly; the majority of fruit growth occurs in the final 6-8 weeks before harvest when vegetative growth is almost complete (Chalmers et al., 1975, 1984) (Figure 1a). This asynchronous growth of fruit and shoots reduces competition for resources at critical stages, and provides a sound basis for the application of the RDI, which relies on water stress during Stage II having a small effect on fruit growth but a significant effect on vegetative growth.
The growth of pear fruit is curvilinear with less than 20 percent occurring by midway from bloom to harvest (Mitchell, 1986). The majority of shoot growth occurs during this period of slow fruit growth (Mitchell et al., 1986). Thus, RDI is applied for the first 70-80 days after bloom. The majority of fruit growth occurs in the remaining 6-8 weeks to harvest (Figure 1b).
Figure 1
Typical shoot and fruit growth pattern for (a) peach and (b) European pear
The above generic descriptions of fruit and shoot growth of peach and pear are useful for explaining the theoretical basis for RDI and the general timing of RDI. However, to implement RDI for a particular variety requires a more accurate description of the growth periods. Stages of fruit growth for different fruit varieties can be readily determined by tagging several fruit and shoots on a tree and making weekly determinations of their circumference (or diameter) and length with a tape measure. Fruit circumference can be converted to relative volume by cubing.
Understanding of when and how to apply RDI has improved substantially over the past 20 years. Scheduling has evolved from the initial recommendations based on US Class A pan evaporation (Epan) toward measuring both soil moisture and tree responses before making management decisions. Although the original simple recommendations may still work for many orchards, the emphasis on measuring soil moisture to estimate orchard water use and tree water stress allows more precise control over vegetative vigour and fruit growth.
Under trickle irrigation, the original recommendation for scheduling RDI was to irrigate daily and calculate irrigation amount from a percent replacement of Epan. The formula used to calculate irrigation run time was:
Replacement amounts were derived from the original RDI experiments at Tatura (Mitchell et al., 1989). For peaches, the recommended replacement was 12.5 percent from flowering until the start of rapid fruit growth. From the start of rapid fruit growth to harvest, the recommended replacement was 100 percent. The start of rapid fruit growth was based on a date for different varieties, e.g. Golden Queen was mid-January. With William Bon Chretien (WBC) pears the strategy was slightly different, consisting of a period of withholding irrigation during spring until attaining a cumulative deficit of 100-125 mm of evaporation from 1 October. After this, a replacement of 20 percent Epan was used until mid-December to calculate required irrigation application. From mid-December to harvest, the recommended replacement was 100-120 percent for pears.
Adapting these recommendations to fit other irrigation systems concentrated on altering the interval between irrigations. During the RDI period, the recommended intervals were 7 days for microjets (40 litres/h/tree in 3x5 m planting) and 21 days for sprinklers (120 litres/h/tree in 6x6 m planting) (Goodwin, 1995). Applying RDI using flood irrigation was based on increasing the interval between irrigations or irrigating every second row.
The next improvement was to estimate irrigation interval for systems other than trickle. Estimates were based on the volume of water in the rootzone and average daily water use, and utilized the measurement of soil moisture to adjust the interval. Calculation of run time was essentially unchanged, although soilmoisture measurements following irrigation were recommended to adjust run time. Mitchell and Goodwin (1996) recommended a formula to calculate interval based on average daily pan evaporation:
Where:
volume of water in rootzone (litres) = width of wetted strip (m) x tree spacing (m) x 0.3 m wetting depth (m) x soil type factor ranging from 60 (sandy soils) to 80 (loams and clays) average daily water use (litres/day) = row spacing (m) x tree spacing (m) x replacement factor x average daily Epan (mm).
This method of scheduling remains well suited to the Goulburn Valley. However, it is not applicable to other soil types and climates. RDI experiments in China on peaches, with root systems up to 2.5 m deep, emphasized the need to measure soil moisture over the entire rootzone depth to trigger the initial irrigation in spring or early summer (Goodwin et al., 1998).
In conjunction with the above formulae to estimate run time and interval based on pan evaporation, recommendations to measure soil moisture were developed to ensure soil dryness was sufficient but not excessive. Measurements of rootzone soil moisture were included in the scheduling of RDI to adjust irrigation interval and run time. Recommendations were based on intensive soil suction monitoring with gypsum blocks in an RDI experiment on pears at Tatura (Goodwin et al., 1992). Under trickle irrigation, soil suction of 400 kPa at 0.1-0.25 m depth, 0.15 m from the emitter, was recommended to trigger irrigations with irrigation run time based on the above formula. Soil moisture measurements after irrigation at 0.6 m from the tree line were recommended to adjust irrigation run time.
Work undertaken on RDI of wine grapes across a range of climates and soil types (Goodwin and Jerie, 1992) highlighted the need for adjustments in soil moisture values to trigger irrigation depending on rootzone depth, soil texture and climate. Recommendations for wine grapes were as follows. In sandy soils with shallow rootzones (<0.4 m) and hot climates (e.g. average January daily evaporation >8 mm), soil suction under RDI should not exceed 100 kPa. In loam soil with intermediate rootzones (0.4-0.8 m) and mild climates (e.g. average January daily evaporation 5-8 mm), soil suction under RDI should not exceed 200 kPa. In clay soil with deep rootzones (>0.8 m) and cool climates (e.g. average January daily evaporation <5 mm), soil suction under RDI should not exceed 400 kPa.
The following is a list of necessary steps implementing RDI successfully:
- at 0.3 m and bottom of rootzone in shallow soil,
- at 0.3 m, 0.6 m and bottom of rootzone in deep soil.
During RDI period
During rapid fruit growth
An understanding of the changes in fruit and shoot growth for different varieties is critical for the timing of RDI. Water stress should be applied only during the vegetative growth period when fruit is growing slowly. Water stress must be avoided or minimized (where water is limited) during rapid fruit growth. The stages of fruit growth for a given variety can be determined by tagging several fruit and shoots and weekly measuring their circumference and length with a tape measure. Converting fruit circumference to volume [volume = 0.02 x (circumference)3] gives a true indication of fruit weight. This technique is simple and the measurements are useful for adjusting irrigations, especially where shoot growth continues despite high soil water deficits.
Root distribution is an important component for RDI scheduling because of thepotential store of available moisture in the soil. The best method for determining root distribution is to dig a pit next to an orchard tree and estimate the amount of roots in 0.2-m depth increments until the bottom of the rootzone (80 percent of roots). Root depth is important for determining the volume of water in the rootzone when the profile is wet from rainfall, and for deciding where to site soil moisture sensors.
It is critical to determine the volume of the wetted rootzone. This can be estimated from the root distribution and the wetted volume of soil. To determine the wetting volume, it is necessary to observe the wetted surface area and depth following an irrigation event.
A hole is dug to observe wetting at depth. The wetted rootzone is then estimated from the volume of roots that are wet following irrigation. The calculation in the following irrigation plan assumes that the wetting pattern is a continuous strip of soil with a wetting depth of 0.3 m. This wetted strip pattern will occur with closely spaced microjets or drippers where the wetting pattern overlaps. For other irrigation systems where the wetting patterns are separate, the wetted rootzone is calculated assuming the shape of a cylinder.
The aim of setting out a season irrigation plan for the approximate interval and run time is to provide a theoretical basis for irrigation scheduling and water budgeting. For each month of a growing season, the interval between irrigations is calculated based on the equation:
At the start of the season, the interval between irrigations is equivalent to the withholding irrigation period where the volume of water in the rootzone (i.e. stored soil moisture) can be calculated by substituting the wetted volume with the root volume:
Volume of water in rootzone (litres/tree) = Lateral root distribution width (m) x Tree spacing (m) x Root depth (m) x Deficit available water ranging from 9 percent (sandy soils) to 13 percent (loams and clays) x 1 000.
Once irrigation commences, the volume of water in the root zone is equivalent to the irrigation amount to be applied:
Volume of water in rootzone (i.e. irrigation amount) (litres/tree) = Width of wetted strip (m) x Tree spacing (m) x 0.3 m wetting depth (m) x Deficit available water ranging from 9 percent (sandy soils) to 13 percent (loams and clays) x 1 000
Run time calculations use the emitter rate per tree and the system irrigation efficiency:
To estimate average daily water use, the plan uses local long-term average USA Class A pan evaporation data and appropriate crop factors for RDI (Mitchell and Goodwin, 1996). Alternatively, it is possible to use ETo and crop coefficients (Kc) (Allen et al., 1998) and appropriate percent replacements for RDI to estimate daily water use.
RDI scheduling requires measurements of soil moisture. In shallow rootzones, soil moisture is measured at two depths (Figure 2). In deep rootzones (>0.6 m), soil moisture is measured at three depths. The aim is to dry out the soil throughout the rootzone to a minimum suction of 200 kPa by withholding irrigation (positions A, B and C). If there is no rain, the soil in the upper rootzone (positions A and B) will become much drier than the soil towards the bottom of the rootzone (position C). If the entire rootzone becomes drier than 200 kPa, stress levels on the tree will cause loss in productivity. Irrigation is necessary.
Figure 2
Position of soil moisture sensors in (a) shallow and (b) deep rootzone soils
Once irrigation commences, the objective is to maintain a moderate level of stress on the trees. This is best achieved by irrigating with less water than the usual full recommendation. Irrigations should aim to wet to 0.3 m depth (position A).
It is necessary to measure soil moisture 6-12 h after irrigation to adjust the amount of water applied in proceeding irrigations. If the soil in the top rootzone (position A) remains dry then the irrigation amount must be increased. If the soil in the mid-rootzone (position B) becomes wet immediately following irrigation then irrigation amount must be cut back.
The gypsum block is preferred over other methods of determining moisture because it measures soil water suction, which relates to the level of water stress on the trees. It is the only instrument capable of measuring soil suction in the range suitable for RDI. It is relatively inexpensive, robust to handle, and simple to install. It requires a portable hand-held meter to measure the resistance between the two electrodes embedded in the block of gypsum. The electronics in the meter convert the resistance automatically to suction. The measurement is simple: requiring the connection of the two wires to the meter and a button to be pushed to directly measure soil suction.
Alternatively, soil samples may be collected with an auger and the moisture content assessed. This is much less accurate than the gypsum block method, but may be useful to assess wetted depth and moisture below the top 0.05 m depth.
As part of an extension programme in the Goulburn Valley, sites were established on growers properties to demonstrate RDI. Growers were interested in controlling vegetative vigour in high-density orchards and saving water. One site consisted of 6-year-old Golden Queen peach trees on Tatura Trellis (van den Ende et al., 1987) irrigated with 45 litres/h microjets (one every second tree). Thirty trees (three rows each of ten trees) received normal irrigation and 30 (also three rows of ten) received the deficit irrigation. Measurements recorded to indicate WUE and vigour control included water applied, soil moisture (tensiometers and gypsum blocks), butt diameter and fruit growth (mm).
RDI was applied from the first week of November to the last week of December, to provide approximately 40 percent of evaporation; control trees received full irrigation. Soil suction was maintained between 0 and 65 kPa on the control treatment and between 0 and 200 kPa on the RDI treatment. For the remainder of the season, soil suction was maintained between 0 and 50 kPa on all of the trees.
Fruit growth was measured over the season (four fruits per tree, 120 per treatment). There was no apparent difference in fruit size between the RDI-treated trees and the controls. Tree butt size was used as an indicator of vigour. The 30 trees irrigated under the RDI strategy exhibited an overall reduction in measured butt diameter at the end of the season. The grower also noted a reduction in tree vigour, with more fruiting wood established. There was a reduction in the water applied under RDI management with a saving of 2.3 Ml/ha: total irrigation for the control was 7.9 Ml/ha, whereas that for the RDI treatment was 5.6 Ml/ha.
The demonstration site showed that RDI can generate considerable savings. Fruit size and yield were maintained, and vegetative vigour appeared to be reduced.
It is evident that root volume is an important factor in the tree growth response to RDI. Some studies have suggested that the success of RDI in controlling vigour and maintaining yield arises from both an adaptation to moderate water stress developed in a shallow soil volume (Jerie et al., 1989) and/or restricted wetted root volume (Richards and Rowe, 1977a, 1977b).
To further explore this effect, an experiment was established to determine the interaction of RDI and root volume on Golden Queen peaches (Boland et al., 1994, 2000a, 2000b). This study demonstrated that the effect of root volume was independent of the RDI water stress response. However, there are important implications for the practical application of RDI under various conditions. In the Goulburn Valley, shallow root volume assists the development of water stress under RDI. In a deep soil with an unrestricted root system, it takes considerably longer to develop water stress; under these conditions it may be necessary to physically restrict the volume of roots.
Therefore, the control of vegetative growth and establishment of RDI depends on the interaction between rainfall/evaporation, available soil volume for root exploration and the readily available water (time taken to develop water stress).
The application of RDI in a saline environment presents potential advantages and disadvantages. Management of orchards irrigated with saline water has traditionally relied on leaching to prevent accumulation of salts, in order to maintain a soil volume that will permit root development. Leaching is regarded as the key to salinity control (Hoffman and van Genutchen, 1983). Although, RDI does not provide the same degree of leaching, it does have the potential to improve salinity management, firstly by a reduction in the importation of salt, and secondly by control of the rising water table (Shalhavet, 1994).
An experiment that assessed the impact of saline irrigation when applying RDI (Boland et al., 1993a) demonstrated significant adverse effects on the productivity of peach trees, with similar results expected on other fruit trees that are generally sensitive to salinity. Therefore, while RDI may lessen the volume of drainage and applied salts, the detrimental effects on productivity would generally outweigh these benefits. Where RDI is applied in a saline environment to either save water or control vegetative vigour, it is necessary to adopt specific management strategies: strategic leaching irrigations (e.g. every five to seven irrigations), and careful monitoring of soil salinity.
Although the control of vegetative vigour in high-density orchards was the original objective of RDI, increased WUE has become a critical issue in areas where water scarcity is a problem. RDI is an ideal water saving technique. Its application and adaptation in various environments have led to improved understanding of the process, the benefits, and the requirements for adoption. Scheduling has evolved to include weather and soil-based monitoring. As a consequence, this wealth of knowledge has enabled the implementation of a practical and achievable programme for grower adoption of RDI.
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