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 References Agrilink. 2001, Avocado Information Kit, Department of Primary Industries QLD. Charlesworth, P. 2005. Soil Water Monitoring – An Information Package. Irrigation Insights No. 1, Second Edition, http://www.lwa.gov.au Department of Agriculture and Food Western Australia. 2005, Irrigation Requirements of Avocados, Farmnote 42/1988 (reviewed July 2005), http://www.agric.wa.gov.au Department of Primary Industries and Fisheries. 2005, Growing Avocados: Before You Start, www.dpi.qld.gov.au/horticulture/4736.html Department of Primary Industries and Fisheries. 2005, Growing Avocados: Common Questions, www.dpi.qld.gov.au/horticulture/4742.html FAO56. 1998, Crop Evapotranspiration: Guidelines for Computing Crop Water Requirements, FAO. Gustafson, D. 1976, ‘Avocado water relations’, Proceedings of the First International Tropical Fruit Short Course: The Avocado, pp. 47-53. Milestone Report No. 1, Sustainable Horticultural Irrigation Project (SHIP), Bundaberg Avocado Trial. Murray Valley Citrus Board. 2005, Drip 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 practical observations in determining water need, irrigation design and frequency scheduling’, Proceedings from Joint Meeting of the Australian Avocado Growers’ Federation Inc and NZ Avocado Growers’ Association Inc, 23-26 September. Plessis, S.F. du 1991, ‘Factors important for optimal irrigation scheduling of avocado orchards’, South African Avocado Growers’ Association Yearbook, pp. 91-93. NSW Agriculture. 2002, Irrigation for Horticulture in the Mallee, NSW. NSW Agriculture. 2003, Avocado Growing, Agfact H6.1.1, www.agric.nsw.gov.au NSW Department of Primary Industries. 2004, Waterwise on the Farm Fact Sheet: Readily Available Water (RAW), Series 1 Irrigation Farm Resources, Dareton. Schaffer, B. & Andersen, P. C (ed) 1994, Handbook of Environmental Physiology of Fruit Crops Volume II Sub-tropical and Tropical Crops, CRC Press, Inc, Florida. SILO. 2007, Evapotranspiration Data, Bureau of Meteorology, Australia. Tim Cummins & Associates. 1998, Irrigation Survival Requirements: A Collation of the Best Available Information on Survival Requirements of Horticultural Crops and Dairy Enterprises During Severe Water Restrictions Lasting One Season, prepared for the Department of Natural Resources and Environment. Turner, D. W., Neuhaus, A. & Colmer, T. 2001, Turning Water into Oil – Physiology 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.
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