written by Dr Mark O’Connell, Agriculture Victoria, Tatura

Water supply and delivery

The availability of water for irrigation is essential for stonefruit production. Supply must be reliable and typical water sources include rivers, channels, ground water, and farm dams. Strategic irrigation management offers growers the ability to maintain yield and fruit quality under water limited conditions (e.g. low irrigation allocation seasons).

Water application

Modern water application methods include drip and micro-sprinkler irrigation. Drip irrigation is considered the most water efficient application method as less water is evaporated from the surface wetted area of the soil to the atmosphere (known as soil evaporation). An irrigation designer is the best option to determine irrigation system design and infrastructure (e.g. pump, filter, control station, submains, emitter rate) requirements.

Scheduling irrigation requires decisions on when and how much to irrigate. Scheduling water application to crop needs is critical to manage irrigation efficiently and achieve high production outcomes. Crop water requirement describes the total amount of water supplied to match maximum transpiration of a crop. Water supply to crops is predominantly derived from irrigation inputs and supplemented from soil storage (rootzone) via rainfall.

For stonefruit crops, approximately 7 megalitres/ha of irrigation is required each season. However, more irrigation is required for long-season cultivars, large canopy sizes and drier climates. Typically, where water supply is ‘on demand’ (i.e. available 24/7), irrigation pumps are run overnight due to the lower cost of power. Depending on farm layout and irrigation system design, crop maturity and weather conditions, orchard blocks are irrigated on a 1­–3 day irrigation cycle determined by factors like residual stored soil water (e.g. recent rainfall) and crop growth stage.

Over- or under irrigation of crops can lead to fruit quality problems, poor yields and crop loss. However, during a growing season, the stages of fruit development (cell division, pit hardening, cell expansion) and vegetative growth have different sensitivities to water stress.

Irrigation strategies linked to fruit growth and development

Deficit irrigation can offer water savings and production benefits. In stonefruit, fruit growth and development follow a double-sigmoidal pattern. Fruit growth are divided into three key stages:

  • Stage I: cell division (rapid fruit growth),
  • Stage II: pit hardening (slow fruit growth, maximum vegetative growth) and,
  • Stage III: cell expansion (rapid fruit growth)

When these different stages occur during the season varies with cultivar. The best approach to determine these growth stages is to monitor fruit size on a weekly basis by measurements of fruit diameter from shuck fall to harvest. The onset of stone pit hardening can be easily monitored by destructive measurements using a knife to detect thickening and lignification of the fruit endocarp.

Regulated Deficit Irrigation (RDI), can provide control on the level of vegetative (shoot) growth without negative effects of water stress on fruit yield or fruit size. RDI typically involves reduced irrigation inputs during stage II of fruit growth (commencement of stone pit hardening), where fruit growth occurs at a slow rate and vegetative growth is at a maximum. Water savings of approximately 30% are achievable under RDI depending on cultivar (e.g. early v. late maturity). Figure 1 outlines fruit growth during each fruit growth stage and recommended irrigation strategy to maintain yield and fruit quality using RDI.

Recent research conducted by Agriculture Victoria at Tatura has shown fruit quality in nectarine is impacted by deficit irrigation management depending on fruit growth stage. Findings from project SF17006 has shown that moderate deficit irrigation (40% of crop water requirements) confirmed earlier RDI studies whereby there is no effect on yield and fruit size when deficit irrigation is applied from the commencement of pit hardening (Stage II of fruit growth). The new research further demonstrated that moderate deficit irrigation (Stage II at 40% of crop water requirements) maintained key fruit quality attributes (sweetness, maturity, firmness, red skin colour). However, deficit irrigation below the recommend level of 30% during Stage II, and/or during Stage I and Stage III periods of fruit growth resulted in reduced fruit size and low yield.

Deficit irrigation can also be applied during the post-harvest period. Excessive irrigation especially after harvest, can result in considerable shoot growth and should be kept in check to maintain fruitfulness and even cropping within the canopy. Irrigation during the post-harvest period at levels to replace 30% of orchard water use is recommended.

Figure 1. Fruit diameter and corresponding irrigation strategies (full and deficit irrigation) during key fruit growth stages for stonefruit to maintain yield and fruit quality (Case study: ‘September Bright’ nectarine, Tatura, Victoria).
Monitoring soil moisture, weather and irrigation system performance

Monitoring the soil moisture levels has traditionally been used to determine when and how much irrigation to apply. Another irrigation scheduling option is to use a weather-based approach where evapotranspiration (calculated from relative humidity, air temperature, wind and radiation data) and a crop coefficient are used.

For the soil-based method to schedule irrigation, sensors include tensiometers (these require frequent monitoring and maintenance) to more expensive logging equipment technologies (e.g. EnvironSCAN, Aquaflex). Further advice on sensing soil moisture can be obtained from your irrigation specialist.

The weather-based irrigation scheduling option uses evapotranspiration data. This is an estimate of the water use by crops and will indicate the amount of water consumed over a period. Reference crop evapotranspiration (ETo) is a grass reference water use.  Daily ETo data are available from some local weather stations, published in local newspapers or available electronically (e.g. on-line, smartphone APPs, http://www.bom.gov.au/watl/eto/). The irrigation water requirements of an orchard requires ETo data to be converted by a crop coefficient to account for the species and tree canopy size.

Monitoring crop water stress is important to assist in deciding when to irrigate. Nowadays, smart sensors can measure crop water stress using plant-based detection of fruit growth, trunk growth and canopy temperature. Further advice on sensing crop water stress can be obtained from your irrigation specialist.

Monitoring of an irrigation system performance is important so that leaks and blockages can be detected. System maintenance is a low-cost strategy that can potentially save water (up to 25%)

with no impact on yield. Simple procedures of cleaning filters, system chlorination, flushing

of pipes and emitter replacement can improve the distribution uniformity of an irrigation system. The emitter rate uniformity of the water application can be checked in irrigation laterals every few seasons to detect blockages and worn dripper/nozzles. New technology for irrigation system diagnostics via smartphone alerts and notifications of water meter flow rate (e.g. low, high, nil) offer growers the capacity to detect real-time leaks and pump (power) outages. Irrigation management courses provide growers the basics of irrigation application and system maintenance (e.g. http://www.irrigationaustralia.com.au/training).

Acknowledgements

Dr Mark O’Connell, Agriculture Victoria, Tatura

This publication was developed under the project: ‘SF17006 Summerfruit Orchard – Phase II’, funded by Hort Innovation using Summerfruit levy and funds from the Australian Government with co-investment from Agriculture Victoria.