Climate adaptation: developing irrigation strategies to combat dry winters

Abstract

Reduced winter rainfall is expected as a consequence of climate change. Low rainfall during winter, resulting in a soil profile that is not full by spring, has been shown to reduce grapevine canopy growth and yield. Building on previous work, irrigation strategies were evaluated that modified the pattern and timing of supplementary soil wetting in winter. Multiple laterals of drippers were used, and the irrigation period extended from winter through bud burst or fruit set. All irrigation treatments (single and multiple laterals, and application at budburst or through until fruit set) yielded significantly higher than the Control. Maintaining soil moisture during winter through irrigation or rainfall is critical, but the extension of this irrigation into spring maximised vineyard productivity.  

Summary

In many Australian wine regions, grapevine production relies on soil moisture stored during the winter in addition to supplementary irrigation during the growing season. Observed declines in winter rainfall and pressure on water resources for irrigation will place increasing strain on these production systems. Observations of seasonal variation and the results of a previous project (SAR 1302) have demonstrated the negative effects of dry soil in spring on vine performance, and the absence of effective irrigation strategies that fully restore yield. In the earlier project even when the soil moisture was maintained during winter using irrigation (drip or sprinkler), yield was reduced compared to the Control vines exposed to winter rainfall. Filling up an empty soil profile at budburst (as opposed to attempting to maintain it throughout winter) resulted in the lowest yield and excessive canopy growth, which impacted negatively on wine phenolic composition and sensory attributes. 

This project aimed to build on the previous work and increase vineyard resilience by developing irrigation strategies to maintain vineyard productivity following dry winters. During the three seasons, from 2018 to 2021, we explored irrigation strategies that aimed to restore vine performance to a similar level to the vines exposed to long-term average patterns of winter and spring rainfall (Control); a goal not attained in the previous project. The best performing (closest yield to the Control) treatment from the previous project was irrigation with micro-sprinklers under the canopy that simulated the pattern of soil wetting of rainfall. However, the majority of Australian vineyards are irrigated using drip-based systems; therefore, one focus of this current project was to evaluate drip-irrigation methods that wet the mid-row space to supplement low winter rainfall, with the aim to maintain yield following dry winters and springs. The second focus was to extend irrigation beyond the start of spring (budburst). As in the previous project, elevated yields (similar and higher than the Control treatments) were observed when the vines received significant rainfall during spring.

An experimental trial was designed and implemented that successfully replicated winter drought conditions in a Shiraz vineyard at Nuriootpa Research Station in the Barossa Valley. Rainout shelters were deployed so that rainfall was excluded during winter, with the covers being installed in May and removed around mid-September, at the time of budburst, to avoid confounding effects associated with growing vines under the covers. Treatments were designed to replicate a winter with reduced water input due to low rainfall or irrigation, followed by additional irrigation during spring. Irrigation was applied either using a single lateral or with an altered soil wetting pattern by using multiple rows of drippers. The winter irrigation (rainfall replacement) period was extended into spring (past budburst), which was later than in the previous project. Treatments were arranged in a replicated factorial experiment combining two methods of irrigation (single and multiple laterals) and two periods of application (budburst to 10 cm shoot length or from budburst to fruit set). In each season, we measured the effect of the treatments on (i) yield, pruning mass and their components, (ii) dynamics of canopy growth, quantified as leaf area index (LAI), and root growth during the season, (iii) vine carbohydrate reserves, (iv) fruit quality parameters, and (v) wine chemical and sensory characteristics. We also assessed growing conditions as affected by the treatments on the dynamics of soil drying and wetting within the season and its implications on vine water potential.

The experiment confirmed expectations on the importance of maintaining soil moisture during winter, but it also contrasted with previous findings. Yield in the irrigated treatments, regardless of the method and timing of water application, was significantly higher than in the Control. Vines irrigated with a single lateral yielded significantly more than the Control and also more than vines irrigated with multiple laterals, indicating limited value from irrigation with multiple laterals to manage dry winters. Irrigation at budburst or from budburst to fruit set increased yield over the Control vines, with some minor differences between timing treatments. In the previous project we found that irrigation at budburst when the soil profile was empty following full exclusion of winter rainfall resulted in the lowest yield and excessive canopy growth. In this current trial we topped up the soil profile at budburst after limited irrigation had also been applied during winter, resulting in a higher yield than Control vines. These findings suggest that the timing of irrigation is less critical than previously thought, but that a minimum level of moisture is critical during winter to balance yield and canopy growth when top-up irrigation is applied from budburst. 

In this trial, both Control and irrigated vines received approx. 247 mm of water from irrigation and effective rain during dormancy and until fruit set. Control vines received 90% during dormancy and 10% from budburst to fruit set, which contrasted with the irrigated vines that received 30% during dormancy and the remaining 70% (165 mm, which represents approx. half of the seasonal water requirement) from budburst to fruit set, resulting in significant (approximately 25%) yield increases compared to the Control vines in dry seasons.

This project was completed as a collaboration between the South Australian Research and Development Institute (SARDI) and the Commonwealth Scientific and Industrial Research Organisation (CSIRO). It was supported by Australian grapegrowers and winemakers through their investment body Wine Australia, with matching funds from the Australian Government. CSIRO and SARDI are members of the Wine Innovation Cluster in Adelaide.