Under or over? Morning or night? Before or during a heatwave?
When to water, and how, has long been the million-dollar question for grapegrowers.
Now, a new study has found some answers.
Heat stress in vineyards is increasingly an issue, as growers battle longer and more severe heat waves brought on by climate change.
However, until now, very few studies have investigated the efficacy of water-based evaporative cooling in vineyards and the effects of cooler, or warmer, grapevines on wine quality.
‘Overhead sprinklers are the current standard practice for inland vineyards. There’s no doubt they’re effective, but they use large amounts of water – much of which is lost through evaporation', explained Dr Vinay Pagay, a lecturer in viticulture and vineyard engineering at the University of Adelaide.
Dr Pagay wanted to evaluate similar, but more efficient, approaches of water-based cooling.
‘Specifically, we wanted to evaluate in-canopy misting and undervine sprinklers, and compare them to additional water applications prior to and during heatwaves.’
In a study funded by Wine Australia, Dr Pagay and his team investigated the different irrigation methods on Sauvignon Blanc vines in the Riverland and Cabernet Sauvignon vines in Adelaide over the 2016–17 and 2017–18 growing seasons.
Grapevine canopies were cooled using water-based evaporative cooling either in the canopy with fine mist applications when temperatures exceeded 35 °C; or with under canopy sprinklers or supplemental drip irrigation during warm nights preceding heatwaves.
The study found:
- Canopy temperatures were 5 °C cooler with lower vapour pressure deficits than ambient in all cooling treatments.
- Cooling resulted in improved vine water status, physiological performance (gas exchange), and yield in many of these treatments. This was especially evident with under canopy sprinklers on Sauvignon Blanc grapevines on the Riverland.
- During heatwaves, supplementary drip irrigation – used in combination with usual irrigation – resulted in in-vine and mid-row temperatures 2–7 °C below those in control vines.
‘Importantly, vines receiving supplemental drip irrigation and under canopy sprinklers consistently had superior water status and gas exchange than controls – and maintained these levels irrespective of prevailing environmental conditions,’ Dr Pagay said.
Supplemental drip irrigation vines also had the highest yields and crop loads in both seasons.
Interestingly, the Riverland site tended to be more responsive to the cooling treatments. This was put down to warmer and longer heatwaves in the Riverland; the fact that the Riverland vines were planted on bare floor (compared to the Adelaide vines that were organically managed with a permanent fescue on the floor); and differing vine size and crop loads.
During the study, Dr Pagay and his team fulfilled their goal of developing an automated system to trigger and control a mister-based cooling system to regulate canopy temperature to 35°C or below. The model is now fully deployable on a portable Raspberry Pi microcontroller.
His message to growers as a result of the study’s findings is to keep the water up – particularly ahead of a heatwave.
‘If growers have under canopy sprinklers, I’d advise them as a first line option. They are not only effective in reducing temperature and humidity – and thus cooling the grapes – but they are also practical and easy to use.’
‘Overhead misters are great at reducing temperatures, but they are cumbersome and have to be moved around during harvest. They also need low-saline water to avoid leaf scorching. Use them if you don’t have under canopy sprinklers – but at night to minimise evaporative loss.’
If growers had neither of the above irrigation solutions, Dr Pagay recommended they just put on the taps, at least one day – but preferably two – before a heatwave.
The project's final report is available here.