Point of View: Vineyard irrigation scheduling

With forecasts calling for a hot, dry summer, irrigation is top of mind for many managers. Ensuring vines receive enough water, without wasting any, is essential as water rights are tightened and minimum river flows for irrigation cutoffs are increased. It is therefore critical for managers to have an irrigation strategy that meets vine and production requirements, while staying within consent. To help with this, there are several monitoring tools to help schedule irrigation.

Soil probes are ubiquitous due to their ease of use and limited input requirement once set up. They measure soil moisture around them relative to field capacity at various depth increments, typically to around a metre deep. Most newer probes have telemetry, providing real-time monitoring. Probes excel at measuring soil moisture content and visualising how water moves through the soil. Understanding water movement helps inform how long an irrigation set should run, and how long the soils can hold onto the water. Most systems also allow users to set ‘stress’ thresholds, which are intended to help visualise a critical soil moisture beyond which vines begin to experience stress, allowing managers to schedule irrigation to manage water relative to such stress. This is where soil probes need a little help. To accurately set stress thresholds, probes require thoughtful placement, because they are limited to measuring the area immediately surrounding the probe. Reviewing normalized difference vegetation index (NDVI), topography, and soil maps can help determine the optimal location to install probes. Even after installation, it is still critical to calibrate any ‘stress’ threshold. Soil probes are limited to reading a set depth, so direct vine water status assessments need to be used to validate when stress occurs relative to the surface soil moisture reported. This also means that the ‘stress’ point for blocks change as they age and the vines grow, both above and below ground.

Technology that directly measures vine water status is probably the best way to assess water requirements. Because they measure vines, these provide a higher level of certainty around whether or not a block needs irrigation. The oldest method to measure vine stress is called a pressure chamber, and has been used in scientific research (and vineyards) for decades, meaning scientists have had a long time to assess stress thresholds using this tool. Leaves are removed from the vines and xylem tension is directly measured by the chamber. The drier the vines, the more tension.

Another tool co-opted from research is called a porometer, which measures stomatal water loss via a small leaf clip. During photosynthesis, leaves absorb CO2 through pores called stomata, which can open and close depending on the vine’s needs, allowing regulation of transpirational water loss when it’s dry. The stomatal openness on any given day, and thus the photosynthetic rate, is affected by light, humidity, temperature, and vine water status. Well-watered vines function with fully open stomata. However, as water becomes limiting, stomata close to reduce losses, which comes at the expense of CO2 entry, so photosynthesis also slows down. When used properly, irrigation can help ensure photosynthesis continues at a rate that matches production goals, and so that photosynthate partitioning within the vine can be better directed. An American colleague of mine almost exclusively uses porometers to schedule irrigation. He works in ultra-premium red wine vineyards that are targeting levels of water stress at specific points in the season. Porometers are excellent for this.

Dr Mark Krasnow

Measurements of xylem sapflow are also used to assess water use. As stomata on vine leaves close to conserve water, the daily transpirational flux through the vines slows, decreasing sapflow. Other technologies have in-field units that measure the canopy temperature relative to air temperature. As stomata close, leaves are less able to evaporatively cool themselves, and heat up. Sapflow and temperature sensors allow a real-time, indirect measure of stomatal conductance. Trunk tensiometers, allowing for real-time monitoring of xylem tension, are the technological improvement on the pressure chamber, but are expensive and not widely used yet. Like probes, the placement of these tools is also critical.

Given these options, what should a grower choose? The ideal choice would be to have a mixture of all options – as they each offer valuable information. However, that’s not an option for most. The real time monitors (soil probes, sapflow, canopy temperature sensors) provide easy data collection and reporting, but are fixed in place, and so are ideal for blocks that have uniform soils and grow few varieties. Probes also require ground-truthing to validate stress points. Handheld options, like the pressure chamber and porometer, allow mobility around the vineyard, but are limited to the midday measuring period, and require labour. Nothing on the market is perfect, unfortunately, and it’s horses for courses depending on your aims.

As a consultant and recovering academic, I use the pressure chamber and porometer for my irrigation recommendations and to ground-truth soil probe stress points. I am confident in their data because of their long history and because of my experience using them. However, limitations on my and my employees’ time mean that I look forward to real-time monitors I trust as much as these time-tested tools. I am currently involved in studies comparing new technologies to pressure chambers and soil probes. Watch this space for an update.

Dr Mark Krasnow runs Thoughtful Viticulture Ltd: thoughtfulviticulture.com