RESEARCH & EVENTS
Matching irrigation to crop demand this summer
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BY THE time summer settles in, Australian almond orchards are working hard. Trees are carrying crops, canopies are full, and evapotranspiration is climbing. This is the point in the season where irrigation decisions carry the most weight. Small miscalculations, repeated over hot days, can quietly turn into yield loss, while well timed adjustments can protect both kernel weight and tree health. At the heart of good summer irrigation are two concepts that growers hear about often, evapotranspiration and crop coefficients. Together, they provide a practical way to match irrigation to crop water use, rather than relying on fixed run times. Reference evapotranspiration, or ET, describes how much water a wellwatered grass surface would use under current weather conditions. It rolls temperature, solar radiation, humidity and wind into a single number. As days get hotter, brighter and windier, ET increases. Almond trees respond to those same conditions, drawing more water through their canopies to keep cool and productive. Crop coefficients, known as Kc values, are what turn reference ET into almond water use. They adjust ET to account for canopy size and growth stage. In Australian orchards, Kc values are typically lower early and later in the season, around 0.43, and rise steadily as the canopy develops. At full canopy, Kc values can reach around 1.54. Pruning style, tree age and local microclimate all influence where a block sits within that range, which is why no two orchards behave the same. Summer is when this information becomes especially powerful. Daily ET data is readily available through local weather stations and online tools, including Bureau of Meteorology products. When growers combine daily ET with an appropriate Kc, they can calculate crop water demand for that day. This allows irrigation schedules to move with the season instead of lagging behind it. Hot spells are where ET based scheduling really earns its keep. A run of days jumping from the low 30s into
In A Nutshell- Spring 2026 Vol 27 Issue 3
the high 30s can lift daily ET sharply. If irrigation does not increase to match that demand, trees can slip into mild stress even though irrigation systems appear to be running well. The numbers make this visible. For example: a day with a reference ET of 9mm and a Kc of 0.85 equates to a crop demand of about 7.65mm. If only 6mm is applied, the shortfall is clear and actionable. Responding does not need to be complicated. Increasing the running time to lift total millimeters applied is often enough. In some situations, splitting a long irrigation into two shorter events across the day can help keep moisture more stable in the root zone and reduce afternoon stress. Checking irrigation systems before peak summer demand also ensures any extra water applied is reaching the whole block. Regulated deficit irrigation still has a place in seasons where water is tight,
but timing matters. Reducing irrigation below calculated water demand is lower risk later in summer as nuts approach maturity. During kernel fill and early nut sizing, staying close to water demand helps protect kernel weight and avoids stress carrying over into future seasons. Irrigation is no longer a set and forget task. ET and Kc provide a clear lens on daily crop demand, helping growers make small, timely adjustments that prevent hidden stress. In a season where water is precious, those adjustments can make all the difference. To support growers in making these decisions, the Almond Board of Australia hosts a range of practical irrigation tools on its Grower Resources webpage. These include a weekly Eto guide prepared by Agriculture Victoria for the Robinvale and Sunraysia region, which track how water demand shifts through the season, and the Almond Irrigation Spreadsheet. The spreadsheet helps growers budget and track irrigation using daily site-specific evaporation data and crop factors. Access the Almond Board of Australia’ s irrigation grower resources online here.