Regional recommendations for blueberry irrigation exist, but few studies report the actual amount of water needed. The goal of this multiyear project was to determine the water requirements for growing northern highbush blueberry (Vaccinium corymbosum) in the U.S. Pacific Northwest. Weighing lysimeters were used to measure crop evapotranspiration (ETc) of three cultivars with various ripening seasons, including ‘Duke’ (early season), ‘Top Shelf’ (mid-season), and ‘Aurora’ (late season). Each lysimeter was constructed from steel and included a large soil tank (1.5-m wide × 0.9-m long × 1.0-m deep) positioned on four hermetically sealed, shear-beam load cells, housed inside an underground chamber. Plants were spaced 0.9 × 3.0 m apart in the lysimeters (one plant/lysimeter) and surrounding field (0.4 ha) and managed using standard industry practices, including raised planting beds mulched with Douglas fir (Pseudotsuga menziesii) sawdust, grass alleyways between the beds, irrigation with two lines of drip tubing per row, and fertigation with nitrogen and other nutrients as needed. During the first year after planting, ETc totaled 94–104 mm, depending on the cultivar, or 18–19% of total potential evapotranspiration (ETo) from initiation of the measurements on 1 June to when irrigation was turned off for the season on 23 September 2023. That year, crop coefficients (Kc), calculated by dividing ETc by ETo obtained from a nearby agricultural weather station, averaged 0.11, 0.15, 0.27, and 0.41 each month from June through September, respectively. The following year, the plants were lightly cropped (0.2–0.8 kg of marketable fruit per plant), and ETc totaled 175–343 mm, or 30–59% of total ETo. At that point, Kc averaged 0.31, 0.34, 0.45, 0.61, and 0.61 each month from 14 May to 30 September, which was the entire timeframe in which the plants were irrigated. This project is ongoing and will continue until the plants reach full production. Once the study is finished, a set of seasonal Kc curves will be developed for each cultivar and uploaded to regional websites called AgriMet and AgWeatherNet, where they will be used to automatically calculate irrigation water requirements using meteorological data from local weather stations.
