The effect of soil type, fruit load and shaded area on ‘Hass’ avocado (Persea americana Mill.) water use and crop coefficients

‘Hass’ avocado tree water use was quantified within orchards located in the three main avocado growing regions of New Zealand, from 2016 to 2019. The three regions, the Bay of Plenty, the Whangarei District and the Aupouri Peninsula in the Far North District, differ in climate and soil type (allophanic, clay, and sandy soils, respectively). At each site, local meteorological conditions were monitored, avocado tree water use was quantified using heat-pulse sap flow measurement, total leaf area was obtained periodically, fruit load was recorded at harvest, and shaded area was measured in mid-summer. Reference evapotranspiration (ETo) calculated by the FAO-56 Penman-Montieth equation was highest in January at 3.8 mm (Whangarei) to 4.3 mm (Bay of Plenty) and lowest in June at 0.7 mm (Bay of Plenty) to 0.8 mm (Whangarei and Far North). The three sites had similar patterns of tree water use relative to ETo, regardless of the differences in soil type, and tree water use was highest in January at 2.7 mm d−1 and lowest in June at 1.2–1.4 mm d−1. Across the three sites, individual tree water use was related to variation in tree spacing and leaf area per tree, but on an orchard area basis was influenced more by fruit load, resulting in variation in estimated crop coefficients (Kc) among individual trees and seasons at all sites. Average monthly Kc was similar across all sites with a value of 0.60–0.65 in mid-summer, as all three orchards were considered mature with fully grown canopies and > 75% orchard shaded area. However, tree water use also increased from light to heavy fruit load. Therefore, when irrigation is applied to avocado, Kc values should be adjusted for variation in fruit load, with a range from 0.50 to 0.85 in mid-summer.