Quantifying evapotranspiration and crop coefficients of California 'Hass' avocado affected by various environmental and plant factors

In California, avocados are primarily grown in southern and central parts of the state along the coast. These regions face uncertain water supplies, mandatory reductions of water use, and the rising cost of water. While efficient use of irrigation water is one of the highest conservation priorities, there is currently a lack of accurate information on crop water use and optimal irrigation strategies hindering the achievement of resource-efficient and profitable avocado production. This study aimed at acquiring relevant information on evapotranspiration and developing more precise and representative crop coefficient values for ‘Hass’ avocados under different environments and cropping systems in California. Extensive data collection was conducted at five avocado sites located in major avocado production regions over a three-year period using cutting-edge ground- and remote-sensing technologies. A combination of eddy covariance and surface renewal techniques were utilized to measure actual crop evapotranspiration (ET_{c act}), which were used to develop actual crop coefficient (K_{c act}) curves that were affected by various climate, crop canopy, slope, elevation, salinity, and soil management features. The results clearly illustrated that the water consumption of avocado orchards is affected by site-specific conditions. Across the experimental sites and study seasons, the seasonal ET_{c act} totals fell within the range of 713 mm to 1028 mm. Considerable variability was found in crop coefficient values of avocado sites, both spatially and temporally. The findings demonstrated greater K_{c act} values during the flower bud development, and flowering through fruit set growth phases than the fruit development phase, ranging from an average of 0.7–0.85 over the season at the site with the highest values. Daily mean ET_{c act} in spring and summer was 3.4- and 3.8-mm d⁻¹, respectively, whilst winter and fall had a similar daily mean ET_{c act} (2.0 mm d⁻¹) at this site. In contrast, the range of K_{c act} was 0.55–0.73 at the avocado site with the lowest values where the coastal climate reduced ET_{c act} relative to standardized reference ET_o. The information developed by this study enables farmers to determine the water needs of avocado orchards in a more reliable, usable, and affordable format, and it assists local water districts with their water delivery and conservation programs.