Long-Term Evapotranspiration from Coastal Avocado/Citrus Orchard

Very limited citrus and avocado orchard evapotranspiration data are available, and practically no published information is available considering citrus/avocado evapotranspiration ETc on steeply sloping fields in coastal climates. Often the data available are from measurements based on limited time periods, and season-to-season orchard water use variability is unknown. The objective of this project was to obtain such long-term-field ETc data. Of particular interest was determination of the monthly variability of this orchard water use through years of drought and heavy winter rains. ETc and ETo measurements (using Bowen ratio and evaporation pan techniques, respectively) and soil-moisture monitoring (using neutron scattering methods) combined with the analysis of 12 more years of water balance data were used to determine citrus/avocado orchard water use at a commercial coastal orchard located northwest of Ventura, Calif. All three approaches indicated that mature citrus/avocado trees utilize approximately 60% of ETo. The greatest variability in water use occurred during the early irrigation season (April through June) because the orchards drew upon residual soil moisture available from winter rains. Such use reduced early irrigation season water applications to substantially less than that indicated by ETo.