Estimation of evapotranspiration of different-sized navel-orange tree orchards using energy balance

Crop evapotranspiration ETc and crop coefficient Kco values of four clean-cultivated navel-orange orchards that wereirrigated with microsprinklers, having different canopy features e.g., age, height, and canopy cover were evaluated. Half-hourly valuesof latent heat flux density were estimated as the residual of the energy balance equation using measured net radiation Rn, soil heat fluxdensity G, and sensible heat flux density H estimated using the surface renewal method. Hourly means of latent heat flux density LEwere calculated and were divided by the latent heat of vaporization L to obtain ETc. Crop coefficients were determined by calculatingthe ratio Kco=ETc/ETo, with reference evapotranspiration ETo determined using the hourly Penman–Monteith equation for shortcanopies. The estimated Kco values ranged from 0.45 to 0.93 for canopy covers having between 3.5 and 70% ground shading. The Kcovalues were compared with Kc values from FAO 24 reported by Doorenbos and Pruitt in 1975 and FAO 56 reported by Allen et al. in1998 and with Kc values from research papers that estimated reference ET from pan evaporation data using the FAO 24 method. Theobserved Kco values were slightly higher than Kc values for clean-cultivated orchards with high-frequency drip irrigation in Arizonaand were slightly lower than for nontilled orchards in Florida. The Kco values were considerably higher than Kc values from FAO 24 andFAO 56 and were higher than Kc values from border-irrigated orchards near Valencia, Spain.