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

Crop evapotranspiration ([ET.sub.c]) and crop coefficient ([K.sub.co]) values of four clean-cultivated navel-orange orchards that were irrigated with microsprinklers, having different canopy features (e.g., age, height, and canopy cover) were evaluated. Half-hourly values of latent heat flux density were estimated as the residual of the energy balance equation using measured net radiation ([R.sub.n]), soil heat flux density (G), and sensible heat flux density (H) estimated using the surface renewal method. Hourly means of latent heat flux density (LE) were calculated and were divided by the latent heat of vaporization (L) to obtain ETC. Crop coefficients were determined by calculating the ratio [K.sub.co]=[ET.sub.c]/[ET.sub.o], with reference evapotranspiration ([ET.sub.o]) determined using the hourly Penman-Monteith equation for short canopies. The estimated [K.sub.co] values ranged from 0.45 to 0.93 for canopy covers having between 3.5 and 70% ground shading. The [K.sub.co] values were compared with KC values from FAO 24 (reported by Doorenbos and Pruitt in 1975) and FAO 56 (reported by Allen et al. in 1998) and with [K.sub.c] values from research papers that estimated reference ET from pan evaporation data using the FAO 24 method. The observed [K.sub.co] values were slightly higher than [K.sub.c] values for clean-cultivated orchards with high-frequency drip irrigation in Arizona and were slightly lower than for nontilled orchards in Florida. The K,.o values were considerably higher than [K.sub.c] values from FAO 24 and FAO 56 and were higher than [K.sub.c] values from border-irrigated orchards near Valencia, Spain. CE Database subject headings: Evatranspiration; Crops; Coefficients; Irrigation.