Estimation of Evapotranspiration of Different-Sized Navel-Orange Tree Orchards Using Energy Balance

Crop evapotranspiration $\left({\mathrm{ET}}_c\right)$ and crop coefficient $\left(K_{\mathit{co}}\right)$ 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 $\left(R_n\right)$ , soil heat flux density $\left(G\right)$ , and sensible heat flux density $\left(H\right)$ 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 $\left(L\right)$ to obtain ${\mathrm{ET}}_c$ . Crop coefficients were determined by calculating the ratio $K_{\mathit{co}}={\mathrm{ET}}_c∕{\mathrm{ET}}_o$ , with reference evapotranspiration $\left({\mathrm{ET}}_o\right)$ determined using the hourly Penman–Monteith equation for short canopies. The estimated $K_{\mathit{co}}$ values ranged from 0.45 to 0.93 for canopy covers having between 3.5 and 70% ground shading. The $K_{\mathit{co}}$ values were compared with $K_c$ values from FAO 24 (reported by Doorenbos and Pruitt in 1975) and FAO 56 (reported by Allen et al. in 1998) and with $K_c$ values from research papers that estimated reference ET from pan evaporation data using the FAO 24 method. The observed $K_{\mathit{co}}$ values were slightly higher than $K_c$ values for clean-cultivated orchards with high-frequency drip irrigation in Arizona and were slightly lower than for nontilled orchards in Florida. The $K_{\mathit{co}}$ values were considerably higher than $K_c$ values from FAO 24 and FAO 56 and were higher than $K_c$ values from border-irrigated orchards near Valencia, Spain.