Energy flux, evapotranspiration, and crop coefficients of drip-irrigated kiwifruit and citrus orchards in Southwest China

Studying crop water use and developing reliable estimation methods are essential for evapotranspiration (ET) estimation and irrigation schedule formulation. Eddy covariance system observations were conducted to investigate the seasonal variations of energy flux, evapotranspiration (ET), crop coefficients (Kc) and basal Kc (Kcb) in drip-irrigated kiwifruit (3 years) and citrus (4 years) widely planted in Southwest China. Energy partitioning showed significant seasonal and diurnal variations during the growing seasons, and most net radiation was consumed by latent heat flux, with ratios of 70.9 % in kiwifruit and 69.9 % in citrus orchard, respectively. The average ET and T were estimated at 2.04 ± 0.18 mm d⁻¹ and 1.15 ± 0.16 mm d−1 for kiwifruit, and 2.49 ± 0.25 mm d−1 and 1.18 ± 0.09 mm d−1 for citrus orchard, respectively. The derived transferable, standard Kc (Kc-standard) for kiwifruit was 0.74 ± 0.01 at the initial stage (Kc-ini), 0.99 ± 0.02 at the mid-season stage (Kc-mid), and 0.82 ± 0.01 at the late-season stage (Kc-end), while the stagewise value for Kcb-standard was 0.24 ± 0.09 (Kcb-ini), 0.58 ± 0.05 (Kcb-mid) and 0.63 ± 0.09 (Kcb-end). The derived Kc-standard values were 0.92 ±0.03, 1.01 ±0.03, and 0.95 ±0.07 at the citrus initial, mid-season, and late-season stages, while the standard citrus Kcb-ini, Kcb-mid and Kcb-end were derived at 0.59 ±0.03, 0.61 ±0.02 and 0.69 ±0.03, respectively. The locally developed daily Kc and Kcb were effectively regulated by leaf area index, growing degree days and underlying conductance in both perennial cash orchards. Seasonal rainfall and vegetation cover greatly affected orchard locally Kc due to the changes in soil surface moisture and coverage, especially in citrus orchards planted with wide rows. The calibrated Kc and Kcb can serve as valuable guidelines for assessing the actual ET and precise irrigation water management of extensively planted kiwifruit and citrus orchards in Southwest China.