FAO-56 dual crop coefficient approach for assessing water use and basal crop coefficients of kiwifruit orchards

Estimating the basal crop coefficient (Kcb) in sparse crops is challenging due to the complex canopy architecture and the partitioning of transpiration and soil evaporation. This study aims to derive Kcb values for modern kiwifruit orchards by integrating observed transpiration with the FAO-56 dual crop coefficient approach. Vine transpiration (Tc obs) was measured using sap-flow sensors and the heat-pulse method, and compared with simulated transpiration (Tc act) from the FAO-56 model. The model partitions actual crop evapotranspiration (ETc act) into vine transpiration and soil evaporation through Kcb and the evaporation coefficient (Ke), respectively. The experiment was conducted on a yellow-fleshed kiwifruit cultivar (Actinidia chinensis var. chinensis ‘Zesy002’) located in Metaponto, Southern Italy (40°24’N, 16°46’E), with data collected over two growing seasons (2021-2022). The reliability of the depletion factor had been independently validated in the same orchard through parallel measurements of soil water content and relative transpiration. Calibration in 2021 minimized the difference between Tc act and Tc obs, enabling the estimation of seasonally varying Kcb values. Validation with independent data from 2022 confirmed the consistency of the calibrated parameters. The resulting Kcb values were 0.21 for the non-growing and initial seasons, 0.70 for the mid-season, and 0.21 for the end season. Across both years, the root mean square error remained below 1 mm d‑1, and the regression slope was close to 1. These findings demonstrate that field observations combined with the FAO-56 dual crop coefficient approach provide reliable estimates of Kcb for kiwifruit orchards, with direct implications for improving irrigation management in modern tree crop systems.