Accurate estimation of crop evapotranspiration (ETa) is essential for optimizing irrigation management, yield, and fruit quality in sustainable European hazelnut production, particularly under increasing evaporative demand in Mediterranean semi-arid climates. This study provides the first comprehensive field-scale quantification of daily ETa, single basal crop coefficients (Kc), and surface energy balance (SEB) components-net radiation (Rn), soil heat flux (G), sensible heat flux (H), and latent heat flux (LE)-in a commercial hazelnut orchard. An eddy covariance (EC) tower was installed to quantify diurnal SEB over three growing seasons (2016-2017, 2017-2018, and 2020-2021) in central Chile. Seasonal Kc ranged from 0.57 to 1.09, peaking in late December and February (summer). LE dominated the SEB partitioning, with mean LE/Rn ratios of 0.56 (+ 0.09), 0.52 (+0.17), and 0.61 (+0.14), respectively, for the three seasons. Maximum ETa reached 6.40 mm day(-1), with a seasonal mean of 3.26 (+1.29) mm day(-1) during the study seasons, indicating that most available energy was converted to ETa during the irrigated period. To our knowledge, this is the first field-scale study to document all SEB components and their diurnal patterns in drip-irrigated hazelnut orchards. These results provide field-based evidence supporting the use of locally derived Kc values to estimate orchard water requirements and improve irrigation water-use efficiency under FAO-based scheduling. SEB partitioning and seasonal ETa variability depend on canopy structure and fractional cover; therefore, the transferability of the reported Kc values should be evaluated across sites, irrigation regimes, and periods of extreme atmospheric demand.