<p>In Tunisia, climate change is expected to exacerbate the prevailing water deficit. Accurate estimation of crop water consumption is essential for the effective management of water resources and the development of adaptive strategies. This study aims to characterize water stress in citrus orchards using operational stress indices and to calibrate and validate the Penman–Monteith (PM) model under varying levels of water stress. Over a five-year period, actual evapotranspiration (ET) was measured using the eddy covariance technique, capturing a wide range of water availability conditions representative of climate change scenarios in Mediterranean citrus orchards. The Bowen Ratio (BR), temperature difference (∆T) index, and Crop Water Stress Index (CWSI) were determined. While ∆T and CWSI detected non-stress and moderate stress conditions (BR &lt; 3.5), only BR was sensitive enough to detect severe water stress. Consequently, BR was used to classify the dataset into five water stress levels. The PM model was calibrated using 70% of the data and applied to both the entire dataset and subsets by stress level. The calibration coefficient was 2.85 for the full dataset and ranged from 0.92 to 4.3 when stratified. When accounting for water stress level, predicted ET (ET_PM) showed strong agreement with observed ET (ET_EC). Model validation showed ET ranging from 0.02 to 0.58&#xa0;mm·h⁻¹, with a relative RMSE below 30%. These results highlight the relevance of incorporating water stress into model calibration to improve ET prediction accuracy.</p>