M.T. Abi Saab - Lebanese Agricultural Research Institute, P.O. Box ۹۰-۱۹۶۵, Fanar, Lebanon.
R. Albrizio - National Research Council of Italy, Institute for Agricultural and Forestry Systems in the Mediterranean (CNR–ISAFOM), Via Patacca, ۸۵, ۸۰۰۵۶, Ercolano (Na), Italy.
V. Nangia - International Center for Agricultural Research in the Dry Areas, P.O. Box ۵۴۶۶, Aleppo, Syria.
F. Karam - International Center for Agricultural Research in the Dry Areas, P.O. Box ۵۴۶۶, Aleppo, Syria.

In a semi-arid environment, the main challenge for crop production is water limitation in space and in time. Considered as appropriate tools, models are used to evaluate the effects of water deficit on crop productivity for better irrigation planning and sustainable yield. The AquaCrop model was tested using data collected during a 4-year experiment on soybean (Glycine max L. Merril) and sunflower (Helianthus annuus L.) in the Bekaa Valley of Lebanon. The model was found to accurately simulate final crop biomass, yield and cumulative evapotranspiration: in fact the Wilmot index of agreement (IoA) values were 0.97, 0.96 and 0.96, respectively, for soybean and 0.93, 0.95 and 0.93, respectively, for sunflower, while the relative RMSE was 0.04, 0.05 and 0.02, respectively, for soybean and 0.04, 0.06 and 0.04, respectively, for sunflower. The analysis of irrigation scenarios showed that the early planting of sunflower could demonstrate a greater efficiency than late sowing. In addition, applying three irrigations, of 100 mm each, prior to flowering, at mid flowering stage and at the beginning of seed formation could lead to highest yields (ranging between 4.51 and 2.34 t ha-1) and crop water productivity (CWP) (ranging between 1.5 and 0.78 kg m-3). Sunflower yields were low (0.42 t ha-1 to 0.37 t ha-1) and unreliable when one single irrigation was performed only at the beginning of seed formation, while highest values (ranging between 1.97 and 1.74 t ha-1) were obtained when it was done prior to flowering. The highest yields and crop water productivity for soybean were obtained when the crop was sown in April and by applying three irrigations, of 100 mm each, at full bloom, at seed enlargement and at mature seeds. Soybean yield values ranged between 3.16 and 2.01 t ha-1, while CWP values varied from 1.05 to 0.67 kg m-3. However, irrigating at seed enlargement and mature seeds, as well as applying only one irrigation of 100 mm at any growth stage, could lead to very low yields and CWP. Keywords: Crop modelling; Deficit irrigation; Glycine max L. Merril; Helianthus annuus L.; Water productivity.