Developing scenarios to assess sunflower and soybean yield under different sowing dates and water regimes in the Bekaa valley (Lebanon): Simulations with Aquacrop

In a semi-arid environment, the main challenge for crop production is waterlimitation in space and in time. Considered as appropriate tools, models are used toevaluate the effects of water deficit on crop productivity for better irrigationplanning and sustainable yield. The AquaCrop model was tested using datacollected during a 4-year experiment on soybean (Glycine max L. Merril) andsunflower (Helianthus annuus L.) in the Bekaa Valley of Lebanon. The model wasfound to accurately simulate final crop biomass, yield and cumulativeevapotranspiration: in fact the Wilmot index of agreement (IoA) values were0.97, 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 ofirrigation scenarios showed that the early planting of sunflower could demonstratea greater efficiency than late sowing. In addition, applying three irrigations, of100 mm each, prior to flowering, at mid flowering stage and at the beginning ofseed 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 onesingle irrigation was performed only at the beginning of seed formation, whilehighest values (ranging between 1.97 and 1.74 t ha-1) were obtained when it wasdone prior to flowering. The highest yields and crop water productivity for soybeanwere 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. Soybeanyield values ranged between 3.16 and 2.01 t ha-1, while CWP values varied from1.05 to 0.67 kg m-3. However, irrigating at seed enlargement and mature seeds, aswell as applying only one irrigation of 100 mm at any growth stage, could lead tovery low yields and CWP.