Crop growth models are important tools to understand the effect of water deficit on crop yield or water productivity and predicting yield to optimize irrigation under limited available water for increased sustainability and profitable production. The objective of this study was to assess the effect of different level of deficit irrigation imposed on grain yield and water productivity. A field experiment was conducted to examine wheat yield response under different water application level in Koga. The experimental setups have seven randomized treatments with three replications. The water application levels considered for the crop at the field in the study were 100 %, 75 % and 50 % and full stress at different growth stages were considered. The agronomic parameters collected from the field were canopy cover, biomass and grain yield that were used to evaluate the crop yield response under different water applications. Aquacrop model was used to analyze the crop response to environmental stress and to test different alternative management practices and optimizing water use efficiency. Measured data in the field were used to evaluate model performance in simulating canopy cover (cc), biomass and yield and water productivity. The result shows that the highest yield was recorded at full (T1) irrigation (3.55 ton/ha), whereas minimum yield was obtained at T1 (1.59 ton/ha) which was subjected to water deficit during mid-season (highly affecting the yield). Skipping irrigation at development growth stage and stress 50 % of the full water demand at all growth stage has significantly affected the canopy, biomass and yield production. While irrigation at 25 % stress at late stage was not significantly affecting the yield and used to save more water for irrigating additional land. The highest (0.93 kg/ha) and the lowest (0.59 kg/ha) water productivity were found under T6 and T4 respectively. Aquacrop model was capable to provided simulation of canopy, biomass and yield and hence the model performance for all treatments resulted in R2 value for yield and biomass as 0.92, 0.81 respectively and. RMSE values for yield and biomass as 0.116 ton/ha and 1.67 ton/ha respectively. This study indicates that deficit irrigation at some stages of the crop growth can bring more water productivity and hence projects can benefit by using this saved water to irrigate more land. Aquacrop model has good capability of simulating the yield of wheat for Koga Irrigation.