Crop growth models are important tools to understand the effect of deficit irrigation on crop yield and predicting yield to optimize irrigation under limited water supply for increased sustainability and profitable production. The objective of this study was to evaluate the effect of deficit irrigation on barley grain yield, canopy cover, biomass and water productivity. A field experiment was conducted to examine barley yield response under different water application and validate Aqua crop at Koga irrigation scheme. The experimental setups have six randomized treatments (T) with three replications. The water application levels for the crop at the field were considered 100 %, 75% and 50% application level in different growth stages. The agronomic parameters were collected (canopy cover, biomass and grain yield) to evaluate the crop yield response of the Aqua crop model. The canopy cover was analyzed by image tracker (LAI-2000) because AquaCrop use canopy cover than leaf area index and above ground biomass was measured by cutting the plant within 0.5 × 0.5m area and oven dried by 650c for 48hr to determine green biomass . The result showed that the highest yield was recorded at full (T1) irrigation (3.58 ton/ha), whereas minimum yield was obtained at T3 (1.58 ton/ha) which was subjected to a 50% water deficit during the whole-season (highly affecting the yield).The Stress of 50% at the mid-growth stage and stress 50 % of the full irrigation water requirement at all growth stage has significantly (α=5%) affected the canopy, biomass and yield production. While irrigation at 50 % stress at an initial and latestage and stress 25% through the whole season was not significantly affecting the yield and yield components. The highest (2.24 kg/m 3 ) and the lowest (1.7 kg/m 3 ) water productivity were found under T3 and T1 respectively. The AquaCrop model performed well in simulating the growth of biomass, canopy cover and yield of barely for most of the treatments ( R2 = 0.84 for yield and 0.81 for biomass . RMSE values for yield and biomass was 0.7 ton/ha and 0.31 ton/ha respectively. This study indicates that deficit irrigation at 25% and 50% stress through the whole crop growth period can bring more water productivity so as to projects can benefit by expanding irrigated land through saved water. Aquacrop model has a good capability of simulating the yield of barley for Koga Irrigation scheme.