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In this study, the agro-hydrological SWAP model was employed for simulation of grain yield, biomass, evapotranspiration (ET), soil water balance components, and water productivity for maize in Shiraz, Iran. The results showed that the irrigation regime is an important factor effecting on the yield and biomass. In general, the model slightly overestimates the yield, biomass, and leaf area index (LAI) for all irrigation treatments. Statistic criteria for the evaluation of the model such as the root mean square error (RMSE) and mean error (ME) were for grain yield (772 kg ha(-1), -9.6), biomass (265.6 Mg ha(-1), -1.5), and evapotranspiration (ET) (25.1 mm, -6). The amount of water productivity for maize based on I, P, T, and ET were in the range of 1.74 to 3.22 kgm(-3). The results indicated that, in general, the SWAP model was an appropriate tool for simulation of grain yield, biomass, and ET with an acceptable precision. The optimization of irrigation management is made to determine the maximum rate of water productivity (WP). The amounts of WP for maize were in the range of 1.74 to 3.22 kgm(-3) based on irrigation (I), precipitation (P), transpiration (T), and ET. The results showed that 500 mm of irrigation depth, water productivity is the highest. Therefore, irrigation management can improve WP and thus the impact of limited water can be reduced.