Agricultural water resources are scarce in Iran, and agricultural water management is essential to overcome this limitation. Determination of the water productivity index (WP) using the methods of optimization of cropping patterns and optimal water allocation is the most effective tool in water management in the agricultural sector. In this study, a non-linear optimization model was used to calculate the maximum WP in the Qazvin plain irrigation network. The proposed model was applied to six different water supply scenarios including 100% and 90%, 80%, 70%, 60%, and 50% of full irrigation volume. The results showed that the scenario of 60% of full irrigation could increase the WP index by 8.5%, while with the implementation of this scenario, the net benefit from the sale of agricultural products was reduced to the amount of 247 × 109 Iranian Rials (IRR), which was 28% less than the existing conditions in the region. Therefore, the 60% water supply was selected as the economical and efficient scenario in the irrigation of Qazvin plain. It can be claimed that the most important index in agricultural water management is the WP index.