An optimal operation model based on the evenshortage water supply hedging rules can be used to solve the timehistory allocation problem in irrigation water quantity because the water demand process is uneven and highly correlated with timehistories. The objective function of the minimum square sum of the water deficit ratio was analyzed based on the meaning of an evenshortage water supply object. The results show that the objective function cannot obtain a water supply process with an even water deficit ratio, and the water deficit ratio is proportional to water demand. Based on the fundamental form of the objective function of the minimum square sum of the water deficit ratio, and considering the constraint of the water balance, the Lagrangian function was built and a new objective function with an even water deficit ratio was derived. Considering the sensitivity extent of crops in different growth stages, the weighting objective function with an even water deficit ratio was constructed by introducing the water sensitivity coefficient. The improved objective function was validated by analyzing a variety of water process examples.