Abstract The manager of an irrigation project needs to know the irrigation time and depth so as to maximize economic return or minimize irrigation cost. This is particularly important for periods of peak demand. This paper describes a procedure to derive the optimal irrigation policy (depth and timing) which will minimize the total cost per unit time during each stage in the growing season of a single crop. The analysis is based on an inventory control model. First, the water depletion by the plant is derived. Then, the economic loss due to the decrease in yield caused by delaying irrigation and the cost of water application are considered. All the variables involved are considered as deterministic. A numerical example is presented to illustrate the use of the proposed method. Finally a sensitivity analysis is used to study the effect of uncertainties in the estimated quantities.