The objective of this paper is to study the problem of management of irrigation water. We analyse farmers' economic rules of tactical irrigation scheduling in a certain or uncertain environment under water scarcity. The optimal plans depend on uncertain weather conditions and expectations. We develop a dynamic economic model of irrigation water applications under certain or uncertain weather conditions and in the context of a limited total quantity of water. We program this economic model, introducing the agronomic model EPIC, which is used to obtain information on the crop yield. The model is used to explain the optimal irrigation management plan of corn in the area of Toulouse (France). This model improves water management and leads to substantial reduction of water consumed. As an of the applications recommended by agronomists.