Irrigation water is produced within the irrigation subprocess of a farm. Water supply is identified for effective field water, which sets the upper bound on water available for plant use. Georgescu-Roegen process analysis concepts are merged with the neoclassical theory of cost as the underlying framework. The approach is illustrated for a permanent overhead system used in a Florida citrus grove. The marginal cost for the 2.54 centimeters application depth dominates all other depths for the higher water levels. Process analysis is an important analytical tool for increasing understanding of the features of irrigation water supply.