The potential benefits of irrigation cannot be achieved unless the collection, storage, transmission, and delivery of water are coordinated with the characteristics and demands of the unit command areas, i.e., the areas where irrigation water is used to increase agricultural production. This study describes the development of the Unit Command Area (UCA) Model, which addresses the twofold problem of (1) determining the aggregate water requirement for a UCA represented by multiple landowners, cropping patterns, soil types, and community water management schemes; and (2) once the water is delivered by the main system, simulating the UCA's response to the water supply. The study explores critical issues related to UCA's and to the interface between them and the main water delivery system, discusses key UCA modeling concepts, and includes a review of the relevant literature. The UCA Model is developed in detail by examining the various modeling methods and options presented in the literature. A case study of Synthabad, India, is then developed to demonstrate the application of the UCA Model and to test its sensitivity to various inputs and assumptions. The model is used, in conjunction with the case study, to investigate various water management options and the effects of water supply reliability. The key value of the UCA Model is that by holding everything constant except the parameters of interest, it allows the behavior of a UCA (in response to different water supplies, irrigation schedules, queuing system priorities, crop mixes, etc.) to be quickly and easily determined in the varying conditions of actual situations. Includes 21 tables, 31 figures, a 106-item bibliography, and three technical appendices.