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In arid zones such as the Southwestern United States, good quality irrigation water is becoming scarce and water costs have tripled in some areas over the past decade. This pattern is repeated throughout many regions in the world. To help growers better manage their irrigation water resources, researchers at the University of Arizona, Department of Agricultural and Biosystems Engineering, have developed AZSCHED, the AriZona irrigation SCHEDuling computer program. The program works on a checkbook balance principle using the Modified-Penman equation to calculate reference evapotranspiration and heat-unit-based crop coefficients. Using these methods makes the program highly adaptable to other climatic conditions. Although AZSCHED is a fairly new program, the interest in using AZSCHED in regions of the world that grow crops similar to those grown in Arizona has increased. To help meet these needs, two programs based on AZSCHED have been developed over the past three years. One program, called EGSCHED, is being tested by researchers in Egypt. Another program, called MEXSCHED, is being tested in Mexico. The paper presented here describes the basic concepts and relationships used in the AZSCHED program. Examples are given on how the program runs and the data requirements for AZSCHED. Also included is how this program was adapted for the development of both EGSCHED and MEXSCHED.