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A water balance irrigation scheduling algorithm and supporting crop evapotranspiration (ET) tables have been available in a paper format for North Dakota and Minnesota irrigators for over 20 years, yet a need exists for explanation and documentation of the algorithms in a computerized format accessible to students, technical personnel, consultants, farmers, and others who do not possess a computer programming background. The objectives of this article are: 1) to present a useable spreadsheet implementation of a checkbook-type water balance algorithm for irrigation scheduling, and 2) to provide documentation sufficient to use the algorithms as teaching and learning tools. The spreadsheet uses rows for days in the growing season and columns for various date, weather, and water balance components. Use of macros was avoided for simplicity, accessibility, and to preclude user concerns about viruses. Advantages of the spreadsheet include relative ease of understanding the underlying algorithms and their implementation, ability to forecast irrigation schedules and examine other "what-if" scenarios, flexibility for note-taking and simple alterations, speed of calculation, and transferability to other regions if local ET data or algorithms are available. Disadvantages of the spreadsheet include demands on the user to collect input data and manage files. Limitations of the algorithms are discussed for situations involving possible contributions of shallow ground water to ET; slowly-drained soils; variable topography; and salinity and sodicity effects on soils, crop growth, and ET. The spreadsheet has been used in instructional and research settings to teach and manage irrigation scheduling tasks for a variety of situations.

ABSTRACT. Using a subsurface drainage system, subirrigation (SI)

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