Abstract: A method and apparatus for using predictive evapotranspiration (ET) and precipitation data in controlling automated and semi-automated irrigation systems. In accordance with the present invention, the conventional use of ET values is modified to take into account predicted or forecast weather events. By using forecast weather data in computing ET values, a substantial savings in water use can be achieved. Past or historical meteorological data is used to make corrective adjustments to the watering schedules that have been predicted using meteorological forecasting. That is, as each day becomes past or historical, adjustments to the overall cumulative depletion of water in the soil need to be made. When an irrigation site is to be updated with a new watering schedule, actual meteorological data is first used to correct any differences between the previously forecast and actual meteorological data to the current day.