The present invention is a method of controlling an irrigation system that minimizes the use of irrigation water comprising defining a stop time for each watering event that is as late as possible to maximize the statistical contribution of predicted or actual rainfall to the proper watering of crops or turf. High rain probabilities cause deferral of the watering event. The present invention includes the embodiments of watering the largest water volume/water flow rate zone last, measuring the actual water applied and using mathematical curve-fitting techniques incorporating historical data from recent watering events to calculate actual plant/soil response to applied water for each irrigated zone.