Temperature, solar radiation, relative humidity, and wind are the parameters used to estimate water use. For a given location these parameters tend to be fairly constant from year to year for time periods greater than a week or two. This would indicate that irrigation scheduling, using long-term weather records, will generally give close approximations to using real-time weather, if the time frame is long enough. The one weather-related parameter that does vary a lot from year to year is rainfall, however, real-time rainfall amounts are used in the Woodruff method, so this is not a problem. In fact, it can be argued that Woodruff Chart users become astute observers of rainfall and effective rainfall amounts allowing them to more accurately input this most varying of the weather factors.

Dates for irrigation applications were calculated using Woodruff charts developed with historic data (30 years of data) and compared to predicted irrigation dates based on the computer program, the Arkansas Scheduler. The data set was from Mexico, MO and included maximum and minimum temperature and precipitation. The crop chosen was May 1-planted corn with a desired deficit target of 1.25 inches. Results showed that 85% of the time the date chosen to commence an irrigation using the Woodruff chart with historic weather was within ± 2 days of when the Arkansas Scheduler program, which used real-time weather, called for an irrigation.

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