Control of irrigation scheduling using temperature-time thresholds
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Irrigation of cotton was scheduled using a minimum three-day irrigation cycle which was adjusted by temperature-time thresholds (TTT). These TTT were measured as the amount of time that canopy temperature exceeded 28° C during one day. The TTT that were tested ranged from 2.5 to 7.0 h in 1991 and from 2 to 8 h in 1992. In both years an increase of 1 h in TTT decreased the number of irrigations by 1.3 for the period day of year (DOY) 184 to DOY 243. Average canopy temperature between DOY 198 and DOY 273 was 1.2° C higher in the 7.5 TTT than in 2.5 TTT in 1991 and in 1992 the 8 TTT treatment was 2.2° C higher than the 2 TTT treatment (averages are based on time when air temperature exceeded 28° C and total radiation exceeded 500 W/m2). The TTT treatments produced consistent differences in irrigation frequency and application amount in both years. Yields were not different among the TTT treatments in either year. In 1991 Verticillium wilt, a severe infestation of aphids, and 26.6 cm of rainfall during July through September reduced yields in all treatments. All irrigated yields were high in 1992 compared to long-term averages in spite of a late planting date. Even a small amount of irrigation in 1992 combined with a full soil water profile prevented significant moisture stress.