CONTROL OF IRRIGATION SCHEDULING USING TEMPERATURE-TIME THRESHOLDS

Irrigation of cotton was scheduled using a minimum three-day irrigatio n cycle which was adjusted by temperature-time thresholds (TTT). These TTT were measured as the amount of time that canopy temperature excee ded 28 degrees 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 incre ase of 1 h in TTT decreased the number of irrigations by 1.3 for the p eriod day of year (DOY) 184 to DOY 243. Average canopy temperature bet ween DOY 198 and DOY 273 was 1.2 degrees C higher in the 7.5 TTT than in 2.5 TTT in 1991 and in 1992 the 8 TTT treatment was 2.2 degrees C h igher than the 2 TTT treatment (averages are based on time when air te mperature exceeded 28 degrees C and total radiation exceeded 500 W/m(2 )). The TTT treatments produced consistent differences in irrigation f requency and application amount in both years. Yields were not differe nt among the TTT treatments in either year. In 1991 Verticillium wilt, a severe infestation of aphids, and 26.6 cm of rainfall during July t hrough September reduced yields in all treatments. All irrigated yield s 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 fill soil water profile prevented significant moisture stress.