ABSTRACT

Subsurface drip irrigation systems have high initial cost, which must be recovered through some combination of higher water use efficiency and yield to increase net return. Canopy temperature (TC) was used in a field study to measure stress time (ST) in cotton, which was the daily accumulation of time when TC > 28 °C. Three levels of ST, 2.5 hr, 5.5 hr, and 7.5 hr, were used as irrigation signal criteria and referred to as time thresholds (TT) that were use to time irrigations. The purpose of the study was to measure the effect of different time thresholds values on the quantity of irrigation applied and their effect on lint yield and efficiency of water use. Daily irrigation decisions were made and 5 mm quantities were applied in response to each irrigation signal. During the entire irrigation period, the 2.5 hr TT treatment received 78 (82% of days) irrigation signals, the 5.5 hr TT generated 60 (63%) irrigation signals, and the 7.5 hr TT had 35 (37%) irrigation signals. For days when an irrigation signal occurred, average daily ST was 353 (5.9 hr), 418 (7.0 hr), and 485 (8.1 hr) min. for 2.5 hr, 5.5 hr, and 7.5 hr TT, respectively. Total irrigation applied during the growing season was 398, 313, and 201 mm, respectively to 2.5, 5.5, and 7.5 hr TT treatments. Total water applied to the treatments was 577, 492, and 380 mm through the completion of crop. Highest lint yields of 1588 kg ha^-1 and 1555 kg h^-1 produced by the 2.5 hr and 5.5 hr treatments, were not statistically different, and both were higher than the 1018 kg ha^-1 yield from the 7.5 hr TT. Dryland yield was 307 kg lint ha^-1. Irrigation water use efficiency was 50 and 51 kg lint ha^-1 cm^-1, respectively, for 5.5 hr TT and 7.5 hr TT, and both were higher than the 40 kg lint ha^-1 cm^-1 measured from 2.5 hr TT. Total water use efficiency was highest for the 5.5 hr TT (32 kg lint ha^-1 cm^-1) and slightly less for the 2.5 hr TT (28 kg lint ha^-1 cm^-1) and 7.5 hr TT (27 kg lint ha^-1 cm^-1). The 5.5 hr TT produced the most favorable results when measured by the combined standards of yield and efficient use of water.