Temperature-Time Threshold irrigation scheduling (TTT) has produced consistent linear responses between time threshold value and either seasonal irrigation quantity or yield with different time thresholds and constant irrigation rates. Under constant irrigation rates, time thresholds controlled soil water at distinct levels. However, pumping capacity of irrigation wells varies from site to site and through the growing season which limits the water available per acre of cropland. Varying well capacity can be the primary limitation on the soil water regime created for crop production. A field study in 2005 evaluated TTT performance with different irrigation rates and a constant time threshold. The objectives were to (a) measure cotton response to varying irrigation application rates in response to irrigation signals generated by a constant time threshold value, and (b) estimate the upper limit of deficit irrigation production where the response between irrigation and yield is no longer linear. A subsurface drip irrigation system was controlled using daily irrigation scheduling. The treatments included application rates of 2 mm, 4 mm, 6 mm, and 8 mm (1.5, 3.0, 4.5, and 6.0 gpm/acre) with all treatments controlled by a constant time threshold of 5.5 hours. The measured system performance factors included irrigation frequency, seasonal irrigation quantity, crop water status as measured by stress time and leaf water potential, crop growth, and lint yield.