Temperature-Time Threshold irrigation scheduling (TTT) has consistently produced linear responses between time threshold value and either seasonal irrigation quantity or yield with different time thresholds and 5 mm irrigation applications. A constant irrigation rate used with different 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. Field studies in 2005 and 2006 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) estimation of the upper limit of deficit irrigation where the yield response of cotton to irrigation is no longer positive. A subsurface drip irrigation system was controlled using daily irrigation scheduling decisions. 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 time threshold of 5.5 hours. Measured system performance factors included irrigation frequency, seasonal irrigation quantity, crop water status measured as stress time, leaf water potential, crop growth, and lint yield.