Technical Abstract: Improvements in the efficient use of applied irrigation water are needed particularly in semiarid regions where sources of irrigation water are limited. Canopy temperature (Tc) provides an easy to acquire indicator of crop water deficit that has been used in irrigation scheduling systems. In order to detect drought stress and schedule irrigations, the stress-time index method of irrigation scheduling accumulates the amount of time during a day that TC is above a specified temperature threshold. Our goal was to quantify the degree of drought stress in terms of leaf-level gas exchange parameters for cotton that was subsurface drip irrigated according to the stress-time index method of irrigation scheduling during two growing seasons. This was accomplished by measuring TC diurnally with hand held infrared thermometers and controlling cuvette leaf temperature TL equal to Tc and then measuring leaf level net assimilation (A) and stomatal conductance (g) at a photosynthetically active radiation level of 1500 µmols (photons) m-2 s-1. In general, leaf level gas exchange tended to decline with rising TL. However, we found that A and g could vary by more than two fold at a given TL indicating that Tc was not a particularly robust indicator of the degree of drought stress. Furthermore, we found the leaf minus air temperature differential (TL-Ta) and vapor pressure deficit calculated based on leaf temperature (VPD) were better predictors of the degree of drought stress than TL alone. Regression of A and g against TL-Ta and VPD indicated that the combination of these two variables accounted for over 79% of the variability in A and g. We conclude that the term Tc-Ta either alone or in combination with VPD should provide a better predictor of the degree of drought stress in cotton than Tc alone.