Improvements in the efficiency of the use of applied irrigation water are needed particularly in semiarid regions where sources of irrigation water are limited. Canopy temperature (T_c) provides an easy to acquire indicator of crop drought stress that has been used in irrigation scheduling systems. In order to quantify the degree of drought stress and schedule irrigations, the stress-time index method of irrigation scheduling accumulates the amount of time during a day that T_c 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 a cotton crop that was subsurface drip irrigated according to the stress-time index method of irrigation scheduling. In this experiment, diurnal measurements of leaf gas exchange were grouped into “Wet” or “Dry” data sets, respectively, depending on whether or not the crop had been irrigated the previous day. At leaf temperatures (T_L) near 28°C, leaf net assimilation (A) and stomatal conductance (g) were reduced by 2 fold and 3 to 5 fold, respectively, in the Dry compared with the Wet data sets. Water use efficiency was higher in the Dry compared with the Wet data set. The ratio of leaf internal to external CO₂ concentration (C_i/C_a) decreased with increasing T_L for the Wet data set while the reverse trend was found for the Dry data set. These opposite trends in C_i/C_a may point to different stomatal vs. non-stomatal mechanisms being responsible for the decline in A and g as both drought stress and T_L increased. Differences between Wet and Dry data sets in A and g trends across common ranges of T_L indicate that T_c alone did not provide a unique and unambiguous measure of the level of drought stress as quantified by leaf-level gas exchange measurements.