A BSTRACT . Upland cotton ( Gossypium hirsutum L.) is increasingly being managed under subsurface drip irrigation (SDI) in the semiarid Texas High Plains (THP). Scheduling irrigation using a thermally based crop water stress index (CWSI) has the potential to overcome limitations of soil water-based methods and facilitate efficient use of water. The objectives of this study were to develop a CWSI calculation procedure using stability correction, determine an appropriate window for averaging CWSI during daytime hours, compare CWSI with soil water-based indicators of crop water stress, and evaluate methods for estimating crop evapotranspiration (ET) using measured canopy temperatures. Infrared thermometer measurements of canopy temperature and corresponding cotton water use derived from soil water measurements were acquired under two irrigation levels (100% and 33%) under SDI during three growing seasons. A theoretical approach was used to calculate the CWSI using the Businger-Dyer stability correction. Basing daily CWSI on short 1- or 2-h periods in the afternoon overestimated crop water stress. In contrast, CWSI averaged during daytime periods with 0.25-h solar radiation and air temperature exceeding 0.3 MJ m-2 and 24 degrees C, respectively, was strongly correlated (R2=0.76) 2 =0.76) with the soil water depletion-based stress coefficient KS S with a plausible lower canopy resistance of 25 s m-1.-1 . The CWSI was weakly correlated (R2=0.48) 2 =0.48) to the fraction of plant available water, with the CWSI exhibiting the greater increase of these two indices in response to irrigation events. Using the proposed CWSI scaling at different locations for canopies with dissimilar cover fractions yielded satisfactory estimates of crop ET (RMSE=0.74 mm d-1)-1 ) compared with soil water balance-calculated ET (ETswb). swb ). When summed over weekly intervals, energy balance predictions of 0.25-h crop transpiration inferred from canopy temperature measurements had an acceptable agreement with ET swb (RMSE=0.92 mm d-1).-1 ). Employing the CWSI to trigger irrigation will require targeted field studies to evaluate thresholds and strategies to optimize water management.