The availability of reliable infrared thermometers has given impetus to research on canopy temperature as an indicator of plant writer stress. Empirical and theoretical approaches have demonstrated that the difference between plant canopy and air temperatures (T(c)-T(a)) is related to plant water status in a number of crops. During daylight periods, the relationship between (T(c)-T(a)) versus vapor pressure deficit (VPD) is linear for well-watered plants transpiring at potential rate. As soil moisture becomes progressively depleted the (T(c)-T(a)) versus VPD relationship deviates from the linear nonstressed baseline condition and this fact was used to develop a crop later stress index (CWSI). CWSI appears to be crop specific and reasonably independent of environmental variability, except for cloud cover.

Most of the research on CWSI has been conducted with plant canopies that completely cover the ground. To the semi-arid, limited irrigation production region of the Southern Great Plains, cotton usually does not achieve 100% ground cover. There are also many days during the growing season that have partly cloudy conditions due to cumulus cloud formation after midday.