Canopy temperature characterizations of corn and cotton water status.

Water stress is a major production factor that determines crop yield. Three temperature-based stress indices were tested using corn and cotton grown under different water levels to determine how well these indices correlated with the amount of water applied and crop yield. Corn and cotton were grown in 1997-1998 field experiments using a range of irrigation levels where canopy temperatures (TC) and field weather were continuously measured. The water levels in 1997 were WL1-dryland, WL2- 1/3*ET, WL3- 2/3*ET, and WL4-1.0*ET, where ET was potential evapotranspiration. The water levels used in 1998 were WL3 and WL4 in corn and WL1 and WL4 with cotton. An average ET value of 7 mm d–1 was used as the 1.0*ET water level of each crop in 1997 and 8 mm d–1 was applied to corn in 1998. The stress time index (ST) quantifies water stress as the daily summation of time when TC exceeds the crop specific optimum temperatures (28°C for both crops). The theoretical, CWSI-T, and empirical, CWSI-TC, forms of the crop water stress index were used. CWSI-T is a physically based description of the energy-exchange processes that determine the temperature of a crop. CWSI-TC is calculated from measured TC of the crop, a well-watered crop (WL4), and a computed TCmax for a completely water stressed crop. The ST index was calculated for the Midday, Daytime, and Entire Day periods, which were determined by threshold levels of net radiation and air temperature. The ST values for the Daytime and Entire Day periods increased as water applied decreased from WL4 to WL1, and were significantly different among water levels. ST values for the Entire Day were higher than during the Daytime period only in 1998. CWSI-T and CWSI-TC values in both years declined as the quantity of water applied increased in both crops. The CWSI value (0.89) for WL1 corn was highest in 1997, which agreed with early senescence from water stress. The CWSI-TC procedure was more accurate than CWSI-T method because it restricted the stress value for well-watered treatment (WL4) to zero, and all values were within the theoretical range of 0 to 1.0. Each water stress index had a unique but common relationship with water applied and yield in both years for cotton, but each year was different for corn. These results demonstrate the usefulness of temperature based indices for characterizing crop stress conditions and their potential for managing irrigation scheduling.