The CWSI variations of a cotton crop in a semi-arid region of Northeast Brazil

A field experiment was conducted with a cotton crop during the growing season of August 1993 to January 1994, in a semi-arid region of Northeast Brazil. The objective of this research is to evaluate the diurnal and seasonal variations of Crop Water Stress Index—CWSI, based on the canopy energy balance. Canopy temperature, air temperature, net radiation, wind speed and psychrometric measurements were made regularly at 10 and 14 h, and several diurnal cycles were also obtained. It was concluded that the irrigation should be initiated when the CWSI approaches 0.3. After a second stress event the crop took 3 days to resume transpiring at normal rate. The CWSI of NSP at 14 h presented more negative values than those obtained at 10 h. The response of CWSI with respect to T c, r c/ r a and R n is evident and probably the most important source of error in CWSI determination.