A field experiment with cotton crop, variety CNPA 8H, has been carried out in Barbalha, CE, a semi-arid site in Northeast Brazil, since August 2001. The main goal of the experiment is the determination of the water requirements of that new cotton variety. Measurements of net radiation, incident and reflected solar radiation, soil heat flux, wind speed at two levels, and dry and wet bulb thermocouples (copper-constantan) at two levels have been obtained in order to determine the energy balance components. The measurements have been obtained once at 5 seconds, and averaged over 15 minutes interval, with a datalogger CR10X of Campbell Scientific Inc., Logan UT. The latent heat flux has been obtained according to the Bowen ratio method. Additional measurements of plant height, fresh matter and leaf area have been made weekly. The experiment was installed on August, 2001, and will last till December. The preliminary results has been obtained and indicat that the actual evapotranspirations has reached 5.7 mm/day. The latent heat flux during the October month represented about 85% of the net radiation.