Field experiments were carried out at the EMBRAPA, Barbalha, CE, during the growing seasons of 2003 and 2005 with the objective of estimating the evapotranspiration and crop coefficient of irrigated BRS-200 Marrom cotton cultivar. For this sensors of solar radiation, net radiation, dry and wet bulbs air temperature and wind speed at the 0.30 and 1.50 m levels above crop canopy and soil heat flux were installed and data were collected by a data acquisition system. Crop (ETc) and reference (ETo) evapotranspiration were estimated by the Bowen ratio and Penman-Monteith methods, respectively, while the crop coefficient (Kc) was estimated by the ratio of ETc to ETo. The ETc changed throughout the phonological cycle of the cotton crop, with mean values of 3.8 mm d-1 in the period from emergence to 10% of soil cover (phase I), 5.0 mm d-1 in the period of vegetative growth (phase II), 5.9 mm d-1 in the period of reproductive development (phase III) and 5.4 mm d-1 in the maturation period (phase IV). The Kc may be estimated as a function of the days after emergence (DAE) as: Kc = -0.00006 DAE² + 0.009 DAE + 0.632.
Gossypium hirsutum L.; irrigation; crop water use; energy balance
