The main objective in this work was to propose the adoption of a thermal crop coefficient by using accumulated degree-days after transplanting to estimate lettuce daily evapotranspiration (ETc) for the cultivars Grand Rapids, Regina and Great Lakes in a NFT hydroponic system under protected environment. Subsequently, the adjusted model to the observed values of Kc was used for estimating ETc during the growing period after transplanting with the following methods: Penman-Monteith parameterized by FÃO in 1998 (PMF); Penman-Monteith modified for aerodynamic characteristics in protected environment (PMAP); simplified Penman-Monteith (PMAPS); Priestley-Taylor (PT); solar radiation (RS); and simplified solar radiation (RSS). It was verified that the sigmoid model with four parameters fitted well to observed values of Kc,giving adjusted coefficients of determination of 0,9874, 0,9784 and 0,9767 for cultivars Grand Rapids, Regina and Great Lakes, respectively. As far as daily ETc estimates are concerned, the methods PMF, PMAP and RS presented better statistical performance as compared to the methods PMAPS, PT and RSS, regardless of the evaluated cultivar. However, the method PMF systematically underestimated ETc in the lettuce initial growth period.
Protected cultivation; thermal time; nutrient film technique
