PROBABILIDADE DE OCORRÊNCIA DE VALORES ESTIMADOS DE EVAPOTRANSPIRAÇÃO DE REFERÊNCIA NO SERTÃO DO CEARÁ/PROBABILITY OF OCCURRENCE OF ESTIMATED VALUES FOR REFERENCE EVAPOTRANSPIRATION IN THE BACKLANDS OF CEARÁ, BRAZIL

Due to the great variation of the meteorological elements in different regions, thus causing dispersion in the estimates of the values of reference evapotranspiration (ET^sub o^), it is necessary the study functions of probability distributions for the correct dimensioning of irrigation systems. Given the context, this work aimed to evaluate the adjustment of monthly values of ET^sub o^ using the FAO-Penman-Monteith method to the probability distributions: Normal, Log-Normal and Gamma for the municipalities Crateús, Quixeramobim and Tauá in the state of Ceará, Brazil. To verify the adherence of empirical distributions to theoretical distribution the Kolmogorov-Smirnov (KS) and Chi-square (χ^sup 2^) tests were used, as the significance level of 5%. The probability distributions Normal, Log-Normal and Gamma showed to be adequate to represent the ET^sub o^ in the municipalities of Crateús, Quixeramobim and Tauá in all months of the year, according to the KS test. It is important to highlight the easy application of the Normal distribution, as well as acquisition of its parameters.