Modeling of probability in obtaining intensity-duration-frequency relationships of rainfall occurrence for Pelotas, RS, Brazil

ABSTRACT Based on historical series, for each locality, equations can characterize the relationship between intensity, duration and frequency of rainfall occurrence. The objective of this study was to present two equations that can describe the occurrence of intense rainfall in Pelotas, RS state, over the period 1982-2015. The two equations were denominated conventional and hybrid, depending on the probabilistic model used. Following the conventional methodology, the parameters of Normal, Log-Normal, Gumbel and Gamma probability distributions were adjusted by the maximum likelihood method for return periods of 2, 5, 10, 20, 25, 50 and 100 years. The maximum intensity values for the hybrid equation were obtained using the empirical model of Weibull, considering return periods of 2, 5, 10, 20 and 25 years. On the other hand, the same theoretical distributions used in the conventional equation were applied to return periods of 50 and 100 years. The Kolmogorov-Smirnov test was used to select the best fitting distribution for the data. In order to verify the information acquired through the Weibull empirical model in comparison to the theoretical distributions, the t-test was applied to the angular coefficients. Significant differences were not verified between the values of maximum rainfall intensities obtained using the two methodologies, for the pre-established durations and return periods. Thus, considering the maximum rainfall intensities values (durations of 5-1440 min) and return periods of 2-100 years in the municipality of Pelotas, RS, Brazil, both the hybrid and the conventional intense rainfall equations can be used. RESUMO Com base em séries históricas, para cada localidade, equações podem caracterizar a relação entre a intensidade, duração e frequência de ocorrência de precipitação pluvial. Objetivou-se apresentar duas equações para a localidade de Pelotas, RS, no período de 1982-2015, uma denominada convencional e outra híbrida, dependendo do modelo probabilístico utilizado. Na metodologia convencional foram ajustados os parâmetros das distribuições de probabilidade Normal, Log-Normal, Gumbel e Gama, pelo método da máxima verossimilhança, para os períodos de retorno de 2, 5, 10, 20, 25, 50 e 100 anos. Para a equação híbrida, os valores de intensidade máxima foram obtidos utilizando o modelo empírico de Weibull, nos períodos de retorno de 2, 5, 10, 20, 25 anos. Por outro lado, as mesmas distribuições teóricas utilizadas na equação convencional fora