Medeiros Pina Ximenes, Patrícia de Souza, et al. “Analysis of Drought Severity in Agreste Pernambucano Using Standardized Precipitation Index”. Sigmae, vol. 8, no. 2, July 2019, pp. 331-9, http://publicacoes.unifal-mg.edu.br/revistas/index.php/sigmae/article/view/950.