Variabilidade climática e ocorrência de cheias na zona semiárida da bacia hidrográfica do São Francisco.

Abstract

In this work extreme rainfall events and flooding downstream of the Sobradinho reservoir in the São Francisco hydrographic basin are analyzed. Meteorological and hydrological aspects of the 1979, 1985 and 2004 floods are evaluated. Special attention is given to the 1985 event. The factorial analysis technique by means of principal components was used to investigate the relative importance of the association of convective systems and extreme rainfall occurrence. The results show that a model with 4 components is adequate to represent the initial structure of the variables in a large-scale domain. The first two components explain 52.51% of the total data variability and indicate that dynamical factors were determinant in the definition of the ambient conditions analyzed. In the local analyses the first component shows that in the 1985 event the variables that have more weight are highly correlated with the low-level moisture content and the degree of atmospheric instability. The second component shows that the variables that have more weight are associated with radiative heating and cooling. The results for 2004 have an inverse pattern. RAI (Rainfall Anomaly Index) values for stations located in the sub-middle São Francisco show that droughts dominate in areas that are equally vulnerable to flooding and inundation. A higher frequency of positive RAI values in March and April indicate that the region is more vulnerable to flooding in this period. Meteorological radar images for 1985 were fundamental in the analysis of the spatial distribution, location, duration and intensity of the precipitating cells. It was verified that the location of the convective systems with respect to the São Francisco tributaries was the determining factor in the process of flood formation.