Impactos dos eventos El Niño na precipitação da bacia do rio São Francisco.

Abstract

The São Francisco river basin is of vital importance for the socio-economic development of the Northeast Brazil regarding generation and hydro-electric power the agricultural lands irrigationin in the river valley. In addition to the above attributions of the river, the transposition of its waters to the states of Pernambuco, Paraíba and Rio Grande do Norte is also of immense importance. Some experts feel that this is a controversial matter and when the transposition of the waters occur, it may effect the existing demands. It is very well known that the phenomena of El Nino and La Nina have their effects on the total volume of the São Francisco river waters. The objective of this research is to verify and analyze the possible effects of these phenomena on the precipitation of the river basin. In other words, investigation of the relation ship (positive or negative) between the phenomena of El Nino - La Nina and the precipitation of the hydrographic basin is the main objective of this study. For this purpose, the correlations were made between the indices of the sea surface temperature anomalies and the monthly total precipitation over the river basin. 237 rainfall stations distributed over the area were used in this study. These stations were selected on the basis of the availability of data for a maximum period, with a minimum of the missing data. It was verified that, when the El Nino phenomenon occured in the area of Nino 1+2, there was an increase of precipitation at the sub-regions of Alto São Francisco (ASF) and Médio São Francisco (MSF). In other words, the rainfall in these sub-regions had a direct relation with the occurrence of anomalies in the area of Nino 1+2. However, in the sub-region of Baixo São Francisco (BSF), the sign of the correlations was reverted. It evidences low precipitation indices in that sub-region during the years of occurrence of the El Nino phenomenon. On the average, 86% of the correlation coefficients in this study were not statistically significant at a=0,05. At the significance level of a=0,10, the percentage rejection free to 48%. It is worth mentioning that the physical significance of the correlations is more important than their absolute values.