Time Trends in Rainfall Records in Amazonia

Abstract

This paper reports the results of statistical analyses for the detection of time trend in 48 rainfall records from sites in the Amazon Basin with more than 15 yr of record. Using a nonparametric test for trend in monthly rainfall, three results emerge: (a) irrespective of the statistical significance of time trends, positive and negative trends occur with approximately equal frequencies over the Brazilian Amazon hydrographic basin; (b) the number of statistically significant time trends, whether positive or negative of statistically significant time trends, whether positive or negative, is very much greater than can be ascribed to chance variation; (c) significantly negative time trends are more common than significantly positive time trends in monthly rainfall. Over the period of approximately 30 yr covered by the records, during which deforestation has been rapid, negative trends seem to have occurred more frequently in two regions of western and central Amazonia, and positive trends more frequently in eastern Amazonia. There is some qualitative agreement between the disposition of contours defining regions of negative trend (reduced rainfall) in the rainfall records, and the contours defining regions of reduced rainfall following 50% deforestation, as predicted by the U.K. Hardley Centre for Climate Prediction and Research. However, the rainfall records show positive trends (increased rainfall, confirming the conclusion of Chu et al.) in some parts of the region where the Hadley Centre reduced rainfall, following deforestation of 50% or more.