TIME TRENDS IN RAINFALL RECORDS IN AMAZONIA

This paper reports the results of statistical analyses for the detecti on 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 sta tistical significance of time trends, positive and negative trends occ ur with approximately equal frequencies over the Brazilian Amazon hydr ographic basin; (b) the number of statistically significant time trend s, whether positive or negative, is very much greater than can be ascr ibed to chance variation; (c) significantly negative time trends are m ore common than significantly positive time trends in monthly rainfall . Over the period of approximately 30 yr covered by the records, durin g which deforestation has been rapid, negative trends seem to have occ urred 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 re gions of negative trend (reduced rainfall) in the rainfall records, an d the contours defining regions of reduced rainfall following 50% defo restation, as predicted by the U.K. Hadley Centre for Climate Predicti on and Research. However, the rainfall records show positive trends (i ncreased rainfall, confirming the conclusion of Chu et al.) in some pa rts of the region where the Hadley Centre predicts reduced rainfall, f ollowing deforestation of 50% or more.