CLIMATE CLASSIFICATION OF THE PARNAÍBA RIVER BASIN FOR THE 1961–1990 AND 1991–2020 CLIMATOLOGICAL NORMAL PERIODS: A PIXEL-BY-PIXEL APPROACH USING REANALYSIS DATA

Abstract

The Parnaíba River Basin (PRB) is characterized by marked geoenvironmental diversity and high vulnerability to meteorological variability. Amid ongoing regional warming and the scarcity of in situ observational data, existing static climate maps have become outdated. This study aimed to update the climate classification of the PRB using the Köppen-Geiger and Thornthwaite systems, applying high-resolution reanalysis data (TerraClimate) in a pixel-by-pixel approach, and to assess climate dynamics by comparing the 1961–1990 and 1991–2020 World Meteorological Organization (WMO) climatological normal periods. Validation of reanalysis estimates against stations of the Brazilian National Institute of Meteorology (INMET) confirmed high correlation (r ≥ 0.90 for precipitation; r close to 0.90 for temperature). Köppen classification results showed that the Aw climate (tropical savanna) dominated 84.8% of the basin during 1961–1990 but retreated to 71.8% in 1991–2020, giving way to the expansion of the BSh climate (hot semi-arid), which increased from 12.6% to 25.7%. Thornthwaite classification revealed even more pronounced changes: the D class (semi-arid) expanded dramatically from 16.9% to 40.0%, humid classes (B1 and B2) were nearly extinguished, and the E class (arid) emerged in the recent period, covering 2.8% of the basin. Comparison with previous mappings demonstrated that approaches based on outdated static averages, sparse station networks, or region-specific empirical methods tend to underestimate the magnitude of current drying or introduce systematic biases in aridity estimation. These findings indicate that the PRB is undergoing an intense and structural aridification process. The use of continuous reanalysis data proved to be a robust methodological alternative, providing an up-to-date territorial basis essential for the formulation of water security policies and agroclimatic zoning in northeastern Brazil.