Abstract Understanding the impacts of climate change on the water balance and extreme hydrological events is essential for effective water resources management and ecosystem conservation. Ecotones are particularly sensitive to climatic variability because they lie at the transition between distinct ecosystems. This study advances the understanding of hydrological and climatic processes in these environments by assessing future changes in the water balance and extreme events in the Upper Paraguaçu River Basin, a transition zone between the Cerrado, Caatinga, and Atlantic Forest biomes in northeastern Brazil. The Soil and Water Assessment Tool+ (SWAT+) was calibrated with historical climate, streamflow and remote sensing evapotranspiration data and driven by bias-corrected projections from four General Circulation Models (GCMs) for three future periods (2021–2040, 2041–2070, and 2071–2100) under two Shared Socioeconomic Pathways (SSP2-4.5 and SSP5-8.5), combined with an analysis of climate extreme indices defined by the Expert Team on Climate Change Detection and Indices (ETCCDI). Results indicate a continuous increase in temperature, with stronger warming under SSP5-8.5. Precipitation projections suggest increases in mean rainfall in the near future, followed by reductions of up to 29% toward the end of the century under SSP5-8.5. Streamflow projections closely follow precipitation patterns, with higher peak flows during the wet season and reduced baseflow during the dry season. Projections of extreme indices indicate an increase in heavy rainfall frequency (R20mm and R30mm) and intensity (RX1day), while the number of rainy days (R1mm) is projected to decrease alongside an increase in consecutive dry days (CDD). Temperature extremes (TXx and TNn) and daily temperature range (DTR) are also projected to increase through the century. The Aridity Index (AI) analysis suggests a progressive transition from a dry sub-humid environment to a predominantly semi-arid by the end of the century. These findings indicate increasing hydroclimatic vulnerability in the basin, amplified by its transitional character, leading to heterogeneous hydrological responses with potential impacts on water availability, agriculture, hydropower generation, and biodiversity, underscoring the need for proactive adaptation and mitigation planning.
SEGANTIN et al. (Tue,) studied this question.