Hydroclimatic factors in the eastern Amazon-Cerrado transition: Implications for coastal systems along Brazil's equatorial margin

At the transition between the Amazon and the semi-arid Northeast of Brazil, a large watershed drain into a macrotidal semi-enclosed coastal system (SECS) where freshwater inflows strongly regulate estuarine dynamics. The São Marcos Estuarine Complex (SMEC) receives runoff from three major basins (Mearim, Pindaré, and Grajaú), which differ in geomorphology and land-use evolution despite being exposed to similar regional hydroclimatic forcing. This study examines how seasonal and interannual hydroclimatic variability (1984–2022) propagates from basin-scale processes to SECS sensitivity. Daily precipitation data of rain gauge stations were homogenized and spatially interpolated, watershed and basins boundaries were derived from the TOPODATA digital elevation model, and land-use data from MapBiomas project. Hydroclimatic variability was assessed using Standardized Precipitation Index (SPI), Seasonal-Trend decomposition using Loess, flow duration curves, and trend analyses (Mann–Kendall and Sen’s slope). Cluster analysis was applied to identify spatially coherent hydroclimatic patterns among basins. Results indicate strong seasonal control of discharge but contrasting long-term trends among basins, reflecting differences in geomorphology and land-use land cover changes. Trends in extreme precipitation and runoff suggest shifts in the balance between high-flow pulses and prolonged low-flow conditions, with implications for freshwater delivery to the SMEC. Such alterations may influence estuarine dynamics in respect to water residence-time and potential pollutant exposure. By explicitly linking basin-scale hydroclimatic diagnostics to estuarine vulnerability in a macrotidal semi-enclosed system, this study advances an operational source-to-sea perspective and provides a framework that can be applied to similar systems worldwide, helping mitigate climate change impacts and protect coastal socio-economic activities. • El Niño Southern Oscillation events drives up to 30% river discharge shifts, with Pindaré River Basin showing highest variability • Since 2012, rising seasonality intensifies extremes, impacting runoff • Low flushing heightens pollutant retention, eutrophication, and seawater intrusion