Quantifying runoff contribution using wavelet–harmonic decomposition in an Amazonian tide-dominated estuary

Understanding how marine and continental forcings redistribute water-level variance along tropical estuaries remains insufficiently quantified, particularly where energetic semi-diurnal tides interact with intense rainfall. We address this problem using a multiresolution discrete wavelet framework applied to 5 years (2020–2024) of high-frequency records from three stations along the Pará River estuary on the Amazon coast. The analysis separated astronomical tidal bands, wave-related high-frequency oscillations, and low-frequency rainfall–runoff variability. Semi-diurnal forcing is dominant, explaining 78–89% of the total variance, whereas diurnal and intermediate constituents together contribute <2%, and waves remain below 0.3%, with progressive inland attenuation. Conversely, the low-frequency component amplifies landward, with runoff influence increasing from ~9% near the mouth to ~20% upstream. Strong synchronization with basin precipitation (r approaching 0.99) and short response lags (0.7–2 h) indicate tight hydrodynamic coupling. Hydrodynamic control therefore reorganizes longitudinally, with tides governing short-term variability while fluvial forcing becomes increasingly relevant inland.