From Daily Fluctuations to Annual Hydrological Cycles: A Wavelet-Based Analysis of Nonstationary Seasonality in Senegal River Hydropower Inflows

Abstract

This study presents a reproducible framework combining Fourier and wavelet analysis to examine the seasonality of daily inflows at three sites on the Senegal River (Bafing Makana, Felou, Gouina), based on 65,631 daily observations spanning nearly 60 years (1961-2020). Using harmonic regression, Welch spectral analysis, stationary wavelet decomposition, Morlet continuous wavelet transforms, trend and change-point tests, and cross-wavelet coherence, the authors show that the annual cycle corresponds mainly to the D8 detail level (roughly 256-512 days), rather than a level-4 approximation. The Haar wavelet was selected as the best fit among four families tested. The D8 component accounts for 18.85-19.89% of total energy, while Fourier harmonic models explain 73.3-78.7% of daily inflow variability. The average seasonal peak occurs in September (around days 252-254), and annual coherence across the three sites is exceptionally high (0.988-0.999), indicating a highly synchronized regional hydrological cycle. No robust monotonic trend was found in seasonality strength, amplitude, or peak timing. However, Pettitt tests detected a statistically significant shift in amplitude or mean inflow around 1971-1976, consistent with a regional regime transition, though confirming this would require independent rainfall and dam-operation data. The framework offers a transparent basis for generating inflow scenarios within MOSSHOOS-Plan4RES and is transferable to other data-scarce hydropower systems.