Trends and Spectral Characteristics in Hydrological Variable Series in the Negro River Basin

ABSTRACT The long‐term trends and prominent cycles in the variability of the Negro River basin hydrological data for 43 years are examined. The basin‐mean precipitation does not show any significant change due to mixed trends at individual stations. The river levels at SM Boiaçu, Cucuí and Manaus stations show a positive trend of the order of 1.0 m in 43 years. The river discharge datasets show negative trends in the Negro sub‐basin and positive trends in the Branco River sub‐basin. Besides the annual, semiannual and ENSO‐related (2–7 years) cycles, there is an interdecadal‐scale (~15 years) oscillation in the annual precipitation, which explains about 12% of the variance. But the river level and discharge data have decadal or near decadal‐scale cycles, indicating possible influence of the Pacific Decadal Oscillation and/or the Atlantic Dipole Oscillation on the hydrology of the basin. The dominating annual cycle in the basin‐mean monthly precipitation explains 31% of the variance. The wavelet analysis shows the same cycles as in the Fourier analysis. The power of the annual cycle in the water level at Manaus increased from the 1980s to the 2020s, showing a widening trend in the annual range of water level. The basin‐mean precipitation has small but significant positive correlation with the Southern Oscillation Index (SOI) and no correlation with either the Atlantic Dipole Index (ADI) or with Southern Annular Mode (SAM). In general, if the water level data at a station correlates significantly with a given climate index, the discharge dataset at the station also correlates significantly. The trends in the variables, along with the amplitude spectra obtained here, constitute the signature of the hydrometeorological variability in the Negro River basin in the 43‐year datasets.