Trend analysis of sediment flux time series from tropical river basins in India using non-parametric tests and multiscale decomposition

Abstract

This paper first analyzed the trend of annual sediment flux time series from seven major basin outlet stations in India using the non-parametric Mann–Kendall (MK) method. Then the non-linear trends in these series were extracted using the empirical mode decomposition (EMD) method. Similar procedure is followed upon annual rainfall of the basin and a comparison of trend is made with that of sediment load. Results of trend analysis study showed a reduction in sediment flux in five out of seven tropical river basins in India despite the increase in rainfall in three among them. Despite the significant reduction in rainfall, the sediment flux from Chennur station in Pennar basin showed an increasing trend, which is attributed to large flow diversion in the upper reaches of the station. The application of continuous wavelet transform (CWT) analysis confirmed that anthropogenic impact is the major driver behind the sediment flux variability of tropical river basins in India. Then for a microscopic examination of trend in monthly sediment flux data from different stations, first the change points in them were identified by the Hubert’s segmentation procedure. For each of the identified segments, the trend is examined using the MK test and EMD method. The nature of trend captured by both methods is similar for most of the cases with discrepancies for few cases. The discrete wavelet transform (DWT) analysis is applied for such segments with discrepancies following extensive sensitivity analysis on mother wavelet type and decomposition levels. The similar nature of trend by DWT as that of EMD confirmed that EMD method is successful in capturing the ‘true’ inherent non-linear trend in sediment flux series.