Determination of Trends and Dominant Modes in 7-Day Annual Minimum Flows: Additive Wavelet Transform–Based Approach

Abstract

The main purpose of this study is to define the most effective periodic components (modes) that produce the trends in 7-day annual minimum flows (AM7) in two important river basins in Turkey, namely, Euphrates and Çoruh. To achieve this aim, additive wavelet transform (AWT) in conjunction with the Mann-Kendall (MK) and the sequential MK tests is applied for the first time. According to the MK test, significant decreasing trends were detected for the AM7 series of the stations located in the middle and lower parts of the Euphrates Basin and in the upper part of the Çoruh Basin. Moreover, the MK test showed significant increasing trends for the AM7 series at the stations located in upper part of the Euphrates Basin. No significant trend was detected for the AM7 series of the stations in the lower and middle parts of the Çoruh Basin. Moreover, the sequential MK test applied to the AM7 series showed that the detected decreasing trends become significant after 1988, whereas the increasing trends become significant after 27, and these significant trends continue during the present time. Then, by using AWT, the original AM7 series at each station were decomposed into their approximation and wavelet subbands, representing 2, 4, 8, and 16 yearly time series. Furthermore, by using AWT coupled with the MK and sequential MK tests, it was found that the 2- and 4-year periodic components are the most dominant modes that produce trends in the original data series of the Euphrates and Çoruh Basins, respectively. The results demonstrated that the AWT-based trend approach can be successively applied for defining the dominant modes in the AM7 series and effectively used in any hydroclimatic time series cases.