A New Approach for Trend Assessment of Annual Streamflows: a Case Study of Hydropower Plants in Serbia

Abstract

The authors propose a new approach for trend assessment that takes into account long-term periodicity of annual flows. In particular, analysis is performed of annual flows recorded at the locations of 30 operating and designed hydropower plants (HPPs) in Serbia, in order to assess the current and future water availability for hydropower generation. The composite annual trend is determined by sliding a fixed time window of 30 years along the observed time series with a one-year time step. Such a linear moving window (LMW) approach enables the identification of the flow trend as a median of all values for each time step. Significant trend harmonics are determined using discrete spectral analysis. The results show an alternation of upward and downward trend phases of different durations, namely: 6787, 33-43 and 21-29 years. On the other hand, the results of the Mann-Kendall test indicate a monotonic downward trend at the studied sites in the Drina River Basin, while statistically insignificant trends are noted at other river basins. The Mann-Kendall test with the Theil-Sen estimator also implies a downward and statistically insignificant flow trend after the observed period, whereas the LMW approach indicates a probable trend increase at all the examined sites. The proposed approach can be used to predict annual flows in order to establish long-term water management plans at hydropower plants.