Stationarity of maximum low-flow periods duration in the right-bank area of the Upper Vistula catchment – Mann-Kendall versus Spearman test

Abstract

This study compares two methods of analyzing the stationarity of time series – the Spearman test and the Mann-Kendall test, with the formula adjusted by Hamed and Rao. Daily discharge series collected at 78 gauging stations were compared for a period of 30 years. The study area consisted of the right-bank area of the Upper Vistula River catchment. Low-flow periods were identified via the pit under threshold method (PUT). Threshold discharge values in the study were represented by Q_70%, Q_80%, and Q_90%. Maximum annual durations T_maxR,i, (i = 1, 2, ..., 30 low-flow periods) were identified for assumed threshold discharge values based on low-flow period duration series for selected gauging stations. Research on the stationarity of T_maxR series for most of the studied water gauging stations has shown an absence of a basis for the rejection of the hypothesis of a lack of a trend for peak low-flow period duration relative to time, independently of assumed threshold discharge. Most of the detected trends are decreasing trends. The lower the threshold discharge value, the larger the number of T_maxR series being non-stationary. The Spearman test detected more non-stationary series than the Mann-Kendall test for the studied gauging stations, independently of assumed threshold discharge values.