Long-term trend analysis for the runoff series in Yulin

In this study, a method based on wavelet transforms (WT) and Mann-Kendall (M-K) test for the runoff series multi-scale analysis in Yulin area is proposed. The result shows that, the runoff exhibits a statistically significant decreasing trend during the last five decades at the level of α=0.05, the Z value of the M-K test gets significantly decreased, what´s more, a significant decreasing tendency started at the early of 1980s; the periodicities locates at 16-year and 29-year scale level are seen during for each continuous spectrum, the major periodicity is located at 16-year scale level, and it can forecast that Yulin exists in low-flow period; we use discrete wavelet (DW) transform coefficients at each decomposition level for non-parametric trend test. It indicates that DW1+DW2+DW4+DW5+approximation has resulted to be the nearest to the real M-K trend values.