Abstract:In this study, the Hailiutu River Basin was chosen as a typical case of a semi-arid sand and grass land area, and the methods of continuous wavelet transform, multi-temporal trend analysis, and discrete wavelet transform and Mann-Kendall test were jointly used to investigate the periods, trends, and basic structure of the trends of the annual mean streamflow in the basin during the period from 1957 to 2011. The results are as follows: (1)There is a main period change in the process of the annual mean streamflow, with a period varying from 12 to 21 years. The first main period is 17 years and the dry period will last from 2010 to 2016 or so, and then the Hailiutu River Basin will enter a wet period. (2)The streamflow of the Hailiutu River Basin exhibits a significantly downward trend, which is consistent with the overall trend of streamflow in the middle reaches of the Yellow River. (3)The results of joint utilization of the discrete wavelet transform and Mann-Kendall test, which can explain the basic structure of the trends, suggest that 8 to 16 years of periodic events account for the observed trend of the annual mean streamflow in the Hailiutu River Basin. The stronger the periods presented in the hydrologic time series are, the greater the influence that the period components exert on the trends observed in the series will be. (4)The trend magnitude and direction depend on the length of the time series and the position of the sub-series within the whole series, because of the existence of the long-term alternating dry and wet periods that probably lead the trend test toward misleading results, and the multi-temporal trend test provides a good way to solve that problem.