Abstract: Based the 1956-2007 annual runoff data from four hydrological stations in the middle reaches of Yellow River from Sanmenxia to Huayuankou (Sanhuajian basin), and by using the statistical methods of trend analysis and wavelet transform, this paper studied the variation characteristics of annual runoff in the basin. There was a distinct decreasing trend of the annual runoff in the basin. On the whole, the annual runoff experienced an alternation of two wet and two dry periods, whose turn points occurred at 1969, 1982, and 1987. The time-frequency variation of the annual runoff was mainly a 25-year cycle or a 2-8-year cycle.

Key words: Chlopyrifos, Microorganism, Degradation