This paper is an attempt to analyze the hydrological phenomena’s non-linearity, randomicity, credibility and similarity. In the same season each year, the flood hydrograph is self-similar and exhibits fractal characteristics. It is an argument about using fractal theory. This paper presents two new approaches to dividing flood sub-seasons by using the fractal theory. Two approaches to calculating fractal dimensions are both the magnitude measurement in the temporal scale and the similarity measurement in the spacial scale. This paper chooses the sample series of the daily maximum discharge in the flood season from 1960 to 2002 at the Zhanghe Reservoir. Both results of the two new approaches to dividing flood sub-seasons by using fractal theory are the same. Both results of dividing flood-periods by using the fractal theory are uniform with the traditional approach. Compared with the traditional approach, fractal approaches have quantitative and objective characteristics and are convenient in calculation, and are widely used in practice.
