Abstract: By using fractal theory and method, the method of calculating antecedent effective precipitation was derived from the fractal relationship between sediment yield frequency and rainfall amount according to 264 single rainfall and sediment yield data during 1974-1989 at Xingshan hydrological station in Xiangxi river watershed. Results reveal that the relationship between cumulative frequency of sediment yield and cumulative rainfall amount in this watershed obeys the power exponential relation. They have different scaling exponents in two different ranges of the rainfall amount. The rainfall amount corresponding to the intersection point of two fitted power correlation curves may be defined as the cumulative rainfall threshold on the boundary causing about 78% of sediment yield frequency. The cumulative rainfall threshold in five time intervals (0, 1, 3, 5 and 7 days before the rainfall) are respectively 26.6 mm, 29.2 mm, 35.3mm, 43.2mm and 47.8mm. This method of calculating the effective antecedent rainfall based on the fractal relation between sediment yield frequency and rainfall amount could better reflect the actual situation.

Key words: sediment yield, fractal, effective antecedent precipitation, rainfall threshold, Xiangxi river watershed