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a b s t r a c t a r t i c l e i n f oThe Soil Conservation Service Curve Number (SCS-CN) method is widely used for predicting direct runoff from rainfall. The ratio of initial abstraction (Ia) to maximum potential retention (S) was assumed in its original development to be equal to 0.2 in SCS-CN method. The constant initial abstraction ratio is the most ambiguous assumption and requires considerable refinement. The objectives of this study were (1) to determine the initial abstraction ratio, in an experimental watershed in the Three Gorges Area of China, by analyzing measured rainfall-runoff events; (2) to compare the performance of the traditional and modified Ia/S values with observed rainfall-runoff data. The dataset consisted of 6 years of rainfall and runoff measurements from the experimental watershed. The results indicated that the Ia/S values, using event rainfall-runoff data, varied from 0.010 to 0.154, with a median of 0.048. The average initial abstraction ratio of the watershed was equal to 0.053. The standard SCS-CN method underestimates large runoff events, yielded a slope of the regression line of 0.559 and an intercept of 0.301. The modified Ia/S value was about 0.05 that better predicted runoff depths with an R 2 of 0.804 and a linear regression slope of 0.834. It also improved model efficiency coefficient (E) to 0.768 compared with 0.482 for traditional Ia/S value. This Ia/S-adjusted SCS-CN method appears to be better appropriate for runoff prediction in the Three Gorges Area of China.

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As a special kind of soil erosion that severely deteriorates the qualities of soil in granite regions, collapsing gully is widely distributed in the tropical and subtropical areas of South China. In particular, collapsing-gully erosion produces a large amount of sediment deposited on the plough layer of alluvial fan farmland and causes increase of desertification, great reduction of nutrients and rapid enhancement of erodibility in the soil. This study was designed to evaluate the effects of different land uses on the soil physic-chemical properties and erodibility of collapsing-gully alluvial fan. Our results show that the physical properties of soil in alluvial fan were greatly improved with smaller bulk density, increased soil porosity, strengthened water holding capacity and a higher particulate matter content. The chemical properties of soil were also significantly improved, including pH, cation exchange capacity, and the contents of organic matter, total and available nitrogen, potassium and phosphorus. All the land uses were proved to improve the soil properties, especially in the tea garden, vegetable land and paddy field. The results of correlation analyses among the properties demonstrate that the physical and chemical properties were significantly correlated, indicating that improving the soil physical properties is an effective method to increase the soil nutrient in the farmland of collapsing-gully alluvial fan. In addition, the application of land uses included grass land, eucalyptus forest land, vegetable land, tea garden, sweet potato land, and paddy field reduced the erodibility of the soil compare to bare land. The erodibility K values of soils in grass land, eucalyptus forest land, vegetable land, tea garden, sweet potato land, and paddy field were 14. 43, 16.97, 45.45, 33.12, 18.94, and 34.01% lower than those of bare land, respectively. The results of multiple linear regression analysis show that the soil erodibility had a significant correlation with the physic-chemical properties, especially the soil texture and the content of organic matter. In conclusion, our results demonstrate that different land use patterns are effective to improve the quality of the soil in collapsing-gully alluvial fan, including the improvements of the soil structure, nutrients and anti-erosion ability. Our findings provide important implications for the soil improvement in the farmland of collapsing-gully alluvial fan.