EFFECTS OF DYNAMIC DISTRIBUTION OF AGGREGATE SIZE ON SPLASH EROSION UNDER RAINFALL IN RED SOILS

Rainfall splash erosion is a critical element in hillslope water erosion.Raindrops splash is a precursor to the start of interrill erosion and surface seal formation.It is the direct results of raindrop strike on top soil,and causes detachment and transportation.And it is usually the first step in soil loss and sediment transport.Soil detachment and transportation occur when erosive forces of raindrops are greater than the inherent strength and particle distribution.The aggregate size distribution is a recognized factor influencing crust characteristics and transport during rainfall erosion events.The red soil hilly regin is located in the tropical and subtropical zones of central China.Because of the plentiful light,heat and efficient rainfall,the area is favourable to erosion.The soil erosion in red soil hilly regin becomes severe.Therefore,it is very disadvantageous to eco-enviromental and agricultural protection.The objects of this study were to improve the understanding of the relationship between aggregate stability and size distribution and rainfall splash erodibility under simulated rainfall.In this study,four red soils derived from quaternary clay and shale were studied.2-5 mm aggregates which were prewetted in alcohol of each soil were exposed to simulated rainfall with an intensity of 60 mm/h.Bissonnais's method was applied to simulate the aggregate breakdown mechanisms.And the relative mechanical breakdown index(R) was used as the aggregate stability index.The results showed that soil splash rate was significantly related to aggregate stability,but the relationship varied with the aggregate stability index(R).The dynamic change of aggregate sizes had great effect on rainfall splash rate.Percentage of 0.25 mm size aggregate(P0.25) and mean weight diameter(M) both decreased at a power function with rainfall time(T),while rainfall splash rate(Dr) increased at a power function with time.The bigger the aggregate mean weight diameter(M) was,the less the rainfall splash rate(Dr) got.The relative mechanical breakdown index(R) was used to quantitatively describe the relationship between different soils M and their rainfall splash rate(Dr) with time(T),and their equation was established with a good correlation coefficient(R2=0.82).The equation revealed that the breakdown process of aggregates was alleviated by increasing aggregate stability,thus the splash rate was decreased,and showed a good prediction performance.This study provides a new approach to reveal the rainfall splash erosion mechanism of red soil,which has great significant for perfection of the physical progress model of rainfall erosion.