【Objective】Soil aggregate is a basic unit in soil structure and its stability is an important
index describing soil’s resistance to breakdown in the process of water erosion. However，in splash erosion
how raindrops function through mechanical impact and slaking effect on soil aggregates and what are the
mechanisms and contribution rates of the two are still unclear. This study is oriented to investigate effects
of mechanical impact and slaking effect of rain drops on breakdown of soil aggregates during splash erosion.
【Method】A series of indoor splash erosion experiments were carried out in the State Key Laboratory ofSoil Erosion and Dryland Farming on the Loess Plateau，Institute of Soil and Water Conservation，Chinese
Academy of Sciences and Ministry of Water Resources，China. Samples of Loessal soil（Sandy loamy soil）
collected from Yan’an and Lou soil（Loamy loam soil）collected from Yangling in Shaanxi province，
two typical soils in the Loess Plateau，were tested in the experiments. A home-made needle type rainfall
simulator，consisting of three parts：water supply apparatus，needle nozzles and support frame，was used
to simulate rain drops of ultra-pure water and alcohol to determine effect of mechanical impact alone and
joint effect of mechanical impact and slaking on soil aggregates，separately. The splash erosion experiments
were designed to have two type of soils and 5 treatments in height for rain drops to fall，i.e. 0.5 m，1 m，1.5
m，2 m and 2.5 m.【Result】Results show that in the two soils soil aggregate stability exhibited an order of
MWDfw＜MWDws＜MWDsw. Slaking effect（Fast wetting）was the major mechanism of the breakdown of soil
aggregates，and followed by mechanical disturbance（Wetting and Shaking），and then chemical slaking
（Slow Wetting）in the end. The soil of loamy clay was higher than the soil of sandy loam in RSI（Relative
Slaking Index），suggesting the former is more susceptible to slaking effect than sandy loam soil，while the
latter is more to mechanical impact. In splash erosion，when rain drops fell from the same height，splash
erosion rate was lower in loamy clay soil than in sandy loam soil，and splash erosion rate caused by rain drops
of pure water through the joint effect of mechanical impact and slaking was higher than that caused by drops
of alcohol through mechanical impact alone in both soils. Regardless of pathways of the rain drops affecting
soil aggregates，splash erosion rate increased with rising kinetic energy of the rain drops，and power function
could be used to well describe the relationship between splash erosion rate and rain drop kinetic energy. The
splash erosion rates caused by slaking effect and/or mechanical impact of rain drops both increased with rising
rain drop kinetic energy or rising height where rain drops fell from. The slaking effect contributed more than
50% to the splash erosion rate，indicting slaking effect was the main factor causing aggregate breakdown
effect，but the slaking effect decreased in contribution to splash erosion rate with rising rain drop kinetic
energy，while the mechanical impact acted reversely. In the cases the same in rain drop kinetic energy，the
contribution of slaking effect was higher in loamy clay soil than in sandy loam soil，but that of mechanical
impact was just the reverse. 【Conclusion】Contribution rates of slaking effect and mechanical impact vary
with rain drop kinetic energy and soil type. All the findings in this study could be of great significance to
evaluation of aggregate stability and to in-depth understanding of the mechanism of aggregate breakdown
during splash erosion.
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