Background The Loess Plateau is one of the most severe regions that suffer from soil and water erosion in China. Soil erosion has been occurred in about 33% coverage of the Loess Plateau. Rainfall splash erosion is the key step of soil water erosion, leading to the breakdown of soil aggregates and movement of soil fragments, which can subsequently aggravate soil erosion.

Methods In the present study, cinnamon soil was chosen and different electrolyte concentrations of NaCl solutions were used as the rainfall materials under two rainfall heights for carrying out rainfall simulation experiments. When the raining was over, total amounts of splash erosion, amounts of splash erosion at different splash distances and contents of aggregates in different sizes were calculated to investigate the size distributions and movement characteristics of splashed soil fragments. The relationships between soil internal/external forces and aggregate stability were explored.

Results 1) With decreasing electrolyte concentration of simulated rainfall, soil internal forces increased sharply first and then remained stable; the aggregate stability decreased sharply first then remained stable; whereas the amounts of splash erosion increased sharply first and then remained stable. Therefore, soil internal force was an important factor affecting aggregate stability and rainfall splash erosion. 2) In the process of rainfall splash, soil aggregates broke down into macroaggregates by soil internal forces, and then microaggregates in different sizes released. After the breakdown, the contents of microaggregates in ≤0.053 mm were the highest. Furthermore, with the increase of soil internal forces, the amounts of microaggregates in ≤0.053 mm increased, while the amounts of aggregates in >0.053 mm decreased. 3) With increasing rainfall height, soil external force increased; the total splash erosion amount increased; and the content of aggregates in >0.053 mm increased. Moreover, with the increase in rainfall height, the splash erosion amount increased with the increase of soil internal force. There was a good exponential relationship between splash erosion amount and electrostatic repulsive force (R²>0.94). 4) The movement characteristics of splashed fragments were affected by the combined effects of soil internal and external forces. Under the same internal force, the amounts of splash erosion decreased with increasing distance. The splashed fragments were distributed mainly within 10 cm. Soil aggregates in ≤0.15 mm were dominant at different splash erosion distances. At the same distance, with the increase of soil internal forces, the amounts of microaggregates released after the macroaggregates breakdown increased, thus splash erosion amounts increased.

Conclusions During the rainfall splash erosion, the particle size distribution and spatial movement characteristics of soil aggregates are affected by the combined effects of soil internal and external forces. The present study provides theoretical references for the establishment of accurate erosion predication model and the development of water erosion control technology.