| Soil erosion by water is the process of detachment, transport and deposition of soil materials which is induced by raindrop impact and the subsequent overland flow generated by rain. Sediment particles with contaminants, such as fertilizers, pesticides, insecticides and other substances, have a great important impact on the ecological environment in erosion area. A better understanding of sediment sorting will improve understanding of erosion and sedimentation processes, which in turn will improve erosion modeling and guide the design and configuration of soil and water conservation measures. Typical soils from the Loess Plateau were chosen in this project, which were collected from Changwu and Suide country, respectively, in Shanxi Province, China. Rainfall simulation experiments were carried out in a 1 m by 5 m box with different steep slopes(10°, 15°, 20° and 25°) at 90 mm/h rainfall intensity. Simulated rainfall experiments were used to explore sediment particle size distribution characteristics and transport mechanism through traditional methods of erosion process monitoring combined laser particle size analysis method. The main results are as follows:(1) Two typical soils from the Loess Plateau under different rainfall conditions were used to explore the regular pattern of runoff and sediment concentration of the erosion process. The time to start runoff of Changwu soils was lower than that of Suide soils under the same rainfall conditions. However, runoff rate, sediment concentration, and soil erosion rate of Changwu soils were higher than that of Suide soils. A significant difference was identified in the time to start runoff, runoff rate between slope gradients(p < 0.01). Runoff rate increased quickly during the initial minutes after runoff initiation and then approached steady state for different slopes in both soils. The variation in sediment concentration with time for different slopes in two soils showed a similar and clear pattern: the sediment concentration declined rapidly from the maximum value after runoff initiation to a relatively stable state.(2) In order to explore the composition of eroded sediment, the effective particle sizes of eroded sediments were measured using a laser diffraction device. The eroding sediment for Changwu soils under different slopes was primarily composed of silt-size particles(0.002 mm- 0.05 mm) during all rainfall simulations, which accounted for approximately 60%- 65% of the sediment load. However, for the Suide soils, the mainly composed of sediment was coarse silt-size(0.02 mm- 0.05 mm) and fine sand-size(0.05 mm- 0.25 mm) with accounted for 65%- 70% of the eroded material. The mean clay content of eroding sediment was 16.4 % in Changwu soils, and 9.9 % in Suide soils. The percentage of silt size sediment nearly kept a steady change during the rainfall events under different slopes in Changwu soils. The percentage of silt size sediment of Suide soils increased gradually after runoff initiation to a relatively stable state. The sand content of soil sediment from the studied soil increased during the initial minutes of a storm and then kept a constant state until the termination of rainfall events.(3) Aggregate ration(AR) and enrich ration(ER) were calculated to research the selectivity characteristics of eroded sediment particle size. The ratio of AR for silt size sediment in both soils was approached 1, indicating that silt size sediment was transported as primary particles. The ratio of AR for sand size particle in both soils was greater than 1, indicating that sand size sediment was transported in the form of aggregates. The ER ratio of clay size sediment in both soils was less than 1, indicating the occurrence of a dispersion of clay size sediment and eroded as aggregates. The ER ratio of sand size sediment was greater than 1, indicating that sand size particles were enriched in erosion process. The silt size particles kept a constant state in the process of erosion, which was consistent with the composition of original soil. The variation of eroded sediment with time in two soils under different slopes showed a similar pattern: the eroded sediment particle was very similar to that of the original soil at final steady state condition.(4) The sediment particle size distribution exhibited a bimodal distribution with peak fractions for the finer size class of < 0.020 mm and the larger size class of > 0.100 mm for the experiments carried out on the Changwu soils. However, the fraction of sediment particle size distribution is highest at the finest class of < 0.09 mm for all sampling times in Suide soils, and the percentage of other eroded sediment size classes decreased gradually with size. The bimodal distribution in particle size of eroded sediment from the experiments carried on Changwu soils resulted from two transport mechanisms of suspension-saltation and rolling, which acted predominantly on particles of different size classes. However, the predominant transport mechanism was suspension-saltation in the experiments from Suide soils in which transport rate reduced uniformly with size. This evidence indicated that the particles size distribution and transport mechanism may be related to soil types. |
