Analysis of sediment particle loss at different gradations on Mollisol hillslopes

Studies of sediment particle losses at different gradations can reveal hillslope soil erosion mechanisms and can reflect soil nutrient loss characteristics. However, there is a lack of process research on the loss rates and ratios of sediment with different sizes in existing research. This study was conducted to study the effects of inflow scour times and slope gradients on the loss characteristics of sediment with different sizes on hillslopes in the Chinese mollisol region based on a successive inflow scour method. The experiments were conducted at the Scientific Research Base of Soil and Water Conservation, which belongs to Jilin Agricultural University, located in the city of Changchun, Jilin Province. This area represents a typical mollisol region of Northeast China. The soil used in this study is classified as mollisol (USDA Taxonomy), with 10.2% sand (>50 μm), 9.6% silt (2-50 μm), 80.2% clay (<2 μm) and 25.6 g/kg soil organic matter. Natural runoff plots (20 m long and 5 m wide) were subjected to 6 successive inflow scour experiments (1 L/min lasting 60 min each) at two representative slope gradients (5° and 10°). The surrounding hydraulic boundary of each plot was made from a galvanized sheet that was molded to provide a greater rigidity. A runoff collector was attached to the base of the field plot to collect the runoff and sediment samples. An overflow tank, which was 0.5 m long, 5 m wide and 0.5 m deep, was attached to the upper end of the field plot to supply the inflow water. There were 10-12 runoff samples collected during the experiment, and they successively passed through a column of sieves of 5, 2, 1, 0.5 and 0.25 mm diameters to quantify the losses of sediment with different sizes. The results showed that the effects of inflow scour times on the loss rates and loss ratios of sediment with different particle sizes were more obvious than those of the slope gradients. With an increase in inflow scour time, the loss rates of <0.25 mm sediments significantly decreased from 1 965.7-6 698.4 g/(m2•h) to 59.5-80.0 g/(m2•h). The loss rates of <0.25 mm sediments in the former inflow scour treatments were 1.6-2.4 and 1.5-3.5 times greater than those in the latter treatments for the hillslopes of 5° and 10°, respectively. Thus, it is critical to prevent the loss of fine sediments during the early stage of rainfall and runoff events. In this study, the loss ratios of <0.25 mm sediments were the largest, followed by those, in descending order, of 1-2, 2-5, 0.5-1, 0.25-0.5 and >5 mm sediments. The effects of slope gradient on loss ratios of sediment with different sizes were relatively complex. As the inflow scour time increased, the loss ratios of the <0.25 mm sediments generally showed a decreasing trend, but the loss ratios of the 1-2 mm sediments generally exhibited an increasing trend. The <0.25 mm and 1-2 mm sediments were the main components of eroded sediments; furthermore, these sediments were complementary to each other, and their changes had obvious regularities. Therefore, the <0.25 mm and 1-2 mm sediments should be given more attention during studies of soil erosion on the Chinese mollisol hillslopes. Meanwhile, selecting proper mulching measures to control the losses of fine sediments on hillslopes of the mollisol region is also necessary.