Selective removal of particles and nutrients by water erosion is a key factor in soil erosion studies. Most agricultural soils are located on gentle slopes where fertility is high; however, until now, the main attention on sediment transport mechanisms was paid to high-slope gradients, where soil erosion is intense, but soils are less productive. Despite the importance of sediment size distribution (SSD) and transport mechanisms under unsteady-state conditions, few studies have been done on this issue. Higher sediment concentrations in the early stages of the runoff indicate the need to deal with unsteady-state conditions. To address this issue, sheet erosion experiments were done using a 0.2 m by 1 m tilting flume under controlled laboratory conditions. Six inflow rates (75, 100, 125, 150, 175, and 200 mL/s) were applied on two contrasting soils, including an adjacent agricultural soil and a not cultivated soil (namely Cropland soil and Control soil, respectively) at two low slope gradients (1.5% and 2%). No rill formation was observed during the experiments. The sediment-laden runoff was sampled during unsteady-state flow to determine the SSDs and sediment transport mechanisms of eroded particles for unsteady-state conditions. The results indicated different trends in the selective removal of sediment particles depending on hydraulic conditions and soil aggregate size. The contribution of suspension-saltation (SS) to the total sediment load for the Cropland and Control soils varied from 22% to 68% and from 35% to 59%, respectively, while up to 78% and 65% of soil particles were transported by bed load (BL), respectively. However, SS and BL indicated a reverse trend with stream power. The Cropland soil showed a single peak in the SSD at low stream powers because of the selective removal of fine particles (0.042 mm), whereas the SSD was shifted to a bimodal distribution at higher stream powers with the selective depletion of both fine (≤ 0.084 mm) and coarse (1.5 mm) particles. The Control soil experienced a unimodal SSD in a range of sizes between 0.109 and 0.175 mm for all stream powers. The findings of the current study highlight the need for further study on the erosion of low-slope agricultural soils, where small stream powers can remove fine and fertile soil particles.

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