An Experimental Study On Soil Detachment And Transport Processes At Loessial Hillslope

Based on the current weakness of research on soil detachment and transport processes and the hotspot of soil erosion, soil detachment and transport processes at loessial hillslope under different erosion conditions were quantitatively studied by using simulated rainfall, simulated upslope runoff and sediment, runoff scouring, and other methods. New progresses on the influences of slope lengths, upslope runoff rates, and0 variable sediment concentrations on detachment and transport processes at loessial hillslope, as well as contributions of rainfall and runoff to hillslope sediment yield have been made. The research results deepen the understanding of soil erosion process and mechanism and provide important theoretical basis for erosion prediction model. The main results were as follows:1. The effect of slope lengths on soil detachment and transport processes at loessial hillslope was elucidated. The results showed that at 15°of slope gradient, 50 and 75 mm/h of rainfall intensities, increment range of runoff rate with an increase of slope length was approximately the same when slope length is within 8 m. At 100 mm/h of rainfall intensity and when slope length was less than 5m, increment range of runoff rate with an increase of slope length was small; but when slope length was greater than 5m, runoff rate remarkably increased with an increase of slope length. Sediment yield was fluctuated obviously when slope length was within 8 m. Sediment regime was also an alternation of detachment-transport dominated and detachment-deposition dominated.2. The influence of upslope runoff rates on detachment and transport processes at loessial hillslope was qualified. At 15°of slope gradient, 50, 75, and 100 mm/h of rainfall intensities, the total sediment yield with upslope runoff input was greater than that without upslope runoff input, i.e., upslope runoff discharging into downslope caused additional sediment (net sediment). Sediment yield was not a simply positive correlation with upslope runoff rate. Upslope runoff rate had a minor influence on sediment yield at downslope when low upslope runoff discharged into downslope. When increasing upslope runoff rate reached to1.6 L/min, sediment yield was remarkably increased with an increase of upslope runoff rate, and the increment range of sediment yield increased with the increase of upslope runoff rate. But when increasing upslope runoff rate was over 3.2 L/min, the increment range of sediment yield decreased with the increase of upslope runoff rate. After upslope runoff discharged into downslope section, sediment regime at downslope was a domination of detachment-transport.3. Soil detachment and transport processes at loessial hillslope at differents upslope runoff with variable sediment concentrations were revealed. Under conditions of 15°of slope gradient, 50 mm/h of rainfall intensity, 1.2 and 4.0 L/min of upslope runoff rates, the net sediment yield caused by upslope runoff decreased with the increase of sediment concentration in upslope runoff. Sediment deposition yield increased with the increase of sediment concentration in upslope runoff. Under conditions of 75 mm/h of rainfall intensity and 5.4 L/min of upslope runoff rate, the net sediment yield caused by upslope runoff increased with the increase of sediment concentration in upslope runoff when sediment concentration was less than 26.03 g/L. The net sediment yield cuaed by upslope runoff decreased with the increase of sediment concentration in upslope runoff when sediment concentration was more than 26.03 g/L. When sediment concentration in upslope runoff was greater than 79.16 g/L, sediment depositopn occurred at downslope and the sediment deposition yield increased with the increase of sediment concentration in upslope runoff. Under conditions of 100 mm/h of rainfall intensity and 3.2 L/min of upslope runoff rate, the net sediment yield caused by upslope runoff decreased with the increase of sediment concentration in upslope runoff. Under conditions of 100 mm/h of rainfall intensity and 5.6 L/min of upslope runoff rate, the net sediment yield caused by upslope runoff initially increased and then decreased with the increase of sediment concentration in upslope runoff. Under the influence of upslope runoff with variable sediment concentration, sediment regime was an alternation of detachment-transport dominated and detachment-deposition dominated, which depended on upslope runoff sediment concentration.4. The contributions of rainfall erosion and runoff erosion to hillslope sediment yield were analyzed. Rainfall not only improved runoff rate, but also significantly increased sediment yield. At treatments of 15°of slope gradient, 50, 75, and 100 mm/h of rainfall intensities, the sediment yields caused by rainfall erosion accounted for 68%-96%, 50%-84%, and 34%-94% of the total sediment delivery, respectively. At low runoff input, rainfall had more effect on sediment yield. At the same runoff rate, sediment yields from loessial hillslope under experiments of simulated rainfall plus runoff scouring were greatly higher than those under experiments of runoff input alone, which reflected combititions of raindrop impact to sedimenr yield. Percentage of sediment yield by raindrop impact decreased with the increment of runoff input rate. At 50, 75, and 100 mm/h of rainfall intensities, the sediment yields caused by raindrop impact accounted for 21%-83%, 20%-90%, and 23%-93% of the total sediment delivery, respectively.