Soil Erosion And Sediment Connectivity Of Slope Gully System In The Hilly And Gully Region On The Loess Plateau

At present,the erosion environment is obviously improved,and the sediment load to the Yellow River is significantly reduced.However,due to the scale dependence of soil erosion processes and sediment deposition along the pathyway,sediment transport can only represent part of the sediment mass in the basin.The proce of soil erosion-sediment yield-sediment transport needs deep understanding.The decrease of sediment in hydrological export area does not indicate that soil erosion on the Loess Plateau has been effectively controlled,and the erosion hotposts or potential risks caused by sediment deposition within the watershed may be ignored.In this study,slope gully system of abandoned farmland without disturbance,slope gully system of abandoned farmland with disturbance and slope gully system of orchard(with terraces of orchard on upper position and sloping farmland on lower position)in Ansai County of the northern Shaanxi Province were selected as experience region.Slope gully systems with different land use patterns in Zizhou County of the northern Shaanxi Province under the"7.26"extreme rainstorm event were also selected.Based on filed soil erosion survy,the unmanned aerial vehicle aerial surveying,runoff and sediment monitoring and other relevant methods of soil erosion,hydrology and landscape ecology,the distribution and characteristics of erosion and deposition of sediment source and sink in slope gully system were analyzed with the concept of sediment connectivity.And the response of opening and closing characteristics of key nodes in sediment transport path to rainfall were studied in different slope gully systems.Moreover,the soil erosion and sediment connectivity in extreme rainstorm were also studied.The effect of rainfall and human activities on runoff and sediment yield was analyzed in slope gully system.Main results were as follow.(1)At present,slope gully systems of small watershed had good vegetation coverage.Sediment source had small area,high landscape fragmentation degree and serious soil erosion.The main sediment sources were steep slope with low vegetation coverage,shallow landslide surface and exposed gully,which accounted for4.05%-23.78%of the total area of slope gully systems.From 2016 to 2019,48 erosive rainfall events occurred in the small watershed,including 12 rainstorms with rainfall of 13.7-72.1 mm and rainfall intensity of 1.5-4.0 mm h^-1.Under a single rainfall,the soil erosion modulus of bare slope could reach 664.33 t km^-2.Shallow landslide surface had the most serious erosion which reached 4237.49 t km^-2,and rill erosion was its main erosion type.Rill network had important effect on runoff redistribution and further rill erosion.The annual erosion modulus of erosion reach of gully was10428.53 t km^-2 a^-1,and the sedimentation modulus of sedimentation reache of gully was 8284.14 t km^-2 a^-1.Under the rainstorm,the sedimentation gully could become the sediment source.(2)The runoff depth and sediment yield modulus of slope gully system were all at a low level,and were significantly affected by rainfall and landscape pattern of sediment source and sink.Average annual runoff depths and average annual sediment yield modulus of the slope gully systems were less than 1.23 mm and 35.86 t km^-2 a^-1.The runoff depth and the sediment yield modulus under rainstorm were 2.56-13.37times and 10.04-142.82 times than that under general erosive rainfall,respectively.More than 70%of the sediment yield occurred in rainstorm in the abandoned farmland with disturbance slope gully system and orchard slope gully system.The relationship between runoff depth,sediment yield modulus and rainfall factors(I₁₅,I₃₀,I₆₀,PI₁₅,PI₃₀,PI₆₀)reached significant level.The runoff depth and sediment yield modulus of slope gully system increased linearly with the aggregation of sediment source patches and the increase of the maximum patch area,and the dispersion of sediment sink patches and the decrease of the maximum patch area.(4)The sediment connectivity of slope gully system was significantly affected by topography,vegetation cover and human activities,and the connectivity state of sediment transport path played a key role in sediment transport.The flat depressions appeared along the downward extension of gullies were the important reasons for the interruption of extreme hot spots and the appearance of extremely cold spots of sediment connectivity which were also important sediment sink.The sediment transport ratio of the abandon farmland slope gully system was only 0.18%,and the response of sediment transport ratio to rainfall was very weak due to the retention effect of litter on runoff and sediment.The sediment transport ratio of abandon farmland with disturbance slope gully system and orchard slope gully system increased linearly with maximum rainfall intensities in a period and rainfall amount,respectively,which could reach 30.09%.Ephemeral gully,gully,production road and the path caused by graze were main sediment transport pathes.Under the rainstorm,the sediment transport path under bare land,between the terraced field and the production road and the path caused by graze were in open state,which significantly affected the sediment transport characteristics of slope gully system,such as sediment delivery ratio,sediment concentration,sediment yield and so on.(5)Under extreme rainstorm events,slope gully systems with different land use pattern had serious soil erosion and were in the state of sediment connectivity.Soil erosion intensity under single rainfall event was 6940-95880 t km^-2.Gully incision,slope failure and rill were main erosion types on slope gully system.The incision in rill and gully greatly increased the sediment connectivity of slope gully system.Huge flow power connected the slope gully system to the downstream channel and transported almost all sediment to the downstream channel.In order to reduce soil erosion risk and sediment connectivity,the new gullies and gullies incision by rainstorm should be filled up in time.