Sediment Sources Of Typical Small Watershed During Ecological Restoration In The Loess Plateau Of Northern Shaanxi

In this study,a typical small watershed in the Shagou Dam of Yulin city was selected as the study area.Based on the historical rainfall data and the erosion and sediment yield information in the sediment rotation of the warping dam,the relative sediment contribution rate of different sediment sources in the small watershed in the past 20 years was quantified by using composite fingerprint identification technology.The interannual variation in erosion and sediment yield and the characteristics of erosion and sediment yield in different stages of the small watershed since the conversion of farmland to forest were analysed.The response mechanism of erosion and sediment yield to rainfall and vegetation cover was revealed.The results show the following:(1)The study area rainfall and rainfall erosivity have an uneven distribution throughout the year,generally appearing in July and August as the center of the skewed normal distribution law of farmland after rainfall events with different levels of rainfall and rainfall increased.The strong precipitation rainfall events,such as storms and synchronous increases in rainfall,are the rainy season in the Shanghai region and are the main cause of serious soil erosion.There is a29-year first main cycle,a 16-year second main cycle and an 8-year third main cycle of rainfall on the 40-year time scale.(2)The vegetation coverage of the small watershed ranged from 34.62% to 45.68%before the conversion of farmland(1981 to 1999)and was less than 30% in 2000.After the implementation of the conversion policy,the vegetation coverage was significantly improved.After the adjustment of the conversion policy in 2007,the vegetation coverage was more than50%,basically stable at approximately 55%.In 2011,FVC exceeded 60%(65.71%)and exceeded 70% for the first time in 2012.Based on the interannual variation in vegetation cover in the study area,this study divided the conversion of cropland to forest into four stages: initial period(2000-2003);vegetation growth and development period(2004-2006);conversion of farmland to forest development period/quality improvement period(2007-2010);and effect of returning farmland to forest/mature and stable period(after 2011).(3)In the 23 years before returning farmland to forest(1977-1999),the accumulated silt depth of the silt dam was 20.58 m,the accumulated storage capacity was 240,700 m3,and the accumulated eroded sediment yield was 362,800 t.In the past 20 years,the accumulated silting depth was only 4.5 m,the accumulated storage capacity was 119,200 m3,and the accumulated silting total was 169,700 t.At present,the height of the dam is approximately 4 m from full silt,the remaining reservoir capacity is approximately 125,600 m3,and the cumulative total reservoir capacity will reach 485,600 m3.The interannual variation in the soil erosion rate in the small watershed was 83.6%.Before returning farmland to forest,the average annual soil erosion rate was 15775 t km-2a-1 in 23 years,which was severe erosion.The soil erosion rate ranged from 0 to 18782 t km-2a-1,and the average annual soil erosion rate was 6238 t km-2a-1,which was decreased by 60.46% compared with that before the conversion,but it still indicated intensive erosion.(4)The results of the multivariate mixed model show that the relative sediment contribution of potential sediment sources is as follows: gully(50.01%)>shrub sandy land(28.61%)> sloping land(9.37%)> forestland and grassland(7.42%)> road(4.59%).The sediment yield of erosion was gully > shrubland sandy land > forestland and grassland land >sloping farmland > road,and the soil erosion modulus was road > shrubland and sandy land >sloping farmland > gully > forest and grassland land.The sediment contributions of forest grassland,shrub sandy land and slope farmland decreased,while the sediment contributions of gullies and roads increased.In recent years,the sediment contribution of roads has increased significantly,and gully sediment(gully wall collapse)showed a lag in the transport process.At the annual scale,the sediment contribution of gully > shrub sandy land > slope farmland >forest grassland > road,and the sediment contribution of gully and shrub sandy land sources accounted for 76% of the whole basin.Since the conversion of farmland to forest,there were 5sediment yield peaks,corresponding to 7 heavy precipitation events,and 7 rainfall events accumulated erosion sediment yields of 69,300 t,accounting for 40.80% of the total sediment yield over 20 years.In the past 20 years,the annual eroded sediment yield of the four source lands showed an overall decreasing trend,except that the annual eroded sediment yield of the road showed an increasing trend.The average annual erosion sediment yield of the four source lands was gully > shrub sandy land > forest and grassland > slope farmland > road.The average annual rate of soil erosion in 20 years was road > slope farmland > shrub sandy land >gully > forest and grass land.The soil erosion rate of roads showed an increasing trend in the time domain,while the soil erosion rate of other source areas and the whole small watershed showed a decreasing trend.(5)Sediment yield is significantly positively correlated with rainfall at both the subrainfall scale and the annual scale.The erosional sediment yield of each potential sediment source area was significantly positively correlated with the rainfall at the 99% confidence level.The sediment yield of forest grassland and road erosion is highly correlated with annual rainstorm rainfall,while there is a moderate correlation between the sediment yield of slope farmland and gully erosion and secondary rainfall.The effects of rainfall on erosion and sediment yield were separated,and there was a significant negative correlation between erosion and sediment yield and vegetation cover change under different rainstorm levels.With the increase in rainfall levels,the sediment reduction effect of vegetation cover became more prominent.With the increase in rainfall level,the sediment reduction effect of vegetation coverage becomes more prominent.The restoration of vegetation from farmland to forest has a good effect on water retention and soil consolidation,and the sediment reduction effect is obviously enhanced when the vegetation coverage is more than 55%.The accurate understanding of the relationship between erosion and sediment yield and the interannual change in rainfall and vegetation succession over a long time scale can provide a scientific basis for the study and evaluation of the implementation effect of the project on the Loess Plateau and provide theoretical support for promoting the scientific management of small watersheds and high-quality development of the Yellow River Basin.