Sediment Connectivity And Its Response To Rainfall In A Catchment In The Hilly And Gully Region On The Loess Plateau

With the project of returning farmland to the forest（returning grassland）on the Loess Plateau,vegetation coverage has improved significantly,and the sediment transport in the Yellow River has dropped significantly.Since the sediment yield monitored at the hydrological outlet treats the entire hydrological system as a"black box",it is difficult to explain the process and extent of soil erosion in the catchment.To deeply understand the complex process of erosion and sediment production at different scales in catchment after large-scale conversion of farmland to the forest（grass）in the loess hilly-gully region,this study took different spatial scales in typical catchment after different conversion of farmland in the loess hilly-gully region as the research object.Through the investigation,observation,and analysis of the connected/unconnected characteristics of sediment transport paths and the features of runoff and sediment production in typical slope runoff plots,slope-channel cascades,and channel scales under different rainfall events to study the characteristic of sediment connectivity at different scales.The study has proved the characteristics of sediment connectivity to varying scales the in the catchment and revealed the sediment connectivity at different scales and its response mechanism to rainfall and underlying surface characteristics and proposed to reduce the sediment source and reasonable measures and layout to reduce sediment connectivity provide a refined management basis for the governance of catchment on the Loess Plateau.The main conclusions are as follows:（1）Vegetation coverage has a significant influence on structural connectivity,the runoff depth and sediment yield modulus of slope runoff plots were well correlated with the terrain-based and impedance-based flow length,and significantly negatively correlated with vegetation coverage,litter coverage,and crust coverage.Compared with bare land,vegetation can reduce surface runoff by 74.46-97.40%and erosion by96.98-99.54%.The correlation between runoff depth and rainfall composite factors PI₁₅and PI₃₀ was better than that of single factors I₃₀ and I₆₀.The sediment yield process of the slope runoff plots is greatly affected by the structural connectivity,there are differences in the correlation between the sediment yield modulus and rainfall factors in different slope plots,the correlation between sediment yield modulus and rainfall factors in woodland and shrubs slope plots are higher than that of bare land and grassland slope plots.（2）Sediment connectivity of slope-channel cascades is affected by its structural connectivity and human disturbance,the larger the structure,the greater the human disturbance,the higher the sediment connectivity.Under different rainfall conditions,the sediment connectivity height and sediment yield on the sediment transport path of the slope-channel cascades showed a consistent variation law.The sediment connectivity height can be used as a surrogate index for functional sediment connectivity.There are also differences in response to rainfall of sediment connectivity on sub-catchments with different areas and land use configurations.Under the general erosion rainfall conditions,the sediment connectivity is low.Under the rainstorm,the sediment connectivity of human disturbances slope-channel cascades increases sharply,and the grassland and woodland slope-channel cascades show a slight growth trend.（3）Landslides play an important role and profoundly impact the slope-channel cascade’s sediment transport process.Landslides generated in the catchment under heavy rain conditions are shallow landslides.The landslide area ranges from 0.2 to1100.07 m².Based on field investigations,the landslides lateral sediment connectivity index（LCI）proposed in our study,which considers landslide area density,transport potential,and landslide connectivity frequency,is a better choice to quantify the effect of landslide lateral sediment connectivity on sediment yield.Regarding slope management,we suggest protecting the gully slope by reducing the lateral sediment connectivity between the upslope area and the gully slope.The border dikes and drainage ditches need to be built for terrace slopes to reduce slope collapse under heavy rainstorms.（4）There is a significant positive correlation between the height of mud marks and the frequency of sediment connectivity in the channel,the higher the height of mud marks on the sediment transport path,the corresponding sediment connectivity frequency is higher.The height of mud marks has a good correlation with rainfall,as the rainfall increases,the mud marks’height also increases,and channels with human disturbance were more sensitive to rainfall.The channel longitudinal connectivity fluctuates from upstream to downstream,sediment connectivity between the upstream due to the surface characteristics of the channel,whether the water area and the channel are straightforward.The channel longitudinal sediment has a good contact when there are no obstacles between the trench monitoring points on the sediment transport path.（5）From 2013 to 2020,the land-use changes in the Fangta catchment were not large,and the impact on the structural connectivity of the catchment was also small.There are differences in the rainfall characteristic of sediment connectivity at different scales.The rainfall eigenvalue of different spatial scale sediment connectivity in the catchment is shown as the catchment export is higher than the outlet of slope-channel cascades and slope（e.g.,the average rainfall is 118.81 mm at the outlet of the catchment,44.60 mm at the outlet of the slope-channel cascades,and 40.30 mm at the slope plots）,and the sediment transport path is shown as slope-channel cascades slightly higher than channels（e.g.rainfall of 61.43 mm and 51.76 mm respectively）,which are affected by the land-use configuration,human disturbance,and sub-catchment area.Under heavy rain conditions,the frequency and level of sediment connectivity in different spatial scales have increased.Under general erosive rainfall conditions,sediment connectivity corresponds to long-time front rainfall events（C-type）were more than frontal rainfall events with thunderstorm characteristics（B-type）.Under the B-type rainfall,the small-scale sediment connectivity is more obvious.Under C-type rainfall,the larger-scale sediment connectivity has a more obvious response to rainfall.