Sediment Budget Balance And Spatial Differentiation Of Nanliu River Basin

After years of the integrated management of small watershed and the conversion of farmland to forest and grass project,great progress has been made in the prevention and control of soil erosion.The ecological environment has gradually improved,and significant changes have been made to runoff and sediment yield has changed significantly as the underlying surface changes.This paper takes the Nanliu River Basin as the research object.Based on the the daily precipitation data from precipitation stations during the period of 1961-2015,the land-use data,DEM data,and the soil data.On the basis of defining the erosive rainfall standard of the watershed.Meantime,the cumulative anomaly and the Mann-Kendall mutation test were applied to analyze the spatial and temporal variations of the rainfall erosivity,and used RUSLE simulating the amount of soil erosion and erosion hotspots in 1990,2000,2010,2015 of the Nanliu River Basin,combined with the hydrological connectivity index to analyze the temporal and spatial changes of the sediment transport ratio,and then the simulate sediment yield of the basin,and compared the measured sediment yield at Changle hydrology station to validate the accuracy of the model,under the concept of sediment budget,the changes in sedimentation and the spatial distribution of erosion sources in the basin during 1990-2015 are discussed.The research results can provide scientific basis for soil erosion risk and ecological restoration and management in the Nanliu River Basin.The main research contents and results of this paper are as follows:(1)Determination of the standard for erosive rainfall.Comparing the correlation between the rainfall erosivity of different rainfall levels and the sediment transport at Changle Station,we found that when the daily rainfall is more than 20mm,the highest correlation between rainfall erosivity and sediment discharge is 0.794.The results show that it is more reasonable to choose daily rainfall greater than 20mm as the erosive rainfall standard in the Nanliu River Basin.(2)Spatio-temporal variation characteristics of rainfall erosivity.The average rainfall erosivity of the basin was 14040.5MJ·mm·hm^-2·h^-1·a^-1 from 1961 to 2015.In2008,there was a mutation point.The change of annual rainfall erosivity was mainly affected by the rainfall in the flood season.the change of annual rainfall erosivity was mainly affected by the rainfall in flood season.In space,rainfall erosivity generally increased from northeast to southwest.The minimum was in Yulin and Beiliu,and the maximum was in Hepu and Changle,which coincides with the central belt of rainfall.Rainfall erosivity increased significantly in Hepu,fluctuated greatly in Changle and Lingshan,and showed no significant growth trend on the whole.Rainfall erosivity fluctuated greatly in spring,summer and autumn,showing an insignificant increasing trend,and showing an insignificant downward trend in winter,(3)Spatio-temporal variation characteristics of soil erosion.The total amount of soil erosion in 1990,2000,2010 and 2015 were 352.14×10⁴t?142.40×10⁴t?381.03×10⁴t?363.98×10⁴t,which showed a rapid decrease,and then increased sharply to a slight decrease,mainly with slight and mild erosion.The area of severe erosion increased significantly from 1990 to 2010.The erosion modulus was the largest among unused land,and eucalyptus was larger than other forest land.The erosion modulus of different land usetypes was unused land>eucalyptus forest>other forest land>economic forest>pine forest>grass land>arable land>construction land>waterarea.At the same time,the water and soil prevention and control work should be strengthened in the 90?500m elevation zone,especially in the 150?300m elevation zone,and the slope is above 10°.Between the different types of erosion,the shift from strong erosion to weak erosion is the main trend.The soil erosion situation in the watershed tends to improve on the whole,and it shows partial deterioration during2000-2010,and the overall trend is improving.Spatially,soil erosion weakened along the northeast-northwest-southwest,and extremely intense and severe erosion were scattered.The erosion hotspots were concentrated in the Bobai area from 1990 to2015 and spread to Yulin,Pubei and other areas.Areas that do not pay attention to low erosion may lead to higher intensity erosion.(4)Spatio-temporal variation characteristics of sediment delivery ratio.The and sediment delivery ratio showed a large value in the valley and a small value on the slope surface,and the range of variation was 0?0.559 of the Nanliu River Basin during 1990-2015.The sediment delivery ratio increased in the middle reaches of the Wuli River and Hongchaojiang Reservoir from 1990 to 2000,and decreased in other regions.From 2000 to 2010,the sediment delivery ratio increased sporadically in Bobai and Yulin,and decreased in other regions.The average value of sediment delivery ratio ranged from 0.236 to 0.435.Compared with 1990,the average value of sediment transport ratio in 2000,2010,and 2015 decreased by 0.148,0.150,0.199.The increase in vegetation coverage and the change in land use are the main reasons for the decrease in sediment delivery ratio.(5)Accuracy Test of Simulation Value of the sediment yield.The sediment delivery ratio and soil erosion are used to the simulate sediment yield,combined with the measured sediment yield of Changle Station for verification.The correlation coefficient and the Nash coefficient are 0.731 and 0.725.The inspection accuracy accords with the requirements and the simulation results of sediment yield is acceptable.(6)Variation characteristics of sediment sink and erosion source.From 1990 to2015,the deposition sediment was greater than the simulate sediment yield.After2000,the gap between the amount of sedimentation and the amount of sediment was more obvious.Sediment sinks are mainly distributed in ditches,valleys,ponds and reservoirs in the basin,and on the floodplains of the main river downstream of the river.From 1990 to 2015,the erosion sources were concentrated in the southeast of Bobai and Xingye,Yulin,County and Hepu reservoir.The erosion source density in Luoyang Mountain in the 00s was smaller than that in the 1990s,but in the upper part of the Liuwan Mountain increased.By 2010-2015,the expansion rate of erosion source in Darong Mountain,Liuwan Mountain,and Luoyang Mountain.