Research On The Spatial And Temporal Characteristics Of Soil Erosion And Its Surface Driving Factors In Yan’an Based On Topographic Factors

Soil erosion is one of the main threats to global soil degradation.The Loess Plateau is the region with the most serious soil erosion in the world.Although the ecological construction has made remarkable achievements in recent years,the ecological environment is still fragile,and the problem of soil erosion in some areas is still serious.Yan’an City is located in the hinterland of the Loess Plateau,belongs to the loess hilly gully area,and the area of soil erosion account for 77.8%of the total area.In order to reduce soil erosion and improve the ecological environment,China began to implement the project of returning farmland to forest(grassland)with ecological construction as the main purpose in 1999.In recent years,this project has shifted from large-scale advancement to results consolidation.Yan’an City,as the"first city of returning farmland to forest(grass)"took the lead in the pilot work of returning farmland to forest(grass).The first(>25°)conversion was conducted in 1999,and the second(>15°)conversion was conducted in 2013,with a cumulative conversion of 7.18×10~3 hm~2.The implementation of the project of returning farmland to forest(grass)in Yan’an City is one of the typical cases that the high intensity of human-land relationship has a positive impact on the ecological environment in the short term.Due to the strong influence of topographic factors on soil and water loss,the slope of about 18°is the critical angle of erosion,and the sediment yield and flow will increase sharply when the slope is about 25°.Therefore,the coupling of topographic factors to explore the high-intensity evolution process of land use and vegetation coverage and the evolution law of soil erosion brought about by them has a typical reference to promote the smooth implementation of the farmland for forest(grass)Project and the improvement of the management system in the loess hilly and gully region,and has an important guiding significance to promote the ecological environment construction in the Loess Plateau.This paper abstained land use,vegetation covers and other information by using random forest,pixel dichotomy and Kriging interpolation method based on remote sensing image,elevation data,meteorological data,soil texture data and other multi-source data in Yan’an City.Based on the RUSLE model,the soil erosion modulus information was obtained by calculating the rainfall erosivity factor(R),soil erodibility factor(K),slope gradient factor(LS),vegetation cover factor(C)and soil and water conservation measures factor(P).The temporal and spatial variation of land use,vegetation covers and soil erosion in Yan’an City from 1988 to 2018 were analyzed by using trendency analysis,transfer matrix,topographic distribution index,gravity center migration and geological information map.CA-Markov model was used to simulate the soil erosion in 2028,and the accuracy verification and change analysis were carried out.The main conclusions are as follows:(1)For land use structure.From 1988 to 2018,the land use structure of Yan’an City had undergone dramatic changes.In all land use types,the change rate of cultivated land and forest land was accelerating,the proportion of cultivated land area was reduced from 29.01%to 11.51%,mainly in areas with slope lower than 15°.The proportion of forest land area was increased from 36.01%to 60.60%.mainly in areas with slope higher than 15°.From 1988 to1998,the distribution dominance of cultivated land decreased in low-altitude areas and increased in high-altitude areas and the distribution dominance of woodland and grassland was opposite to that of cultivated land.From 1998 to 2008,the change characteristics of the dominance of cultivated land and grassland at each elevation were opposite to those of the previous period.From 2008 to 2018,the dynamic changes of forest land,arable land and grassland reached the maximum.(2)For vegetation coverage.From 1988 to 2018,about 50%of the vegetation in Yan’an city improved,26.85%of the vegetation degraded slightly,and the high vegetation coverage area of secondary forest growth remained stable.On each elevation and slope gradient,the distribution dominance increases with time.The gravity of vegetation coverage generally shifted to the north and west.During 1988 to 1998,the vegetation as a whole was degraded,and after 1998,the vegetation was improved in a large area.From 2008 to 2018,the percentage of vegetation degraded area decreased with the increase of slope,and the percentage of improved area increased with the increase of slope.(3)For soil erosion.From 1988 to 2018,there were 62.27%of soil erosion areas in Yan’an remain stable,and 6.94%of the regions showed one-way changes.The maximum pattern evolution type was“severe erosion→strong erosion→strong erosion→strong erosion”.The distribution of each grade of erosion on different elevation gradients was polarized.The advantage of low grade erosion was small when elevation gradient was small.The distribution characteristics of high-grade erosion were opposite.With the increase of time,the dominance level of each grade erosion in different terrain gradients in Yan’an City was gradually averaging.Soil erosion in Yan’an City aggravated from 1988 to 1998,and improved greatly after 1998.The area of severe erosion increased slightly from 1998 to 2008,but decreased from 2008 to 2018.(4)Soil erosion prediction results show that soil erosion in Yan’an City in 2028 has improved compared with 2018.The higher grade erosion mostly transfers to the lower grade erosion.Most of the lower-grade erosion will transfer to the second and fifth-grade elevations and the third and fourth-grade slopes,while the higher-grade erosion will mainly transfer to the fourth and fifth-grade elevations and slopes.With the increase of erosion grade,the differentiation of transfer area in each slope grade will increase gradually.