Research On The Stability And Spatial Differentiation Characteristics Of The Approximation Index Of The Loess Plateau In Northern Shaanxi Based On DEM

As the most active part of material transport and energy conversion in loess gully,the gully source point,along with the gully shoulder-line,gully line and watershed divide line,constitutes the basic framework of loess topography.The development process of loess watershed geomorphology can be abstractly generalized into the process of countless gully source points formed by the intersection of countless gully lines and gully shoulder-line,and the process of approaching the watershed divide line.The characteristic points,line space combination and structural model in this process are important signs to depict the development process of loess geomorphology,and also important foundation to reveal the types of loess geomorphology.Therefore,the length ratio between the gully source point,the upstream branch line and the downstream gully line is an important entry point to characterize the spatial structure of the topographic points and lines of loess,as well as a breakthrough to measure the degree of gully erosion.In order to study the proximity degree of gully source point to watershed divide line in Loess Plateau of Northern Shaanxi,and to determine the development stage of gully and the dominant erosion mode at present,this paper constructed the core factorproximity index,which quantifies the proximity degree of gully source point to watershed divide line from horizontal and vertical dimensions.It includes the horizontal proximity index of the horizontal dimension and the vertical proximity index of the vertical dimension,and the proximity index variation derived from the proximity index of the two dimensions.Based on the data of 5-meter resolution digital elevation model,42 small watersheds were selected as experimental sample areas in the Loess Plateau of Northern Shaanxi.These 42 small watersheds belong to 15 types of loess geomorphologic regions left by the exclusion of sandy land,grassland and bedrock in the Loess Plateau of Northern Shaanxi.Using the method of digital terrain analysis,this paper mainly analyzes the mean horizontal and vertical proximity index of 42 sample areas under 15 geomorphic types in the Loess Plateau of Northern Shaanxi,as well as the statistical characteristics and spatial differentiation law of the proximity index variation.Combining with the loess thickness,land use type,normalized vegetation index and mean average rainfall data,this paper discusses the proximity degree index and valley development spatial distribution of influencing factors.In addition,Chunhua,Yijun,Ganquan and Suide,which are representative of loess tableland,loess residual tableland,loess ridge,and loess hill respectively,are selected as the key experimental areas.Taking gully and watershed as the research object,the stability analysis on the watershed scale of proximity index is carried out.The stability analysis of the resolution scale of the proximity index between 5m and 30m with 5m interval.The main conclusions of this study are as follows:1.The proximity index of Loess Plateau of Northern Shaanxi has certain spatial differentiation law.The mean horizontal proximity index increases from south to north and then decreases,and increases from west to east.The mean vertical proximity index decreases first and then increases from southwest to northeast,and decreases gradually from northwest to southeast.On the horizontal and vertical two dimensions proximity index,whole presents one increases the other decreases.The absolute value of the proximity index variation can well quantify the variation trend of the approximation degree in the north-south sequence and provide a basis for judging the development state of gully.2.Different watershed scales and different resolution scales will cause the variation of proximity index value.In the stability analysis of watershed scale proximity index with gully as the research object,it is found that watershed level has a great influence on the approximation degree.In complete five levels in the basin,the proximity index value of watershed divide line as the research object than in gully as the research object.The proximity index of digital elevation model data varies with different scales,but the higher the data resolution,the greater the gully proximity index.When the data condition is limited,the low-resolution data can also be used to calculate the proximity index,because although the resolution is reduced,the variation trend of the typical sample region on the north-south sequence remains consistent.3.The positive and negative of the proximity variation index are related to the development stage and type sensitivity of the gully.When the proximity variation index is negative,the gully geomorphic type is loess plateau or residual plateau;When the proximity variation index is positive,it indicates that the gully is located in the loess ridge,and loess hill.The range of the proximity variation index is-1 to 1.When the absolute value tends to 1,it indicates that the gully development is in the early stage or the later stage of maturity.4.There is a spatial distribution relationship between loess plateau proximity and loess thickness,land use type,normalized vegetation index and mean annual rainfall data.Among them,the influence of loess thickness difference on the mean vertical proximity index is less than that on the mean horizontal proximity index.The mean horizontal proximity index of low coverage grassland is the largest but the mean vertical proximity index is smaller.Although vegetation is the decisive factor affecting the development of slope erosion,the indicative effect of normalized vegetation index on the degree of approximation is not particularly obvious in the development of erosion of gully source points.In addition,the relationship between mean annual precipitation and proximity index is not significant for a single factor.As a quantitative index to reflect the degree of approaching from the gully source point to watershed divide line,the proximity index can not only reflect the development stage of gully,but also judge the dominant erosion mode of gully at present stage.This can not only provide the basis for the division of geomorphic types,but also provide a reference for the comprehensive control of soil and water conservation.