Study On The Connectivity Of Loess Gully Network Based On Graph Theory

The loess gully is one of the areas with the most frequent material exchange in the loess landform.The development process of the gully deeply affects the development and evolution of the loess landform.The gully network is the basic framework of the gully system,the link connecting each part of the gully system,and also the main way of material,energy and information transmission within the system.The spatial difference of material quality in the gully system depends on the spatial structure of the network.The current research on loess gully mainly includes the quantitative expression of gully morphological characteristics based on statistical characteristics and attribute characteristics of terrain feature elements,and the study on the development and evolution of landforms based on gully morphological characteristics,ignoring the spatial structure characteristics of topographic feature elements in the gully system,and the quantification of "network effect" is to evaluate the impact of the spatial structure of the gully network on the quality of the material transferred to the outlet of the basin.In this study,the influence of the spatial structure of the gully network on the material transportation in the gully system was defined as the gully network connectivity,which is used to describe the difficulty of the material transport from the source to the sink in the gully system.The connectivity of the gully network controls the process of runoff confluence and the migration of erosion sediment,affects the process of erosion and accumulation in the gully landform,and then affects the shaping process of the gully landform.Therefore,the connectivity of the gully network contains abundant and important information about the development and evolution of the loess gully landform.The study of the connectivity of the gully network is helpful to deepen the understanding of the development and evolution of the loess gully landform.Based on graph theory and digital terrain analysis,the concept of the connectivity of the loess gully network was defined in this study,a quantitative assessment method for the connectivity of the gully network was implemented.Taking 6 typical small watershed sample areas of the Loess Plateau and simulated small watershed as the research objects,the variation characteristics of connectivity of gully network in different geomorphic types and different stages of gully development were analyzed,which deepens the understanding of the evolution process and mechanism of the loess gully landform development.The main research content and conclusions of the thesis are as follows:(1)According to the related concepts of connectivity in geomorphology,the concept of gully network connectivity was proposed.Based on the basic principle of graph theory,the topographic features and their relationships in the loess gully network were abstracted and generalized into the gully network graph model,and the construction method of the gully network model was designed and realized.From the overall connectivity and node connectivity of the gully network,the average node strength,Accessibility from source to sinks(Shimbel index)and Potential fluxes(Fi)and the Network Structural Connectivity index(NSC)were selected to evaluate the gully network connectivity,and the calculation method and meaning of each connectivity index were described in detail.(2)The connectivity of loess gully network of 6 typical small watershed sample areas located in different loess landform types was studied,and the gully network graph models were constructed respectively to calculate the connectivity indexes.The results of the connectivity indexes of the gully network were analyzed from the perspectives of spatial distribution characteristics and quantitative statistical characteristics.From Shenmu to Yanchuan sample area,with the gradual maturity of loess gully landform development,the overall connectivity of gully network is enhanced,while from Yanchuan to Chunhua sample area,the development degree of gully landform is gradually reduced,and the overall connectivity of gully network is weakened.T The average values of the three connectivity indexes also show a similar trend.The results show that the spatial distribution of the connectivity index values of the network nodes in the six typical sample areas shows the agglomeration characteristics,but the spatial agglomeration characteristics of the nodes of the gully network in Suide,Yanchuan,Ganquan and Yijun sample areas with higher degree of gully geomorphology development are more significant than those in Shenmu and Chunhua sample areas with lower degree of gully geomorphology development.(3)The connectivity characteristics of gully network of the multi-stage indoor simulation small watershed were studied,and the gully network graph models were constructed respectively to calculate the connectivity indexes.The calculation results of the connectivity indexes of the gully network were analyzed from the perspective of spatial distribution characteristics and quantitative statistical characteristics.With the continuous development of the gully,the overall connectivity of the gully network of small watershed from the fifth stage to the ninth stage showed a trend of gradual enhancement.In each period of watershed,the calculated results of the connectivity index of the nodes of the gully network also showed the spatial agglomeration characteristics.However,with the continuous increase of the development degree of the gully in the small watershed,the spatial agglomeration and distribution characteristics of the connectivity of the nodes of the gully network became more significant.This study focuses on quantifying the structural connectivity,revealing the impact of the spatial structure of the gully network on the material transport,and analyzing the evolution of the connectivity of the gully network in different geomorphic types and different stages of gully development.