Research On Occurrence And Spatial Differentiation Of Hillslope Erosion Gullies Based On Topographic Indexes In Loess Plateau

Soil erosion is a critical environment issue in Loess Plateau.Gully erosion is one of the important contents of soil erosion research.The gully development is complex,and the gully erosion is serious in Loess Plateau.The volume of hillslope erosion gully is small,but it is the most active and the contribution to erosion is the largest.It is of great theoretical and practical significance to simulate the occurrence area of hillslope erosion gully by modern technology comprehensively,and to explore the influence of topography on the hillslope erosion gully.Based on the theory and method of digital terrain analysis,this study takes ephemeral gullies(EGs)and permanent gullies(PGs)of hillslope erosion gully as research object,discussing the prediction method of erosion gully occurrence area of loess hillslope,analyzing the influence mode and degree of Digital Elevation Model(DEM)resolution on it,and expounding the difference of terrain simulation parameters and influencing factors of hillslope erosion gully in different geomorphic areas.High-resolution Unmanned Aerial Vehicle(UAV)photography and high-precision field measures were utilized as data sources.Research shows:(1)The topographic indexes extracted from the high resolution UAV aerial survey data can effectively simulate the erosion area of hillslope erosion gully.Based on DEM,critical topographic threshold model of EGs or PGs and the relationship curve of S-A(slope and catchment area)can be constructed.Then,the sensitivity area of gully head was formed by simulating the EGs and PGs.The accuracy of prediction is 91.43%for EG head,71.79%for PG head,and the prediction accuracy of non-gully head area is 98.44%.The length of hillslope erosion gully can be simulated by topographic indexes,such as Stream Power Index(SPI),a variation of stream power index(AS~2),Compound Topographic Index(CTI)and Topographic Wetness Index(TWI).The simulation effect of SPI is the best for EGs,and AS~2is the best for PGs.(2)The resolution of the basic DEM data has a great influence on the simulation of hillslope erosion gully.The resolution of the effective simulation area of the erosion groove along the hillslope where the EGs developed should be at least 4m,and the formation area of the PGs should be at least 13m.The simulation results of DEM with resolution of 1m～30m show that the watershed S-A curve is the best at 1m resolution.As resolution becomes coarser,the accuracy of simulating the length of EGs with SPI,AS~2,CTI and TWI is"rising first and then falling",and the highest resolution does not correspond to the optimal simulation results.The better resolution of EGs length simulation is 4m～9m,which is related to the interference of local terrain.However,the simulation of PGs length is less affected by local terrain.The highest resolution is better when three indexes except TWI are applied.Generally,the resolution for better simulation should be 1m～14m.(3)The occurrence of EGs and PGs in different geomorphic types of Loess Plateau has strong spatial differentiation law.Three watersheds were selected Wangwugou watershed in Zizhou County,Anjiagou watershed in Dingxi City,Gansu Province,and Getuodian watershed in Zhunge’er Banner,Inner Mongolia.The simulation results of the gully head show that the values of a and b in the gully head S-A(slope and catchment area)relationship model are influenced by land use type,soil type and climate.The S-A curve of three watersheds has strong spatial differentiation characteristics,and the partition boundary of each erosion process is different in the S-A curve.The simulation results of hillslope erosion gully length of SPI and AS~2 are relatively good.And AS~2 has a wider applicability for PGs simulation when the difference of land use type,soil type and climate is larger.