Study Of The Distribution Characteristic And Evolution Of Headward Erosionon Dongzhiyuan Of Loess Gully Region

Dongzhiyuan is the best preserved and largest Plateau, which is known as the "the firstPlateau in the world" and "granary of eastern Gansu" in south-central of Qingyang City,Gansu Province.The headward erosion is serious which caused by water erosion and gravityerosion,resulting in destroying fertile farmland and causing plateau surface atrophy."Controlgully to protect tableland" plays an important role on the regional food and ecological securityin the Loess Gully Area. From the subject development,the backward of headward erosion inloess plateau leads to getting data acquisition difficulty and possesing rarely literaturedata,while headward erosion plays an important role to gully development and takes a largeproportion in gully region of Loess Plateau. In this study,we take headward erosion onDongzhiyuan as the research object to investigate its distribution characteristics and forwardspeed by using field survey and GIS analysis.We also study on the causes of headwarderosion difference between the north-south and west-east direction,reveal the relationshipbetween headward erosion and geometric features of catchment, discuss internal relationbetween headward erosion and its influencing factors to provide a scientific comprehensivetreatment for Dongzhiyuan. The main conclusions are as follows:(1) The headward erosion has typical regional difference on Dongzhiyuan which mainlyreflected on the topography and the human factors.Its average rate of headward erosion in50years is0.319m／a, the strength of headward erosion follows Xifengqu＞Ningxian＞Qingchengxian＞Heshuixian,and xifengqu is the most serious erosion region which also hasthe largest number of erosion gully.(2) The morphological of gullyhead has an impact on headward erosion whichgully-heads are clsaaified as active are characterised by plunge edges,sharp edges, tensioncracks, undercut, piping, recently deposited sediment and flow marks.The not activegully-heads have rounded edges, gully head wall, vegetation on gully walls and bed.(3) The headward erosion intensity on Dongzhiyuan can be divided into5grades, whichare slight(79.59km~2,8.3%), light(427.11km~2,44.5%), medium (333.74km~2,34.8%),strong(65.08km~2,6.8%) and ultra strong(54.54km~2,5.6%). (4) In addition to the impact of rainfall, the catchment area of the gully head, drainagelength and shape factor also have effect on headward erosion. There is a typical powerfunction relationship between catchment area and forward length, and the correlationcoefficient is0.675. The larger watershed cause scouring erosion on gully-head by influxing alarge number of run-off, on the other hand,it can make some adjustments to the flood causedby rainstorm.For a typical watershed, the larger of catchment area, the more easily lead toheadward erosion. There is regular relationship between drainage length and forwardlength,when the gully head forward to a length greater than50m, inverse relation exists,whenis less than50m in length, proportional relation exists. There is a typical power linearrelationship between shape factor and forward length, and the correlation coefficient is0.659.The forward length increases with the increase of shape factor, the larger of shape factor, thefaster of water concentration,and too much water inflow causes a bigger strength of headwarderosion.(5) The headward erosion on Dongzhiyuan has significant differences between thenorth-south and east-west direction, the headward erosion intensity in the east-west directionis bigger than the north-south direction. Analysis on topography, rainfall runoff andconfluence of water system shows that the highland in east-west direction has more obviousaffluence terrain and its affluence ability is bigger than the southern,while the east highlandhas much more obvious affluence terrain than the west one.The distribution of flow directionis mainly in the eastern part of the norther and souther regions and northwest of central andthe the minimum value is in the southern region.The distribution of maximum value ofconfluence cumulant is in the eastern direction.Catchment flow is mainly concentrated in theeast and west sides of the plateau in the surface,and the eastern plateau surface is higher thanthe western.The distribution of maximum value of flow length is in the eastern and westerndirection,while the soil erosion intensity is obvious in east and west sides of the plateau.