Analysis Of The Formation Of Dunhuang Yardang Landform Based On Remote Sensing And WRF Wind Simulating

The study area locates in the northeast of Tibet Plateau,the border of Gansu Province and Xinjiang Province.It lies in the faulted basin between the Altyn Tagh Mountain and Beishan Mountain,and in the southwest is the Kumtagh Desert.It is puzzling that two types of yardangs with very different alignment co-exist within an area of 350 square kilometers.The two alignments are almost in 90 degree intersection,with one alignment is from north to south,and another is from east to west.In between there are linear sand dunes with alignment from northeast to southwest.The formation of the yardangs has been controversial.Yardang is a wind erosion landform,the orientation of yardangs always aligns withthe prevailing wind directions.The mean directions of yardangs in northern area coincide with the most frequent wind directions,there is little debate in academia it is the wind erosion landform.The view,that the formative force of yardangs in south Dunhuang is fluvial process(not wind),is dominating ever since its proposal in 1987.They believed yardangs in the northern area were in line with the wind direction,those in the southern area were in line with the Shulehe River,concluded that yardangs in the northern area were dominated by wind erosion,while yardangs in the southern area were dominated by fluvial erosion.This doctoral thesis intends to use remote sensing and wind field simulation to explore the formation of Dunhuang yardang landform.Extracting of geomorphological characteristic parameters(e.g.area,length,width,length/width ratio,long-axis strike etc.)using remote sensing methods are critical factors for research on yardangs,as they reflect different stages of yardangs'developments and evolution,but there are few low-cost,accurate,and automatic or semi-automatic methods.For the research of the controlling factor of Dunhuang yardang,the most important aspect is to understand the role of wind erosion.We first apply object-based method and WRF(Weather Research and Forecasting Model)wind simulating for the research of yardangs in our study area.Based on geomorphological characteristic of Dunhuang yardang landform,previous wind direction data,geomorphologic characteristics of yardangs and WRF wind simulating results,the main controlling factors of Dunhuang yardang landform were discussed.The following conclusions are drawn:(1)Based on Google Earth image,we proposed to conduct quantitative research on Dunhuang yardang landform by using object-based image analysis(OBIA)method and Canny edge detection(CED)algorithm,and obtained high precision morphological parameters.OBIA method achieved the highest overall accuracy of 92.26%with a Kappa of 0.82under the smaller segmentation scale(SC=52).Using CED method,resampling the image subset to a series of lower spatial resolution ones can effectively reduce noise.Despite being unable to extract shadows,CED method achieved an overall accuracy of89.23%with a kappa coefficient of 0.72,similar to that of OBIA method under medium segmentation scale(SC=138).For OBIA method,segmentation scale is the key factor to determine the size ofyardangs.With the increase of segmentation scale,the number of the three types of yardangs decreases,micro yardangs(area?100 m~2)can only be identified using the smaller segmentation scale(SC=34,52,80),and only several meso yardangs(100 m~2<area<1000 m~2)can be extracted with the highest segmentation scale(SC=257).For CED method,the total length of the yardang boundaries shows a logarithmically decreasing(R2=0.904)tendency with decreasing spatial resolution.There is also a linear relationship between the median widths of yardangs and spatial resolutions(R2=0.95).(2)WRF simulating results show that there are two sets of nearly vertical winds in the study area,two directions of winds formed two sets of yardangs which perpendicular to each other in orientations,and the collision of the two winds formed the linear sand dunes in the southwest.The wind condition data in the study area can be obtained from the previous measured data,dune shape and WRF wind field simulation.The wind field simulation results of WRF are basically the same as those of the previous observed field data in autumn and winter,but different in spring and summer.Mongolian high,westerly anticyclone,Indian Ocean thermal low and topography are the main causes of wind regime in the study area.(3)Yardangs in the southern area of Dunhuang are also formed by wind erosion.Wind erosion is the main controlling factor for yardangs evolution.Fluvial erosion is largely responsible for the density of yardangs as a result of destructing yardangs during storm events,which accelerated the demise of yardangs.The distribution of fluvial network is closely related to the density of yardangs.The distribution and morphologies of yardangs vary in response to the diversity of the exogenetic factors.The main external forces in the three sub-areas of Dunhuang yardang area are different:yardangs in the north are mainly affected by north-south wind,yardangs in the south are affected by east-west wind and fluvial erosion,while yardangs in the middle are affected by two groups of wind and fluvial erosion.To sum up,two semi-automatic,accurate and low-cost methods were proposed for extracting of yardangs in this paper,and the morphological characteristics,distribution and classification of Dunhuang yardangs were discussed in detail.WRF wind simulating was first applied for the study of wind conditions in the yardang area.We put forward a new view that yardangs in the southern of Dunhuang in the east-west direction were formed by wind erosion,rather than by water.