| The Mu Us Sandy Land is located in the transition area from the Ordos Plateau to the Loess Plateau.As an important ecological barrier in China,its ecological environment is fragile.The strong aeolian activities have caused serious harm to the local ecological environment,agricultural and animal husbandry production.In recent years,the degree of sandy desertification in the Mu Us Sandy Land has changed obviously,and great achievements have been made in vegetation reconstruction,which has inhibited the activation of dune to a certain extent.However,according to previous research results and remote sensing image analysis,it is found that there is still activation phenomenon of dune in the northwest of the Mu Us Sandy Land.Mobiledune and semi-fixed dune are distributed alternately,and there is a great difference in vegetation coverage.In order to explore the relationship between the activation degree of dune and the vegetation coverage on the activities of dune in the northwest of the Mu Us Sandy Land,based on field investigations,remote sensing image analysis,and sediment grain size analysis,this paper deeply studied the morphological evolution process,migration laws and development environment of dune in the northwest of the Mu Us Sandy Land,and analyzed the vegetation coverage characteristics of different types of dune fields in July and November 2020.The main conclusions were drawn as follows:1.According to the monitoring of dune morphology based on the Google Earth images,it is found that the dune morphology in the northwest of the Mu Us Sandy Land has a trend from "barchan shape" to "parabolic shape".The mean migration speed of the dune is 2.74 m·a-1,the mean migration direction is 159°,and there is a certain angle with the resultant drift direction,and the angle is 30°.Among them,the barchan dune is the fastest migration dune(4.47 m·a-1),which is 2.5 times higher than barchan-to-parabolic dune and 2.3 times higher than the parabolic dune,respectively.There is little difference between its migration direction and the resultant sand transport direction.The included angle is 17°,and the direction of migration is inclined toward the ESE direction.While the other two types of dunes have a larger angel with the resultant sand transport direction(22° and 86°,respectively),and the direction of migration tends toward the SSE direction and the SW direction.The morphological evolution of the dune during the migration is as follows:the height of the dune with barchan shape decreases,the windward slope is elongated,and the ratio of length to width(Lu/W)becomes larger,and the "normal" barchan shape is still maintained.From barchan dune to barchan-to-parabolic dune,the height decreases,which is not much different from that of the barchan dune in the same period.The windward slope is significantly elongated,the two wings are shortened,the curvature of the ridge line becomes smaller,the ratio of length to width(Lu/W)becomes larger,and the dune becomes "fat".The dune evolve from the barchan dune tothe parabolic dune,with its two wings tending toward the windward direction,its ridge line is elongated,and convex to the downward direction,and its shape is "elliptical",but it does not develop into a typical "parabolic" shape.2.UAV were used to measure the dune morphology in the study area,and it was found that under the same wind speed conditions,the three types of dune morphology were significantly different,the highest point and the ridge line were separated,and the dune strike difference was within 30°.Among them,the barchan dune is the highest in height and the largest in volume.The height of the barchan-to-parabolic dune is slightly lower than that of the barchan dune,and the expansion degree of its two wings is greater than that of the barchan dune.The wing angles bend upward,and there is a bifurcation phenomenon.The parabolic dune is the lowest in height,and its two wings are of equal length.It develops into a "leaf-shaped" parabolic dune.3.By comparing the vegetation coverage of the three types of dunes,it is found that parabolic dune have the largest vegetation coverage,followed by barchan-to-parabolic dune and barchan dune.The vegetation coverage in July is greater than that in November,and the barchan dune have the largest vegetation change rate.The spatial distribution characteristics of vegetation:the vegetation of the barchan dune is mainly distributed at the bottom of the dune and inter-dune.Shrubs are scattered on the surface of the dune,and trees are on the two wings.There are more vegetation in the transitional zone between the windward slope and the inter-dune of the barchan-to-parabolic dune.The coverage of the south side is more than that of the north side.A small amount of vegetation is distributed in the lower part of the windward slope.A large number of herbs are distributed on the two wings of the dune and the sand arms left by the migration.The vegetation of the parabolic dune is mainly concentrated on the two wings of the dune and the middle and lower parts of the windward and leeward slopes.A small amount of vegetation is distributed on the top of dune with large shape.The growth rate was fast from 1982 to 2006,and the growth rate was slower from 2006 to 2018.4.The surface sediments of the dune in the study area are mainly medium sand and fine sand,with a good degree of sorting.The frequency curve is nearly symmetrical distribution,and the kurtosis is medium.From mobile dune to semi-fixed dune,the overall particle size has little difference.Parabolic dune have the coarsest particle size and the worst sorting.The mean grain size of the longitudinal section of the windward slope of the barchan dune becomes coarser gradually.The sorting effect is the best at the top,while the particle size of the leeward slope becomes finer from the top to the bottom of thedune,and the sorting becomes better.The grain size at the top of the barchan-to-parabolic dune is coarser,and the sorting is better.The grain size in the middle of the slope is finer,and the bottom of the dune becomes coarser,and the sorting becomes worse.The mean particle size in the middle and lower part of the windward slope of the parabolic dune is coarser and the sorting is poor.The particle size of the leeward slope is smaller than that of the windward slope. |
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