Study On Evolution Process And Development Mechanism Of Blowouts In Typical Sandy Grassland

Blowout is the initial form of sand belt or sand dune in sandy grassland area.If which has continuous development,it will lead to expansion of desertification and deterioration of ecological environment.The thesis analyzed change characteristics of spatial pattern for blowouts in study area,which is typical wind erosion area and is located in Northern part of Hulunbuir grassland with area of about 305 km²,remotely sensed images were used such as MODIS data with 250 m spatial resolution and Landsat TM data with 30 m spatial resolution as main data sources,and NDVI value,landscape pattern index and correlation coefficient were used by software applications of ArcGIS,ENVI,Fragstats and SPSS etc.The sample pits at different development stages of blowouts include bare land,active stage,fixed stage,extinction stage and reactivation stage,which were compared with the surface of grassland,the characteristics of wind-sand flow structure and soil physical and chemical properties of blowouts are analyzed.Especially,the evolution process of blowouts in active development stage is emphatically researched.Research conclusion of this thesis provides scientific basis for monitoring and controlling grassland desertification.The main conclusions are as follows:（1）The areas with better vegetation coverage mainly concentrated in the northeast of Chen Barag Banner,the eastern and middle Ewenke Banner and the southeast of Xin Barag Left Banner.Vegetation coverage showed a fluctuation improvement trend during the past 15 years,but the NDVI value of many patches between the northern sand belt and the Hailar River in the sandy grassland area was between 0.2 and 0.35,and the vegetation coverage was very poor,which indicated that the existence of blowout affects the growth of vegetation,and there are almost no vegetation growth in some blowouts at active development stage.Annual precipitation showed increasing trend and the summer temperature was not significantly reduced after 2010,the changes were conducive to vegetation growth.Precipitation was the main influence factor of vegetation cover,and the impact of temperature on vegetation cover was also great.（2）The distribution of blowouts in the study area was relatively dense,accounting for 4.12%of the landscape area,and the patch density was 0.28 hm^-2.The count of patches with area of less than 10 hm²and the area of 10-100 hm²were basically equal.There were significant correlations between the patch area of the blowout,the shape index and fractal dimension.The long axis of blowouts was basically consistent with the local prevailing wind direction.With the increase of blowouts area,the time series of blowouts shape pattern can be summarized as in order of from saucer shape to trough shape and to irregular shape.The area of the largest blowout in the study area was 96.59hm²,which reflected the seriousness of wind erosion and desertification in Hulun Buir Grassland.There was a wide distribution of blowouts,but it had a local aggregation distribution in certain areas,and its landscape fragmentation was larger.The style of blowout was typical,and there was a risk of forming large area of sandy land or partially broken sand dune landforms with blowouts.Therefore,corresponding measures for vegetation restoration should be adopted at the early stage of blowout to prevent further development of grassland desertification.（3）Under the alternation of wind from northwest,west and southwest,the blowout extended along the main wind direction.The relative relationship between the direction of the air flow and the long axis of the blowout affected the erosion characteristics in the blowout,and the longitudinal air flow into the blowout and eroded the blowout bottom to make it deeper,the oblique air flowed to the inner wall of the blowout and the pitting of the bottom of the blowout.Therefore,the blowout had deepened and extended longitudinally and expanded along the lateral direction.The rear edge of blowout was eroded by the wind-sand flow,and the blowout extended along the main wind direction.The sand was deposited on the grassland in the rear of the blowout from the blowout,spreading fan-shaped to the southeast,east and northeast.Saucer blowouts were constantly eroded by the strong air flow in the air inlet,inner wall and bottom of the blowout.Wind erosion gradually extended along the wind direction and eventually developed into a trough blowout.More than two adjacent saucer and trough blowouts were connected and combined into composite blowouts in the process of the blowout expansion.With the increasing area of the blowout,the shape characteristics of the blowout were connected from saucer shape or trough shape,then to make a compound form.According to the morphological change of the blowouts,the growth rate of blowout length was the fastest,followed by the width of blowout,and the growth rate of blowout depth was the slowest.（4）Wind profiles in the blowout,active development,fixed stage and reactivation stage basically followed the logarithmic distribution rule,while the air flow showed"S"distribution disturbed by the underlying surface in the grassland and the extinction stage of blowouts.The optimal fitting model between sand flux and height was the negative exponential model in each development stage of blowouts.The sand flux was significantly different in every development stages of the blowouts.More than 95%of the sand flux was on the surface of 0-30 cm,and 63.97%-90.96%of the sand flux concentrated in the 10cm height from the surface.The saltation height of wind-sand flow was positively correlated with wind speed,and their order from big to small was:Active development>Reactivation stage>Sand patch>Fixed stage>Extinction stage>Grassland.The sand fluxes coefficient of wind sand flow was analyzed,it was concluded that the sand flux tends to move towards high level in the stage of sand patch,active development and reactivation,while in other stages,the sand flux was mostly concentrated in the near ground layer because of the high vegetation coverage.（5）The surface soil particles at each stage of blowout were mainly sand,and the content of fine sand and medium sand was the most.Sand concentration distribution from poor to good order:Grassland<Decaying stage<Fixing stage<The patch stage<Active development stage<Activation stage,according to the classification standard of kurtosis was medium range.The fractal dimension of soil particle was as follows:Grassland﹥The patch stage﹥Decaying stage﹥Fixing stage﹥Activation stage﹥Active development stage.The grain size composition of sediments was uniform in the study area,and the distribution curve of the surface of blowouts basically consistent,and the curve was biased towards the side of coarse sands with the increase of wind erosion intensity.The intensity of wind erosion activity and the increase of vegetation destruction caused the surface sand coarsening in blowouts.With the continuous development and evolution of the blowout,the soil bulk density decreased,soil compaction decreased,soil moisture content was less affected by desertification,and it might be affected by other factors such as vegetation.Soil organic matter and other nutrients decreased with the coarsening of soil mechanical composition.The comprehensive evaluation of soil quality was obtained.The comprehensive nutrient quality of the grassland was the best,while the blowout was the worst in the active development stage.